



Internet Engineering Task Force (IETF)                     D. K. Gillmor
Request for Comments: 9788                American Civil Liberties Union
Updates: 8551                                               B. Hoeneisen
Category: Standards Track                                    pEp Project
ISSN: 2070-1721                                              A. Melnikov
                                                               Isode Ltd
                                                             August 2025


        Header Protection for Cryptographically Protected Email

Abstract

   S/MIME version 3.1 introduced a mechanism to provide end-to-end
   cryptographic protection of email message headers.  However, few
   implementations generate messages using this mechanism, and several
   legacy implementations have revealed rendering or security issues
   when handling such a message.

   This document updates the S/MIME specification (RFC 8551) to offer a
   different mechanism that provides the same cryptographic protections
   but with fewer downsides when handled by legacy clients.
   Furthermore, it offers more explicit usability, privacy, and security
   guidance for clients when generating or handling email messages with
   cryptographic protection of message headers.

   The Header Protection scheme defined here is also applicable to
   messages with PGP/MIME (Pretty Good Privacy with MIME) cryptographic
   protections.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9788.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

Table of Contents

   1.  Introduction
     1.1.  Update to RFC 8551
       1.1.1.  Problems with RFC 8551 Header Protection
     1.2.  Risks of Header Protection for Legacy MUA Recipients
     1.3.  Motivation
       1.3.1.  Backward Compatibility
       1.3.2.  Deliverability
     1.4.  Other Protocols to Protect Email Header Fields
     1.5.  Applicability to PGP/MIME
     1.6.  Requirements Language
     1.7.  Terms
     1.8.  Document Scope
       1.8.1.  In Scope
       1.8.2.  Out of Scope
     1.9.  Example
   2.  Internet Message Format Extensions
     2.1.  Content-Type Parameters
       2.1.1.  Content-Type Parameter: hp
       2.1.2.  Content-Type Parameter: hp-legacy-display
     2.2.  HP-Outer Header Field
       2.2.1.  HP-Outer Header Field Definition
   3.  Header Confidentiality Policy
     3.1.  HCP Definition
       3.1.1.  HCP Avoids Changing addr-spec of From Header Field
     3.2.  Initial Registered HCPs
       3.2.1.  Baseline Header Confidentiality Policy
       3.2.2.  Shy Header Confidentiality Policy
       3.2.3.  No Header Confidentiality Policy
     3.3.  Default Header Confidentiality Policy
     3.4.  HCP Evolution
       3.4.1.  Offering More Ambitious Header Confidentiality
       3.4.2.  Expert Guidance for Registering Header Confidentiality
               Policies
   4.  Rendering Guidance (Receiving Side)
     4.1.  Identifying That a Message Has Header Protection
     4.2.  Extracting Protected Header Fields From an Encrypted
            Message
       4.2.1.  HeaderSetsFromMessage
     4.3.  Updating the Cryptographic Summary
       4.3.1.  HeaderFieldProtection
     4.4.  Handling Mismatch of From Header Fields
       4.4.1.  Definitions
       4.4.2.  Warning for From Header Field Mismatch
       4.4.3.  From Header Field Rendering
       4.4.4.  Handling the Protected From Header Field When
               Responding
       4.4.5.  Matching addr-specs
     4.5.  Rendering a Message with Header Protection
       4.5.1.  Example Signed-Only Message
       4.5.2.  Example Signed-and-Encrypted Message
       4.5.3.  Do Not Render Legacy Display Elements
     4.6.  Implicitly Rendered Header Fields
     4.7.  Handling Undecryptable Messages
     4.8.  Guidance for Automated Message Handling
       4.8.1.  Only Interpret Protected Header Fields
       4.8.2.  Ignore Legacy Display Elements
     4.9.  Affordances for Debugging and Troubleshooting
     4.10. Handling RFC8551HP Messages (Backward Compatibility)
       4.10.1.  Identifying an RFC8551HP Message
       4.10.2.  Rendering or Responding to an RFC8551HP Message
     4.11. Rendering Other Schemes
   5.  Composing Guidance (Sending Side)
     5.1.  Composing a Cryptographically Protected Message Without
           Header Protection
       5.1.1.  ComposeNoHeaderProtection
     5.2.  Composing a Message with Header Protection
       5.2.1.  Compose
       5.2.2.  Adding a Legacy Display Element to a text/plain Part
       5.2.3.  Adding a Legacy Display Element to a text/html Part
       5.2.4.  Only Add a Legacy Display Element to Main Body Parts
       5.2.5.  Do Not Add a Legacy Display Element to Other
               Content-Types
   6.  Replying and Forwarding Guidance
     6.1.  Avoid Leaking Encrypted Header Fields in Replies and
           Forwards
       6.1.1.  The Respond Function
       6.1.2.  ReferenceHCP
     6.2.  Avoid Misdirected Replies
   7.  Unprotected Header Fields Added in Transit
     7.1.  Mailing List Header Fields: List-* and Archived-At
   8.  Email Ecosystem Evolution
     8.1.  Dropping Legacy Display Elements
     8.2.  More Ambitious Default HCP
     8.3.  Deprecation of Messages Without Header Protection
   9.  Usability Considerations
     9.1.  Mixed Protections Within a Message Are Hard to Understand
     9.2.  Users Should Not Have to Choose a Header Confidentiality
           Policy
   10. Security Considerations
     10.1.  From Address Spoofing
       10.1.1.  From Rendering Reasoning
     10.2.  Avoid Cryptographic Summary Confusion from the hp
            Parameter
     10.3.  Caution About Composing with Legacy Display Elements
     10.4.  Plaintext Attacks
   11. Privacy Considerations
     11.1.  Leaks When Replying
     11.2.  Encrypted Header Fields Are Not Always Private
       11.2.1.  Encrypted Header Fields Can Leak Unwanted Information
               to the Recipient
       11.2.2.  Encrypted Header Fields Can Be Inferred from External
               or Internal Metadata
       11.2.3.  Encrypted Header Fields May Not Be Fully Masked by HCP
     11.3.  A Naive Recipient May Overestimate the Cryptographic
            Status of a Header Field in an Encrypted Message
     11.4.  Privacy and Deliverability Risks with Bcc and Encrypted
            Messages
   12. IANA Considerations
     12.1.  Registration of the HP-Outer Header Field
     12.2.  Reference Update for the Content-Type Header Field
     12.3.  New Mail Header Confidentiality Policies Registry
   13. References
     13.1.  Normative References
     13.2.  Informative References
   Appendix A.  Table of Pseudocode Listings
   Appendix B.  Possible Problems with Legacy MUAs
     B.1.  Problems Viewing Messages in a List View
     B.2.  Problems When Rendering a Message
     B.3.  Problems When Replying to a Message
   Appendix C.  Test Vectors
     C.1.  Baseline Messages
       C.1.1.  No Cryptographic Protections over a Simple Message
       C.1.2.  S/MIME Signed-Only signedData over a Simple Message, No
               Header Protection
       C.1.3.  S/MIME Signed-Only multipart/signed over a Simple
               Message, No Header Protection
       C.1.4.  S/MIME Signed-and-Encrypted over a Simple Message, No
               Header Protection
       C.1.5.  No Cryptographic Protections over a Complex Message
       C.1.6.  S/MIME Signed-Only signedData over a Complex Message,
               No Header Protection
       C.1.7.  S/MIME Signed-Only multipart/signed over a Complex
               Message, No Header Protection
       C.1.8.  S/MIME Signed-and-Encrypted over a Complex Message, No
               Header Protection
     C.2.  Signed-Only Messages
       C.2.1.  S/MIME Signed-Only signedData over a Simple Message,
               Header Protection
       C.2.2.  S/MIME Signed-Only multipart/signed over a Simple
               Message, Header Protection
       C.2.3.  S/MIME Signed-Only signedData over a Complex Message,
               Header Protection
       C.2.4.  S/MIME Signed-Only multipart/signed over a Complex
               Message, Header Protection
       C.2.5.  S/MIME Signed-Only signedData over a Complex Message,
               Legacy RFC 8551 Header Protection
       C.2.6.  S/MIME Signed-Only multipart/signed over a Complex
               Message, Legacy RFC 8551 Header Protection
     C.3.  Signed-and-Encrypted Messages
       C.3.1.  S/MIME Signed-and-Encrypted over a Simple Message,
               Header Protection with hcp_baseline
       C.3.2.  S/MIME Signed-and-Encrypted over a Simple Message,
               Header Protection with hcp_baseline (+ Legacy Display)
       C.3.3.  S/MIME Signed-and-Encrypted over a Simple Message,
               Header Protection with hcp_shy
       C.3.4.  S/MIME Signed-and-Encrypted over a Simple Message,
               Header Protection with hcp_shy (+ Legacy Display)
       C.3.5.  S/MIME Signed-and-Encrypted Reply over a Simple
               Message, Header Protection with hcp_baseline
       C.3.6.  S/MIME Signed-and-Encrypted Reply over a Simple
               Message, Header Protection with hcp_baseline (+ Legacy
               Display)
       C.3.7.  S/MIME Signed-and-Encrypted Reply over a Simple
               Message, Header Protection with hcp_shy
       C.3.8.  S/MIME Signed-and-Encrypted Reply over a Simple
               Message, Header Protection with hcp_shy (+ Legacy
               Display)
       C.3.9.  S/MIME Signed-and-Encrypted over a Complex Message,
               Header Protection with hcp_baseline
       C.3.10. S/MIME Signed-and-Encrypted over a Complex Message,
               Header Protection with hcp_baseline (+ Legacy Display)
       C.3.11. S/MIME Signed-and-Encrypted over a Complex Message,
               Header Protection with hcp_shy
       C.3.12. S/MIME Signed-and-Encrypted over a Complex Message,
               Header Protection with hcp_shy (+ Legacy Display)
       C.3.13. S/MIME Signed-and-Encrypted Reply over a Complex
               Message, Header Protection with hcp_baseline
       C.3.14. S/MIME Signed-and-Encrypted Reply over a Complex
               Message, Header Protection with hcp_baseline (+ Legacy
               Display)
       C.3.15. S/MIME Signed-and-Encrypted Reply over a Complex
               Message, Header Protection with hcp_shy
       C.3.16. S/MIME Signed-and-Encrypted Reply over a Complex
               Message, Header Protection with hcp_shy (+ Legacy
               Display)
       C.3.17. S/MIME Signed-and-Encrypted over a Complex Message,
               Legacy RFC 8551 Header Protection with hcp_baseline
   Appendix D.  Composition Examples
     D.1.  New Message Composition
       D.1.1.  Unprotected Message
       D.1.2.  Encrypted with hcp_baseline and Legacy Display
     D.2.  Composing a Reply
       D.2.1.  Unprotected Message
       D.2.2.  Encrypted with hcp_no_confidentiality and Legacy
               Display
   Appendix E.  Rendering Examples
     E.1.  Example text/plain Cryptographic Payload with Legacy
           Display Elements
     E.2.  Example text/html Cryptographic Payload with Legacy Display
           Elements
   Appendix F.  Other Header Protection Schemes
     F.1.  Original RFC 8551 Header Protection
     F.2.  Pretty Easy Privacy (pEp)
     F.3.  "draft-autocrypt" Protected Headers
   Acknowledgements
   Authors' Addresses

1.  Introduction

   Privacy and security issues regarding email Header Protection in S/
   MIME and PGP/MIME have been identified for some time.  Most current
   implementations of cryptographically protected email protect only the
   Body of the message, which leaves significant room for attacks
   against otherwise-protected messages.  For example, lack of Header
   Protection allows an attacker to substitute the message subject and/
   or author.

   This document describes how to cryptographically protect message
   headers and provides guidance for the implementer of a Mail User
   Agent (MUA) that generates, interprets, and replies to such a
   message.  It uses the term "Legacy MUA" to refer to an MUA that does
   not implement this specification.  This document takes particular
   care to ensure that messages interact reasonably well with Legacy
   MUAs.

1.1.  Update to RFC 8551

   An older scheme for Header Protection was specified in S/MIME 3.1
   [RFC8551], which involves wrapping a message/rfc822 MIME object with
   a Cryptographic Envelope around the message to protect it.  This
   document refers to that scheme as "RFC 8551 Header Protection", or
   "RFC8551HP".  Substantial testing has shown that RFC8551HP does not
   interact well with some Legacy MUAs (see Section 1.1.1).

   This specification supersedes RFC8551HP, effectively replacing the
   final two paragraphs of Section 3.1 of [RFC8551].

   In this specification, all Header Fields gain end-to-end
   cryptographic integrity and authenticity by being copied directly
   into the Cryptographic Payload without using an intervening
   message/rfc822 MIME object.  In an encrypted message, some Header
   Fields can also be made confidential by removing or obscuring them
   from the Outer Header Section.

   This specification also offers substantial security, privacy, and
   usability guidance for composing and rendering MUAs that was not
   considered in [RFC8551].

1.1.1.  Problems with RFC 8551 Header Protection

   Several Legacy MUAs have difficulty rendering a message that uses
   RFC8551HP.  These problems can appear on signed-only messages, as
   well as signed-and-encrypted messages.

   In some cases, some MUAs cannot render message/rfc822 message
   subparts at all, which is in violation of baseline MIME requirements
   as defined in requirement 6 of Section 2 of [RFC2049].  A message
   using RFC8551HP is unreadable by any recipient using such an MUA.

   In other cases, the user sees an attachment suggesting a forwarded
   email message that -- in fact -- contains the protected email message
   that should be rendered directly.  In most of these cases, the user
   can click on the attachment to view the protected message.

   However, viewing the protected message as an attachment in isolation
   may strip it of any security indications, leaving the user unable to
   assess the cryptographic properties of the message.  Worse, for
   encrypted messages, interacting with the protected message in
   isolation may leak contents of the cleartext, for example, if the
   reply is not also encrypted.

   Furthermore, RFC8551HP lacks any discussion of the following points,
   all of which are provided in this specification:

   *  Which Header Fields should be given end-to-end cryptographic
      integrity and authenticity protections (this specification
      mandates protection of all Header Fields that the composing MUA
      knows about).

   *  How to securely indicate the composer's intent to offer Header
      Protection and encryption, which lets a rendering MUA detect
      messages whose cryptographic properties may have been modified in
      transit (see Section 2.1.1).

   *  Which Header Fields should be given end-to-end cryptographic
      confidentiality protections in an encrypted message and how (see
      Section 3).

   *  How to securely indicate the composer's choices about which Header
      Fields were made confidential, which lets a rendering MUA reply or
      forward an encrypted message safely without accidentally leaking
      confidential material (see Section 2.2).

   These stumbling blocks with Legacy MUAs, missing mechanisms, and
   missing guidance create a strong disincentive for existing MUAs to
   generate messages using RFC8551HP.  Because few messages have been
   produced, there has been little incentive for those MUAs capable of
   upgrading to bother interpreting them better.

   In contrast, the mechanisms defined here are safe to adopt and
   produce messages with very few problems for Legacy MUAs.  And
   Section 4.10 provides useful guidance for rendering and replying to
   RFC8551HP messages.

1.2.  Risks of Header Protection for Legacy MUA Recipients

   Producing a signed-only message using this specification has no
   additional risks (compared to producing a signed-only message without
   Header Protection).  Such a message will render in the same way on
   any Legacy MUA as a Legacy Signed Message (that is, a signed message
   without Header Protection).  An MUA conformant to this specification
   that encounters such a message will be able to gain the benefits of
   end-to-end cryptographic integrity and authenticity for all Header
   Fields.

   An encrypted message produced according to this specification that
   has some User-Facing Header Fields removed or obscured may not render
   as desired in a Legacy MUA.  In particular, those Header Fields that
   were made confidential will not be visible to the user of a Legacy
   MUA.  For example, if the Subject Header Field outside the
   Cryptographic Envelope is replaced with [...], a Legacy MUA will
   render the [...] anywhere the Subject is normally seen.  This is the
   only additional risk of producing an encrypted message according to
   this specification (compared to producing an encrypted message
   without confidentiality for any Header Field).

   A workaround "Legacy Display" mechanism is provided in this
   specification (see Section 2.1.2).  Legacy MUAs will render "Legacy
   Display Elements" to the user, albeit not in the same location that
   the Header Fields would normally be rendered.

   Alternately, if the composer of an encrypted message is particularly
   concerned about the experience of a recipient using a Legacy MUA, and
   they are willing to accept leaking the User-Facing Header Fields,
   they can simply adopt the No Header Confidentiality Policy (see
   Section 3.2.3).  A signed-and-encrypted message composed using the No
   Header Confidentiality Policy offers no usability risk for a reader
   using a Legacy MUA and retains end-to-end cryptographic integrity and
   authenticity properties for all Header Fields for any reader using a
   conformant MUA.  Of course, such a message has the same (non-
   existent) confidentiality properties for all Header Fields as a
   Legacy Encrypted Message (that is, an encrypted message made without
   Header Protection).

1.3.  Motivation

   Ordinary Users generally do not understand the distinction between
   email message Body and Header Section.  When an email message has
   cryptographic protections that cover the message Body but not the
   Header Fields, several attacks become possible.

   For example, a Legacy Signed Message has a signature that covers the
   Body but not the Header Fields.  An attacker can therefore modify the
   Header Fields (including Subject) without invalidating the signature.
   Since most readers consider a message Body in the context of the
   message's Subject, the meaning of the message itself could change
   drastically (under the attacker's control) while still retaining the
   same cryptographic indicators of integrity and authenticity.

   In another example, a Legacy Encrypted Message has its Body
   effectively hidden from an adversary that snoops on the message.  But
   if the Header Fields are not also encrypted, significant information
   about the message (such as the message Subject) will leak to the
   inspecting adversary.

   However, if the composing and rendering MUAs ensure that
   cryptographic protections cover the message Header Section as well as
   the message Body, these attacks are defeated.

1.3.1.  Backward Compatibility

   If the composing MUA is unwilling to generate such a fully protected
   message due to the potential for rendering, usability,
   deliverability, or security issues, these defenses cannot be
   realized.

   The composer cannot know what MUA (or MUAs) the recipient will use to
   handle the message.  Thus, an outbound message format that is
   backward compatible with as many legacy implementations as possible
   is a more effective vehicle for providing the whole-message
   cryptographic protections described above.

   This document aims for backward compatibility with Legacy MUAs to the
   extent possible.  In some cases, like when a user-visible Header
   Field like the Subject is cryptographically hidden, a Legacy MUA will
   not be able to render or reply to the message exactly the same way as
   a conformant MUA would.  But accommodations are described here (in
   particular, Section 2.1.2) that ensure a rough semantic equivalence
   for a Legacy MUA even in these cases.

1.3.2.  Deliverability

   A message with perfect cryptographic protections that cannot be
   delivered is less useful than a message with imperfect cryptographic
   protections that can be delivered.  Senders want their messages to
   reach the intended recipients.

   Given the current state of the Internet mail ecosystem, encrypted
   messages in particular cannot shield all of their Header Fields from
   visibility and still be guaranteed delivery to their intended
   recipient.

   This document accounts for this concern by providing a mechanism
   (Section 3) that prioritizes initial deliverability (at the cost of
   some header leakage) while facilitating future message variants that
   shield more header metadata from casual inspection.

1.4.  Other Protocols to Protect Email Header Fields

   A separate pair of protocols also provides some cryptographic
   protection for the email message header integrity: DomainKeys
   Identified Mail (DKIM) [RFC6376], as used in combination with Domain-
   based Message Authentication, Reporting, and Conformance (DMARC)
   [RFC7489].  This pair of protocols provides a domain-based reputation
   mechanism that can be used to mitigate some forms of unsolicited
   email (spam).

   However, the DKIM+DMARC suite provides cryptographic protection at a
   different scope, as it is usually applied by and evaluated by a mail
   transport agent (MTA).  DKIM+DMARC typically provide MTA-to-MTA
   protection, whereas this specification provides MUA-to-MUA
   protection.  This is because DKIM+DMARC are typically applied to
   messages by (and interpreted by) MTAs, whereas the mechanisms in this
   document are typically applied and interpreted by MUAs.

   A rendering MUA that relies on DKIM+DMARC for sender authenticity
   should note Section 10.1.

   Furthermore, the DKIM+DMARC suite only provides cryptographic
   integrity and authentication, not encryption.  So cryptographic
   confidentiality is not available from that suite.

   The DKIM+DMARC suite can be used on any message, including messages
   formed as defined in this document.  There should be no conflict
   between DKIM+DMARC and the specification here.

   Though not strictly email, similar protections have been in use on
   Usenet for the signing and verification of message Header Fields for
   years.  See [PGPCONTROL] and [PGPVERIFY-FORMAT] for more details.
   Like DKIM, these Usenet control protections offer only integrity and
   authentication, not confidentiality.

1.5.  Applicability to PGP/MIME

   This document specifies end-to-end cryptographic protections for
   email messages in reference to S/MIME [RFC8551].

   Comparable end-to-end cryptographic protections can also be provided
   by PGP/MIME [RFC3156].

   The mechanisms in this document should be applicable in the PGP/MIME
   protections as well as S/MIME protections, but analysis and
   implementation in this document focuses on S/MIME.

   To the extent that any divergence from the mechanism defined here is
   necessary for PGP/MIME, that divergence is out of scope for this
   document.

1.6.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

1.7.  Terms

   The following terms are defined for the scope of this document:

   S/MIME:  Secure/Multipurpose Internet Mail Extensions (see [RFC8551])

   PGP/MIME:  Pretty Good Privacy with MIME (see [RFC3156])

   Message:  An email message consisting of Header Fields (collectively
      called "the Header Section of the message") optionally followed by
      a message Body; see [RFC5322].

   Header Field:  A Header Field includes a field name, followed by a
      colon (":"), followed by a field Body (value), and is terminated
      by CRLF; see Section 2.2 of [RFC5322] for more details.

   Header Section:  The Header Section is a sequence of lines of
      characters with special syntax as defined in [RFC5322].  The
      Header Section of a message contains the Header Fields associated
      with the message itself.  The Header Section of a MIME part (that
      is, a subpart of a message) typically contains Header Fields
      associated with that particular MIME part.

   Outer Header Section:  The unprotected Header Section that MTAs and
      MUAs unaware of Header Protection treat as the Header Section of
      the Message.

   Inner Header Section:  The Header Section at the root of the
      Cryptographic Payload.  An MUA that implements Header Protection
      renders Header Fields from this section for the user.

   Body:  The Body is the part of a message that follows the Header
      Section and is separated from the Header Section by an empty line
      (that is, a line with nothing preceding the CRLF); see [RFC5322].
      It is the (bottom) section of a message containing the payload of
      a message.  Typically, the Body consists of a (possibly multipart)
      MIME [RFC2045] construct.

   Header Protection (HP):  The cryptographic protection of email Header
      Sections (or parts of it) by means of signatures and/or
      encryption.

   Legacy MUA:  An MUA that does not understand Header Protection as
      defined in this document.  A Legacy Non-Crypto MUA is incapable of
      doing any end-to-end cryptographic operations.  A Legacy Crypto
      MUA is capable of doing cryptographic operations but does not
      understand or generate messages with Header Protection.

   Legacy Signed Message:  An email message that was signed by a Legacy
      MUA and therefore has no cryptographic authenticity or integrity
      protections on its Header Fields.

   Legacy Encrypted Message:  An email message that was signed and
      encrypted by a Legacy MUA and therefore has no cryptographic
      authenticity, integrity, or confidentiality protections on any of
      its Header Fields.

   Header Confidentiality Policy (HCP):  A functional specification of
      which Header Fields should be removed or obscured when composing
      an encrypted message with Header Protection.  An HCP is considered
      more "conservative" when it removes or obscures fewer Header
      Fields.  When it removes or obscures more Header Fields, it is
      more "ambitious".  See Section 3.

   Ordinary User:  A user of an MUA who follows a simple and minimal
      experience, focused on sending and receiving emails.  A user who
      opts into advanced configuration, expert mode, or the like is not
      an "Ordinary User".

   Respond Function:  A function found in most MUAs that defines how to
      pre-populate the Header Fields of a new message in response to
      another message.  See Section 6.1.1.

   Additionally, Cryptographic Layer, Cryptographic Payload,
   Cryptographic Envelope, Cryptographic Summary, Structural Header
   Fields, Non-Structural Header Fields, Main Body Part, User-Facing
   Header Fields, and MUA are all used as defined in [RFC9787].

   The policies "Specification Required" and "IETF Review" that appear
   in this document when used to describe namespace allocation are to be
   interpreted as described in [RFC8126].

   Note: To avoid ambiguity, this document avoids using the terms
   "Header" or "Headers" in isolation, but instead always uses "Header
   Field" to refer to the individual field and "Header Section" to refer
   to the entire collection.

1.8.  Document Scope

   This document describes sensible, simple behavior for a program that
   generates an email message with standard end-to-end cryptographic
   protections, following the guidance in [RFC9787].  An implementation
   conformant to this document will produce messages that have
   cryptographic protection that covers the message's Header Fields as
   well as its Body.

1.8.1.  In Scope

   This document also describes sensible, simple behavior for a program
   that interprets such a message in a way that can take advantage of
   these protections covering the Header Fields as well as the Body.

   The message generation guidance aims to minimize negative
   interactions with any Legacy rendering MUA while providing actionable
   cryptographic properties for modern rendering MUAs.

   In particular, this document focuses on two standard types of
   cryptographic protection that cover the entire message:

   *  a cleartext message with a single signature and

   *  an encrypted message that contains a single cryptographic
      signature.

1.8.2.  Out of Scope

   The message composition guidance in this document (in Section 5.2)
   aims to provide minimal disruption for any Legacy MUA that renders
   such a message.  However, by definition, a Legacy MUA does not
   implement any of the guidance here.  Therefore, the document does not
   attempt to provide guidance for Legacy MUAs directly.

   Furthermore, this document does not explicitly contemplate other
   variants of cryptographic message protections, including any of
   these:

   *  encrypted-only message (without a cryptographic signature; see
      Section 5.3 of [RFC9787])

   *  triple-wrapped message

   *  signed message with multiple signatures

   *  encrypted message with a cryptographic signature outside the
      encryption

   All such messages are out of scope of this document.

1.9.  Example

   This section provides an example of MIME messages with Header
   Protection.

   Consider the following MIME message:

   A └┬╴application/pkcs7-mime; smime-type="enveloped-data"
      ╧ (decrypts to)
   B  └┬╴application/pkcs7-mime; smime-type="signed-data"
       ┴ (unwraps to)
   C   └┬╴multipart/alternative; hp="cipher"
   D    ├─╴text/plain; hp-legacy-display="1"
   E    └─╴text/html; hp-legacy-display="1"

   Observe that:

   *  Nodes A and B are collectively called the Cryptographic Envelope.
      Node C (including its subnodes D and E) is called the
      Cryptographic Payload [RFC9787].

   *  Node A contains the (unprotected) outer Header Fields.  Node C
      contains the (protected) inner Header Fields.

   *  The presence of the hp attribute (see Section 2.1.1) on the
      Content-Type of node C allows the renderer to know that the
      composer applied Header Protection.  Its value allows the renderer
      to distinguish whether the composer intended for the message to be
      confidential (hp="cipher") or not (hp="clear"), since encryption
      may have been added in transit (see Section 10.2).

   The Outer Header Section on node A looks as follows:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: [...]
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: application/pkcs7-mime; smime-type="enveloped-data"
   MIME-Version: 1.0

   The Inner Header Section on node C looks as follows:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: Handling the Jones contract
   Keywords: Contract, Urgent
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: multipart/alternative; hp="cipher"
   MIME-Version: 1.0
   HP-Outer: Date: Wed, 11 Jan 2023 16:08:43 -0500
   HP-Outer: From: Bob <bob@example.net>
   HP-Outer: To: Alice <alice@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <20230111T210843Z.1234@lhp.example>

   Observe that:

   *  Between node C and node A, some Header Fields are copied as is
      (Date, From, To, Message-ID), some are obscured (Subject), and
      some are removed (Keywords).

   *  The HP-Outer Header Fields (see Section 2.2) of node C contain a
      protected copy of the Header Fields in node A.  The copy allows
      the renderer to recompute for which Header Fields the composer
      provided confidentiality by removing or obscuring them.

   *  The copying/removing/obscuring and the HP-Outer only apply to Non-
      Structural Header Fields, not to Structural Header Fields like
      Content-Type or MIME-Version (see Section 1.1.1 of [RFC9787]).

   *  If the composer intends no confidentiality and doesn't encrypt the
      message, it doesn't remove or obscure Header Fields.  All Non-
      Structural Header Fields are copied as is.  No HP-Outer Header
      Fields are present.

   Node D looks as follows:

   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";

   Subject: Handling the Jones contract
   Keywords: Contract, Urgent

   Please review and approve or decline by Thursday, it's critical!

   Thanks,
   Bob

   --
   Bob Gonzalez
   ACME, Inc.

   Observe that:

   *  The composer adds the removed and obscured User-Facing Header
      Fields (see Section 1.1.2 of [RFC9787]) to the main Body (note the
      empty line after the Content-Type).  This is called the Legacy
      Display Element.  It allows a user with a Legacy MUA that doesn't
      implement this document to understand the message, since the
      Header Fields will be shown as part of the main Body.

   *  The hp-legacy-display="1" attribute (see Section 2.1.2) indicates
      that the composer added a Legacy Display Element.  This allows
      renderers that implement this document to recognize the Legacy
      Display Element and distinguish it from user-added content.  The
      renderer then hides the Legacy Display Element and doesn't display
      it to the user.

   *  hp-legacy-display is added to the node to which it applies, not on
      any outer nodes (e.g., not to node C).

   For more examples, see Appendices D and E.

2.  Internet Message Format Extensions

   This section describes relevant, backward-compatible extensions to
   the Internet Message Format [RFC5322].  Subsequent sections offer
   concrete guidance for an MUA to make use of these mechanisms,
   including policy decisions and recommended pseudocode.

2.1.  Content-Type Parameters

   This document introduces two parameters for the Content-Type Header
   Field, which have distinct semantics and use cases.

2.1.1.  Content-Type Parameter: hp

   This specification defines a parameter for the Content-Type Header
   Field named hp (for Header Protection).  This parameter is only
   relevant on the Content-Type Header Field at the root of the
   Cryptographic Payload.  The presence of this parameter at the root of
   the Cryptographic Payload indicates that the composer intends for
   this message to have end-to-end cryptographic protections for the
   Header Fields.

   The parameter's defined values describe the composer's cryptographic
   intent when producing the message:

   +========+==============+=========+=================+===============+
   |hp Value| Authenticity |Integrity| Confidentiality | Description   |
   +========+==============+=========+=================+===============+
   |"clear" | yes          |yes      | no              | This          |
   |        |              |         |                 | message has   |
   |        |              |         |                 | been signed   |
   |        |              |         |                 | by the        |
   |        |              |         |                 | composer,     |
   |        |              |         |                 | with Header   |
   |        |              |         |                 | Protection.   |
   +--------+--------------+---------+-----------------+---------------+
   |"cipher"| yes          |yes      | yes             | This          |
   |        |              |         |                 | message has   |
   |        |              |         |                 | been signed   |
   |        |              |         |                 | by the        |
   |        |              |         |                 | composer,     |
   |        |              |         |                 | with Header   |
   |        |              |         |                 | Protection,   |
   |        |              |         |                 | and is        |
   |        |              |         |                 | encrypted     |
   |        |              |         |                 | to the        |
   |        |              |         |                 | recipients.   |
   +--------+--------------+---------+-----------------+---------------+

            Table 1: hp Parameter for Content-Type Header Field

   A composing implementation MUST NOT produce a Cryptographic Payload
   with parameter hp="cipher" for an unencrypted message (that is, where
   none of the Cryptographic Layers in the Cryptographic Envelope of the
   message provide encryption).  Likewise, if a composing implementation
   is constructing an encrypted message with Header Protection, it MUST
   emit an hp="cipher" parameter, regardless of which Header Fields were
   made confidential.

   Note that hp="cipher" indicates that the message itself has been
   encrypted by the composer to the recipients but makes no assertions
   about which Header Fields have been removed or obscured.  This can be
   derived from the Cryptographic Payload itself (see Section 4.2).

   A rendering implementation MUST NOT mistake the presence of an
   hp="cipher" parameter in the Cryptographic Payload for the actual
   presence of a Cryptographic Layer that provides encryption.

2.1.2.  Content-Type Parameter: hp-legacy-display

   This specification also defines an hp-legacy-display parameter for
   the Content-Type Header Field.  The only defined value for this
   parameter is 1.

   This parameter is only relevant on a leaf MIME node of Content-Type
   text/html or text/plain within a well-formed message with end-to-end
   cryptographic protections.  Its presence indicates that the MIME node
   it is attached to contains a decorative "Legacy Display Element".
   The Legacy Display Element itself is used for backward-compatible
   visibility of any removed or obscured User-Facing Header Field in a
   Legacy MUA.

   Such a Legacy Display Element need not be rendered to the user of an
   MUA that implements this specification, because the MUA already knows
   the correct Header Field information and can render it to the user in
   the appropriate part of the MUA's user interface rather than in the
   Body of the message.

   See Section 5.2.2 for how to insert a Legacy Display Element into a
   text/plain Main Body Part.  See Section 5.2.3 for how to insert a
   Legacy Display Element into a text/html Main Body Part.  See
   Section 4.5.3 for how to avoid rendering a Legacy Display Element.

2.2.  HP-Outer Header Field

   This document also specifies a new Header Field: HP-Outer.

   This Header Field is used only in the Header Section of the
   Cryptographic Payload of an encrypted message.  It is not relevant
   for signed-only messages.  It documents, with the same cryptographic
   guarantees shared by the rest of the message, the composer's choices
   about Header Field confidentiality.  It does so by embedding a copy
   within the Cryptographic Envelope of every Non-Structural Header
   Field that the composer put outside the Cryptographic Envelope.  This
   Header Field enables the MUA rendering the encrypted message to
   reliably identify whether the composing MUA intended to make a Header
   Field confidential (see also Section 11.3).

   The HP-Outer Header Fields in a message's Cryptographic Payload are
   useful for ensuring that any confidential Header Field will not be
   automatically leaked in the clear if the user replies to or forwards
   the message.  They may also be useful for an MUA that indicates the
   confidentiality status of any given Header Field to the user.

   An implementation that composes encrypted email MUST include a copy
   of all Non-Structural Header Fields deliberately exposed to the
   outside of the Cryptographic Envelope using a series of HP-Outer
   Header Fields within the Cryptographic Payload.  These HP-Outer MIME
   Header Fields should only ever appear directly within the Header
   Section of the Cryptographic Payload of a Cryptographic Envelope
   offering confidentiality.  They MUST be ignored for the purposes of
   evaluating the message's Header Protection if they appear in other
   places.

   Each instance of HP-Outer contains a Non-Structural Header Field name
   and the value that this Header Field was set to within the
   (unprotected) Outer Header Section.  The HP-Outer Header Field can
   appear multiple times in the Header Section of a Cryptographic
   Payload.

   If a Non-Structural Header Field named Z is present in Header
   Section of the Cryptographic Payload but doesn't appear in an HP-
   Outer Header Field value at all, then the composer is effectively
   asserting that every instance of Z was made confidential by removal
   from the Outer Header Section.  Specifically, it means that no Header
   Field Z was included on the outside of the message's Cryptographic
   Envelope by the composer at the time the message was injected into
   the mail system.

   See Section 5.2 for how to insert HP-Outer Header Fields into an
   encrypted message.  See Section 4.3 for how to determine the end-to-
   end confidentiality of a given Header Field from an encrypted message
   with Header Protection using HP-Outer.  See Section 6.1 for how an
   MUA can safely reply to (or forward) an encrypted message without
   leaking confidential Header Fields by default.

2.2.1.  HP-Outer Header Field Definition

   The syntax of this Header Field is defined using the following ABNF
   [RFC5234], where field-name, WSP, VCHAR, and FWS are defined in
   [RFC5322]:

   hp-outer     =   "HP-Outer:" [FWS] field-name ": "
                       hp-outer-value CRLF

   hp-outer-value  =   (*([FWS] VCHAR) *WSP)

   Note that hp-outer-value is the same as unstructured from
   Section 3.2.5 of [RFC5322] but without the obsolete obs-unstruct
   option.

3.  Header Confidentiality Policy

   An MUA composing an encrypted message according to this specification
   may make any given Header Field confidential by removing it from the
   Header Section outside the Cryptographic Envelope or by obscuring it
   by rewriting it to a different value in that Outer Header Section.
   The composing MUA faces a choice for any new message: Which Header
   Fields should be made confidential, and how?

   This section defines the "Header Confidentiality Policy" (or HCP) as
   a well-defined abstraction to encourage MUA developers to consider,
   document, and share reasonable policies across the community.  It
   establishes a registry of known HCPs, defines a small number of
   simple HCPs in that registry, and makes a recommendation for a
   reasonable default.

   Note that such a policy is only needed when the end-to-end
   protections include encryption (confidentiality).  No comparable
   policy is needed for other end-to-end cryptographic protections
   (integrity and authenticity), as they are simply uniformly applied so
   that all Header Fields known by the composer have these protections.

   This asymmetry is a consequence of complexities in existing message
   delivery systems, some of which may reject, drop, or delay messages
   where all Header Fields are removed from the top-level MIME object.

   Note that no representation of the HCP itself ever appears "on the
   wire".  However, the consumer of the encrypted message can see the
   decisions that were made by the composer's HCP via the HP-Outer
   Header Fields (see Section 2.2).

3.1.  HCP Definition

   In this document, we represent that HCP as a function hcp:

   *  hcp(name, val_in) -> val_out: This function takes a Non-Structural
      Header Field identified by name with the initial value val_in as
      arguments and returns a replacement Header Field value val_out.
      If val_out is the special value null, it means that the Header
      Field in question should be removed from the set of Header Fields
      visible outside the Cryptographic Envelope.

   In the pseudocode descriptions of various choices of HCP in this
   document, any comparison with the name input is done case-
   insensitively.  This is appropriate for Header Field names, as
   described in [RFC5322].

   Note that hcp is only applied to Non-Structural Header Fields.  When
   composing a message, Structural Header Fields are dealt with
   separately, as described in Section 5.2.

   As an example, an MUA that obscures the Subject Header Field by
   replacing it with the literal string "[...]", hides all Cc'ed
   recipients, and does not offer confidentiality to any other Header
   Fields would be represented as (in pseudocode):

   hcp_example_hide_cc(name, val_in) → val_out:
       if lower(name) is 'subject':
           return '[...]'
       else if lower(name) is 'cc':
           return null
       else:
           return val_in

   For alignment with common practice as well as the ABNF in
   Section 2.2.1 for HP-Outer, val_out MUST be one of the following:

   *  identical to val_in,

   *  the special value null (meaning that the Header Field will be
      removed from the outside of the message), or

   *  a sequence of printable 7-bit clean ASCII characters (of course,
      non-ASCII text can be encoded as ASCII using the encoded-word
      construct from [RFC2047]) and ASCII whitespace (specifically,
      space (0x20) and tab (0x09)).

   The HCP can compute val_out using any technique describable in
   pseudocode, such as copying a fixed string or invocations of other
   pseudocode functions.  If it alters the value, it MUST NOT include
   control or NUL characters in val_out. val_out SHOULD match the
   expected ABNF for the Header Field identified by name.

3.1.1.  HCP Avoids Changing addr-spec of From Header Field

   The From Header Field should also be treated specially by the HCP to
   enable defense against possible email address spoofing (see
   Section 10.1).  In particular, for hcp("From", val_in), the addr-spec
   of val_in and the addr-spec of val_out SHOULD match according to
   Section 4.4.5, unless the composing MUA has additional knowledge
   coordinated with the rendering MUA about more subtle addr-spec
   equivalence or certificate validity.

3.2.  Initial Registered HCPs

   This document formally defines three Header Confidentiality Policies
   with known and reasonably well-understood characteristics as a way to
   compare and contrast different possible behavioral choices for a
   composing MUA.  These definitions are not meant to preclude the
   creation of other HCPs.

   The purpose of the registry of HCPs is to facilitate HCP evolution
   and interoperability discussion among MUA developers and MTA
   operators.

   (The example hypothetical HCP, hcp_example_hide_cc, described in
   Section 3.1 above is deliberately not formally registered, as it has
   not been evaluated in practice.)

3.2.1.  Baseline Header Confidentiality Policy

   The most conservative recommended HCP only provides confidentiality
   for Informational Fields, as defined in Section 3.6.5 of [RFC5322].
   These fields are "only human-readable content" and thus their content
   should not be relevant to transport agents.  Since most Internet
   messages today do have a Subject Header Field, and some filtering
   engines might object to a message without a Subject, this policy is
   conservative and merely obscures that Header Field by replacing it
   with a fixed string [...].  By contrast, Comments and Keywords Header
   Fields are comparatively rare, so these fields are removed entirely
   from the Outer Header Section.

   hcp_baseline(name, val_in) → val_out:
       if lower(name) is 'subject':
           return '[...]'
       else if lower(name) is in ['comments', 'keywords']:
           return null
       else:
           return val_in

   hcp_baseline is the recommended default HCP, as it provides
   meaningful confidentiality protections and is unlikely to cause
   deliverability or usability problems.

3.2.2.  Shy Header Confidentiality Policy

   Alternately, a slightly more ambitious (and therefore more privacy-
   preserving) HCP might avoid leaking human-interpretable data that
   MTAs generally don't care about.  The additional protected data isn't
   related to message routing or transport but might reveal sensitive
   information about the composer or their relationship to the
   recipients.  This "shy" HCP builds on hcp_baseline but also:

   *  avoids revealing the display-name of each identified email address
      and

   *  avoids leaking the composer's locally configured time zone in the
      Date Header Field.

   hcp_shy(name, val_in) → val_out:
      if lower(name) is 'from':
         if val_in is an RFC 5322 mailbox:
            return the RFC 5322 addr-spec part of val_in
      if lower(name) in ['to', 'cc']:
         if val_in is an RFC 5322 mailbox-list:
            let val_out be an empty mailbox-list
            for each mailbox in val_in:
               append the RFC 5322 addr-spec part of mailbox to val_out
            return val_out
      if lower(name) is 'date':
         if val_in is an RFC 5322 date-time:
             return the UTC form of val_in
      else if lower(name) is 'subject':
         return '[...]'
      else if lower(name) is in ['comments', 'keywords']:
         return null
      return val_in

   hcp_shy requires more sophisticated parsing and Header Field
   manipulation and is not recommended as a default HCP.

3.2.3.  No Header Confidentiality Policy

   Legacy MUAs can be conceptualized as offering a "No Header
   Confidentiality" Policy, which offers no confidentiality protection
   to any Header Field:

   hcp_no_confidentiality(name, val_in) → val_out:
       return val_in

   A conformant MUA that is not modified by local policy or
   configuration MUST NOT use hcp_no_confidentiality by default.

3.3.  Default Header Confidentiality Policy

   An MUA MUST have a default HCP that offers confidentiality for the
   Subject Header Field at least.  Local policy and configuration may
   alter this default, but the MUA SHOULD NOT require the user to select
   an HCP.

   hcp_baseline provides confidentiality for the Subject Header Field by
   replacing it with the literal string "[...]".  It also provides
   confidentiality for the other less common Informational Header Fields
   (Comments and Keywords) by removing them entirely from the Outer
   Header Section.  This is a sensible default because most users treat
   the Informational Fields of a message (particularly the Subject) the
   same way that they treat the Body, and they are surprised to find
   that the Subject of an encrypted message is visible.

3.4.  HCP Evolution

   This document does not mandate any particular HCP, though it offers
   guidance for MUA implementers in selecting one in Section 3.3.
   Future documents may recommend or mandate such a policy for an MUA
   with specific needs.  Such a recommendation might be motivated by
   descriptions of metadata-derived attacks, stem from research about
   message deliverability, or describe new signaling mechanisms, but
   these topics are out of scope for this document.

3.4.1.  Offering More Ambitious Header Confidentiality

   An MUA MAY offer even more ambitious confidentiality for Header
   Fields of an encrypted message than defined in Section 3.2.2.  For
   example, it might implement an HCP that removes the To and Cc Header
   Fields entirely, relying on the SMTP envelope to ensure proper
   routing.  Or it might remove References and In-Reply-To so that
   message threading is not visible to any MTA.  Any more ambitious
   choice might result in deliverability, rendering, or usability issues
   for the relevant messages, so testing and documentation will be
   valuable to get this right.

   The authors of this document hope that implementers with deployment
   experience will document their chosen HCP and the rationale behind
   their choice.

3.4.2.  Expert Guidance for Registering Header Confidentiality Policies

   There is no formal syntax specified for the HCP, but any attempt to
   specify an HCP for inclusion in the registry needs to provide:

   *  a stable reference document clearly indicating the distinct name
      for the proposed HCP,

   *  pseudocode that other implementers can clearly and unambiguously
      interpret,

   *  a clear explanation of why this HCP is different from all other
      registered HCPs, and

   *  any relevant considerations related to deployment of the HCP (for
      example, known or expected deliverability, rendering, or privacy
      challenges and possible mitigations).

   When the proposed HCP produces any non-null output for a given Header
   Field name, val_out SHOULD match the expected ABNF for that Header
   Field.  If the proposed HCP does not match the expected ABNF for that
   Header Field, the documentation should explicitly identify the
   relevant circumstances and provide a justification for the deviation.

   An entry should not be marked as "Recommended" unless it has been
   shown to offer confidentiality or privacy improvements over the
   status quo and have minimal or mitigable negative impact on messages
   to which it is applied, considering factors such as message
   deliverability and security.  Only one entry in the table
   (hcp_baseline) is initially marked as "Recommended".  In the future,
   more than one entry may be marked as "Recommended".

4.  Rendering Guidance (Receiving Side)

   An MUA that receives a cryptographically protected email will render
   it for the user.

   The rendering MUA will render the message Body, render a selected
   subset of Header Fields, and (as described in Section 3 of [RFC9787])
   provide a summary of the cryptographic properties of the message.

   Most MUAs only render a subset of Header Fields by default.  For
   example, most MUAs render the From, To, Cc, Date, and Subject Header
   Fields to the user, but few render Message-Id or Received.

   An MUA that knows how to handle a message with Header Protection
   makes the following four changes to its behavior when rendering a
   message:

   *  If the MUA detects that an incoming message has protected Header
      Fields:

      -  For a Header Field that is present in the protected Header
         Section, the MUA SHOULD render the protected value and ignore
         any unprotected counterparts that may be present (with a
         special exception for the From Header Field (see Section 4.4)).

      -  For a Header Field that is present only in the Outer Header
         Section, the MUA SHOULD NOT render that value.  If it does
         render the value, the MUA SHOULD indicate that the rendered
         value is unprotected.  For an exception to this, see Section 7
         for a discussion of some specific Header Fields that are known
         to be added in transit and therefore are not expected to have
         end-to-end cryptographic protections.

   *  The MUA SHOULD include information in the message's Cryptographic
      Summary to indicate the types of protection that applied to each
      rendered Header Field (if any).

   *  If any Legacy Display Elements are present in the Body of the
      message, it does not render them.

   *  When replying to (or forwarding) a message with confidential
      Header Fields, the replying (or forwarding) MUA avoids leaking any
      Header Fields that were confidential in the original into the
      cleartext of the reply (or forwarded message).  It does this even
      if its own HCP would not have treated those Header Fields as
      confidential.  See Section 6 for more details.

   Note that an MUA that handles a message with Header Protection does
   _not_ need to render any new Header Fields that it did not render
   before.

4.1.  Identifying That a Message Has Header Protection

   An incoming message can be identified as having Header Protection
   using the following test:

   *  The Cryptographic Payload has parameter hp set to "clear" or
      "cipher".  See Section 4.5 for rendering guidance.

   When consuming a message, an MUA MUST ignore the hp parameter to
   Content-Type when it encounters it anywhere other than the root of
   the message's Cryptographic Payload.

4.2.  Extracting Protected Header Fields From an Encrypted Message

   When a message is encrypted and uses Header Protection, the rendering
   MUA extracts two lists of Header Fields (names and values):

   *  The list of Header Fields that the composing MUA applied to the
      protected message.

   *  Those Header Fields added by the composing MUA to the
      (unprotected) Outer Header Section of the message, intended for
      interpretation by MTAs and Legacy MUAs.

   The following algorithm takes referenced message refmsg as input,
   which is encrypted with Header Protection as described in this
   document (that is, the Cryptographic Envelope includes a
   Cryptographic Layer that provides encryption, and the hp parameter
   for the Content-Type Header Field of the Cryptographic Payload is
   cipher).  It produces as output a pair of lists of (h,v) Header
   Fields.

4.2.1.  HeaderSetsFromMessage

   Method signature:

   HeaderSetsFromMessage(refmsg) -> (refouter, refprotected)

   Procedure:

   1.  Let refheaders be the list of (h,v) protected Header Fields found
       in the root of the Cryptographic Payload of refmsg.

   2.  Let refouter be an empty list of Header Field names and values.

   3.  Let refprotected be an empty list of Header Field names and
       values.

   4.  For each (h,v) in refheaders:

       i.   If h is HP-Outer:

            a.  Split v into (h1,v1) on the first colon (:), followed by
                any amount of whitespace.

            b.  Append (h1,v1) to refouter.

       ii.  Else:

            a.  Append (h,v) to refprotected.

   5.  Return refouter, refprotected.

   Note that this algorithm is independent of the Outer Header Section.
   It derives its output only from the normal Header Fields and the HP-
   Outer Header Fields, both contained inside the Cryptographic Payload.

4.3.  Updating the Cryptographic Summary

   Regardless of whether a cryptographically protected message has
   protected Header Fields, the Cryptographic Summary of the message
   should be modified to indicate what protections the Header Fields
   have.  This field-by-field status is complex and isn't necessarily
   intended to be presented in full to the user.  Rather, it represents
   the state of the message internally within the MUA and may be used to
   influence behavior like replying to or forwarding the message (see
   Section 6.1).

   Each Header Field individually has exactly one of the following
   protection states:

   *  unprotected (has no Header Protection)

   *  signed-only (bound into the same validated signature as the
      enclosing message, but also visible in transit)

   *  encrypted-only (only appears within the Cryptographic Payload; the
      corresponding external Header Field was either removed or
      obscured)

   *  signed-and-encrypted (same as encrypted-only, but additionally is
      under a validated signature)

   If the message does not have Header Protection (as determined by
   Section 4.1), then all of the Header Fields are by definition
   unprotected.

   If the message has Header Protection, an MUA SHOULD use the following
   algorithm to compute the protection state of a protected Header Field
   (h,v):

4.3.1.  HeaderFieldProtection

   Method signature:

   HeaderFieldProtection(msg, h, v) -> protection_state

   Procedure:

   1.  Let ct be the Content-Type of the root of the Cryptographic
       Payload of msg.

   2.  Compute (refouter, refprotected) from HeaderSetsFromMessage(msg).

   3.  If (h, v) is not in refprotected:

       i.  Abort, v is not a valid value for Header Field h.

   4.  Let is_sig_valid be false.

   5.  If the message is signed:

       i.  Let is_sig_valid be the result of validating the signature.

   6.  If the message is encrypted, and if ct has a parameter
       hp="cipher", and if (h,v) is not in refouter:

       i.  Return signed-and-encrypted if is_sig_valid, otherwise return
           encrypted-only.

   7.  Return signed-only if is_sig_valid, otherwise return unprotected.

   Note that:

   *  This algorithm is independent of the unprotected Header Fields.
      It derives the protection state only from (h,v) and the set of HP-
      Outer Header Fields, both of which are inside the Cryptographic
      Envelope.

   *  If the signature fails validation, the MUA lowers the affected
      state to unprotected or encrypted-only without any additional
      warning to the user (see also Section 3.1 of [RFC9787]).

   *  Data from signed-and-encrypted and encrypted-only Header Fields
      may still not be fully private (see Section 11.2).

   *  Encryption may have been added in transit to an originally signed-
      only message.  Thus, only consider Header Fields to be
      confidential if the composer indicates it with the hp="cipher"
      parameter.

   *  The protection state of a Header Field may be weaker than that of
      the message Body.  For example, a message Body can be signed-and-
      encrypted, but a Header Field that is copied unmodified to the
      Outer Header Section is signed-only.

   If the message has Header Protection, the Header Fields that are not
   in refprotected (e.g., because they were added in transit) are
   unprotected.

   Rendering the cryptographic status of each Header Field is likely to
   be complex and messy -- users may not understand it.  It is beyond
   the scope of this document to suggest any specific graphical
   affordances or user experience.  Future work should include examples
   of successful rendering of this information.

4.4.  Handling Mismatch of From Header Fields

   End-to-end (MUA-to-MUA) Header Protection is good for authenticity,
   integrity, and confidentiality, but it potentially introduces new
   issues when an MUA depends on its MTA to authenticate parts of the
   Header Section.  The latter is typically the case in modern email
   systems.

   In particular, when an MUA depends on its MTA to ensure that the
   email address in the (unprotected) From Header Field is authentic,
   but the MUA renders the email address of the protected From Header
   Field that differs from the address visible to the MTA, this could
   create a risk of sender address spoofing (see Section 10.1).  This
   potential risk applies to signed-only messages as well as signed-and-
   encrypted messages.

4.4.1.  Definitions

4.4.1.1.  From Header Field Mismatch

   "From Header Field Mismatch" is defined as follows:

   The addr-spec of the inner From Header Field doesn't match the addr-
   spec of the outer From Header Field (see Section 4.4.5).

   Note: The unprotected From Header Field used in this comparison is
   the actual Header Field found in the Outer Header Section (as seen by
   the MTA), not the value indicated by any potential inner HP-Outer
   Header Field.

4.4.1.2.  No Valid and Correctly Bound Signature

   "No Valid and Correctly Bound Signature" is defined as follows:

   There is no valid signature made by a certificate for which the MUA
   has a valid binding to the protected From address.  This includes:

   *  the message has no signature

   *  the message has a broken signature

   *  the message has a valid signature, but the rendering MUA does not
      see any valid binding between the signing certificate and the
      addr-spec of the inner From Header Field

   Note: There are many possible ways that an MUA could choose to
   validate a certificate-to-address binding.  For example, the MUA
   could ensure the certificate is issued by one of a set of trusted
   certification authorities, it could rely on the user to do a manual
   out-of-band comparison, it could rely on a DNSSEC signal ([RFC7929]
   or [RFC8162]), and so on.  It is beyond the scope of this document to
   describe all possible ways an MUA might validate the certificate-to-
   address binding or to choose among them.

4.4.2.  Warning for From Header Field Mismatch

   To mitigate the above described risk of sender address spoofing, an
   MUA SHOULD warn the user whenever both of the following conditions
   are met:

   *  From Header Field Mismatch (as defined in Section 4.4.1.1) and

   *  No Valid and Correctly Bound Signature (as defined in
      Section 4.4.1.2)

   This warning should be comparable to the MUA's warning about messages
   that are likely spam or phishing, and it SHOULD show both of the non-
   matching From Header Fields.

4.4.3.  From Header Field Rendering

   Furthermore, a rendering MUA that depends on its MTA to authenticate
   the (unprotected) outer From Header Field SHOULD render the outer
   From Header Field (as an exception to the guidance in the beginning
   of Section 4) if both of the following conditions are met:

   *  From Header Field Mismatch (as defined in Section 4.4.1.1) and

   *  No Valid and Correctly Bound Signature (as defined in
      Section 4.4.1.2)

   An MUA MAY apply a local preference to render a different display
   name (e.g., from an address book).

   See Section 10.1.1 for a detailed explanation of this rendering
   guidance.

4.4.4.  Handling the Protected From Header Field When Responding

   When responding to a message, an MUA has different ways to populate
   the recipients of the new message.  Depending on whether it is a
   Reply, a Reply All, or a Forward, an MUA may populate the composer
   view using a combination of the referenced message's From, To, Cc,
   Reply-To, or Mail-Followup-To Header Fields as well as any other
   signals.

   When responding to a message with Header Protection, an MUA MUST only
   use the protected Header Fields when populating the recipients of the
   new message.

   This avoids compromise of message confidentiality when a machine-in-
   the-middle (MITM) attacker modifies the unprotected From address of
   an encrypted message, attempting to learn the contents through a
   misdirected reply.  Note that with the rendering guidance above, a
   MITM attacker can cause the unprotected From Header Field to be
   displayed.  Thus, when responding, the populated To address may
   differ from the rendered From address.  However, this change in
   addresses should not cause more user confusion than the address
   change caused by a Reply-To in a Legacy Message does.

4.4.5.  Matching addr-specs

   When generating (Section 3.1.1) or consuming (Section 4.4) a
   protected From Header Field, the MUA considers the equivalence of two
   different addr-spec values.

   First, the MUA MUST check whether the domain part of an addr-spec
   being compared contains a U-label [RFC5890].  If it does, it MUST be
   converted to the A-label form as described in [RFC5891].  We call a
   domain converted in this way (or the original domain if it didn't
   contain any U-label) "the ASCII version of the domain part".  Second,
   the MUA MUST compare the ASCII version of the domain part of the two
   addr-specs by standard DNS comparison: Assume ASCII text and compare
   alphabetic characters case-insensitively, as described in Section 3.1
   of [RFC1035].  If the domain parts match, then the two local-parts
   are matched against each other.  The simplest and most common
   comparison for the local-part is also an ASCII-based, case-
   insensitive match.  If the MUA has special knowledge about the domain
   and, when composing, it can reasonably expect the rendering MUAs to
   have the same information, it MAY match the local-part using a more
   sophisticated and inclusive matching algorithm.

   It is beyond the scope of this document to recommend a more
   sophisticated and inclusive matching algorithm.

4.5.  Rendering a Message with Header Protection

   When the Cryptographic Payload's Content-Type has the parameter hp
   set to "clear" or "cipher", the values of the protected Header Fields
   are drawn from the Header Fields of the Cryptographic Payload, and
   the Body that is rendered is the content of the Cryptographic Payload
   itself.

4.5.1.  Example Signed-Only Message

   Consider a message with this structure, where the MUA is able to
   validate the cryptographic signature:

   A └┬╴application/pkcs7-mime; smime-type="signed-data"
      ┴ (unwraps to)
   B  └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
   C   ├─╴text/plain
   D   └─╴text/html

   The message Body should be rendered the same way as this message:

   B └┬╴multipart/alternative
   C  ├─╴text/plain
   D  └─╴text/html

   The MUA should render Header Fields taken from part B.

   Its Cryptographic Summary should indicate that the message was signed
   and all rendered Header Fields were included in the signature.

   Because this message is signed-only, none of its parts will have a
   Legacy Display Element.

   The MUA should ignore Header Fields from part A for the purposes of
   rendering.

4.5.2.  Example Signed-and-Encrypted Message

   Consider a message with this structure, where the MUA is able to
   validate the cryptographic signature:

   E └┬╴application/pkcs7-mime; smime-type="enveloped-data"
      ╧ (decrypts to)
   F  └┬╴application/pkcs7-mime; smime-type="signed-data"
       ┴ (unwraps to)
   G   └┬╴multipart/alternative [Cryptographic Payload + Rendered Body]
   H    ├─╴text/plain
   I    └─╴text/html

   The message Body should be rendered the same way as this message:

   G └┬╴multipart/alternative
   H  ├─╴text/plain
   I  └─╴text/html

   It should render Header Fields taken from part G.

   Its Cryptographic Summary should indicate that the message is signed-
   and-encrypted.

   When rendering the Cryptographic Status of a Header Field and when
   composing a reply (or forward), each Header Field found in G should
   be considered against all HP-Outer Header Fields found in G.  If an
   HP-Outer Header Field that matches both the name and value is found,
   the Header Field's Cryptographic Status is just signed-only, even
   though the message itself is signed-and-encrypted.  If no matching
   HP-Outer Header Field is found, the Header Field's Cryptographic
   Status is signed-and-encrypted, like the rest of the message (see
   Section 4.3).

   If any of the User-Facing Header Fields are removed or obscured, the
   composer of this message may have placed Legacy Display Elements in
   parts H and I.

   The MUA should ignore Header Fields from part E for the purposes of
   rendering.

4.5.3.  Do Not Render Legacy Display Elements

   As described in Section 2.1.2, a message with cryptographic
   confidentiality protection MAY include Legacy Display Elements for
   backward compatibility with Legacy MUAs.  These Legacy Display
   Elements are strictly decorative and unambiguously identifiable and
   will be discarded by compliant implementations.

   The rendering MUA MUST completely avoid rendering the identified
   Legacy Display Elements to the user, since it is aware of Header
   Protection and can render the actual protected Header Fields.

   If a text/html or text/plain part within the Cryptographic Envelope
   is identified as containing Legacy Display Elements, those elements
   MUST be hidden when rendering and MUST be dropped when generating a
   draft reply or inline forwarded message.  Whenever a message or a
   MIME subtree is exported, downloaded, or otherwise further processed,
   if there is no need to retain a valid cryptographic signature, the
   implementer MAY drop the Legacy Display Elements.

4.5.3.1.  Identifying a Part with Legacy Display Elements

   A rendering MUA acting on a message that contains an encrypting
   Cryptographic Layer identifies a MIME subpart within the
   Cryptographic Payload as containing Legacy Display Elements based on
   the Content-Type of the subpart.  The subpart's Content-Type:

   *  contains a parameter hp-legacy-display with value set to 1 and

   *  is either text/plain (see Section 4.5.3.2) or text/html (see
      Section 4.5.3.3).

   Note that the term "subpart" above is used in the general sense: If
   the Cryptographic Payload is a single part, that part itself may
   contain a Legacy Display Element if it is marked with the hp-legacy-
   display="1" parameter.

4.5.3.2.  Omitting Legacy Display Elements from text/plain

   If a text/plain part within the Cryptographic Payload has the
   Content-Type parameter hp-legacy-display="1", it should be processed
   before rendering in the following fashion:

   *  Discard the leading lines of the content of the MIME part up to
      and including the first entirely blank line.

   Note that implementing this strategy is dependent on the charset used
   by the MIME part.

   See Appendix E.1 for an example.

4.5.3.3.  Omitting Legacy Display Elements from text/html

   If a text/html part within the Cryptographic Payload has the Content-
   Type parameter hp-legacy-display="1", it should be processed before
   rendering in the following fashion:

   *  If any element of the HTML <body> is a <div> with class attribute
      header-protection-legacy-display, that entire element should be
      omitted.

   This cleanup could be done, for example, as a custom rule in the
   MUA's HTML sanitizer, if one exists.  Another implementation strategy
   for an HTML-capable MUA would be to add an entry to the [CSS] style
   sheet for such a part:

   body div.header-protection-legacy-display { display: none; }

4.6.  Implicitly Rendered Header Fields

   While the From, To, Cc, Subject, and Date Header Fields are often
   explicitly rendered to the user, some Header Fields do affect message
   display without being explicitly rendered.

   For example, the Message-Id, References, and In-Reply-To Header
   Fields may collectively be used to place a message in a "thread" or
   series of messages.

   In another example, Section 6.2 notes that the value of the Reply-To
   Header Field can influence the draft reply message.  So while the
   user may never see the Reply-To Header Field directly, it is
   implicitly "rendered" when the user interacts with the message by
   replying to it.

   An MUA that depends on any implicitly rendered Header Field in a
   message with Header Protection MUST use the value from the protected
   Header Field and SHOULD NOT use any value found outside the
   cryptographic protection unless it is known to be a Header Field
   added in transit, as specified in Section 7.

4.7.  Handling Undecryptable Messages

   An MUA might receive an apparently encrypted message that it cannot
   currently decrypt.  For example, when an MUA does not have regular
   access to the secret key material needed for decryption, it cannot
   know the cryptographically protected Header Fields or even whether
   the message has any cryptographically protected Header Fields.

   Such an undecrypted message will be rendered by the MUA as a message
   without any Header Protection.  This means that the message summary
   may well change how it is rendered when the user is finally able to
   supply the secret key.

   For example, the rendering of the Subject Header Field in a mailbox
   summary might change from [...] to the real message subject when the
   message is decrypted.  Or the message's placement in a message thread
   might change if, say, References or In-Reply-To have been removed or
   obscured (see Section 4.6).

   Additionally, if the MUA does not retain access to the decrypting
   secret key, and it drops the decrypted form of a message, the
   message's rendering may revert to the encrypted form.  For example,
   if an MUA follows this behavior, the Subject Header Field in a
   mailbox summary might change from the real message subject back to
   [...].  Or the message might be displayed outside of its current
   thread if the MUA loses access to a removed References or In-Reply-To
   Header Field.

   These behaviors are likely to surprise the user.  However, an MUA has
   several possible ways of reducing or avoiding all of these surprises,
   including:

   *  Ensuring that the MUA always has access to decryption-capable
      secret key material.

   *  Rendering undecrypted messages in a special quarantine view until
      the decryption-capable secret key material is available.

   To reduce or avoid the surprises associated with a decrypted message
   with removed or obscured Header Fields becoming undecryptable, the
   MUA could also:

   *  Securely cache metadata from a decrypted message's protected
      Header Fields so that its rendering doesn't change after the first
      decryption.

   *  Securely store the session key associated with a decrypted message
      so that attempts to read the message when the long-term secret key
      is unavailable can proceed using only the session key itself.  For
      example, see the discussion about stashing session keys in
      Section 9.1 of [RFC9787].

4.8.  Guidance for Automated Message Handling

   Some automated systems have a control channel that is operated by
   email.  For example, an incoming email message could subscribe
   someone to a mailing list, initiate the purchase of a specific
   product, approve another message for redistribution, or adjust the
   state of some shared object.

   To the extent that such a system depends on end-to-end cryptographic
   guarantees about the email control message, Header Protection as
   defined in this document should improve the system's security.  This
   section provides some specific guidance for systems that use email
   messages as a control channel that want to benefit from these
   security improvements.

4.8.1.  Only Interpret Protected Header Fields

   Consider the situation where an email-based control channel depends
   on the message's cryptographic signature and the action taken depends
   on some Header Field of the message.

   In this case, the automated system MUST rely on information from the
   Header Field that is protected by the mechanism defined in this
   document.  It MUST NOT rely on any Header Field found outside the
   Cryptographic Payload.

   For example, consider an administrative interface for a mailing list
   manager that only accepts control messages that are signed by one of
   its administrators.  When an inbound message for the list arrives, it
   is queued (waiting for administrative approval) and the system
   generates and listens for two distinct email addresses related to the
   queued message -- one that approves the message and one that rejects
   it.  If an administrator sends a signed control message to the
   approval address, the mailing list verifies that the protected To
   Header Field of the signed control message contains the approval
   address before approving the queued message for redistribution.  If
   the protected To Header Field does not contain that address, or there
   is no protected To Header Field, then the mailing list logs or
   reports the error and does not act on that control message.

4.8.2.  Ignore Legacy Display Elements

   Consider the situation where an email-based control channel expects
   to receive an end-to-end encrypted message -- for example, where the
   control messages need confidentiality guarantees -- and where the
   action taken depends on the contents of some MIME part within the
   message Body.

   In this case, the automated system that decrypts the incoming
   messages and scans the relevant MIME part MUST identify when the MIME
   part contains a Legacy Display Element (see Section 4.5.3.1), and it
   MUST parse the relevant MIME part with the Legacy Display Element
   removed.

   For example, consider an administrative interface of a confidential
   issue tracking software.  An authorized user can confidentially
   adjust the status of a tracked issue by a specially formatted first
   line of the message Body (for example, severity #183 serious).  When
   the user's MUA encrypts a plaintext control message to this issue
   tracker, depending on the MUA's HCP and its choice of legacy value,
   it may add a Legacy Display Element.  If it does so, then the first
   line of the message Body will contain a decorative copy of the
   confidential Subject Header Field.  The issue tracking software
   decrypts the incoming control message, identifies that there is a
   Legacy Display Element in the part (see Section 4.5.3.1), strips the
   lines comprising the Legacy Display Element (including the first
   blank line), and only then parses the remaining top line to look for
   the expected special formatting.

4.9.  Affordances for Debugging and Troubleshooting

   Note that advanced users of an MUA may need access to the original
   message, for example, to troubleshoot problems with the rendering MUA
   itself or problems with the SMTP transport path taken by the message.

   An MUA that applies these rendering guidelines SHOULD ensure that the
   full original source of the message as it was received remains
   available to such a user for debugging and troubleshooting.

   If a troubleshooting scenario demands information about the
   cryptographically protected values of Header Fields, and the message
   is encrypted, the debugging interface SHOULD also provide a "source"
   view of the Cryptographic Payload itself, alongside the full original
   source of the message as received.

4.10.  Handling RFC8551HP Messages (Backward Compatibility)

   Section 1.1.1 describes some drawbacks to the Header Protection
   scheme defined in [RFC8551], referred to here as RFC8551HP.  An MUA
   MUST NOT generate an RFC8551HP message.  However, for backward
   compatibility, an MUA MAY try to render or respond to such a message
   as though the message has standard Header Protection.

   The following two sections contain guidance for identifying,
   rendering, replying to, and forwarding RFC8551HP messages.
   Corresponding test vectors are provided in Appendices C.2.5, C.2.6,
   and C.3.17.

4.10.1.  Identifying an RFC8551HP Message

   An RFC8551HP message can be identified by its MIME structure, given
   that all of the following conditions are met:

   *  It has a well-formed Cryptographic Envelope consisting of at least
      one Cryptographic Layer as the outermost MIME object.

   *  The Cryptographic Payload is a single message/rfc822 object.

   *  The message that constitutes the Cryptographic Payload does not
      itself have a well-formed Cryptographic Envelope; that is, its
      outermost MIME object is not a Cryptographic Layer.

   *  No Content-Type parameter of hp= is set on either the
      Cryptographic Payload or its immediate MIME child.

   Here is the MIME structure of an example signed-and-encrypted
   RFC8551HP message:

   A └┬╴application/pkcs7-mime; smime-type="enveloped-data"
      ╧ (decrypts to)
   B  └┬╴application/pkcs7-mime; smime-type="signed-data"
       ┴ (unwraps to)
   C   └┬╴message/rfc822 [Cryptographic Payload]
   D    └┬╴multipart/alternative [Rendered Body]
   E     ├─╴text/plain
   F     └─╴text/html

   This meets the definition of an RFC8551HP message because:

   *  Cryptographic Layers A and B form the Cryptographic Envelope.

   *  The Cryptographic Payload, rooted in part C, has Content-Type:
      message/rfc822.

   *  Part D (the MIME root of the message at C) is itself not a
      Cryptographic Layer.

   *  Neither part C nor part D have any hp parameters set on their
      Content-Type.

4.10.2.  Rendering or Responding to an RFC8551HP Message

   When an MUA has precisely identified a message as an RFC8551HP
   message, the MUA MAY render or respond to that message as though it
   were a message with Header Protection as defined in this document by
   making the following adjustments:

   *  Rather than rendering the message Body as the Cryptographic
      Payload itself (part C in the example above), render the RFC8551HP
      message's Body as the MIME subtree that is the Cryptographic
      Payload's immediate child (part D).

   *  Make a comparable modification to HeaderSetsFromMessage
      (Section 4.2.1) and HeaderFieldProtection (Section 4.3.1): Both
      algorithms currently look for the protected Header Fields on the
      Cryptographic Payload (part C), but they should instead look at
      the Cryptographic Payload's immediate child (part D).

   *  If the Cryptographic Envelope is signed-only, behave as though
      there is an hp="clear" parameter for the Cryptographic Payload; if
      the Envelope contains encryption, behave as though there is an
      hp="cipher" parameter.  That is, infer the composer's
      cryptographic intent from the structure of the message.

   *  If the Cryptographic Envelope contains encryption, further modify
      HeaderSetsFromMessage to derive refouter from the actual Outer
      Header Section (those Header Fields found in part A in the example
      above) rather than looking for HP-Outer Header Fields with the
      other protected Header Fields.  That is, infer Header Field
      confidentiality based on the unprotected Header Fields.

   The inferences in the above modifications are not based on any strong
   end-to-end guarantees.  An intervening MTA may tamper with the
   message's Outer Header Section or wrap the message in an encryption
   layer to undetectably change the recipient's understanding of the
   confidentiality of the message's Header Fields or the message Body
   itself.

4.11.  Rendering Other Schemes

   Other MUAs may have generated different structures of messages that
   aim to offer end-to-end cryptographic protections that include Header
   Protection.  This document is not normative for those schemes, and it
   is NOT RECOMMENDED to generate these other schemes, as they can
   either have structural flaws or simply render poorly on Legacy MUAs.
   A conformant MUA MAY attempt to infer Header Protection when
   rendering an existing message that appears to use some other scheme
   not documented here.  Pointers to some known other schemes can be
   found in Appendix F.

5.  Composing Guidance (Sending Side)

   This section describes the process an MUA should use to apply
   cryptographic protection to an email message with Header Protection.

   When composing a message with end-to-end cryptographic protections,
   an MUA SHOULD apply Header Protection.

   When generating such a message, an MUA MUST add the hp parameter (see
   Section 2.1.1) only to the Content-Type Header Field at the root of
   the message's Cryptographic Payload.  The value of the parameter MUST
   indicate whether the Cryptographic Envelope contains a layer that
   provides encryption.

5.1.  Composing a Cryptographically Protected Message Without Header
      Protection

   For contrast, we first consider the typical message composition
   process of a Legacy Crypto MUA, which does not provide any Header
   Protection.

   This process is described in Section 5.1 of [RFC9787].  We replicate
   it here for reference.  The inputs to the algorithm are:

   *  origbody: The unprotected message Body as a well-formed MIME tree
      (possibly just a single MIME leaf part).  As a well-formed MIME
      tree, origbody already has Structural Header Fields (Content-*)
      present.

   *  origheaders: The intended Non-Structural Header Fields for the
      message, represented here as a list of (h,v) pairs, where h is a
      Header Field name and v is the associated value.  Note that these
      are Header Fields that the MUA intends to be visible to the
      recipient of the message.  In particular, if the MUA uses the Bcc
      Header Field during composition but plans to omit it from the
      message (see Section 3.6.3 of [RFC5322]), it will not be in
      origheaders.

   *  crypto: The series of cryptographic protections to apply (for
      example, "sign with the secret key corresponding to X.509
      certificate X, then encrypt to X.509 certificates X and Y").  This
      is a routine that accepts a MIME tree as input (the Cryptographic
      Payload), wraps the input in the appropriate Cryptographic
      Envelope, and returns the resultant MIME tree as output.

   The algorithm returns a MIME object that is ready to be injected into
   the mail system.

5.1.1.  ComposeNoHeaderProtection

   Method signature:

   ComposeNoHeaderProtection(origbody, origheaders, crypto) ->
   mime_message

   Procedure:

   1.  Apply crypto to MIME part origbody, producing MIME tree output.

   2.  For each Header Field name and value (h,v) in origheaders:

       i.  Add Header Field h to output with value v.

   3.  Return output.

5.2.  Composing a Message with Header Protection

   To compose a message using Header Protection, the composing MUA uses
   the following inputs:

   *  all the inputs described in Section 5.1

   *  hcp: an HCP, as defined in Section 3

   *  respond: if the new message is a response to another message, the
      MUA's Respond Function corresponding to the user's action (see
      Section 6.1.1), otherwise null

   *  refmsg: if the new message is a response to another message, the
      message being responded to, otherwise null

   *  legacy: a boolean value, indicating whether any recipient of the
      message is believed to have a Legacy MUA.  If all recipients are
      known to implement this document, legacy should be set to false.
      (How an MUA determines the value of legacy is out of scope for
      this document; an initial implementation can simply set it to
      true.)

   To enable visibility of User-Facing but now removed/obscured Header
   Fields for decryption-capable Legacy MUAs, the Header Fields are
   included as a decorative Legacy Display Element in specially marked
   parts of the message (see Section 2.1.2).  This document recommends
   two mechanisms for such a decorative adjustment: one for a text/plain
   Main Body Part (see Section 5.2.2) and one for a text/html Main Body
   Part (see Section 5.2.3) of the email message.  This document does
   not recommend adding a Legacy Display Element to any other part.

   Please see Section 7.1 of [RFC9787] for guidance on identifying the
   parts of a message that are a Main Body Part.

5.2.1.  Compose

   Method signature:

   Compose(origbody, origheaders, crypto, hcp, respond, refmsg, legacy)
   -> mime_message

   Procedure:

   1.  Let newbody be a copy of origbody.

   2.  If crypto contains encryption and legacy is true:

       i.    Create ldlist, an empty list of (header, value) pairs.

       ii.   For each Header Field name and value (h,v) in origheaders:

             a.  If h is User-Facing (see Section 1.1.2 of [RFC9787]):

                 I.  If hcp(h,v) is not v:

                     A.  Add (h,v) to ldlist.

       iii.  If ldlist is not empty:

             a.  Identify each leaf MIME part of newbody that represents
                 a "Main Body Part" of the message.

             b.  For each "Main Body Part" bodypart of type text/plain
                 or text/html:

                 I.  Adjust bodypart by inserting a Legacy Display
                     Element Header Field list ldlist into its content
                     and adding a Content-Type parameter hp-legacy-
                     display with value 1 (see Section 5.2.2 for text/
                     plain and Section 5.2.3 for text/html).

   3.  For each Header Field name and value (h,v) in origheaders:

       i.  Add Header Field h to MIME part newbody with value v.

   4.  If crypto does not contain encryption:

       i.   Set the hp parameter on the Content-Type of MIME part
            newbody to clear.

       ii.  Let newheaders be a copy of origheaders.

   5.  Else (if crypto contains encryption):

       i.    Set the hp parameter on the Content-Type of MIME part
             newbody to cipher.

       ii.   Let response_hcp be an ephemeral HCP, the output of
             ReferenceHCP(refmsg, respond) (see Section 6.1.2).

       iii.  Create a new empty list of Header Field names and values
             newheaders.

       iv.   For each Header Field name and value (h,v) in origheaders:

             a.  Let newval be hcp(h,v).

             b.  If newval is v:

                 I.  Let newval be response_hcp(h,v).

             c.  If newval is not null:

                 I.  Add (h,newval) to newheaders.

       v.    For each Header Field name and value (h,v) in newheaders:

             a.  Let string record be the concatenation of h, a literal
                 ": " (ASCII colon (0x3A) followed by ASCII space
                 (0x20)), and v.

             b.  Add Header Field "HP-Outer" to MIME part newbody with
                 value record.

   6.  Apply crypto to MIME part newbody, producing MIME tree output.

   7.  For each Header Field name and value (h,v) in newheaders:

       i.  Add Header Field h to output with value v.

   8.  Return output.

   Note that both new parameters (hcp and legacy) are effectively
   ignored if crypto does not contain encryption.  This is by design,
   because they are irrelevant for signed-only cryptographic
   protections.

5.2.2.  Adding a Legacy Display Element to a text/plain Part

   For a list of obscured and removed User-Facing Header Fields
   represented as (header, value) pairs, concatenate them as a set of
   lines, with one newline at the end of each pair.  Add an additional
   trailing newline after the resultant text, and prepend the entire
   list to the content of the text/plain part.

   The MUA MUST also add a Content-Type parameter of hp-legacy-display
   with value 1 to the MIME part to indicate that a Legacy Display
   Element was added.

   For example, if the list of obscured Header Fields was [("Cc",
   "alice@example.net"), ("Subject", "Thursday's meeting")], then a
   text/plain Main Body Part that originally looked like this:

   Content-Type: text/plain; charset=UTF-8

   I think we should skip the meeting.

   would become:

   Content-Type: text/plain; charset=UTF-8; hp-legacy-display=1

   Subject: Thursday's meeting
   Cc: alice@example.net

   I think we should skip the meeting.

   Note that the Legacy Display Element (the lines beginning with
   Subject: and Cc:) is part of the content of the MIME part in
   question.

   This example assumes that the Main Body Part in question is not the
   root of the Cryptographic Payload.  For instance, it could be a leaf
   of a multipart/alternative Cryptographic Payload.  This is why there
   are no additional Header Fields in the MIME part of this example.

5.2.3.  Adding a Legacy Display Element to a text/html Part

   Adding a Legacy Display Element to a text/html part is similar to how
   it is added to a text/plain part (see Section 5.2.2).  Instead of
   adding the obscured or removed User-Facing Header Fields to a block
   of text delimited by a blank line, the composing MUA injects them in
   an HTML <div> element annotated with a class attribute of header-
   protection-legacy-display.

   The content and formatting of this decorative <div> have no strict
   requirements, but they MUST represent all the obscured and removed
   User-Facing Header Fields in a readable fashion.  A simple approach
   is to assemble the text in the same way as Section 5.2.2, wrap it in
   a verbatim <pre> element, and put that element in the annotated
   <div>.

   The annotated <div> should be placed as close to the start of the
   <body> as possible, where it will be visible when viewed with a
   standard HTML renderer.

   The MUA MUST also add a Content-Type parameter of hp-legacy-display
   with value 1 to the MIME part to indicate that a Legacy Display
   Element was added.

   For example, if the list of obscured Header Fields was [("Cc",
   "alice@example.net"), ("Subject", "Thursday's meeting")], then a
   text/html Main Body Part that originally looked like this:

   Content-Type: text/html; charset=UTF-8

   <html><head><title></title></head><body>
   <p>I think we should skip the meeting.</p>
   </body></html>

   would become:

   Content-Type: text/html; charset=UTF-8; hp-legacy-display=1

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>Subject: Thursday's meeting
   Cc: alice@example.net</pre></div>
   <p>I think we should skip the meeting.</p>
   </body></html>

   This example assumes that the Main Body Part in question is not the
   root of the Cryptographic Payload.  For instance, it could be a leaf
   of a multipart/alternative Cryptographic Payload.  This is why there
   are no additional Header Fields in the MIME part of this example.

5.2.3.1.  Step-by-Step Example for Inserting a Legacy Display Element
          into text/html

   A composing MUA MAY insert the Legacy Display Element anywhere
   reasonable within the message as long as it prioritizes visibility
   for the reader using a Legacy MUA that is capable of decryption.
   This decision may take into account special message-specific HTML
   formatting expectations if the MUA is aware of them.  However, some
   MUAs may not have any special insight into the user's preferred HTML
   formatting and still want to insert a Legacy Display Element.  This
   section offers a non-normative, simple, and minimal step-by-step
   approach for a composing MUA that has no other information or
   preferences to fall back on.

   The process below assumes that the MUA already has the full HTML
   object that it intends to send, including all of the text supplied by
   the user.

   1.  Assemble the text exactly as specified for text/plain (see
       Section 5.2.2).

   2.  Wrap that text in a verbatim <pre> element.

   3.  Wrap that <pre> element in a <div> element annotated with the
       class header-protection-legacy-display.

   4.  Find the <body> element of the full HTML object.

   5.  Insert the <div> element as the first child of the <body>
       element.

5.2.4.  Only Add a Legacy Display Element to Main Body Parts

   Some messages may contain a text/plain or text/html subpart that is
   _not_ a Main Body Part.  For example, an email message might contain
   an attached text file or a downloaded web page.  Attached documents
   need to be preserved as intended in the transmission, without
   modification.

   The composing MUA MUST NOT add a Legacy Display Element to any part
   of the message that is not a Main Body Part.  In particular, if a
   part is annotated with Content-Disposition: attachment, or if it does
   not descend via the first child of any of its multipart/mixed or
   multipart/related ancestors, it is not a Main Body Part and MUST NOT
   be modified.

   See Section 7.1 of [RFC9787] for more guidance about common ways to
   distinguish Main Body Parts from other MIME parts in a message.

5.2.5.  Do Not Add a Legacy Display Element to Other Content-Types

   The purpose of injecting a Legacy Display Element into each Main Body
   Part is to enable rendering of otherwise obscured Header Fields in
   Legacy MUAs that are capable of message decryption but don't know how
   to follow the rest of the guidance in this document.

   The authors are unaware of any Legacy MUA that would render any MIME
   part type other than text/plain and text/html as the Main Body.  A
   generating MUA SHOULD NOT add a Legacy Display Element to any MIME
   part with any other Content-Type.

6.  Replying and Forwarding Guidance

   An MUA might create a new message in response to another message,
   thus acting both as a rendering MUA and as a composing MUA.  For
   example, the user of an MUA viewing any given message might take an
   action like "Reply", "Reply All", "Forward", or some comparable
   action to start the composition of a new message.  The new message
   created this way effectively references the original message that was
   viewed at the time.

   For encrypted messages, special guidance applies, because information
   can leak in at least two ways: leaking previously confidential Header
   Fields and leaking the entire message by sending the reply or forward
   to the wrong party.

6.1.  Avoid Leaking Encrypted Header Fields in Replies and Forwards

   As noted in Section 5.4 of [RFC9787], an MUA in this position MUST
   NOT leak previously encrypted content in the clear in a follow-up
   message.  The same is true for protected Header Fields.

   Values from any Header Field that was identified as either encrypted-
   only or signed-and-encrypted based on the steps outlined in
   Section 4.3 MUST NOT be sent in cleartext in a reply or forwarded
   message.

   For example, if Subject was encrypted, and it is copied into the
   draft encrypted reply's Subject, the replying MUA will automatically
   obscure the reply's Subject Header Field.

   When crafting the Header Fields for a reply or forwarded message, the
   composing MUA SHOULD make use of the HP-Outer Header Fields from
   within the Cryptographic Envelope of the referenced message to ensure
   that Header Fields derived from the referenced message do not leak in
   the reply or forwarded message.

   On a high level, this can be achieved as follows: Consider a Header
   Field in a reply message that is generated by derivation from a
   Header Field in the referenced message.  For example, the To Header
   Field is typically derived from the referenced message's Reply-To or
   From Header Fields.  When generating this Header Field for the Outer
   Header Section, the composing MUA first applies its own HCP.  If the
   Header Field's value is changed by that HCP, then the resulting value
   is used for the Outer Header Section.  If the Header Field's value is
   unchanged, the composing MUA re-generates the Header Field using the
   Header Fields that had been in the Outer Header Section of the
   original message at composition time.  These are inferred from the
   HP-Outer Header Fields located within the Cryptographic Payload of
   the referenced message.  If the value is itself different than the
   protected value, then it is applied to the Outer Header Section.  If
   the value is the same as the protected value, then it is simply
   copied to the Outer Header Section directly.  As long as the
   resulting value is not null, it is noted (whether identical to the
   protected value or not) in the protected Header Section using HP-
   Outer, as described in Section 2.2.1.

   See Appendix D.2 for a simple worked example of this process.

   Below we describe a supporting algorithm to handle this.  It produces
   a list of Header Fields that should be obscured or removed in the new
   message even if the composer's choice of HCP wouldn't normally remove
   or obscure the Header Field in question.  This is effectively a
   single-use HCP.  The normal composing guidance in Section 5.2 applies
   this single-use HCP to implement the high-level guidance above.

6.1.1.  The Respond Function

   The mechanism described below depends on an abstraction referred to
   in this document as a Respond Function.

   The Respond Function takes a list of Header Fields from a referenced
   message as input and generates a list of initial candidate message
   Header Field names and values that are used to populate the message
   composition interface.

   Something like this function already exists in most MUAs, though it
   may differ across responsive actions.  For example, the Respond
   Function that implements "Reply All" is likely to be different from
   the Respond Function that implements "Reply", which is in turn
   different from the Respond Function that implements "Forward".

6.1.2.  ReferenceHCP

   The algorithm takes two inputs:

   *  refmsg: a single referenced message

   *  respond: the MUA's Respond Function associated with the user's
      action (see Section 6.1.1)

   As an output, it produces an ephemeral single-use HCP, specific to
   this kind of response to this specific message.

   Method signature:

   ReferenceHCP(refmsg, respond) -> response_hcp

   Procedure:

   1.  If respond is null, refmsg is null, or refmsg is not encrypted
       with Header Protection:

       i.  Return hcp_no_confidentiality (there is no header
           confidentiality in any referenced message that needs
           protection).

   2.  Extract refouter, refprotected from refmsg as described in
       Section 4.2.

   3.  Let genprotected be a list of (h,v) pairs generated by
       respond(refprotected).

   4.  Let genouter be a list of (h,v) pairs generated by
       respond(refouter).

   5.  For each (h,v) in genprotected:

       i.  If (h,v) is in genouter:

           a.  Remove (h,v) from both genprotected and genouter (this
               Header Field does not need additional confidentiality).

   6.  Let confmap be a mapping from a Header Field name and value (h,v)
       to either a string or the special value null (this mapping is
       initially empty).

   7.  For each (h,v) remaining in genprotected:

       i.    Set result to the special value null.

       ii.   For each (h1,v1) in genouter:

             a.  If h1 is h:

                 I.  Set result to v1.

       iii.  Insert (h,v) -> result into confmap.

   8.  Return a new HCP from confmap that tests whether the
       (name,val_in) tuple is in confmap; if so, return
       confmap[(name,val_in)]; otherwise, return val_in.

   Note that the key idea here is to reuse the MUA's existing Respond
   Function.  The algorithm simulates how the MUA would pre-populate a
   reply to (or forward of) two messages whose Header Fields have the
   values refouter and refprotected, respectively (independent of any
   cryptographic protections).  Then, it uses the difference to derive a
   one-time HCP.  This HCP takes into account both the referenced
   message's composer's preferences and the derivations that can happen
   to Header Field values when responding.  Note that while some of
   these derivations are straightforward (e.g., In-Reply-To is usually
   derived from Message-ID), others are non-trivial.  For example, the
   From address may be derived from To, Cc, or the MUA's local address
   preference (especially when the MUA received the referenced message
   via Bcc).  Similarly, To may be derived from To, From, and/or Cc
   Header Fields depending on the MUA implementation and depending on
   whether the user clicked "Reply", "Reply All", "Forward", or any
   other action that generates a response to a message.  Reusing the
   MUA's existing Respond Function incorporates these nuances without
   requiring any extra configuration choices or additional maintenance
   burden.

6.2.  Avoid Misdirected Replies

   When replying to a message, the composing MUA typically decides who
   to send the reply to based on:

   *  the Reply-To, Mail-Followup-To, or From Header Fields

   *  optionally, the other To or Cc Header Fields (if the user chose to
      "Reply All")

   When a message has Header Protection, the replying MUA MUST populate
   the destination fields of the draft message using the protected
   Header Fields and ignore any unprotected Header Fields.

   This mitigates against an attack where Mallory gets a copy of an
   encrypted message from Alice to Bob and then replays the message to
   Bob with an additional Cc to Mallory's own email address in the
   message's (unprotected) Outer Header Section.

   If Bob knows Mallory's certificate already, and he replies to such a
   message without following the guidance in this section, it's likely
   that his MUA will encrypt the cleartext of the message directly to
   Mallory.

7.  Unprotected Header Fields Added in Transit

   Some Header Fields are legitimately added in transit and could not
   have been known to the composer at message composition time.

   The most common of these Header Fields are Received and DKIM-
   Signature, neither of which are typically rendered, either explicitly
   or implicitly.

   If a rendering MUA has specific knowledge about a given Header Field,
   including that:

   *  the Header Field would not have been known to the original
      composer and

   *  the Header Field might be rendered explicitly or implicitly,

   then the MUA MAY decide to operate on the value of that Header Field
   from the Outer Header Section, even though the message has Header
   Protection.

   The MUA MAY prefer to verify that the Header Fields in question have
   additional transit-derived cryptographic protections before rendering
   or acting on them.  For example, the MUA could verify whether these
   Header Fields are covered by an appropriate and valid ARC-
   Authentication-Results (see [RFC8617]) or DKIM-Signature (see
   [RFC6376]) Header Field.

   Specific examples of Header Fields added in transit that are
   meaningful to the user can be found in the following section.

7.1.  Mailing List Header Fields: List-* and Archived-At

   If the message arrives through a mailing list, the list manager
   itself may inject Header Fields (most have a List- prefix) in the
   message.  Header Fields commonly added by list managers include:

   *  List-Archive

   *  List-Subscribe

   *  List-Unsubscribe

   *  List-Id

   *  List-Help

   *  List-Post

   *  Archived-At

   Some MUAs render these Header Fields implicitly by providing buttons
   for actions like "Subscribe", "View Archived Version", "Reply List",
   "List Info", etc.

   An MUA rendering a message with Header Protection that contains any
   of these Header Fields in the Outer Header Section and that has
   reason to believe the message arrived through a mailing list MAY
   decide to render them to the user (explicitly or implicitly) even
   though they are not protected.

8.  Email Ecosystem Evolution

   The email ecosystem is the set of client-side and server-side
   software and policies that are used in the creation, transmission,
   storage, rendering, and indexing of email over the Internet.

   This document is intended to offer tooling needed to improve the
   state of the email ecosystem in a way that can be deployed without
   significant disruption.  Some elements of this specification are
   present for transitional purposes but would not exist if the system
   were designed from scratch.

   This section describes these transitional mechanisms, as well as some
   suggestions for how they might eventually be phased out.

8.1.  Dropping Legacy Display Elements

   Any decorative Legacy Display Element added to an encrypted message
   that uses Header Protection is present strictly for enabling Header
   Field visibility (most importantly, the Subject Header Field) when
   the message is viewed with a decryption-capable Legacy MUA.

   Eventually, the hope is that most decryption-capable MUAs will
   conform to this specification and there will be no need for injection
   of Legacy Display Elements in the message Body.  A survey of widely
   used decryption-capable MUAs might be able to establish when most of
   them do support this specification.

   At that point, a composing MUA could set the legacy parameter defined
   in Section 5.2 to false by default or could even hard-code it to
   false, yielding a much simpler message construction set.

   Until that point, an end user might want to signal that their
   rendering MUAs are conformant to this document so that a peer
   composing a message to them can set legacy to false.  A signal
   indicating capability of handling messages with Header Protection
   might be placed in the user's cryptographic certificate or in
   outbound messages.

   This document does not attempt to define the syntax or semantics of
   such a signal.

8.2.  More Ambitious Default HCP

   This document defines a few different forms of HCP.  An MUA
   implementing an HCP for the first time SHOULD deploy hcp_baseline as
   recommended in Section 3.3.  This HCP offers the most commonly
   expected protection (obscuring the Subject Header Field) without
   risking deliverability or rendering issues.

   The HCPs proposed in this document are relatively conservative and
   still leak a significant amount of metadata for encrypted messages.
   This is largely done to ensure deliverability (see Section 1.3.2) and
   usability (see Section 2 of [RFC9787] and Section 9), as messages
   without some critical Header Fields are more likely to not reach
   their intended recipient.

   In the future, some mail transport systems may accept and deliver
   messages with even less publicly visible metadata.  Many MTA
   operators today would ask for additional guarantees about such a
   message to limit the risks associated with abusive or spam mail.

   This specification offers the HCP formalism itself as a way for MUA
   developers and MTA operators to describe their expectations around
   message deliverability.  MUA developers can propose a more ambitious
   default HCP and ask MTA operators (or simply test) whether their MTAs
   would be likely to deliver or reject encrypted mail with that HCP
   applied.  Proponents of a more ambitious HCP should explicitly
   document the HCP and name it clearly and unambiguously to facilitate
   this kind of interoperability discussion.

   Reaching widespread consensus around a more ambitious global default
   HCP is a challenging problem of coordinating many different actors.
   A piecemeal approach might be more feasible, where some signaling
   mechanism allows a message recipient, MTA operator, or third-party
   clearinghouse to announce what kinds of HCPs are likely to be
   deliverable for a given recipient.  In such a situation, the default
   HCP for an MUA might involve consulting the signaled acceptable HCPs
   for all recipients and combining them (along with a default for when
   no signal is present) in some way.

   If such a signal were to reach widespread use, it could also be used
   to guide reasonable statistical default HCP choices for recipients
   with no signal.

   This document does not attempt to define the syntax or semantics of
   such a signal.

8.3.  Deprecation of Messages Without Header Protection

   At some point, when the majority of MUA clients that can generate
   cryptographically protected messages can do so with Header
   Protection, it should be possible to deprecate any cryptographically
   protected message that does not have Header Protection.

   For example, as noted in Section 9.1, it's possible for an MUA to
   render a signed-only message that has no Header Protection the same
   as an unprotected message.  And a signed-and-encrypted message
   without Header Protection could likewise be marked as not fully
   protected.

   These stricter rules could be adopted immediately for all messages.
   Or an MUA developer could roll them out immediately for any new
   message but still treat an old message (based on the Date Header
   Field and cryptographic signature timestamp) more leniently.

   A decision like this by any popular rendering MUA could drive
   adoption of this standard for composing MUAs.

9.  Usability Considerations

   This section describes concerns for MUAs that are interested in easy
   adoption of Header Protection by normal users.

   While they are not protocol-level artifacts, these concerns motivate
   the protocol features described in this document.

   See also the usability commentary in Section 2 of [RFC9787].

9.1.  Mixed Protections Within a Message Are Hard to Understand

   When rendering a message to the user, the ideal circumstance is to
   present a single cryptographic status for any given message.
   However, when message Header Fields are present, some message Header
   Fields do not have the same cryptographic protections as the main
   message.

   Representing such a mixed set of protection statuses is very
   difficult to do in a way that an Ordinary User can understand.  There
   are at least three scenarios that are likely to be common and poorly
   understood:

   *  A signed message with no Header Protection.

   *  A signed-and-encrypted message with no Header Protection.

   *  A signed-and-encrypted message with Header Protection as defined
      in this document, where some User-Facing Header Fields have
      confidentiality but some do not.

   An MUA should have a reasonable strategy for clearly communicating
   each of these scenarios to the user.  For example, an MUA operating
   in an environment where it expects most cryptographically protected
   messages to have Header Protection could use the following rendering
   strategy:

   *  When rendering a message with a signed-only cryptographic status
      but no Header Protection, an MUA may decline to indicate a
      positive security status overall and only indicate the
      cryptographic status to a user in a message properties or
      diagnostic view.  That is, the message may appear identical to an
      unsigned message except if a user verifies the properties through
      a menu option.

   *  When rendering a message with a signed-and-encrypted or encrypted-
      only cryptographic status but no Header Protection, overlay a
      warning flag on the typical cryptographic status indicator.  That
      is, if a typical signed-and-encrypted message displays a lock
      icon, display a lock icon with a warning sign (e.g., an
      exclamation point in a triangle) overlaid.  For example, see the
      graphics in [chrome-indicators].

   *  When rendering a message with a signed-and-encrypted or encrypted-
      only cryptographic status with Header Protection but where the
      Subject Header Field has not been removed or obscured, place a
      warning sign on the Subject line.

   Other simple rendering strategies could also be reasonable.

9.2.  Users Should Not Have to Choose a Header Confidentiality Policy

   This document defines the abstraction of an HCP object for the sake
   of communication between implementers and deployments.

   Most email users are unlikely to understand the trade-offs between
   different policies.  In particular, the potential negative side
   effects (e.g., poor deliverability) may not be easily attributable by
   a normal user to a particular HCP.

   Therefore, MUA implementers should be conservative in their choice of
   default HCP and should not require the Ordinary User to make an
   incomprehensible choice that could cause unfixable, undiagnosable
   problems.  The safest option is for the MUA developer to select a
   known, stable HCP (this document recommends hcp_baseline in
   Section 3.3) on the user's behalf.  An MUA should not expose the
   Ordinary User to a configuration option where they are expected to
   manually select (let alone define) an HCP.

10.  Security Considerations

   Header Protection improves the security of cryptographically
   protected email messages.  Following the guidance in this document
   improves security for users by more directly aligning the underlying
   messages with user expectations about confidentiality, authenticity,
   and integrity.

   Nevertheless, helping the user distinguish between cryptographic
   protections of various messages remains a security challenge for
   MUAs.  This is exacerbated by the fact that many existing messages
   with cryptographic protections do not employ Header Protection.  MUAs
   encountering these messages (e.g., in an archive) will need to handle
   older forms (without Header Protection) for quite some time, possibly
   forever.

   For any MUA that offers S/MIME cryptographic protections, the
   security considerations from Section 6 of [RFC8551] (S/MIME),
   Section 3 of [RFC5083] (Authenticated-Enveloped-Data in Cryptographic
   Message Syntax (CMS)), and Section 14 of [RFC5652] (CMS more broadly)
   continue to apply.  Likewise, for any MUA that offers PGP/MIME
   cryptographic protections, the security considerations from Section 8
   of [RFC3156] (PGP with MIME) as well as Section 13 of [RFC9580]
   (OpenPGP itself) continue to apply.  In addition, these underlying
   security considerations are now also applicable to the contents of
   the message Header Section, not just the message Body.

10.1.  From Address Spoofing

   For a rendering MUA that depends on its MTA to authenticate the
   origin of the message, applying this specification could enable
   sender address spoofing.

   To prevent sender spoofing, many rendering MUAs implicitly rely on
   their receiving MTA to inspect the Outer Header Section and verify
   that the From Header Field is authentic.  If a rendering MUA displays
   a From address (from the protected part) that doesn't match the From
   address the MTA used to authenticate and/or filter (see also
   Section 4.4.1.1), the MUA may be vulnerable to spoofing.

   Consider a malicious MUA that sets the following Header Fields on an
   encrypted message with Header Protection:

   *  Outer: From: <alice@example.com>

   *  Inner: HP-Outer: From: <alice@example.com>

   *  Inner: From: <bob@example.org>

   During sending, the MTA of example.com validates that the sending MUA
   is authorized to send from alice@example.com.  Since the message is
   encrypted, the sending and receiving MTAs cannot see the protected
   Header Fields.  A naive rendering MUA might follow the algorithms in
   this document without special consideration for the From Header
   Field.  Such an MUA might display the email as coming from
   bob@example.org to the user, resulting in a spoofed address.

   This problem applies both between domains and within a domain.

   This problem always applies to signed-and-encrypted messages.  This
   problem also applies to signed-only messages because MTAs typically
   do not look at the protected Header Fields when confirming From
   address authenticity.

   Sender address spoofing is relevant for two distinct security
   properties:

   *  Sender authenticity: relevant for rendering the message (which
      address to show the user?)

   *  Message confidentiality: relevant when replying to a message (a
      reply to the wrong address can leak the message contents)

10.1.1.  From Rendering Reasoning

   Section 4.4.3 provides guidance for rendering the From Header Field.
   It recommends a rendering MUA that depends on its MTA to authenticate
   the (unprotected) outer From Header Field to render the outer From
   Header Field if both of the following conditions are met:

   *  From Header Field Mismatch (as defined in Section 4.4.1.1) and

   *  No Valid and Correctly Bound Signature (as defined in
      Section 4.4.1.2)

   Note: The second condition effectively means that the inner (expected
   to be protected) From Header Field appears to have insufficient
   protection.

   This may seem surprising since it causes the MUA to render a mix of
   both protected and unprotected values.  This section provides an
   argument as to why this guidance makes sense.

   We proceed by case distinction:

   *  Case 1: Malicious composing MUA.

      -  Attack situation: The composing MUA puts a different inner From
         Header Field to spoof the sender address.

      -  In this case, it is "better" to fall back and render the outer
         From Header Field because this is what the receiving MTA can
         validate.  Otherwise, this document would introduce a new way
         for senders to spoof the From address of the message.

      -  This does not preclude a future document from updating this
         document to specify a protocol for legitimate sender address
         hiding.

   *  Case 2: Malicious sending/transiting/receiving MTA (or anyone
      meddling between MTAs).

      -  Attack situation: An on-path attacker changes the outer From
         Header Field (possibly with other meddling to invalidate the
         signature; see below).  Their goal is to get the rendering MUA
         to show a different From address than the composing MUA
         intended (breaking MUA-to-MUA sender authenticity).

      -  Case 2.a: The composing MUA submitted an unsigned or encrypted-
         only message to the email system.  In this case, there can be
         no sender authenticity anyway.

      -  Case 2.b: The composing MUA submitted a signed-only message to
         the email system.

         o  Case 2.b.i: The attacker removes or invalidates the
            signature.  In this case, the attacker can also modify the
            inner From Header Field to their liking.

         o  Case 2.b.ii: The signature is valid, but the rendering MUA
            does not see any valid binding between the signing
            certificate and the addr-spec of the inner From Header
            Field.  In this case, there can be no sender authenticity
            anyways (the certificate could have been generated by the
            on-path attacker).  This case is indistinguishable from a
            malicious composing MUA; hence, it is "better" to fall back
            to the outer From Header Field that the MTA can validate.
            Note that once the binding is validated (e.g., after an out-
            of-band comparison), the rendering may change from showing
            the outer From address (and a warning) to showing the inner,
            now validated From address.  In some cases, the binding may
            be instantly validated even for previously unseen
            certificates (e.g., if the certificate is issued by a
            trusted certification authority).

      -  Case 2.c: The composing MUA submitted a signed-and-encrypted
         message to the email system.

         o  Case 2.c.i: The attacker removes or invalidates the
            signature.  Note that the signature is inside the ciphertext
            (see Section 5.2 of [RFC9787]).  Thus, assuming the
            encryption is non-malleable, any on-path attacker cannot
            invalidate the signature while ensuring that the message
            still decrypts successfully.

         o  Case 2.c.ii: The signature is valid, but the rendering MUA
            does not see any valid binding between the signing
            certificate and the addr-spec of the inner From Header
            Field.  See case 2.b.ii.

   As the case distinction shows, the outer From Header Field is either
   the preferred fallback (in particular, to avoid introducing a new
   spoofing channel) or just as good (because just as modifiable) as the
   inner From Header Field.

   Rendering the outer From Header Field does carry the risk of a
   "temporary downgrade attack" in cases 2.b.ii and 2.c.ii, where a
   malicious MTA keeps the signature intact but modifies the outer From
   Header Field.  The MUA can resolve this temporary downgrade by
   validating the certificate-to-addr-spec binding.  If the MUA never
   does this validation, the entire message could be fake.

   If there were a signaling channel where the MTA can tell the MUA
   whether it authenticated the From Header Field, an MUA could use this
   in its rendering decision.  In the absence of such a signal, and when
   end-to-end authenticity is unavailable, this document prefers to fall
   back to the outer From Header Field.  This default is based on the
   assumption that most MTAs apply some filtering based on the outer
   From Header Field (whether the MTA can authenticate it or not).
   Rendering the unprotected outer From Header Field (instead of the
   protected inner one) in case of a mismatch retains this ability for
   MTAs.

   If the MUA decides not to rely on the MTA to authenticate the outer
   From Header Field, it may prefer the inner From Header Field.

10.2.  Avoid Cryptographic Summary Confusion from the hp Parameter

   When parsing a message, the recipient MUA infers the message's
   Cryptographic Status from the Cryptographic Layers, as described in
   Section 4.6 of [RFC9787].

   The Cryptographic Layers that make up the Cryptographic Envelope
   describe an ordered list of cryptographic properties as present in
   the message after it has been delivered.  By contrast, the hp
   parameter to the Content-Type Header Field contains a simpler
   indication: whether the composer originally tried to encrypt the
   message or not (see Section 2.1.1).  In particular, for a message
   with Header Protection, the Cryptographic Payload MUST have a hp
   parameter of cipher if the message is encrypted (in addition to
   signed) and clear if no encryption is present (that is, the message
   is signed-only).

   As noted in Section 2.1.1, the rendering implementation MUST NOT
   inflate its estimation of the confidentiality of the message or its
   Header Fields based on the composer's intent if it can see that the
   message was not actually encrypted.  A signed-only message that
   happens to have an hp parameter of cipher is still signed-only.

   Conversely, since the encrypting Cryptographic Layer is typically
   outside the signature layer (see Section 5.2 of [RFC9787]), an
   originally signed-only message could have been wrapped in an
   encryption layer by an intervening party before receipt to appear
   encrypted.

   If a message appears to be wrapped in an encryption layer, and the hp
   parameter is present but is not set to cipher, then it is likely that
   the encryption layer was not added by the original composer.  For
   such a message, the lack of any HP-Outer Header Field (see
   Section 2.2) in the Header Section of the Cryptographic Payload MUST
   NOT be used to infer that all Header Fields were removed from the
   Outer Header Section by the original composer.  In such a case, the
   rendering MUA SHOULD treat every Header Field as though it was not
   confidential.

10.3.  Caution About Composing with Legacy Display Elements

   When composing a message, it's possible for a Legacy Display Element
   (see Section 2.1.2) to contain risky data that could trigger errors
   in a rendering client.

   For example, if the value for a Header Field to be included in a
   Legacy Display Element within a given Body part contains folding
   whitespace, it SHOULD be "unfolded" before generating the Legacy
   Display Element: All contiguous folding whitespace SHOULD be replaced
   with a single space character.  Likewise, if the Header Field value
   was originally encoded per [RFC2047], it SHOULD be decoded first to a
   standard string and re-encoded using the charset appropriate to the
   target part.

   When including a Legacy Display Element in a text/plain part (see
   Section 5.2.2), if the decoded Subject Header Field contains a pair
   of newlines (e.g., if it is broken across multiple lines by encoded
   newlines), the composing MUA MUST strip any newline from the Legacy
   Display Element.  If the pair of newlines is not stripped, a
   rendering MUA that follows the guidance in Section 4.5.3.2 might
   leave the later part of the Legacy Display Element in the rendered
   message.

   When including a Legacy Display Element in a text/html part (see
   Section 5.2.3), any material in the Header Field values MUST be
   explicitly HTML escaped to avoid being rendered as part of the HTML.
   At a minimum, the characters <, >, ', ", and & MUST be escaped to
   &lt;, &gt;, &apos;, &quot;, and &amp;, respectively (for example, see
   [HTML-ESCAPES]).  If unescaped characters from removed or obscured
   Header Field values end up in the Legacy Display Element, a rendering
   MUA that follows the guidance in Section 4.5.3.3 might fail to
   identify the boundaries of the Legacy Display Element, cutting out
   more than it should or leaving remnants visible.  And a Legacy MUA
   parsing such a message might misrender the entire HTML stream,
   depending on the content of the removed or obscured Header Field
   values.

   The Legacy Display Element is a decorative addition solely to enable
   visibility of obscured or removed Header Fields in decryption-capable
   Legacy MUAs.  When it is produced, it should be generated minimally
   and strictly, as described above, to avoid damaging the rest of the
   message.

10.4.  Plaintext Attacks

   An encrypted email message using S/MIME or PGP/MIME tends to have
   some amount of predictable plaintext.  For example, the standard MIME
   Header Fields of the Cryptographic Payload of a message are often a
   predictable sequence of bytes, even without Header Protection, when
   they only include the Structural Header Fields MIME-Version and
   Content-Type.  This is a potential risk for known-plaintext attacks.

   Including protected Header Fields as defined in this document
   increases the amount of known plaintext.  Since some of those Header
   Fields in a reply will be derived from the message being replied to,
   this also creates a potential risk for chosen-plaintext attacks, in
   addition to known-plaintext attacks.  This potential risk also
   applies in a similar manner to forwarded messages.

   Modern message encryption mechanisms are expected to be secure
   against both known-plaintext attacks and chosen-plaintext attacks.
   An MUA composing an encrypted message should ensure that it is using
   such a mechanism, regardless of whether it does Header Protection.

11.  Privacy Considerations

11.1.  Leaks When Replying

   The encrypted Header Fields of a message may accidentally leak when
   replying to the message.  See the guidance in Section 6.

11.2.  Encrypted Header Fields Are Not Always Private

   For encrypted messages, depending on the composer's HCP, some Header
   Fields may appear both within the Cryptographic Envelope and on the
   outside of the message (e.g., Date might exist identically in both
   places).  Section 4.3 identifies such a Header Field as signed-only.
   These Header Fields are clearly _not_ private at all, despite a copy
   being inside the Cryptographic Envelope.

   A Header Field whose name and value are not matched verbatim by any
   HP-Outer Header Field from the same part will have an encrypted-only
   or signed-and-encrypted status.  But even Header Fields with these
   stronger levels of cryptographic confidentiality protection might not
   be as private as the user would like.

   See the examples below.

   This concern is true for any encrypted data, including the Body of
   the message, not just the Header Fields: If the composer isn't
   careful, the message contents or session keys can leak in many ways
   that are beyond the scope of this document.  The message recipient
   has no way in principle to tell whether the apparent confidentiality
   of any given piece of encrypted content has been broken via channels
   that they cannot perceive.  Additionally, an active intermediary
   aware of the recipient's public key can always encrypt a cleartext
   message in transit to give the recipient a false sense of security
   (see also Section 10.2).

11.2.1.  Encrypted Header Fields Can Leak Unwanted Information to the
         Recipient

   For an encrypted message, even with an ambitious HCP that
   successfully obscures most Header Fields from all transport agents,
   Header Fields will be ultimately visible to each intended recipient.
   This can be especially problematic for a Header Field that is not
   User-Facing; the composer may not expect such a Header Field to be
   injected by their MUA.  Consider the three following examples:

   *  The MUA may inject a User-Agent Header Field that describes itself
      to every recipient, even though the composer may not want a
      recipient to know the exact version of their OS, hardware
      platform, or MUA.

   *  The MUA may have an idiosyncratic way of generating a Message-ID
      Header Field, which could embed the choice of MUA, time zone,
      hostname, or other subtle information to a knowledgeable
      recipient.

   *  The MUA may erroneously include a Bcc Header Field in the
      origheaders of a copy of a message sent to a named recipient,
      defeating the purpose of using Bcc instead of Cc (see Section 11.4
      for more details about risks related to Bcc).

   Clearly, no end-to-end cryptographic protection of any Header Field
   as defined in this document will hide such a sensitive field from an
   intended recipient.  Instead, the composing MUA MUST populate the
   origheaders list for any outbound message with only information each
   recipient should have access to.  This is true for any message
   without any cryptographic protection as well, of course, and it is
   even worse there: Such a leak is exposed to the transport agents as
   well as all recipients.  An encrypted message with Header Protection
   and a more ambitious HCP avoids these leaks that expose information
   to the transport agents, but it cannot defend against such a leak to
   a recipient.

11.2.2.  Encrypted Header Fields Can Be Inferred from External or
         Internal Metadata

   For example, if the To and Cc Header Fields are removed from the
   Outer Header Section, the values in those fields might still be
   inferred with high probability by an adversary who looks at the
   message either in transit or at rest.  For example, if the message is
   found in a mailbox, or being delivered to a mailbox, and the mailbox
   is known to be associated with the email address bob@example.org,
   it's likely that Bob was in either To or Cc.  Furthermore, encrypted
   message ciphertext may hint at the recipients: For S/MIME messages,
   the RecipientInfo, and for PGP/MIME messages, the key ID in the
   Public Key Encrypted Session Key (PKESK) packets will all hint at a
   specific set of recipients.  Additionally, an MTA that handles the
   message may add a Received Header Field (or some other custom Header
   Field) that leaks some information about the nature of the delivery.

11.2.3.  Encrypted Header Fields May Not Be Fully Masked by HCP

   In another example, if the HCP modifies the Date Header Field to mask
   out high-resolution timestamps (e.g., rounding to the most recent
   hour), some information about the date of delivery will still be
   attached to the email.  At the very least, the low-resolution, global
   version of the date will be present on the message.  Additionally,
   Header Fields like Received that are added during message delivery
   might include higher-resolution timestamps.  And if the message lands
   in a mailbox that is ordered by time of receipt, even its placement
   in the mailbox and the unobscured Date Header Fields of the
   surrounding messages could leak this information.

   Some Header Fields like From may be impossible to fully obscure, as
   many modern message delivery systems depend on at least domain
   information in the From Header Field for determining whether a
   message is coming from a domain with "good reputation" (that is, from
   a domain that is not known for leaking spam).  So even if an
   ambitious HCP opts to remove the human-readable part from any From
   Header Field and to standardize/genericize the local part of the From
   address, the domain will still leak.

11.3.  A Naive Recipient May Overestimate the Cryptographic Status of a
       Header Field in an Encrypted Message

   When an encrypted (or signed-and-encrypted) message is in transit, an
   active intermediary can strip or tamper with any Header Field that
   appears outside the Cryptographic Envelope.  A rendering MUA that
   naively infers cryptographic status from differences between the
   external Header Fields and those found in the Cryptographic Envelope
   could be tricked into overestimating the protections afforded to some
   Header Fields.

   For example, if the original composer's HCP passes through the Cc
   Header Field unchanged, a cleanly delivered message would indicate
   that the Cc Header Field has a cryptographic status of signed.  But
   if an intermediary attacker simply removes the Header Field from the
   Outer Header Section before forwarding the message, then the naive
   recipient might believe that the field has a cryptographic status of
   signed-and-encrypted.

   This document offers protection against such an attack by way of the
   HP-Outer Header Fields (see Section 2.2) that can be found on the
   Cryptographic Payload.  If a Header Field appears to have been
   obscured by inspection of the Outer Header Section but an HP-Outer
   Header Field matches it exactly, then the rendering MUA can indicate
   to the user that the Header Field in question may not have been
   confidential.

   In such a case, a cautious MUA may render the Header Field in
   question as signed (because the composer did not hide it) but still
   treat it as signed-and-encrypted during reply to avoid accidental
   leakage of the cleartext value in the reply message, as described in
   Section 6.1.

11.4.  Privacy and Deliverability Risks with Bcc and Encrypted Messages

   As noted in Section 9.3 of [RFC9787], handling Bcc when generating an
   encrypted email message can be particularly tricky.  With Header
   Protection, there is an additional wrinkle.  When an encrypted email
   message with Header Protection has a Bcc'ed recipient, and the
   composing MUA explicitly includes the Bcc'ed recipient's address in
   their copy of the message (see the "second method" in Section 3.6.3
   of [RFC5322]), that Bcc Header Field will always be visible to the
   Bcc'ed recipient.

   In this scenario, though, the composing MUA has one additional
   choice: whether or not to hide the Bcc Header Field from intervening
   message transport agents by returning null when the HCP is invoked
   for Bcc. If the composing MUA's rationale for including an explicit
   Bcc in the copy of the message sent to the Bcc recipient is to ensure
   deliverability via a message transport agent that inspects message
   Header Fields, then stripping the Bcc field during encryption may
   cause the intervening transport agent to drop the message entirely.
   This is why Bcc is not explicitly stripped in hcp_baseline.

   On the other hand, if deliverability to a Bcc'ed recipient is not a
   concern, the most privacy-preserving option is to simply omit the Bcc
   Header Field from the protected Header Section in the first place.
   An MUA that is capable of receiving and processing such a message can
   infer that since their user's address was not mentioned in any To or
   Cc Header Field, they were likely a Bcc recipient.

   Please also see Section 9.4 of [RFC9787] for more discussion about
   Bcc and encrypted messages.

12.  IANA Considerations

   This document registers an email Header Field, describes parameters
   for the Content-Type Header Field, and establishes a registry for
   Header Confidentiality Policies to facilitate HCP evolution.

12.1.  Registration of the HP-Outer Header Field

   IANA has registered the following Header Field in the "Permanent
   Message Header Field Names" registry within the "Message Headers"
   registry group <https://www.iana.org/assignments/message-headers> in
   accordance with [RFC3864].

        +===================+==========+==========+===============+
        | Header Field Name | Protocol | Status   | Reference     |
        +===================+==========+==========+===============+
        | HP-Outer          | mail     | standard | Section 2.2.1 |
        |                   |          |          | of RFC 9788   |
        +-------------------+----------+----------+---------------+

          Table 2: Addition to the Permanent Message Header Field
                               Names Registry

   Note that the Template and Trace columns are empty and therefore not
   included in the table.

   The Author/Change Controller (Section 4.5 of [RFC3864]) for this
   entry is the IETF.

12.2.  Reference Update for the Content-Type Header Field

   This document defines the Content-Type parameters known as hp (in
   Section 2.1.1) and hp-legacy-display (in Section 2.1.2).
   Consequently, IANA has added this document as a reference for
   Content-Type in the "Permanent Message Header Field Names" registry
   as shown below.

         +===================+==========+========================+
         | Header Field Name | Protocol | Reference              |
         +===================+==========+========================+
         | Content-Type      | MIME     | [RFC4021] and RFC 9788 |
         +-------------------+----------+------------------------+

           Table 3: Permanent Message Header Field Names Registry

   Note that the Template and Trace columns are empty and therefore not
   included in the table.

12.3.  New Mail Header Confidentiality Policies Registry

   IANA has created a new registry titled "Mail Header Confidentiality
   Policies" within the "MAIL Parameters" registry group
   <https://www.iana.org/assignments/mail-parameters/> with the
   following content:

   +========================+=================+=============+=========+
   | Header Confidentiality | Description     | Recommended |Reference|
   | Policy Name            |                 |             |         |
   +========================+=================+=============+=========+
   | hcp_no_confidentiality | No header       | N           |Section  |
   |                        | confidentiality |             |3.2.3 of |
   |                        |                 |             |RFC 9788 |
   +------------------------+-----------------+-------------+---------+
   | hcp_baseline           | Confidentiality | Y           |Section  |
   |                        | for             |             |3.2.1 of |
   |                        | Informational   |             |RFC 9788 |
   |                        | Header Fields:  |             |         |
   |                        | Subject Header  |             |         |
   |                        | Field is        |             |         |
   |                        | obscured,       |             |         |
   |                        | Keywords and    |             |         |
   |                        | Comments are    |             |         |
   |                        | removed         |             |         |
   +------------------------+-----------------+-------------+---------+
   | hcp_shy                | Obscure         | N           |Section  |
   |                        | Subject, remove |             |3.2.2 of |
   |                        | Keywords and    |             |RFC 9788 |
   |                        | Comments,       |             |         |
   |                        | remove the time |             |         |
   |                        | zone from Date, |             |         |
   |                        | and remove      |             |         |
   |                        | display-names   |             |         |
   |                        | from From, To,  |             |         |
   |                        | and Cc          |             |         |
   +------------------------+-----------------+-------------+---------+

          Table 4: Mail Header Confidentiality Policies Registry

   Note that hcp_example_hide_cc is offered as an example in Section 3.1
   but is not formally registered by this document.

   The following textual note has been added to this registry:

   |  Adding an entry to this registry with an N in the "Recommended"
   |  column follows the registration policy of Specification Required.
   |  Adding an entry to this registry with a Y in the "Recommended"
   |  column or changing the "Recommended" column in an existing entry
   |  (from N to Y or vice versa) requires IETF Review.

   Note that during IETF Review, the designated expert must be
   consulted.  Guidance for the designated expert can be found in
   Section 3.4.2.

   Additionally, this textual note has been added to the registry:

   |  The Header Confidentiality Policy Name never appears on the wire.
   |  This registry merely tracks stable references to implementable
   |  descriptions of distinct policies.  Any addition to this registry
   |  should be governed by guidance in Section 3.4.2 of RFC 9788.

13.  References

13.1.  Normative References

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
              <https://www.rfc-editor.org/info/rfc2045>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3864]  Klyne, G., Nottingham, M., and J. Mogul, "Registration
              Procedures for Message Header Fields", BCP 90, RFC 3864,
              DOI 10.17487/RFC3864, September 2004,
              <https://www.rfc-editor.org/info/rfc3864>.

   [RFC5083]  Housley, R., "Cryptographic Message Syntax (CMS)
              Authenticated-Enveloped-Data Content Type", RFC 5083,
              DOI 10.17487/RFC5083, November 2007,
              <https://www.rfc-editor.org/info/rfc5083>.

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5322]  Resnick, P., Ed., "Internet Message Format", RFC 5322,
              DOI 10.17487/RFC5322, October 2008,
              <https://www.rfc-editor.org/info/rfc5322>.

   [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
              RFC 5652, DOI 10.17487/RFC5652, September 2009,
              <https://www.rfc-editor.org/info/rfc5652>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8551]  Schaad, J., Ramsdell, B., and S. Turner, "Secure/
              Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
              Message Specification", RFC 8551, DOI 10.17487/RFC8551,
              April 2019, <https://www.rfc-editor.org/info/rfc8551>.

   [RFC9580]  Wouters, P., Ed., Huigens, D., Winter, J., and Y. Niibe,
              "OpenPGP", RFC 9580, DOI 10.17487/RFC9580, July 2024,
              <https://www.rfc-editor.org/info/rfc9580>.

   [RFC9787]  Gillmor, D. K., Ed., Melnikov, A., Ed., and B. Hoeneisen,
              Ed., "Guidance on End-to-End Email Security", RFC 9787,
              DOI 10.17487/RFC9787, August 2025,
              <https://www.rfc-editor.org/info/rfc9787>.

13.2.  Informative References

   [chrome-indicators]
              Schechter, E., "Evolving Chrome's security indicators",
              Chromium Blog, May 2018,
              <https://blog.chromium.org/2018/05/evolving-chromes-
              security-indicators.html>.

   [CSS]      Bos, B., Ed., "Cascading Style Sheets Level 2 Revision 2
              (CSS 2.2) Specification", W3C First Public Working Draft,
              12 April 2016,
              <https://www.w3.org/TR/2016/WD-CSS22-20160412/>.  Latest
              version available at <https://www.w3.org/TR/CSS22/>.

   [HTML-ESCAPES]
              W3C, "Using character escapes in markup and CSS", 12
              August 2010, <https://www.w3.org/International/questions/
              qa-escapes#use>.

   [PEP-EMAIL]
              Marques, H. and B. Hoeneisen, "pretty Easy privacy (pEp):
              Email Formats and Protocols", Work in Progress, Internet-
              Draft, draft-pep-email-03, 22 May 2025,
              <https://datatracker.ietf.org/doc/html/draft-pep-email-
              03>.

   [PEP-GENERAL]
              Birk, V., Marques, H., and B. Hoeneisen, "pretty Easy
              privacy (pEp): Privacy by Default", Work in Progress,
              Internet-Draft, draft-pep-general-03, 22 May 2025,
              <https://datatracker.ietf.org/doc/html/draft-pep-general-
              03>.

   [PGPCONTROL]
              UUNET Technologies, Inc., "Authentication of Usenet Group
              Changes", 27 October 2016,
              <https://ftp.isc.org/pub/pgpcontrol/>.

   [PGPVERIFY-FORMAT]
              Lawrence, D. C., "Signing Control Messages, Verifying
              Control Messages",
              <https://www.eyrie.org/~eagle/usefor/other/pgpverify>.

   [PROTECTED-HEADERS]
              Einarsson, B. R., juga, and D. K. Gillmor, "(Deprecated)
              Protected E-mail Headers", Work in Progress, Internet-
              Draft, draft-autocrypt-lamps-protected-headers-03, 16
              April 2025, <https://datatracker.ietf.org/doc/html/draft-
              autocrypt-lamps-protected-headers-03>.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
              November 1987, <https://www.rfc-editor.org/info/rfc1035>.

   [RFC2047]  Moore, K., "MIME (Multipurpose Internet Mail Extensions)
              Part Three: Message Header Extensions for Non-ASCII Text",
              RFC 2047, DOI 10.17487/RFC2047, November 1996,
              <https://www.rfc-editor.org/info/rfc2047>.

   [RFC2049]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part Five: Conformance Criteria and
              Examples", RFC 2049, DOI 10.17487/RFC2049, November 1996,
              <https://www.rfc-editor.org/info/rfc2049>.

   [RFC3156]  Elkins, M., Del Torto, D., Levien, R., and T. Roessler,
              "MIME Security with OpenPGP", RFC 3156,
              DOI 10.17487/RFC3156, August 2001,
              <https://www.rfc-editor.org/info/rfc3156>.

   [RFC3851]  Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
              Extensions (S/MIME) Version 3.1 Message Specification",
              RFC 3851, DOI 10.17487/RFC3851, July 2004,
              <https://www.rfc-editor.org/info/rfc3851>.

   [RFC4021]  Klyne, G. and J. Palme, "Registration of Mail and MIME
              Header Fields", RFC 4021, DOI 10.17487/RFC4021, March
              2005, <https://www.rfc-editor.org/info/rfc4021>.

   [RFC5751]  Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
              Mail Extensions (S/MIME) Version 3.2 Message
              Specification", RFC 5751, DOI 10.17487/RFC5751, January
              2010, <https://www.rfc-editor.org/info/rfc5751>.

   [RFC5890]  Klensin, J., "Internationalized Domain Names for
              Applications (IDNA): Definitions and Document Framework",
              RFC 5890, DOI 10.17487/RFC5890, August 2010,
              <https://www.rfc-editor.org/info/rfc5890>.

   [RFC5891]  Klensin, J., "Internationalized Domain Names in
              Applications (IDNA): Protocol", RFC 5891,
              DOI 10.17487/RFC5891, August 2010,
              <https://www.rfc-editor.org/info/rfc5891>.

   [RFC6376]  Crocker, D., Ed., Hansen, T., Ed., and M. Kucherawy, Ed.,
              "DomainKeys Identified Mail (DKIM) Signatures", STD 76,
              RFC 6376, DOI 10.17487/RFC6376, September 2011,
              <https://www.rfc-editor.org/info/rfc6376>.

   [RFC7489]  Kucherawy, M., Ed. and E. Zwicky, Ed., "Domain-based
              Message Authentication, Reporting, and Conformance
              (DMARC)", RFC 7489, DOI 10.17487/RFC7489, March 2015,
              <https://www.rfc-editor.org/info/rfc7489>.

   [RFC7929]  Wouters, P., "DNS-Based Authentication of Named Entities
              (DANE) Bindings for OpenPGP", RFC 7929,
              DOI 10.17487/RFC7929, August 2016,
              <https://www.rfc-editor.org/info/rfc7929>.

   [RFC8162]  Hoffman, P. and J. Schlyter, "Using Secure DNS to
              Associate Certificates with Domain Names for S/MIME",
              RFC 8162, DOI 10.17487/RFC8162, May 2017,
              <https://www.rfc-editor.org/info/rfc8162>.

   [RFC8617]  Andersen, K., Long, B., Ed., Blank, S., Ed., and M.
              Kucherawy, Ed., "The Authenticated Received Chain (ARC)
              Protocol", RFC 8617, DOI 10.17487/RFC8617, July 2019,
              <https://www.rfc-editor.org/info/rfc8617>.

   [RFC9216]  Gillmor, D. K., Ed., "S/MIME Example Keys and
              Certificates", RFC 9216, DOI 10.17487/RFC9216, April 2022,
              <https://www.rfc-editor.org/info/rfc9216>.

Appendix A.  Table of Pseudocode Listings

   This document contains guidance with pseudocode descriptions.  Each
   algorithm is listed here for easy reference.

    +===========================+=========================+===========+
    | Method Name               | Description             | Reference |
    +===========================+=========================+===========+
    | HeaderSetsFromMessage     | Derive "outer" and      | Section   |
    |                           | "protected" sets of     | 4.2.1     |
    |                           | Header Fields from a    |           |
    |                           | given message           |           |
    +---------------------------+-------------------------+-----------+
    | HeaderFieldProtection     | Calculate cryptographic | Section   |
    |                           | protections for a       | 4.3.1     |
    |                           | Header Field in a given |           |
    |                           | message                 |           |
    +---------------------------+-------------------------+-----------+
    | ReferenceHCP              | Produce an ephemeral    | Section   |
    |                           | HCP to use when         | 6.1.2     |
    |                           | responding to a given   |           |
    |                           | message                 |           |
    +---------------------------+-------------------------+-----------+
    | ComposeNoHeaderProtection | Legacy Message          | Section   |
    |                           | composition with end-   | 5.1.1     |
    |                           | to-end cryptographic    |           |
    |                           | protections (but no     |           |
    |                           | Header Protection)      |           |
    +---------------------------+-------------------------+-----------+
    | Compose                   | Compose a message with  | Section   |
    |                           | end-to-end              | 5.2.1     |
    |                           | cryptographic           |           |
    |                           | protections including   |           |
    |                           | Header Protection       |           |
    +---------------------------+-------------------------+-----------+

                   Table 5: Table of Pseudocode Listings

Appendix B.  Possible Problems with Legacy MUAs

   When an email message with end-to-end cryptographic protection is
   rendered by an MUA, the user might experience many different possible
   problematic interactions.  A message with Header Protection may
   introduce new forms of user experience failure.

   In this section, the authors enumerate different kinds of failures we
   have observed when reviewing, rendering, and replying to messages
   with different forms of Header Protection in different Legacy MUAs.
   Different Legacy MUAs demonstrate different subsets of these
   problems.

   A conformant MUA would not exhibit any of these problems.  An
   implementer updating their Legacy MUA to be compliant with this
   specification should consider these concerns and try to avoid them.

   Recall that "protected" refers to the values of the inner Header
   Fields, e.g., the real Subject, and "unprotected" refers to the
   values of the outer Header Fields, e.g., the replacement Subject.

B.1.  Problems Viewing Messages in a List View

   *  Unprotected Subject, Date, From, and To Header Fields are visible
      (instead of being replaced by protected values)

   *  Threading is not visible

B.2.  Problems When Rendering a Message

   *  Unprotected Subject is visible

   *  Protected Subject (on its own) is visible in the Body

   *  Protected Subject, Date, From, and To Header Fields are visible in
      the Body

   *  User interaction needed to view the whole message

   *  User interaction needed to view the message Body

   *  User interaction needed to view the protected Subject

   *  Impossible to view the protected Subject

   *  Nuisance alarms during user interaction

   *  Impossible to view the message Body

   *  Appears as a forwarded message

   *  Appears as an attachment

   *  Security indicators not visible

   *  Security indicators do not identify the protection status of
      Header Fields

   *  User has multiple different methods to reply (e.g., reply to
      outer, reply to inner)

   *  User sees English "Subject:" in Body despite message itself being
      in non-English

   *  Security indicators do not identify the protection status of
      Header Fields

   *  Header Fields in the Body render with local Header Field names
      (e.g., showing "Betreff" instead of "Subject") and dates (TZ,
      locale)

B.3.  Problems When Replying to a Message

   Note that the use case here is:

   *  User views a message, to the point where they can read it

   *  User then replies to the message, and they are shown a message
      composition window, which has some UI elements

   *  If the MUA has multiple different methods to reply to a message,
      each way may need to be evaluated separately

   This section also uses the shorthand UI:x to mean "the UI element
   that the user can edit that they think of as x".

   *  Unprotected Subject is in UI:subject (instead of the protected
      Subject)

   *  Protected Subject is quoted in UI:body (from Legacy Display
      Element)

   *  Protected Subject leaks when the reply is serialized into MIME

   *  Protected Subject is not anywhere in UI

   *  Message Body is _not_ visible/quoted in UI:body

   *  User cannot reply while viewing protected message

   *  Reply is not encrypted by default (but is for legacy signed-and-
      encrypted messages without Header Protection)

   *  Unprotected From or Reply-To Header Field is in UI:To (instead of
      the protected From or Reply-To Header Field)

   *  User's locale (lang, TZ) leaks in quoted Body

   *  Header Fields not protected (and in particular, Subject is not
      obscured) by default

Appendix C.  Test Vectors

   This section contains sample messages using the specification defined
   above.  Each sample contains a MIME object, a textual and
   diagrammatic view of its structure, and examples of how an MUA might
   render it.

   The cryptographic protections used in this document use the S/MIME
   standard, and keying material and certificates come from [RFC9216].

   These messages should be accessible to any IMAP client at
   imap://bob@header-protection.cmrg.net/ (any password should
   authenticate to this read-only IMAP mailbox).

   Copies of these test vectors can also be downloaded separately at
   <https://header-protection.cmrg.net>.

   If any of the messages downloaded differ from those offered here,
   this document is the canonical source.

C.1.  Baseline Messages

   These messages offer no Header Protection at all and can be used as a
   baseline.  They are provided in this document as a counterexample.
   An MUA implementer can use these Messages to verify that the reported
   Cryptographic Summary of the Message indicates no Header Protection.

C.1.1.  No Cryptographic Protections over a Simple Message

   This message uses no cryptographic protection at all.  Its Body is a
   text/plain message.

   It has the following structure:

   └─╴text/plain 152 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit
   Subject: no-crypto
   Message-ID: <no-crypto@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:00:02 -0500
   User-Agent: Sample MUA Version 1.0

   This is the
   no-crypto
   message.

   This message uses no cryptographic protection at all.  Its Body
   is a text/plain message.

   --
   Alice
   alice@smime.example

C.1.2.  S/MIME Signed-Only signedData over a Simple Message, No Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message.  It uses no Header Protection.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 3856 bytes
    ┴ (unwraps to)
    └─╴text/plain 206 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part
   Message-ID: <smime-one-part@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:01:02 -0500
   User-Agent: Sample MUA Version 1.0

   MIILGQYJKoZIhvcNAQcCoIILCjCCCwYCAQExDTALBglghkgBZQMEAgEwggFCBgkq
   hkiG9w0BBwGgggEzBIIBL01JTUUtVmVyc2lvbjogMS4wDQpDb250ZW50LVR5cGU6
   IHRleHQvcGxhaW47IGNoYXJzZXQ9InV0Zi04Ig0KQ29udGVudC1UcmFuc2Zlci1F
   bmNvZGluZzogN2JpdA0KDQpUaGlzIGlzIHRoZQ0Kc21pbWUtb25lLXBhcnQNCm1l
   c3NhZ2UuDQoNClRoaXMgaXMgYSBzaWduZWQtb25seSBTL01JTUUgbWVzc2FnZSB2
   aWEgUEtDUyM3IHNpZ25lZERhdGEuICBUaGUNCnBheWxvYWQgaXMgYSB0ZXh0L3Bs
   YWluIG1lc3NhZ2UuIEl0IHVzZXMgbm8gSGVhZGVyIFByb3RlY3Rpb24uDQoNCi0t
   IA0KQWxpY2UNCmFsaWNlQHNtaW1lLmV4YW1wbGUNCqCCB6YwggPPMIICt6ADAgEC
   AhMPLSW9ETmXSs5CVIeh7j00Boq0MA0GCSqGSIb3DQEBDQUAMFUxDTALBgNVBAoT
   BElFVEYxETAPBgNVBAsTCExBTVBTIFdHMTEwLwYDVQQDEyhTYW1wbGUgTEFNUFMg
   UlNBIENlcnRpZmljYXRpb24gQXV0aG9yaXR5MCAXDTE5MTEyMDA2NTQxOFoYDzIw
   NTIwOTI3MDY1NDE4WjA7MQ0wCwYDVQQKEwRJRVRGMREwDwYDVQQLEwhMQU1QUyBX
   RzEXMBUGA1UEAxMOQWxpY2UgTG92ZWxhY2UwggEiMA0GCSqGSIb3DQEBAQUAA4IB
   DwAwggEKAoIBAQCalSn6i8Gi44/oAVAn5GnCk4PHHNjrSfWUnnelN41KImVaTC3D
   9zFCrS3i4Pa9ZgHyA5Qf8JW3ZmnVz5q7M8onZm7mZjqQeb6FUH4i2GMt4jse2Dqs
   165ernT9O5NLFflHUjURca3ynqEBBV4DmhnZp8eDhv3t6dXyCjNHT82S6DgCReZu
   TtMc1zy++MxQlqdn9WZLhOAOpeNZKGmVwjeVy+8FkyzC3jX/Qcm+ZLCqlLqhBwDH
   dZ5qDTII2PVX1X3K7/cONxhvBbaUl/k1swdszUtjhflyFZ80RuQ3qFC6vL/PGeWy
   6SCf58duq/AOEksCAWlb+MD8QH9Yj7CFSmq1AgMBAAGjga8wgawwDAYDVR0TAQH/
   BAIwADAXBgNVHSAEEDAOMAwGCmCGSAFlAwIBMAEwHgYDVR0RBBcwFYETYWxpY2VA
   c21pbWUuZXhhbXBsZTATBgNVHSUEDDAKBggrBgEFBQcDBDAOBgNVHQ8BAf8EBAMC
   BSAwHQYDVR0OBBYEFKJTQdVEPIApFXwBI/Dnjq/N83cPMB8GA1UdIwQYMBaAFJEw
   jnwHFwyn8QkoZTYaZxxodvRZMA0GCSqGSIb3DQEBDQUAA4IBAQCBSXignLEynBak
   DKU68ro0RsyXWAPkfXgQLgy7GrW7SrZeBc5IEcjoN9f/gsOx/Ht9Ii6zyBZVjdao
   x644DsiLOQEP4YMS7y4q94RFFdmdzEbDLYx9sfUhvdTxDNOOoHz53PYDBh4zE4Na
   r2inC0D+VM6RGDy66K9l+D+bl8Wj9CyGUc1ppMNURexTg+z3web/eDOdu+F2MVtl
   uLihne0Bp1GUTkr0mJBolg6dSYal8Hw8/ANHpyExl56BJABb744gqoeuD9YSHjKK
   49+qYC9faFmQ+mK80lh1M9RdNI7srjn0LKpuob6w06jaRzWdNeXzlEc2tUpAr4vR
   hZjVD6FYMIIDzzCCAregAwIBAgITN0EFee11f0Kpolw69Phqzpqp1zANBgkqhkiG
   9w0BAQ0FADBVMQ0wCwYDVQQKEwRJRVRGMREwDwYDVQQLEwhMQU1QUyBXRzExMC8G
   A1UEAxMoU2FtcGxlIExBTVBTIFJTQSBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eTAg
   Fw0xOTExMjAwNjU0MThaGA8yMDUyMDkyNzA2NTQxOFowOzENMAsGA1UEChMESUVU
   RjERMA8GA1UECxMITEFNUFMgV0cxFzAVBgNVBAMTDkFsaWNlIExvdmVsYWNlMIIB
   IjANBgkqhkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAtPSJ6Fg4Fj5Nmn9PkrYo0jTk
   fCv4TfA/pdO/KLpZbJOAEr0sI7AjaO7B1GuMUFJeSTulamNfCwDcDkY63PQWl+DI
   Ls7GxVwXurhYdZlaV5hcUqVAckPvedDBc/3rz4D/esFfs+E7QMFtmd+K04s+A8TC
   NO12DRVBDpbP4JFD9hsc8prDtpGmFk7rd0q8gqnhxBW2RZAeLqzJOMayCQtws1q7
   ktkNBR2wZX5ICjecF1YJFhX4jrnHwp/iELGqqaNXd3/Y0pG7QFecN7836IPPdfTM
   SiPR+peCrhJZwLSewbWXLJe3VMvbvQjoBMpEYlaJBUIKkO1zQ1Pq90njlsJLOwID
   AQABo4GvMIGsMAwGA1UdEwEB/wQCMAAwFwYDVR0gBBAwDjAMBgpghkgBZQMCATAB
   MB4GA1UdEQQXMBWBE2FsaWNlQHNtaW1lLmV4YW1wbGUwEwYDVR0lBAwwCgYIKwYB
   BQUHAwQwDgYDVR0PAQH/BAQDAgbAMB0GA1UdDgQWBBS79syyLR0GEhyXrilqkBDT
   IGZmczAfBgNVHSMEGDAWgBSRMI58BxcMp/EJKGU2GmccaHb0WTANBgkqhkiG9w0B
   AQ0FAAOCAQEAc4miNqfOqaBpI3f+CpJDhxtuZ2P9HjQEQ+v6BdP7GKJ19naIs3Bj
   JOd64roAKHAp+c284VvyVXWJ99FMX8q2ZUQMxH+xh6oAfzcozmnd6XaVWHg4eHIj
   So27PmhKE1oAJKKhDbdbEcZXL2+x1V+duGymWtaD01DZZukKYr7agyHahiXRn/C9
   cy31wbqNsy9x0fjPQg6+DqatiQpMz9EIae6aCHHBhOiPU7IPkazgPYgkLD59fk4P
   GHnYxs1FhdO6zZk9E8zwlc1ALgZa/iSbczisqckN3qGehD2s16jMhwFXLJtBiN+u
   CDgNG/D0qyTbY4fgKieUHx/tHuzUszZxJjGCAgAwggH8AgEBMGwwVTENMAsGA1UE
   ChMESUVURjERMA8GA1UECxMITEFNUFMgV0cxMTAvBgNVBAMTKFNhbXBsZSBMQU1Q
   UyBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3JpdHkCEzdBBXntdX9CqaJcOvT4as6a
   qdcwCwYJYIZIAWUDBAIBoGkwGAYJKoZIhvcNAQkDMQsGCSqGSIb3DQEHATAcBgkq
   hkiG9w0BCQUxDxcNMjEwMjIwMTUwMTAyWjAvBgkqhkiG9w0BCQQxIgQg+APzZJl4
   pcksifU3FOYwAUqexbFmtbnUdg8eCFIklg8wDQYJKoZIhvcNAQEBBQAEggEARlZH
   lulQA7h4AzGUznSRv1TB3w2u4oXQBgxTTaUFXvezPsEacndc16K4ESz8IpjsLEqC
   lhFU6haOKz3OZnab6A8sCqozqAoCpJI35L3D0XwlqucqRDMQoNDZf1AZw1/2rvhl
   BA4+YVc1vNjwbFF7T8bz6ttkXBdseesPV8zy01tsPVBSEr9A8QtVGTPw/BLEV/sV
   d6QtbPMCqdVDjRAa5onUPyZvXkt+Qkt5Wcqxfwbotg/u7ecLhqnK0rC2SZkGDjtZ
   a6BuLu88DxA9T90G+L3hhL5VPdEdkdRCounTb9McyGWWmnK0PYind/sKBATP5ouF
   jj3rLaMfllxGB0xn3A==

C.1.2.1.  S/MIME Signed-Only signedData over a Simple Message, No Header
          Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message. It uses no Header Protection.

   --
   Alice
   alice@smime.example

C.1.3.  S/MIME Signed-Only multipart/signed over a Simple Message, No
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a text/plain message.  It uses no
   Header Protection.

   It has the following structure:

   └┬╴multipart/signed 4187 bytes
    ├─╴text/plain 224 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="e19";
    micalg="sha-256"
   Subject: smime-multipart
   Message-ID: <smime-multipart@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:02:02 -0500
   User-Agent: Sample MUA Version 1.0

   --e19
   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a text/plain
   message. It uses no Header Protection.

   --
   Alice
   alice@smime.example

   --e19
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

   MIIJ4AYJKoZIhvcNAQcCoIIJ0TCCCc0CAQExDTALBglghkgBZQMEAgEwCwYJKoZI
   hvcNAQcBoIIHpjCCA88wggK3oAMCAQICEw8tJb0ROZdKzkJUh6HuPTQGirQwDQYJ
   KoZIhvcNAQENBQAwVTENMAsGA1UEChMESUVURjERMA8GA1UECxMITEFNUFMgV0cx
   MTAvBgNVBAMTKFNhbXBsZSBMQU1QUyBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3Jp
   dHkwIBcNMTkxMTIwMDY1NDE4WhgPMjA1MjA5MjcwNjU0MThaMDsxDTALBgNVBAoT
   BElFVEYxETAPBgNVBAsTCExBTVBTIFdHMRcwFQYDVQQDEw5BbGljZSBMb3ZlbGFj
   ZTCCASIwDQYJKoZIhvcNAQEBBQADggEPADCCAQoCggEBAJqVKfqLwaLjj+gBUCfk
   acKTg8cc2OtJ9ZSed6U3jUoiZVpMLcP3MUKtLeLg9r1mAfIDlB/wlbdmadXPmrsz
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   --e19--

C.1.4.  S/MIME Signed-and-Encrypted over a Simple Message, No Header
        Protection

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses no Header Protection.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 6720 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 3960 bytes
     ┴ (unwraps to)
     └─╴text/plain 241 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: smime-signed-enc
   Message-ID: <smime-signed-enc@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:03:02 -0500
   User-Agent: Sample MUA Version 1.0

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   BiD2T2KmI8jOgLaSmuAnyw==

C.1.4.1.  S/MIME Signed-and-Encrypted over a Simple Message, No Header
          Protection, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

   MIILPAYJKoZIhvcNAQcCoIILLTCCCykCAQExDTALBglghkgBZQMEAgEwggFlBgkq
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C.1.4.2.  S/MIME Signed-and-Encrypted over a Simple Message, No Header
          Protection, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: text/plain; charset="utf-8"
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses no Header Protection.

   --
   Alice
   alice@smime.example

C.1.5.  No Cryptographic Protections over a Complex Message

   This message uses no cryptographic protection at all.  Its Body is a
   multipart/alternative message with an inline image/png attachment.

   It has the following structure:

   └┬╴multipart/mixed 1402 bytes
    ├┬╴multipart/alternative 794 bytes
    │├─╴text/plain 206 bytes
    │└─╴text/html 304 bytes
    └─╴image/png inline 232 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="0cf"
   Subject: no-crypto-complex
   Message-ID: <no-crypto-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:00:02 -0500
   User-Agent: Sample MUA Version 1.0

   --0cf
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="6e6"

   --6e6
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   no-crypto-complex
   message.

   This message uses no cryptographic protection at all.  Its Body
   is a multipart/alternative message with an inline image/png
   attachment.

   --
   Alice
   alice@smime.example
   --6e6
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>no-crypto-complex</b>
   message.</p>
   <p>This message uses no cryptographic protection at all.  Its Body
   is a multipart/alternative message with an inline image/png
   attachment.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --6e6--

   --0cf
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --0cf--

C.1.6.  S/MIME Signed-Only signedData over a Complex Message, No Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline image/png
   attachment.  It uses no Header Protection.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 5253 bytes
    ┴ (unwraps to)
    └┬╴multipart/mixed 1288 bytes
     ├┬╴multipart/alternative 882 bytes
     │├─╴text/plain 260 bytes
     │└─╴text/html 355 bytes
     └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-complex
   Message-ID: <smime-one-part-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:01:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.1.6.1.  S/MIME Signed-Only signedData over a Complex Message, No
          Header Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="db0"

   --db0
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="51d"

   --51d
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part-complex
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses no Header Protection.

   --
   Alice
   alice@smime.example
   --51d
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-one-part-complex</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses no Header Protection.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --51d--

   --db0
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --db0--

C.1.7.  S/MIME Signed-Only multipart/signed over a Complex Message, No
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a multipart/alternative message
   with an inline image/png attachment.  It uses no Header Protection.

   It has the following structure:

   └┬╴multipart/signed 5230 bytes
    ├┬╴multipart/mixed 1344 bytes
    │├┬╴multipart/alternative 938 bytes
    ││├─╴text/plain 278 bytes
    ││└─╴text/html 376 bytes
    │└─╴image/png inline 232 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="872";
    micalg="sha-256"
   Subject: smime-multipart-complex
   Message-ID: <smime-multipart-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:02:02 -0500
   User-Agent: Sample MUA Version 1.0

   --872
   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="757"

   --757
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="3ff"

   --3ff
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart-complex
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no Header Protection.

   --
   Alice
   alice@smime.example
   --3ff
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-multipart-complex</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no Header Protection.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --3ff--

   --757
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --757--

   --872
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   --872--

C.1.8.  S/MIME Signed-and-Encrypted over a Complex Message, No Header
        Protection

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses no
   Header Protection.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8710 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 5434 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 1356 bytes
      ├┬╴multipart/alternative 950 bytes
      │├─╴text/plain 295 bytes
      │└─╴text/html 390 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: smime-signed-enc-complex
   Message-ID: <smime-signed-enc-complex@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:03:02 -0500
   User-Agent: Sample MUA Version 1.0

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   w/CDpK5lffK0VMX62Dce+3QefqFVifhmXQfYRxgJGSh/qGYeLLLdiOWrdeZrFrvD

C.1.8.1.  S/MIME Signed-and-Encrypted over a Complex Message, No Header
          Protection, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   AQ0FAAOCAQEAc4miNqfOqaBpI3f+CpJDhxtuZ2P9HjQEQ+v6BdP7GKJ19naIs3Bj
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   cy31wbqNsy9x0fjPQg6+DqatiQpMz9EIae6aCHHBhOiPU7IPkazgPYgkLD59fk4P
   GHnYxs1FhdO6zZk9E8zwlc1ALgZa/iSbczisqckN3qGehD2s16jMhwFXLJtBiN+u
   CDgNG/D0qyTbY4fgKieUHx/tHuzUszZxJjGCAgAwggH8AgEBMGwwVTENMAsGA1UE
   ChMESUVURjERMA8GA1UECxMITEFNUFMgV0cxMTAvBgNVBAMTKFNhbXBsZSBMQU1Q
   UyBSU0EgQ2VydGlmaWNhdGlvbiBBdXRob3JpdHkCEzdBBXntdX9CqaJcOvT4as6a
   qdcwCwYJYIZIAWUDBAIBoGkwGAYJKoZIhvcNAQkDMQsGCSqGSIb3DQEHATAcBgkq
   hkiG9w0BCQUxDxcNMjEwMjIwMTcwMzAyWjAvBgkqhkiG9w0BCQQxIgQgup+VC4mf
   BVNHPJS0b9oKX/dVMKiR3JOz5AXfqv/YG0AwDQYJKoZIhvcNAQEBBQAEggEAJ2XX
   xojAdRnBTCRahPos057TnArr1wju76pnJSWXK1flGWjEsSpHVro2t9LRKALqwTnX
   YLM1PbrPoMyivqfhFik1h1dR9J2aXisS4FfZB3jj1c8XkD1yZb8qTBBRQ4v17MFS
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   aaY3k5UwyHNxRjFkkQAsnMe7HAiVnwsDLYCDOXACbg/DOwOCFK9vzDYkD5HjnqK2
   wrhkTs1R4OZW+gWXPhFYClf3fMvrGZvr9rCwgjnwMvrpQjugZi5QGoi/sEdHO5T5
   edT2/t+0u3oJtCflrQ==

C.1.8.2.  S/MIME Signed-and-Encrypted over a Complex Message, No Header
          Protection, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="363"

   --363
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="f27"

   --f27
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no Header Protection.

   --
   Alice
   alice@smime.example
   --f27
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses no Header Protection.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --f27--

   --363
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --363--

C.2.  Signed-Only Messages

   These messages are signed-only, using different schemes of Header
   Protection and different S/MIME structures.  They use no HCP because
   the HCP is only relevant when a message is encrypted.

C.2.1.  S/MIME Signed-Only signedData over a Simple Message, Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message.  It uses the Header Protection
   scheme from RFC 9788.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 4189 bytes
    ┴ (unwraps to)
    └─╴text/plain 232 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-hp
   Message-ID: <smime-one-part-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:06:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.2.1.1.  S/MIME Signed-Only signedData over a Simple Message, Header
          Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-one-part-hp
   Message-ID: <smime-one-part-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:06:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="clear"

   This is the
   smime-one-part-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a text/plain message. It uses the Header Protection
   scheme from RFC 9788.

   --
   Alice
   alice@smime.example

C.2.2.  S/MIME Signed-Only multipart/signed over a Simple Message,
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a text/plain message.  It uses
   the Header Protection scheme from RFC 9788.

   It has the following structure:

   └┬╴multipart/signed 4434 bytes
    ├─╴text/plain 249 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="54f";
    micalg="sha-256"
   Subject: smime-multipart-hp
   Message-ID: <smime-multipart-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:07:02 -0500
   User-Agent: Sample MUA Version 1.0

   --54f
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-multipart-hp
   Message-ID: <smime-multipart-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:07:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="clear"

   This is the
   smime-multipart-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788.

   --
   Alice
   alice@smime.example

   --54f
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   --54f--

C.2.3.  S/MIME Signed-Only signedData over a Complex Message, Header
        Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline image/png
   attachment.  It uses the Header Protection scheme from RFC 9788.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 5643 bytes
    ┴ (unwraps to)
    └┬╴multipart/mixed 1568 bytes
     ├┬╴multipart/alternative 932 bytes
     │├─╴text/plain 286 bytes
     │└─╴text/html 381 bytes
     └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-complex-hp
   Message-ID: <smime-one-part-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:06:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.2.3.1.  S/MIME Signed-Only signedData over a Complex Message, Header
          Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-one-part-complex-hp
   Message-ID: <smime-one-part-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:06:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="ab8"; hp="clear"

   --ab8
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="0f4"

   --0f4
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part-complex-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the Header Protection scheme from
   RFC 9788.

   --
   Alice
   alice@smime.example
   --0f4
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-one-part-complex-hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the Header Protection scheme from
   RFC 9788.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --0f4--

   --ab8
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --ab8--

C.2.4.  S/MIME Signed-Only multipart/signed over a Complex Message,
        Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a multipart/alternative message
   with an inline image/png attachment.  It uses the Header Protection
   scheme from RFC 9788.

   It has the following structure:

   └┬╴multipart/signed 5518 bytes
    ├┬╴multipart/mixed 1626 bytes
    │├┬╴multipart/alternative 988 bytes
    ││├─╴text/plain 303 bytes
    ││└─╴text/html 401 bytes
    │└─╴image/png inline 232 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="a64";
    micalg="sha-256"
   Subject: smime-multipart-complex-hp
   Message-ID: <smime-multipart-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:07:02 -0500
   User-Agent: Sample MUA Version 1.0

   --a64
   MIME-Version: 1.0
   Subject: smime-multipart-complex-hp
   Message-ID: <smime-multipart-complex-hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:07:02 -0500
   User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="550"; hp="clear"

   --550
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="fcd"

   --fcd
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart-complex-hp
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788.

   --
   Alice
   alice@smime.example
   --fcd
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-multipart-complex-hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --fcd--

   --550
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --550--

   --a64
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   Bsa2Y/uhoez/pldX/5A7Rv+JX2vdt71C2BZAk4166wvDhhlHf9pVCWXdKXSh99c6
   Do1TzpnakOm4bKSzPMXTrz1p5GcfDzO94kbNImkcdr8yAdcB

   --a64--

C.2.5.  S/MIME Signed-Only signedData over a Complex Message, Legacy RFC
        8551 Header Protection

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline image/png
   attachment.  It uses the legacy RFC 8551 Header Protection
   (RFC8551HP) scheme.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 5696 bytes
    ┴ (unwraps to)
    └┬╴message/rfc822 1660 bytes
     └┬╴multipart/mixed 1612 bytes
      ├┬╴multipart/alternative 974 bytes
      │├─╴text/plain 296 bytes
      │└─╴text/html 394 bytes
      └─╴image/png inline 232 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"
   Subject: smime-one-part-complex-rfc8551hp
   Message-ID: <smime-one-part-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:26:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.2.5.1.  S/MIME Signed-Only signedData over a Complex Message, Legacy
          RFC 8551 Header Protection, Unwrapped

   The S/MIME signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: message/rfc822

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="fcc"
   Subject: smime-one-part-complex-rfc8551hp
   Message-ID: <smime-one-part-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:26:02 -0500
   User-Agent: Sample MUA Version 1.0

   --fcc
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="0f8"

   --0f8
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-one-part-complex-rfc8551hp
   message.

   This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the legacy RFC 8551 Header
   Protection (RFC8551HP) scheme.

   --
   Alice
   alice@smime.example
   --0f8
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-one-part-complex-rfc8551hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 signedData.  The
   payload is a multipart/alternative message with an inline
   image/png attachment. It uses the legacy RFC 8551 Header
   Protection (RFC8551HP) scheme.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --0f8--

   --fcc
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --fcc--

C.2.6.  S/MIME Signed-Only multipart/signed over a Complex Message,
        Legacy RFC 8551 Header Protection

   This is a signed-only S/MIME message via PKCS#7 detached signature
   (multipart/signed).  The payload is a multipart/alternative message
   with an inline image/png attachment.  It uses the legacy RFC 8551
   Header Protection (RFC8551HP) scheme.

   It has the following structure:

   └┬╴multipart/signed 5624 bytes
    ├┬╴message/rfc822 1718 bytes
    │└┬╴multipart/mixed 1670 bytes
    │ ├┬╴multipart/alternative 1030 bytes
    │ │├─╴text/plain 324 bytes
    │ │└─╴text/html 422 bytes
    │ └─╴image/png inline 232 bytes
    └─╴application/pkcs7-signature [smime.p7s] 3429 bytes

   Its contents are:

   MIME-Version: 1.0
   Content-Type: multipart/signed;
    protocol="application/pkcs7-signature"; boundary="740";
    micalg="sha-256"
   Subject: smime-multipart-complex-rfc8551hp
   Message-ID: <smime-multipart-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:27:02 -0500
   User-Agent: Sample MUA Version 1.0

   --740
   MIME-Version: 1.0
   Content-Type: message/rfc822

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="cf8"
   Subject: smime-multipart-complex-rfc8551hp
   Message-ID: <smime-multipart-complex-rfc8551hp@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:27:02 -0500
   User-Agent: Sample MUA Version 1.0

   --cf8
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="e8a"

   --e8a
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-multipart-complex-rfc8551hp
   message.

   This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 Header Protection
   (RFC8551HP) scheme.

   --
   Alice
   alice@smime.example
   --e8a
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-multipart-complex-rfc8551hp</b>
   message.</p>
   <p>This is a signed-only S/MIME message via PKCS#7 detached
   signature (multipart/signed).  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 Header Protection
   (RFC8551HP) scheme.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --e8a--

   --cf8
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --cf8--

   --740
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-signature; name="smime.p7s"

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   URqYcZhz+4DJc9iim5CqXRZzIF6t6fioS8lCBalaWRy4AaEJ

   --740--

C.3.  Signed-and-Encrypted Messages

   These messages are signed and encrypted.  They use PKCS#7 signedData
   inside envelopedData, with different Header Protection schemes and
   different Header Confidentiality Policies.

C.3.1.  S/MIME Signed-and-Encrypted over a Simple Message, Header
        Protection with hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_baseline Header Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 7825 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 4786 bytes
     ┴ (unwraps to)
     └─╴text/plain 330 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:09:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.3.1.1.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.1.2.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline
   Message-ID: <smime-signed-enc-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:09:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-baseline@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:09:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-baseline
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_baseline` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.2.  S/MIME Signed-and-Encrypted over a Simple Message, Header
        Protection with hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_baseline Header Confidentiality Policy with a "Legacy Display"
   element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8085 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 4972 bytes
     ┴ (unwraps to)
     └─╴text/plain 418 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:10:02 -0500
   User-Agent: Sample MUA Version 1.0

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   oOAsZzzqd5R1io5ABgZD5A==

C.3.2.1.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_baseline (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   4RtIRPzk8ehj5ko6LULT

C.3.2.2.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_baseline (+ Legacy Display), Decrypted and
          Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline-legacy
   Message-ID: <smime-signed-enc-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:10:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-baseline-legacy@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:10:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-baseline-legacy

   This is the
   smime-signed-enc-hp-baseline-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_baseline` Header Confidentiality Policy with a "Legacy
   Display" element.

   --
   Alice
   alice@smime.example

C.3.3.  S/MIME Signed-and-Encrypted over a Simple Message, Header
        Protection with hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_shy Header Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 7760 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 4732 bytes
     ┴ (unwraps to)
     └─╴text/plain 320 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:12:02 +0000
   User-Agent: Sample MUA Version 1.0

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C.3.3.1.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.3.2.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy
   Message-ID: <smime-signed-enc-hp-shy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:12:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-shy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:12:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-shy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_shy` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.4.  S/MIME Signed-and-Encrypted over a Simple Message, Header
        Protection with hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_shy Header Confidentiality Policy with a "Legacy Display"
   element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8190 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 5050 bytes
     ┴ (unwraps to)
     └─╴text/plain 506 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy-legacy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:13:02 +0000
   User-Agent: Sample MUA Version 1.0

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C.3.4.1.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.4.2.  S/MIME Signed-and-Encrypted over a Simple Message, Header
          Protection with hcp_shy (+ Legacy Display), Decrypted and
          Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy-legacy
   Message-ID: <smime-signed-enc-hp-shy-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:13:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-shy-legacy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:13:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-shy-legacy
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:13:02 -0500

   This is the
   smime-signed-enc-hp-shy-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_shy` Header Confidentiality Policy with a "Legacy
   Display" element.

   --
   Alice
   alice@smime.example

C.3.5.  S/MIME Signed-and-Encrypted Reply over a Simple Message, Header
        Protection with hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_baseline Header Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8300 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 5136 bytes
     ┴ (unwraps to)
     └─╴text/plain 336 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline@example>
   References: <smime-signed-enc-hp-baseline@example>

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   A7Ihrk0TXY6pECERvfrAhWhVQsxrBQqND3Fbc2Nk6vc=

C.3.5.1.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.5.2.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline-reply
   Message-ID: <smime-signed-enc-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline@example>
   References: <smime-signed-enc-hp-baseline@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-baseline-reply@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:15:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-hp-baseline@example>
   HP-Outer: References: <smime-signed-enc-hp-baseline@example>
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-baseline-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_baseline` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.6.  S/MIME Signed-and-Encrypted Reply over a Simple Message, Header
        Protection with hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_baseline Header Confidentiality Policy with a "Legacy Display"
   element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8625 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 5376 bytes
     ┴ (unwraps to)
     └─╴text/plain 430 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-baseline-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline-legacy@example>
   References: <smime-signed-enc-hp-baseline-legacy@example>

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C.3.6.1.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_baseline (+ Legacy Display),
          Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.6.2.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_baseline (+ Legacy Display),
          Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-baseline-legacy-reply
   Message-ID: <smime-signed-enc-hp-baseline-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-baseline-legacy@example>
   References: <smime-signed-enc-hp-baseline-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-baseline-legacy-reply@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 10:16:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer:
    In-Reply-To: <smime-signed-enc-hp-baseline-legacy@example>
   HP-Outer:
    References: <smime-signed-enc-hp-baseline-legacy@example>
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-baseline-legacy-reply

   This is the
   smime-signed-enc-hp-baseline-legacy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_baseline` Header Confidentiality Policy with a "Legacy
   Display" element.

   --
   Alice
   alice@smime.example

C.3.7.  S/MIME Signed-and-Encrypted Reply over a Simple Message, Header
        Protection with hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_shy Header Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8190 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 5054 bytes
     ┴ (unwraps to)
     └─╴text/plain 326 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:18:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy@example>
   References: <smime-signed-enc-hp-shy@example>

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   jc7o/O8cONg3kbHyYbGUlaIKBBap285GWNSotQSChkaDo3hT6S6cjPVsrPoqMN8j
   PQNPARqKYRriyI3ej8msK311VRTjAGKWwEVgxn4nF02bvg9HBqv/TiFrTrFLSsd8
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   MTd1VoPxWmlvj/z2NahU05eC12yx29sJN9Oqz4DB2juUaLFSA45+rJMQc9Vj8fFr
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   d3DBHNGszistpi7cUoOgYVMvFwuTSPgvLfyb3CiyUHlK3TIEqox2BJun3tXL2P9s
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   3Z4937SMWwtg+oZYcd+ndW1OnEVyGqTmWB2UKhJhfIhL1YzpkS6444tdlIV2LKeK
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C.3.7.1.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   ZWIgMjAyMSAxMDoxODowMiAtMDUwMA0KVXNlci1BZ2VudDogU2FtcGxlIE1VQSBW
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   eUBleGFtcGxlPg0KUmVmZXJlbmNlczogPHNtaW1lLXNpZ25lZC1lbmMtaHAtc2h5
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   DQptZXNzYWdlLg0KDQpUaGlzIGlzIGEgc2lnbmVkLWFuZC1lbmNyeXB0ZWQgUy9N
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   UG9saWN5Lg0KDQotLSANCkFsaWNlDQphbGljZUBzbWltZS5leGFtcGxlDQqgggem
   MIIDzzCCAregAwIBAgITDy0lvRE5l0rOQlSHoe49NAaKtDANBgkqhkiG9w0BAQ0F
   ADBVMQ0wCwYDVQQKEwRJRVRGMREwDwYDVQQLEwhMQU1QUyBXRzExMC8GA1UEAxMo
   U2FtcGxlIExBTVBTIFJTQSBDZXJ0aWZpY2F0aW9uIEF1dGhvcml0eTAgFw0xOTEx
   MjAwNjU0MThaGA8yMDUyMDkyNzA2NTQxOFowOzENMAsGA1UEChMESUVURjERMA8G
   A1UECxMITEFNUFMgV0cxFzAVBgNVBAMTDkFsaWNlIExvdmVsYWNlMIIBIjANBgkq
   hkiG9w0BAQEFAAOCAQ8AMIIBCgKCAQEAmpUp+ovBouOP6AFQJ+RpwpODxxzY60n1
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   hVB+IthjLeI7Htg6rNeuXq50/TuTSxX5R1I1EXGt8p6hAQVeA5oZ2afHg4b97enV
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   /0HJvmSwqpS6oQcAx3Weag0yCNj1V9V9yu/3DjcYbwW2lJf5NbMHbM1LY4X5chWf
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   BgNVHSMEGDAWgBSRMI58BxcMp/EJKGU2GmccaHb0WTANBgkqhkiG9w0BAQ0FAAOC
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   sfx7fSIus8gWVY3WqMeuOA7IizkBD+GDEu8uKveERRXZncxGwy2MfbH1Ib3U8QzT
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   98Hm/3gznbvhdjFbZbi4oZ3tAadRlE5K9JiQaJYOnUmGpfB8PPwDR6chMZeegSQA
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C.3.7.2.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy-reply
   Message-ID: <smime-signed-enc-hp-shy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:18:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy@example>
   References: <smime-signed-enc-hp-shy@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-hp-shy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:18:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-hp-shy@example>
   HP-Outer: References: <smime-signed-enc-hp-shy@example>
   Content-Type: text/plain; charset="utf-8"; hp="cipher"

   This is the
   smime-signed-enc-hp-shy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_shy` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example

C.3.8.  S/MIME Signed-and-Encrypted Reply over a Simple Message, Header
        Protection with hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message.  It uses the Header Protection scheme from RFC 9788 with the
   hcp_shy Header Confidentiality Policy with a "Legacy Display"
   element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 8690 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 5422 bytes
     ┴ (unwraps to)
     └─╴text/plain 518 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-hp-shy-legacy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 15:19:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy-legacy@example>
   References: <smime-signed-enc-hp-shy-legacy@example>

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C.3.8.1.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.8.2.  S/MIME Signed-and-Encrypted Reply over a Simple Message,
          Header Protection with hcp_shy (+ Legacy Display), Decrypted
          and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Subject: smime-signed-enc-hp-shy-legacy-reply
   Message-ID: <smime-signed-enc-hp-shy-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:19:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-hp-shy-legacy@example>
   References: <smime-signed-enc-hp-shy-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-hp-shy-legacy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 15:19:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-hp-shy-legacy@example>
   HP-Outer: References: <smime-signed-enc-hp-shy-legacy@example>
   Content-Type: text/plain; charset="utf-8";
    hp-legacy-display="1"; hp="cipher"

   Subject: smime-signed-enc-hp-shy-legacy-reply
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 10:19:02 -0500

   This is the
   smime-signed-enc-hp-shy-legacy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a text/plain
   message. It uses the Header Protection scheme from RFC 9788 with
   the `hcp_shy` Header Confidentiality Policy with a "Legacy
   Display" element.

   --
   Alice
   alice@smime.example

C.3.9.  S/MIME Signed-and-Encrypted over a Complex Message, Header
        Protection with hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_baseline Header
   Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 10035 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 6416 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2054 bytes
      ├┬╴multipart/alternative 1126 bytes
      │├─╴text/plain 384 bytes
      │└─╴text/html 479 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:09:02 -0500
   User-Agent: Sample MUA Version 1.0

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   XGazzMVR4ksU8jkHPdw1nA==

C.3.9.1.  S/MIME Signed-and-Encrypted over a Complex Message, Header
          Protection with hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   ybK5q4kRvQ==

C.3.9.2.  S/MIME Signed-and-Encrypted over a Complex Message, Header
          Protection with hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline
   Message-ID: <smime-signed-enc-complex-hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:09:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-complex-hp-baseline@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:09:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="3a3"; hp="cipher"

   --3a3
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="f31"

   --f31
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-baseline
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --f31
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-baseline</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --f31--

   --3a3
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --3a3--

C.3.10.  S/MIME Signed-and-Encrypted over a Complex Message, Header
         Protection with hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_baseline Header
   Confidentiality Policy with a "Legacy Display" element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 10640 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 6870 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2373 bytes
      ├┬╴multipart/alternative 1423 bytes
      │├─╴text/plain 480 bytes
      │└─╴text/html 640 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
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   Subject: [...]
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:10:02 -0500
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   X20oNkEZ4yhYoiSOTp/yfWOphJ9iDxfXO0RVHSrO2Aw=

C.3.10.1.  S/MIME Signed-and-Encrypted over a Complex Message, Header
           Protection with hcp_baseline (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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   8g==

C.3.10.2.  S/MIME Signed-and-Encrypted over a Complex Message, Header
           Protection with hcp_baseline (+ Legacy Display), Decrypted
           and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline-legacy
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:10:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:10:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="3c5"; hp="cipher"

   --3c5
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="af3"

   --af3
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-baseline-legacy

   This is the
   smime-signed-enc-complex-hp-baseline-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy with a
   "Legacy Display" element.

   --
   Alice
   alice@smime.example
   --af3
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-baseline-legacy
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-baseline-legacy</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy with a
   "Legacy Display" element.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --af3--

   --3c5
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --3c5--

C.3.11.  S/MIME Signed-and-Encrypted over a Complex Message, Header
         Protection with hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_shy Header
   Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 9945 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 6346 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2005 bytes
      ├┬╴multipart/alternative 1106 bytes
      │├─╴text/plain 374 bytes
      │└─╴text/html 469 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-shy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:12:02 +0000
   User-Agent: Sample MUA Version 1.0

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C.3.11.1.  S/MIME Signed-and-Encrypted over a Complex Message, Header
           Protection with hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.11.2.  S/MIME Signed-and-Encrypted over a Complex Message, Header
           Protection with hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy
   Message-ID: <smime-signed-enc-complex-hp-shy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:12:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <smime-signed-enc-complex-hp-shy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:12:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="eb4"; hp="cipher"

   --eb4
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="aab"

   --aab
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-shy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --aab
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-shy</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --aab--

   --eb4
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --eb4--

C.3.12.  S/MIME Signed-and-Encrypted over a Complex Message, Header
         Protection with hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_shy Header
   Confidentiality Policy with a "Legacy Display" element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 10945 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 7084 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2525 bytes
      ├┬╴multipart/alternative 1605 bytes
      │├─╴text/plain 568 bytes
      │└─╴text/html 740 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-shy-legacy@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:13:02 +0000
   User-Agent: Sample MUA Version 1.0

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   6yI9K5QPEcFa9AErInwKFQ==

C.3.12.1.  S/MIME Signed-and-Encrypted over a Complex Message, Header
           Protection with hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.12.2.  S/MIME Signed-and-Encrypted over a Complex Message, Header
           Protection with hcp_shy (+ Legacy Display), Decrypted and
           Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy-legacy
   Message-ID: <smime-signed-enc-complex-hp-shy-legacy@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:13:02 -0500
   User-Agent: Sample MUA Version 1.0
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-complex-hp-shy-legacy@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:13:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   Content-Type: multipart/mixed; boundary="88b"; hp="cipher"

   --88b
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="6bd"

   --6bd
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-shy-legacy
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:13:02 -0500

   This is the
   smime-signed-enc-complex-hp-shy-legacy
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy with a "Legacy
   Display" element.

   --
   Alice
   alice@smime.example
   --6bd
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-shy-legacy
   From: Alice &lt;alice@smime.example&gt;
   To: Bob &lt;bob@smime.example&gt;
   Date: Sat, 20 Feb 2021 12:13:02 -0500
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-shy-legacy</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy with a "Legacy
   Display" element.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --6bd--

   --88b
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --88b--

C.3.13.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
         Header Protection with hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_baseline Header
   Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 10575 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 6820 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2343 bytes
      ├┬╴multipart/alternative 1138 bytes
      │├─╴text/plain 390 bytes
      │└─╴text/html 485 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-baseline@example>
   References: <smime-signed-enc-complex-hp-baseline@example>

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C.3.13.1.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.13.2.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_baseline, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline-reply
   Message-ID: <smime-signed-enc-complex-hp-baseline-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:15:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-baseline@example>
   References: <smime-signed-enc-complex-hp-baseline@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-baseline-reply@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:15:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer:
    In-Reply-To: <smime-signed-enc-complex-hp-baseline@example>
   HP-Outer:
    References: <smime-signed-enc-complex-hp-baseline@example>
   Content-Type: multipart/mixed; boundary="8ec"; hp="cipher"

   --8ec
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="bce"

   --bce
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-baseline-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --bce
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-baseline-reply</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --bce--

   --8ec
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --8ec--

C.3.14.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
         Header Protection with hcp_baseline (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_baseline Header
   Confidentiality Policy with a "Legacy Display" element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 11205 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 7286 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2668 bytes
      ├┬╴multipart/alternative 1427 bytes
      │├─╴text/plain 482 bytes
      │└─╴text/html 642 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-lgc-rpl@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   References:
    <smime-signed-enc-complex-hp-baseline-legacy@example>

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   GpZyX492SC9oN3dOJZELsQ==

C.3.14.1.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_baseline (+ Legacy Display),
           Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.14.2.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_baseline (+ Legacy Display),
           Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-baseline-lgc-rpl
   Message-ID:
    <smime-signed-enc-complex-hp-baseline-lgc-rpl@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:16:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   References:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-baseline-lgc-rpl@example>
   HP-Outer: From: Alice <alice@smime.example>
   HP-Outer: To: Bob <bob@smime.example>
   HP-Outer: Date: Sat, 20 Feb 2021 12:16:02 -0500
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   HP-Outer: References:
    <smime-signed-enc-complex-hp-baseline-legacy@example>
   Content-Type: multipart/mixed; boundary="bed"; hp="cipher"

   --bed
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="828"

   --828
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-baseline-lgc-rpl

   This is the
   smime-signed-enc-complex-hp-baseline-lgc-rpl
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy with a
   "Legacy Display" element.

   --
   Alice
   alice@smime.example
   --828
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-baseline-lgc-rpl
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-baseline-lgc-rpl</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_baseline` Header Confidentiality Policy with a
   "Legacy Display" element.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --828--

   --bed
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --bed--

C.3.15.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
         Header Protection with hcp_shy

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_shy Header
   Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 10445 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 6720 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2273 bytes
      ├┬╴multipart/alternative 1118 bytes
      │├─╴text/plain 380 bytes
      │└─╴text/html 475 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID: <smime-signed-enc-complex-hp-shy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:18:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy@example>
   References: <smime-signed-enc-complex-hp-shy@example>

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C.3.15.1.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_shy, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.15.2.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_shy, Decrypted and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy-reply
   Message-ID: <smime-signed-enc-complex-hp-shy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:18:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy@example>
   References: <smime-signed-enc-complex-hp-shy@example>
   HP-Outer: Subject: [...]
   HP-Outer:
    Message-ID: <smime-signed-enc-complex-hp-shy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:18:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer: In-Reply-To: <smime-signed-enc-complex-hp-shy@example>
   HP-Outer: References: <smime-signed-enc-complex-hp-shy@example>
   Content-Type: multipart/mixed; boundary="230"; hp="cipher"

   --230
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="4c8"

   --4c8
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-signed-enc-complex-hp-shy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --4c8
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-signed-enc-complex-hp-shy-reply</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --4c8--

   --230
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
   MAgS739nO3TpRw20dqpbfARQEjOywiwYnCtkDKnbcLk66sqlT+zt9cidkE+6KwkZ
   sgrzfcqVMpL2jo0447gYDpeArk+OnJHkIhAfTPRicihAf5YJrw7vjv0ZWRWM/uli
   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --230--

C.3.16.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
         Header Protection with hcp_shy (+ Legacy Display)

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   Header Protection scheme from RFC 9788 with the hcp_shy Header
   Confidentiality Policy with a "Legacy Display" element.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 11530 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 7520 bytes
     ┴ (unwraps to)
     └┬╴multipart/mixed 2834 bytes
      ├┬╴multipart/alternative 1629 bytes
      │├─╴text/plain 580 bytes
      │└─╴text/html 752 bytes
      └─╴image/png inline 236 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-signed-enc-complex-hp-shy-legacy-reply@example>
   From: alice@smime.example
   To: bob@smime.example
   Date: Sat, 20 Feb 2021 17:19:02 +0000
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy-legacy@example>
   References: <smime-signed-enc-complex-hp-shy-legacy@example>

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   5P6nHYB/zZ7qHM/LPSZdWA==

C.3.16.1.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_shy (+ Legacy Display), Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.16.2.  S/MIME Signed-and-Encrypted Reply over a Complex Message,
           Header Protection with hcp_shy (+ Legacy Display), Decrypted
           and Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Subject: smime-signed-enc-complex-hp-shy-legacy-reply
   Message-ID:
    <smime-signed-enc-complex-hp-shy-legacy-reply@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:19:02 -0500
   User-Agent: Sample MUA Version 1.0
   In-Reply-To: <smime-signed-enc-complex-hp-shy-legacy@example>
   References: <smime-signed-enc-complex-hp-shy-legacy@example>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID:
    <smime-signed-enc-complex-hp-shy-legacy-reply@example>
   HP-Outer: From: alice@smime.example
   HP-Outer: To: bob@smime.example
   HP-Outer: Date: Sat, 20 Feb 2021 17:19:02 +0000
   HP-Outer: User-Agent: Sample MUA Version 1.0
   HP-Outer:
    In-Reply-To: <smime-signed-enc-complex-hp-shy-legacy@example>
   HP-Outer:
    References: <smime-signed-enc-complex-hp-shy-legacy@example>
   Content-Type: multipart/mixed; boundary="242"; hp="cipher"

   --242
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="da7"

   --da7
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/plain; charset="us-ascii";
    hp-legacy-display="1"

   Subject: smime-signed-enc-complex-hp-shy-legacy-reply
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:19:02 -0500

   This is the
   smime-signed-enc-complex-hp-shy-legacy-reply
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy with a "Legacy
   Display" element.

   --
   Alice
   alice@smime.example
   --da7
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit
   Content-Type: text/html; charset="us-ascii";
    hp-legacy-display="1"

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>
   Subject: smime-signed-enc-complex-hp-shy-legacy-reply
   From: Alice &lt;alice@smime.example&gt;
   To: Bob &lt;bob@smime.example&gt;
   Date: Sat, 20 Feb 2021 12:19:02 -0500
   </pre>
   </div><p>This is the
   <b>smime-signed-enc-complex-hp-shy-legacy-reply</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the Header Protection scheme from RFC 9788
   with the `hcp_shy` Header Confidentiality Policy with a "Legacy
   Display" element.</p>
   <p><tt>-- <br>Alice<br>alice@smime.example</tt></p></body></html>
   --da7--

   --242
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --242--

C.3.17.  S/MIME Signed-and-Encrypted over a Complex Message, Legacy RFC
         8551 Header Protection with hcp_baseline

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a multipart/
   alternative message with an inline image/png attachment.  It uses the
   legacy RFC 8551 Header Protection (RFC8551HP) scheme with the
   hcp_baseline Header Confidentiality Policy.

   It has the following structure:

   └┬╴application/pkcs7-mime [smime.p7m] 9580 bytes
    ╧ (decrypts to)
    └┬╴application/pkcs7-mime [smime.p7m] 6082 bytes
     ┴ (unwraps to)
     └┬╴message/rfc822 1876 bytes
      └┬╴multipart/mixed 1828 bytes
       ├┬╴multipart/alternative 1168 bytes
       │├─╴text/plain 393 bytes
       │└─╴text/html 491 bytes
       └─╴image/png inline 232 bytes

   Its contents are:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   Subject: [...]
   Message-ID:
    <smime-enc-signed-complex-rfc8551hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:28:02 -0500
   User-Agent: Sample MUA Version 1.0

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C.3.17.1.  S/MIME Signed-and-Encrypted over a Complex Message, Legacy
           RFC 8551 Header Protection with hcp_baseline, Decrypted

   The S/MIME enveloped-data layer unwraps to this signed-data part:

   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="signed-data"

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C.3.17.2.  S/MIME Signed-and-Encrypted over a Complex Message, Legacy
           RFC 8551 Header Protection with hcp_baseline, Decrypted and
           Unwrapped

   The inner signed-data layer unwraps to:

   MIME-Version: 1.0
   Content-Type: message/rfc822

   MIME-Version: 1.0
   Content-Type: multipart/mixed; boundary="144"
   Subject: smime-enc-signed-complex-rfc8551hp-baseline
   Message-ID:
    <smime-enc-signed-complex-rfc8551hp-baseline@example>
   From: Alice <alice@smime.example>
   To: Bob <bob@smime.example>
   Date: Sat, 20 Feb 2021 12:28:02 -0500
   User-Agent: Sample MUA Version 1.0

   --144
   MIME-Version: 1.0
   Content-Type: multipart/alternative; boundary="579"

   --579
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   This is the
   smime-enc-signed-complex-rfc8551hp-baseline
   message.

   This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 Header Protection
   (RFC8551HP) scheme with the `hcp_baseline` Header
   Confidentiality Policy.

   --
   Alice
   alice@smime.example
   --579
   Content-Type: text/html; charset="us-ascii"
   MIME-Version: 1.0
   Content-Transfer-Encoding: 7bit

   <html><head><title></title></head><body>
   <p>This is the
   <b>smime-enc-signed-complex-rfc8551hp-baseline</b>
   message.</p>
   <p>This is a signed-and-encrypted S/MIME message using PKCS#7
   envelopedData around signedData.  The payload is a
   multipart/alternative message with an inline image/png
   attachment. It uses the legacy RFC 8551 Header Protection
   (RFC8551HP) scheme with the `hcp_baseline` Header
   Confidentiality Policy.</p>
   <p><tt>-- <br/>Alice<br/>alice@smime.example</tt></p></body></html>
   --579--

   --144
   Content-Type: image/png
   Content-Transfer-Encoding: base64
   Content-Disposition: inline

   iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAYAAACNiR0NAAAAcElEQVR42uVTOxbA
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   vdPf1QZ2kDD9xppd8wAAAABJRU5ErkJggg==

   --144--

Appendix D.  Composition Examples

   This section offers step-by-step examples of message composition.

D.1.  New Message Composition

   A typical MUA composition interface offers the user a place to
   indicate the message recipients, subject, and content of the message.
   Consider a composition window filled out by the user like so:

    .------------------------------------------------------.
   |                 Composing New Message          .----.  |
   |          +---------------------------------+  | Send | |
   |      To: | Alice <alice@example.net>       |   '----'  |
   |          +---------------------------------+---------+ |
   | Subject: | Handling the Jones contract               | |
   |          +-------------------------------------------+ |
   +--------------------------------------------------------+
   | Please review and approve or decline by Thursday, it's |
   | critical!                                              |
   |                                                        |
   | Thanks,                                                |
   | Bob                                                    |
   |                                                        |
   | --                                                     |
   | Bob Gonzalez                                           |
   | ACME, Inc.                                             |
   |                                                        |
   +--------------------------------------------------------+

              Figure 1: Example Message Composition Interface

   When Bob clicks "Send", his MUA generates values for the Message-ID,
   From, and Date Header Fields and converts the message content into
   the appropriate format.

D.1.1.  Unprotected Message

   The resulting message would look something like this if it was sent
   without cryptographic protections:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: Handling the Jones contract
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0

   Please review and approve or decline by Thursday, it's critical!

   Thanks,
   Bob

   --
   Bob Gonzalez
   ACME, Inc.

D.1.2.  Encrypted with hcp_baseline and Legacy Display

   Now consider the message to be generated if it is to be
   cryptographically signed and encrypted, using HCP hcp_baseline, and
   the legacy variable is set.

   For each Header Field, Bob's MUA passes its name and value through
   hcp_baseline.  This returns the same value for every Header Field,
   except that:

   hcp_baseline("Subject", "Handling the Jones contract") yields
   "[...]".

D.1.2.1.  Cryptographic Payload

   The Cryptographic Payload that will be signed and then encrypted is
   very similar to the unprotected message in Appendix D.1.1.  Note the
   addition of:

   *  the hp="cipher" parameter for the Content-Type

   *  the appropriate HP-Outer Header Field for Subject

   *  the hp-legacy-display="1" parameter for the Content-Type

   *  the Legacy Display Element (the simple pseudo-header and its
      trailing newline) in the Main Body Part

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: Handling the Jones contract
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   MIME-Version: 1.0
   HP-Outer: Date: Wed, 11 Jan 2023 16:08:43 -0500
   HP-Outer: From: Bob <bob@example.net>
   HP-Outer: To: Alice <alice@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <20230111T210843Z.1234@lhp.example>

   Subject: Handling the Jones contract

   Please review and approve or decline by Thursday, it's critical!

   Thanks,
   Bob

   --
   Bob Gonzalez
   ACME, Inc.

D.1.2.2.  Outer Header Section

   The Cryptographic Payload from Appendix D.1.2.1 is then wrapped in
   the appropriate Cryptographic Layers.  For this example using S/MIME,
   it is wrapped in an application/pkcs7-mime; smime-type="signed-data"
   layer, which is in turn wrapped in an application/pkcs7-mime; smime-
   type="enveloped-data" layer.

   Then, an Outer Header Section is applied to the outer MIME object,
   which looks like this:

   Date: Wed, 11 Jan 2023 16:08:43 -0500
   From: Bob <bob@example.net>
   To: Alice <alice@example.net>
   Subject: [...]
   Message-ID: <20230111T210843Z.1234@lhp.example>
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   MIME-Version: 1.0

   Note that the Subject Header Field has been obscured appropriately by
   hcp_baseline.  The output of the CMS enveloping operation is base64
   encoded and forms the Body of the message.

D.2.  Composing a Reply

   Next, we consider a typical MUA reply interface, where we see Alice
   replying to Bob's message from Appendix D.1.

   When Alice clicks "Reply" to Bob's signed-and-encrypted message with
   Header Protection, she might see something like this:

    .--------------------------------------------------------.
   |  Replying to Bob ("Handling the Jones Contract") .----.  |
   |          +-----------------------------------+  | Send | |
   |      To: | Bob <bob@example.net>             |   '----'  |
   |          +-----------------------------------+---------+ |
   | Subject: | Re: Handling the Jones contract             | |
   |          +---------------------------------------------+ |
   +----------------------------------------------------------+
   | On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:           |
   |                                                          |
   | > Please review and approve or decline by Thursday,      |
   | > it's critical!                                         |
   | >                                                        |
   | > Thanks,                                                |
   | > Bob                                                    |
   | >                                                        |
   | > --                                                     |
   | > Bob Gonzalez                                           |
   | > ACME, Inc.                                             |
   |                                                          |
   | --                                                       |
   | Alice Jenkins                                            |
   | ACME, Inc.                                               |
   |                                                          |
   +----------------------------------------------------------+

            Figure 2: Example Message Reply Interface (Unedited)

   Note that because Alice's MUA is aware of Header Protection, it knows
   what the correct Subject Header Field is, even though it was
   obscured.  It also knows to avoid including the Legacy Display
   Element in the quoted/attributed text that it includes in the draft
   reply.

   Once Alice has edited the reply message, it might look something like
   this:

    .--------------------------------------------------------.
   |  Replying to Bob ("Handling the Jones Contract") .----.  |
   |          +-----------------------------------+  | Send | |
   |      To: | Bob <bob@example.net>             |   '----'  |
   |          +-----------------------------------+---------+ |
   | Subject: | Re: Handling the Jones contract             | |
   |          +---------------------------------------------+ |
   +----------------------------------------------------------+
   | On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:           |
   |                                                          |
   | > Please review and approve or decline by Thursday,      |
   | > it's critical!                                         |
   |                                                          |
   | I'll get right on it, Bob!                               |
   |                                                          |
   | Regards,                                                 |
   | Alice                                                    |
   |                                                          |
   | --                                                       |
   | Alice Jenkins                                            |
   | ACME, Inc.                                               |
   |                                                          |
   +----------------------------------------------------------+

             Figure 3: Example Message Reply Interface (Edited)

   When Alice clicks "Send", the MUA generates values for the Message-
   ID, From, and Date Header Fields, populates the In-Reply-To and
   References Header Fields, and also converts the reply content into
   the appropriate format.

D.2.1.  Unprotected Message

   The resulting message would look something like this if it were to be
   sent without any cryptographic protections:

   Date: Wed, 11 Jan 2023 16:48:22 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Re: Handling the Jones contract
   Message-ID: <20230111T214822Z.5678@lhp.example>
   In-Reply-To: <20230111T210843Z.1234@lhp.example>
   References: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"
   MIME-Version: 1.0

   On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:

   > Please review and approve or decline by Thursday,
   > it's critical!

   I'll get right on it, Bob!

   Regards,
   Alice

   --
   Alice Jenkins
   ACME, Inc.

   Of course, this would leak not only the contents of Alice's message
   but also the contents of Bob's initial message, as well as the
   Subject Header Field!  So Alice's MUA won't do that; it is going to
   create a signed-and-encrypted message to submit to the network.

D.2.2.  Encrypted with hcp_no_confidentiality and Legacy Display

   This example assumes that Alice's MUA uses hcp_no_confidentiality,
   not hcp_baseline.  That is, by default, it does not obscure or remove
   any Header Fields, even when encrypting.

   However, it follows the guidance in Section 6.1 and will make use of
   the HP-Outer field in the Cryptographic Payload of Bob's original
   message (Appendix D.1.2.1) to determine what to obscure.

   When crafting the Cryptographic Payload, its baseline HCP
   (hcp_no_confidentiality) leaves each field untouched.  To uphold the
   confidentiality of the composer's values when replying, the MUA
   executes the following steps (for brevity, only Subject and Message-
   ID/In-Reply-To are shown):

   *  Extract the referenced Header Fields (see Section 4.2):

      -  refouter contains:

         o  Date: Wed, 11 Jan 2023 16:08:43 -0500

         o  From: Bob <bob@example.net>

         o  To: Alice <alice@example.net>

         o  Subject: [...]

         o  Message-ID: <20230111T210843Z.1234@lhp.example>

      -  refprotected contains:

         o  Date: Wed, 11 Jan 2023 16:08:43 -0500

         o  From: Bob <bob@example.net>

         o  To: Alice <alice@example.net>

         o  Subject: Handling the Jones contract

         o  Message-ID: <20230111T210843Z.1234@lhp.example>

   *  Apply the response function:

      -  respond(refouter) contains:

         o  From: Alice <alice@example.net>

         o  To: Bob <bob@example.net>

         o  Subject: Re: [...]

         o  In-Reply-To: <20230111T210843Z.1234@lhp.example>

         o  References: <20230111T210843Z.1234@lhp.example>

      -  respond(refprotected) contains:

         o  From: Alice <alice@example.net>

         o  To: Bob <bob@example.net>

         o  Subject: Re: Handling the Jones contract

         o  In-Reply-To: <20230111T210843Z.1234@lhp.example>

         o  References: <20230111T210843Z.1234@lhp.example>

   *  Compute the ephemeral response_hcp (see Section 6.1):

      -  Note that all Header Fields except Subject are the same.

      -  confmap contains only ("Subject", "Re: Handling the Jones
         contract") -> "Re: [...]"

   Thus, all Header Fields that were signed are passed through
   untouched.  The reply's Subject is obscured as Subject: Re: [...] if
   and only if the user does not edit the Subject line from that
   initially proposed by the MUA's reply interface.  If the user edits
   the Subject line, e.g., to Subject: Re: Handling the Jones contract
   ASAP, the response_hcp will _not_ obscure it and instead pass it
   through in the clear.

   For stronger header confidentiality, the replying MUA should use a
   reasonable HCP (not hcp_no_confidentiality).  Also recall that the
   local HCP is applied first and that response_hcp is only applied to
   what is left unchanged by the local HCP.

D.2.2.1.  Cryptographic Payload

   Consequently, the Cryptographic Payload for Alice's reply looks like
   this:

   Date: Wed, 11 Jan 2023 16:48:22 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Re: Handling the Jones contract
   Message-ID: <20230111T214822Z.5678@lhp.example>
   In-Reply-To: <20230111T210843Z.1234@lhp.example>
   References: <20230111T210843Z.1234@lhp.example>
   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   MIME-Version: 1.0
   HP-Outer: Date: Wed, 11 Jan 2023 16:48:22 -0500
   HP-Outer: From: Alice <alice@example.net>
   HP-Outer: To: Bob <bob@example.net>
   HP-Outer: Subject: Re: [...]
   HP-Outer: Message-ID: <20230111T214822Z.5678@lhp.example>
   HP-Outer: In-Reply-To: <20230111T210843Z.1234@lhp.example>
   HP-Outer: References: <20230111T210843Z.1234@lhp.example>

   Subject: Re: Handling the Jones contract

   On Wed, 11 Jan 2023 16:08:43 -0500, Bob wrote:

   > Please review and approve or decline by Thursday,
   > it's critical!

   I'll get right on it, Bob!

   Regards,
   Alice

   --
   Alice Jenkins
   ACME, Inc.

   Note the following features:

   *  the hp="cipher" parameter to Content-Type

   *  the appropriate HP-Outer Header Field for Subject

   *  the hp-legacy-display="1" parameter for the Content-Type

   *  the Legacy Display Element (the simple pseudo-header and its
      trailing newline) in the Main Body Part

D.2.2.2.  Outer Header Section

   The Cryptographic Payload from Appendix D.2.2.1 is then wrapped in
   the appropriate Cryptographic Layers.  For this example using S/MIME,
   it is wrapped in an application/pkcs7-mime; smime-type="signed-data"
   layer, which is in turn wrapped in an application/pkcs7-mime; smime-
   type="enveloped-data" layer.

   Then, an Outer Header Section is applied to the outer MIME object,
   which looks like this:

   Date: Wed, 11 Jan 2023 16:48:22 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Re: [...]
   Message-ID: <20230111T214822Z.5678@lhp.example>
   In-Reply-To: <20230111T210843Z.1234@lhp.example>
   References: <20230111T210843Z.1234@lhp.example>
   Content-Transfer-Encoding: base64
   Content-Type: application/pkcs7-mime; name="smime.p7m";
    smime-type="enveloped-data"
   MIME-Version: 1.0

   Note that the Subject Header Field has been obscured appropriately
   even though hcp_no_confidentiality would not have touched it by
   default.  The output of the CMS enveloping operation is base64
   encoded and forms the Body of the message.

Appendix E.  Rendering Examples

   This section offers example Cryptographic Payloads (the content
   within the Cryptographic Envelope) that contain Legacy Display
   Elements.

E.1.  Example text/plain Cryptographic Payload with Legacy Display
      Elements

   Here is a simple one-part Cryptographic Payload (Header Section and
   Body) of a message that includes Legacy Display Elements:

   Date: Fri, 21 Jan 2022 20:40:48 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Dinner plans
   Message-ID: <text-plain-legacy-display@lhp.example>
   MIME-Version: 1.0
   Content-Type: text/plain; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   HP-Outer: Date: Fri, 21 Jan 2022 20:40:48 -0500
   HP-Outer: From: Alice <alice@example.net>
   HP-Outer: To: Bob <bob@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <text-plain-legacy-display@lhp.example>

   Subject: Dinner plans

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

   A compatible MUA will recognize the hp-legacy-display="1" parameter
   and render the Body of the message as:

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

   A legacy decryption-capable MUA that is unaware of this mechanism
   will ignore the hp-legacy-display="1" parameter and instead render
   the Body including the Legacy Display Elements:

   Subject: Dinner plans

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

E.2.  Example text/html Cryptographic Payload with Legacy Display
      Elements

   Here is a modern one-part Cryptographic Payload (Header Section and
   Body) of a message that includes Legacy Display Elements:

   Date: Fri, 21 Jan 2022 20:40:48 -0500
   From: Alice <alice@example.net>
   To: Bob <bob@example.net>
   Subject: Dinner plans
   Message-ID: <text-html-legacy-display@lhp.example>
   MIME-Version: 1.0
   Content-Type: text/html; charset="us-ascii"; hp-legacy-display="1";
    hp="cipher"
   HP-Outer: Date: Fri, 21 Jan 2022 20:40:48 -0500
   HP-Outer: From: Alice <alice@example.net>
   HP-Outer: To: Bob <bob@example.net>
   HP-Outer: Subject: [...]
   HP-Outer: Message-ID: <text-html-legacy-display@lhp.example>

   <html><head><title></title></head><body>
   <div class="header-protection-legacy-display">
   <pre>Subject: Dinner plans</pre>
   </div>
   <p>
   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.
   </p>
   </body>
   </html>

   A compatible MUA will recognize the hp-legacy-display="1" parameter
   and mask out the Legacy Display div, rendering the Body of the
   message as a simple paragraph:

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

   A legacy decryption-capable MUA that is unaware of this mechanism
   will ignore the hp-legacy-display="1" parameter and instead render
   the Body including the Legacy Display Elements:

   Subject: Dinner plans

   Let's meet at Rama's Roti Shop at 8pm and go to the park
   from there.

Appendix F.  Other Header Protection Schemes

   Other Header Protection schemes have been proposed in the past.
   However, those typically have drawbacks such as sparse
   implementation, known problems with legacy interoperability (in
   particular with rendering), lack of clear signaling of composer
   intent, and/or incomplete cryptographic protections.  This section
   lists such schemes known at the time of the publication of this
   document out of historical interest.

F.1.  Original RFC 8551 Header Protection

   S/MIME [RFC8551] (as well as its predecessors [RFC5751] and
   [RFC3851]) defined a form of cryptographic Header Protection that has
   never reached wide adoption and has significant drawbacks compared to
   the mechanism in this document.  See Section 1.1.1 for more
   discussion of the differences and Section 4.10 for guidance on how to
   handle such a message.

F.2.  Pretty Easy Privacy (pEp)

   The pretty Easy privacy (pEp) [PEP-GENERAL] project specifies two
   different MIME schemes that include Header Protection for Signed-and-
   Encrypted email messages in [PEP-EMAIL]: One scheme -- referred as
   pEp Email Format 1 (PEF-1) -- is generated towards MUAs not known to
   be pEp-capable, while the other scheme -- referred as PEF-2 -- is
   used between MUAs discovered to be compatible with pEp.  Signed-only
   messages are not recommended in pEp.

   Although the PEF-2 scheme is only meant to be used between MUAs
   compatible with PEF-2, a PEF-2 message may end up at an MUA unaware
   of PEF-2 (in which case, it typically renders badly).  This is due to
   signaling mechanism limitations.

   As the PEF-2 scheme is an enhanced variant of the RFC8551HP scheme
   (with an additional MIME Layer), it is similar to the RFC8551HP
   scheme (see Section 4.10).  The basic PEF-2 MIME structure looks as
   follows:

   A └┬╴multipart/encrypted [Outer Message]
   B  ├─╴application/pgp-encrypted
   C  └┬╴application/octet-stream inline [Cryptographic Payload]
   D   ╧ (decrypts to)
   E   └┬╴multipart/mixed
   F    ├─╴text/plain
   G    ├┬╴message/rfc822
   H    │└─╴[Inner Message]
   I    └─╴application/pgp-keys

   The MIME structure at part H contains the Inner Message to be
   rendered to the user.

   It is possible for a normal MUA to accidentally produce a message
   that happens to have the same MIME structure as used for PEF-2
   messages.  Therefore, a PEF-2 message cannot be identified by the
   MIME structure alone.

   The lack of a mechanism comparable to HP-Outer (see Section 2.2)
   makes it impossible for the recipient of a PEF-2 message to safely
   determine which Header Fields are confidential or not while
   forwarding or replying to a message (see Section 6).

   Note: As this document is not normative for PEF-2 messages, it does
   not provide any guidance for handling them.  Please see [PEP-EMAIL]
   for more guidance.

F.3.  "draft-autocrypt" Protected Headers

   [PROTECTED-HEADERS] describes a scheme similar to the Header
   Protection scheme specified in this document.  However, instead of
   adding Legacy Display Elements to existing MIME parts (see
   Section 5.2.2), [PROTECTED-HEADERS] suggests injecting a new MIME
   element "Legacy Display Part", thus modifying the MIME structure of
   the Cryptographic Payload.  These modified Cryptographic Payloads
   cause significant rendering problems on some common Legacy MUAs.

   The lack of a mechanism comparable to hp="cipher" and hp="clear" (see
   Section 2.1.1) means the recipient of an encrypted message as
   described in [PROTECTED-HEADERS] cannot be cryptographically certain
   whether the composer intended for the message to be confidential or
   not.  The lack of a mechanism comparable to HP-Outer (see
   Section 2.2) makes it impossible for the recipient of an encrypted
   message as described in [PROTECTED-HEADERS] to safely determine which
   Header Fields are confidential or not while forwarding or replying to
   a message (see Section 6).

Acknowledgements

   Alexander Krotov identified the risk of From address spoofing (see
   Section 10.1) and helped provide guidance to MUAs.

   Thore Göbel identified significant gaps in earlier draft versions of
   this document and proposed concrete, substantial improvements.
   Thanks to his contributions, the document is clearer, and the
   protocols described herein are more useful.

   Additionally, the authors would like to thank the following people
   who have provided helpful comments and suggestions for this document:
   Berna Alp, Bernhard E. Reiter, Bron Gondwana, Carl Wallace, Claudio
   Luck, Daniel Huigens, David Wilson, Éric Vyncke, Hernani Marques,
   juga, Kelly Bristol, Krista Bennett, Lars Rohwedder, Michael StJohns,
   Nicolas Lidzborski, Orie Steele, Paul Wouters, Peter Yee, Phillip
   Tao, Robert Williams, Rob Sayre, Rohan Mahy, Roman Danyliw, Russ
   Housley, Sofia Balicka, Steve Kille, Volker Birk, Warren Kumari, and
   Wei Chuang.

Authors' Addresses

   Daniel Kahn Gillmor
   American Civil Liberties Union
   125 Broad St.
   New York, NY 10004
   United States of America
   Email: dkg@fifthhorseman.net


   Bernie Hoeneisen
   pEp Project
   Oberer Graben 4
   CH-8400 Winterthur
   Switzerland
   Email: bernie@ietf.hoeneisen.ch
   URI:   https://pep-project.org/


   Alexey Melnikov
   Isode Ltd
   14 Castle Mews
   Hampton, Middlesex
   TW12 2NP
   United Kingdom
   Email: alexey.melnikov@isode.com
