



Internet Engineering Task Force (IETF)                       G. Selander
Request for Comments: 9528                             J. Preuß Mattsson
Category: Standards Track                                   F. Palombini
ISSN: 2070-1721                                                 Ericsson
                                                              March 2024


               Ephemeral Diffie-Hellman Over COSE (EDHOC)

Abstract

   This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
   very compact and lightweight authenticated Diffie-Hellman key
   exchange with ephemeral keys.  EDHOC provides mutual authentication,
   forward secrecy, and identity protection.  EDHOC is intended for
   usage in constrained scenarios, and a main use case is to establish
   an Object Security for Constrained RESTful Environments (OSCORE)
   security context.  By reusing CBOR Object Signing and Encryption
   (COSE) for cryptography, Concise Binary Object Representation (CBOR)
   for encoding, and Constrained Application Protocol (CoAP) for
   transport, the additional code size can be kept very low.

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/rfc9528.

Copyright Notice

   Copyright (c) 2024 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
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   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.  Motivation
     1.2.  Message Size Examples
     1.3.  Document Structure
     1.4.  Terminology and Requirements Language
   2.  EDHOC Outline
   3.  Protocol Elements
     3.1.  General
     3.2.  Method
     3.3.  Connection Identifiers
     3.4.  Transport
     3.5.  Authentication Parameters
     3.6.  Cipher Suites
     3.7.  Ephemeral Public Keys
     3.8.  External Authorization Data (EAD)
     3.9.  Application Profile
   4.  Key Derivation
     4.1.  Keys for EDHOC Message Processing
     4.2.  Keys for EDHOC Applications
   5.  Message Formatting and Processing
     5.1.  EDHOC Message Processing Outline
     5.2.  EDHOC Message 1
     5.3.  EDHOC Message 2
     5.4.  EDHOC Message 3
     5.5.  EDHOC Message 4
   6.  Error Handling
     6.1.  Success
     6.2.  Unspecified Error
     6.3.  Wrong Selected Cipher Suite
     6.4.  Unknown Credential Referenced
   7.  EDHOC Message Deduplication
   8.  Compliance Requirements
   9.  Security Considerations
     9.1.  Security Properties
     9.2.  Cryptographic Considerations
     9.3.  Cipher Suites and Cryptographic Algorithms
     9.4.  Post-Quantum Considerations
     9.5.  Unprotected Data and Privacy
     9.6.  Updated Internet Threat Model Considerations
     9.7.  Denial of Service
     9.8.  Implementation Considerations
   10. IANA Considerations
     10.1.  EDHOC Exporter Label Registry
     10.2.  EDHOC Cipher Suites Registry
     10.3.  EDHOC Method Type Registry
     10.4.  EDHOC Error Codes Registry
     10.5.  EDHOC External Authorization Data Registry
     10.6.  COSE Header Parameters Registry
     10.7.  Well-Known URI Registry
     10.8.  Media Types Registry
     10.9.  CoAP Content-Formats Registry
     10.10. Resource Type (rt=) Link Target Attribute Values Registry
     10.11. Expert Review Instructions
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Use with OSCORE and Transfer over CoAP
     A.1.  Deriving the OSCORE Security Context
     A.2.  Transferring EDHOC over CoAP
   Appendix B.  Compact Representation
   Appendix C.  Use of CBOR, CDDL, and COSE in EDHOC
     C.1.  CBOR and CDDL
     C.2.  CDDL Definitions
     C.3.  COSE
   Appendix D.  Authentication-Related Verifications
     D.1.  Validating the Authentication Credential
     D.2.  Identities
     D.3.  Certification Path and Trust Anchors
     D.4.  Revocation Status
     D.5.  Unauthenticated Operation
   Appendix E.  Use of External Authorization Data
   Appendix F.  Application Profile Example
   Appendix G.  Long PLAINTEXT_2
   Appendix H.  EDHOC_KeyUpdate
   Appendix I.  Example Protocol State Machine
     I.1.  Initiator State Machine
     I.2.  Responder State Machine
   Acknowledgments
   Authors' Addresses

1.  Introduction

1.1.  Motivation

   Many Internet of Things (IoT) deployments require technologies that
   are highly performant in constrained environments [RFC7228].  IoT
   devices may be constrained in various ways, including memory,
   storage, processing capacity, and power.  The connectivity for these
   settings may also exhibit constraints, such as unreliable and lossy
   channels, highly restricted bandwidth, and dynamic topology.  The
   IETF has acknowledged this problem by standardizing a range of
   lightweight protocols and enablers designed for the IoT, including
   CoAP [RFC7252], CBOR [RFC8949], and Static Context Header Compression
   (SCHC) [RFC8724].

   The need for special protocols targeting constrained IoT deployments
   extends also to the security domain [LAKE-REQS].  Important
   characteristics in constrained environments are the number of round
   trips and protocol message sizes, which (if kept low) can contribute
   to good performance by enabling transport over a small number of
   radio frames, reducing latency due to fragmentation, duty cycles,
   etc.  Another important criterion is code size, which may be
   prohibitively large for certain deployments due to device
   capabilities or network load during firmware updates.  Some IoT
   deployments also need to support a variety of underlying transport
   technologies, potentially even with a single connection.

   Some security solutions for such settings exist already.  COSE
   [RFC9052] specifies basic application-layer security services
   efficiently encoded in CBOR.  Another example is OSCORE [RFC8613],
   which is a lightweight communication security extension to CoAP using
   CBOR and COSE.  In order to establish good quality cryptographic keys
   for security protocols such as COSE and OSCORE, the two endpoints may
   run an authenticated Diffie-Hellman key exchange protocol, from which
   shared secret keying material can be derived.  Such a key exchange
   protocol should also be lightweight to prevent bad performance in
   case of repeated use, e.g., due to device rebooting or frequent
   rekeying for security reasons or to avoid latencies in a network
   formation setting with many devices authenticating at the same time.

   This document specifies Ephemeral Diffie-Hellman Over COSE (EDHOC), a
   lightweight authenticated key exchange protocol providing good
   security properties including forward secrecy, identity protection,
   and cipher suite negotiation.  Authentication can be based on raw
   public keys (RPKs) or public key certificates and requires the
   application to provide input on how to verify that endpoints are
   trusted.  This specification supports the referencing of credentials
   in order to reduce message overhead, but credentials may
   alternatively be embedded in the messages.  EDHOC does not currently
   support Pre-Shared Key (PSK) authentication as authentication with
   static Diffie-Hellman (DH) public keys by reference produces equally
   small message sizes but with much simpler key distribution and
   identity protection.

   EDHOC makes use of known protocol constructions, such as SIGn-and-MAc
   [SIGMA], the Noise XX pattern [Noise], and Extract-and-Expand
   [RFC5869].  EDHOC uses COSE for cryptography and identification of
   credentials (including COSE_Key, CBOR Web Token (CWT), CWT Claims Set
   (CCS), X.509, and CBOR-encoded X.509 (C509) certificates; see
   Section 3.5.2).  COSE provides crypto agility and enables the use of
   future algorithms and credential types targeting IoT.

   EDHOC is designed for highly constrained settings, making it
   especially suitable for low-power networks [RFC8376] such as Cellular
   IoT, IPv6 over the TSCH mode of IEEE 802.15.4e (6TiSCH), and LoRaWAN.
   A main objective for EDHOC is to be a lightweight authenticated key
   exchange for OSCORE, i.e., to provide authentication and session key
   establishment for IoT use cases such as those built on CoAP [RFC7252]
   involving 'things' with embedded microcontrollers, sensors, and
   actuators.  By reusing the same lightweight primitives as OSCORE
   (CBOR, COSE, and CoAP), the additional code size can be kept very
   low.  Note that while CBOR and COSE primitives are built into the
   protocol messages, EDHOC is not bound to a particular transport.

   A typical setting is when one of the endpoints is constrained or in a
   constrained network and the other endpoint is a node on the Internet
   (such as a mobile phone).  Thing-to-thing interactions over
   constrained networks are also relevant since both endpoints would
   then benefit from the lightweight properties of the protocol.  EDHOC
   could, e.g., be run when a device connects for the first time or to
   establish fresh keys that are not revealed by a later compromise of
   the long-term keys.

1.2.  Message Size Examples

   Examples of EDHOC message sizes are shown in Table 1, which use
   different kinds of authentication keys and COSE header parameters for
   identification, including static Diffie-Hellman keys or signature
   keys, either in CWT/CCS [RFC8392] identified by a key identifier
   using 'kid' [RFC9052] or in X.509 certificates identified by a hash
   value using 'x5t' [RFC9360].  EDHOC always uses ephemeral-ephemeral
   key exchange.  As a comparison, in the case of RPK authentication and
   when transferred in CoAP, the EDHOC message size can be less than 1/7
   of the DTLS 1.3 handshake [RFC9147] with Ephemeral Elliptic Curve
   Diffie-Hellman (ECDHE) and connection ID; see [CoAP-SEC-PROT].

              +===========+================+================+
              |           | Static DH Keys | Signature Keys |
              +===========+==========+=====+==========+=====+
              |           |      kid | x5t |      kid | x5t |
              +===========+==========+=====+==========+=====+
              | message_1 |       37 |  37 |       37 |  37 |
              +-----------+----------+-----+----------+-----+
              | message_2 |       45 |  58 |      102 | 115 |
              +-----------+----------+-----+----------+-----+
              | message_3 |       19 |  33 |       77 |  90 |
              +-----------+----------+-----+----------+-----+
              | Total     |      101 | 128 |      216 | 242 |
              +-----------+----------+-----+----------+-----+

                Table 1: Examples of EDHOC Message Sizes in
                                   Bytes

1.3.  Document Structure

   The remainder of the document is organized as follows: Section 2
   outlines EDHOC authenticated with signature keys; Section 3 describes
   the protocol elements of EDHOC, including formatting of the ephemeral
   public keys; Section 4 specifies the key derivation; Section 5
   specifies message processing for EDHOC authenticated with signature
   keys or static Diffie-Hellman keys; Section 6 describes the error
   messages; Section 7 describes EDHOC support for transport that does
   not handle message duplication; and Section 8 lists compliance
   requirements.  Note that normative text is also used in appendices,
   in particular Appendix A.

1.4.  Terminology and 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.

   Readers are expected to be familiar with the terms and concepts
   described in CBOR [RFC8949], CBOR Sequences [RFC8742], COSE
   Structures and Processing [RFC9052], COSE Algorithms [RFC9053], CWT
   and CCS [RFC8392], and the Concise Data Definition Language (CDDL)
   [RFC8610], which is used to express CBOR data structures.  Examples
   of CBOR and CDDL are provided in Appendix C.1.  When referring to
   CBOR, this specification always refers to Deterministically Encoded
   CBOR, as specified in Sections 4.2.1 and 4.2.2 of [RFC8949].  The
   single output from authenticated encryption (including the
   authentication tag) is called "ciphertext", following [RFC5116].

2.  EDHOC Outline

   EDHOC supports different authentication methods of the ephemeral-
   ephemeral Diffie-Hellman key exchange.  This document specifies
   authentication methods based on signature keys and static Diffie-
   Hellman keys.  This section outlines the signature-key-based method.
   Further details of protocol elements and other authentication methods
   are provided in the remainder of this document.

   SIGn-and-MAc (SIGMA) is a family of theoretical protocols with a
   number of variants [SIGMA].  Like in Internet Key Exchange Protocol
   Version 2 (IKEv2) [RFC7296] and (D)TLS 1.3 [RFC8446] [RFC9147], EDHOC
   authenticated with signature keys is built on a variant of the SIGMA
   protocol, SIGMA-I, which provides identity protection against active
   attacks on the party initiating the protocol.  Also like IKEv2, EDHOC
   implements the MAC-then-Sign variant of the SIGMA-I protocol.  The
   message flow (excluding an optional fourth message) is shown in
   Figure 1.

   Initiator                                                   Responder
   |                                G_X                                |
   +------------------------------------------------------------------>|
   |                                                                   |
   |      G_Y, Enc( ID_CRED_R, Sig( R; MAC( CRED_R, G_X, G_Y ) ) )     |
   |<------------------------------------------------------------------+
   |                                                                   |
   |        AEAD( ID_CRED_I, Sig( I; MAC( CRED_I, G_Y, G_X ) ) )       |
   +------------------------------------------------------------------>|
   |                                                                   |

      Figure 1: MAC-then-Sign Variant of the SIGMA-I Protocol Used by
                             the EDHOC Method 0

   The parties exchanging messages in an EDHOC session are called the
   Initiator (I) and the Responder (R), where the Initiator sends
   message_1 (see Section 3).  They exchange ephemeral public keys,
   compute a shared secret session key PRK_out, and derive symmetric
   application keys used to protect application data.

   *  G_X and G_Y are the Elliptic Curve Diffie-Hellman (ECDH) ephemeral
      public keys of I and R, respectively.

   *  CRED_I and CRED_R are the authentication credentials containing
      the public authentication keys of I and R, respectively.

   *  ID_CRED_I and ID_CRED_R are used to identify and optionally
      transport the credentials of I and R, respectively.

   *  Sig(I; . ) and Sig(R; . ) denote signatures made with the private
      authentication key of I and R, respectively.

   *  Enc(), AEAD(), and MAC() denote encryption, Authenticated
      Encryption with Associated Data, and Message Authentication Code
      -- crypto algorithms applied with keys derived from one or more
      shared secrets calculated during the protocol.

   In order to create a "full-fledged" protocol, some additional
   protocol elements are needed.  This specification adds:

   *  transcript hashes (hashes of message data), TH_2, TH_3, and TH_4,
      used for key derivation and as additional authenticated data,

   *  computationally independent keys derived from the ECDH shared
      secret and used for authenticated encryption of different
      messages,

   *  an optional fourth message giving key confirmation to I in
      deployments where no protected application data is sent from R to
      I,

   *  a keying material exporter and a key update function with forward
      secrecy,

   *  secure negotiation of the cipher suite,

   *  method types, error handling, and padding,

   *  the selection of connection identifiers, C_I and C_R, which may be
      used in EDHOC to identify the protocol state, and

   *  transport of external authorization data.

   EDHOC is designed to encrypt and integrity protect as much
   information as possible.  Symmetric keys and random material used in
   EDHOC are derived using EDHOC_KDF with as much previous information
   as possible; see Figure 6.  EDHOC is furthermore designed to be as
   compact and lightweight as possible, in terms of message sizes,
   processing, and the ability to reuse already existing CBOR, COSE, and
   CoAP libraries.  Like in (D)TLS, authentication is the responsibility
   of the application.  EDHOC identifies (and optionally transports)
   authentication credentials and provides proof-of-possession of the
   private authentication key.

   To simplify for implementors, the use of CBOR, CDDL, and COSE in
   EDHOC is summarized in Appendix C.  Test vectors, including CBOR
   diagnostic notation, are provided in [RFC9529].

3.  Protocol Elements

3.1.  General

   The EDHOC protocol consists of three mandatory messages (message_1,
   message_2, and message_3), an optional fourth message (message_4),
   and an error message, between an Initiator (I) and a Responder (R).
   The odd messages are sent by I, the even by R.  Both I and R can send
   error messages.  The roles have slightly different security
   properties that should be considered when the roles are assigned; see
   Section 9.1.  All EDHOC messages are CBOR Sequences [RFC8742] and are
   defined to be deterministically encoded CBOR as specified in
   Section 4.2.1 of [RFC8949].  Figure 2 illustrates an EDHOC message
   flow with the optional fourth message as well as the content of each
   message.  The protocol elements in the figure are introduced in
   Sections 3 and 5.  Message formatting and processing are specified in
   Sections 5 and 6.

   Application data may be protected using the agreed application
   algorithms (AEAD, hash) in the selected cipher suite (see
   Section 3.6), and the application can make use of the established
   connection identifiers C_I and C_R (see Section 3.3).  Media types
   that may be used for EDHOC are defined in Section 10.8.

   The Initiator can derive symmetric application keys after creating
   EDHOC message_3; see Section 4.2.1.  Protected application data can
   therefore be sent in parallel or together with EDHOC message_3.
   EDHOC message_4 is typically not sent.

   Initiator                                                   Responder
   |                 METHOD, SUITES_I, G_X, C_I, EAD_1                 |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |       G_Y, Enc( C_R, ID_CRED_R, Signature_or_MAC_2, EAD_2 )       |
   |<------------------------------------------------------------------+
   |                             message_2                             |
   |                                                                   |
   |            AEAD( ID_CRED_I, Signature_or_MAC_3, EAD_3 )           |
   +------------------------------------------------------------------>|
   |                             message_3                             |
   |                                                                   |
   |                           AEAD( EAD_4 )                           |
   |<- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - +
   |                             message_4                             |

     Figure 2: EDHOC Message Flow Including the Optional Fourth Message

3.2.  Method

   The data item METHOD in message_1 (see Section 5.2.1) is an integer
   specifying the authentication method.  EDHOC currently supports
   authentication with signature or static Diffie-Hellman keys, as
   defined in the four authentication methods: 0, 1, 2, and 3; see
   Table 2.  When using a static Diffie-Hellman key, the authentication
   is provided by a Message Authentication Code (MAC) computed from an
   ephemeral-static ECDH shared secret that enables significant
   reductions in message sizes.  Note that, also in the static Diffie-
   Hellman-based authentication methods, there is an ephemeral-ephemeral
   Diffie-Hellman key exchange.

   The Initiator and Responder need to have agreed on a single method to
   be used for EDHOC; see Section 3.9.

      +===================+====================+====================+
      | Method Type Value | Initiator          | Responder          |
      |                   | Authentication Key | Authentication Key |
      +===================+====================+====================+
      |                 0 | Signature Key      | Signature Key      |
      +-------------------+--------------------+--------------------+
      |                 1 | Signature Key      | Static DH Key      |
      +-------------------+--------------------+--------------------+
      |                 2 | Static DH Key      | Signature Key      |
      +-------------------+--------------------+--------------------+
      |                 3 | Static DH Key      | Static DH Key      |
      +-------------------+--------------------+--------------------+
      |                23 | Reserved           | Reserved           |
      +-------------------+--------------------+--------------------+

               Table 2: Authentication Keys for Method Types

   EDHOC does not have a dedicated message field to indicate the
   protocol version.  Breaking changes to EDHOC can be introduced by
   specifying and registering new methods.

3.3.  Connection Identifiers

   EDHOC includes the selection of connection identifiers (C_I and C_R)
   identifying a connection for which keys are agreed.

   Connection identifiers may be used to correlate EDHOC messages and
   facilitate the retrieval of protocol state during an EDHOC session
   (see Section 3.4) or may be used in applications of EDHOC, e.g., in
   OSCORE (see Section 3.3.3).  The connection identifiers do not have
   any cryptographic purpose in EDHOC and only facilitate the retrieval
   of security data associated with the protocol state.

   Connection identifiers in EDHOC are intrinsically byte strings.  Most
   constrained devices only have a few connections for which short
   identifiers may be sufficient.  In some cases, minimum length
   identifiers are necessary to comply with overhead requirements.
   However, CBOR byte strings -- with the exception of the empty byte
   string h'', which encodes as one byte (0x40) -- are encoded as two or
   more bytes.  To enable one-byte encoding of certain byte strings
   while maintaining CBOR encoding, EDHOC represents certain identifiers
   as CBOR integers on the wire; see Section 3.3.2.

3.3.1.  Selection of Connection Identifiers

   C_I and C_R are chosen by I and R, respectively.  The Initiator
   selects C_I and sends it in message_1 for the Responder to use as a
   reference to the connection in communications with the Initiator.
   The Responder selects C_R and sends it in message_2 for the Initiator
   to use as a reference to the connection in communications with the
   Responder.

   If connection identifiers are used by an application protocol for
   which EDHOC establishes keys, then the selected connection
   identifiers SHALL adhere to the requirements for that protocol; see
   Section 3.3.3 for an example.

3.3.2.  Representation of Byte String Identifiers

   To allow identifiers with minimal overhead on the wire, certain byte
   strings used in connection identifiers and credential identifiers
   (see Section 3.5.3) are defined to have integer representations.

   The integers with one-byte CBOR encoding are -24, ..., 23; see
   Figure 3.

   Integer:  -24  -23  ... -11  ...  -2   -1    0    1  ...  15  ...  23
   Encoding:  37   36  ...  2A  ...  21   20   00   01  ...  0F  ...  17

                  Figure 3: One-Byte CBOR-Encoded Integers

   The byte strings that coincide with a one-byte CBOR encoding of an
   integer MUST be represented by the CBOR encoding of that integer.
   Other byte strings are simply encoded as CBOR byte strings.

   For example:

   *  0x21 is represented by 0x21 (CBOR encoding of the integer -2), not
      by 0x4121 (CBOR encoding of the byte string 0x21).

   *  0x0D is represented by 0x0D (CBOR encoding of the integer 13), not
      by 0x410D (CBOR encoding of the byte string 0x0D).

   *  0x18 is represented by 0x4118 (CBOR encoding of the byte string
      0x18).

   *  0x38 is represented by 0x4138 (CBOR encoding of the byte string
      0x38).

   *  0xABCD is represented by 0x42ABCD (CBOR encoding of the byte
      string 0xABCD).

   One may view this representation of byte strings as a transport
   encoding, i.e., a byte string that parses as the one-byte CBOR
   encoding of an integer (i.e., integer in the interval -24, ..., 23)
   is just copied directly into the message, and a byte string that does
   not is encoded as a CBOR byte string during transport.

      |  Implementation Note: When implementing the byte string
      |  identifier representation, in some programming languages, it
      |  can help to define a new type or other data structure, which
      |  (in its user-facing API) behaves like a byte string but when
      |  serializing to CBOR produces a CBOR byte string or a CBOR
      |  integer depending on its value.

3.3.3.  Use of Connection Identifiers with OSCORE

   For OSCORE, the choice of connection identifier results in the
   endpoint selecting its Recipient ID (see Section 3.1 of [RFC8613])
   for which certain uniqueness requirements apply (see Section 3.3 of
   [RFC8613]).  Therefore, the Initiator and Responder MUST NOT select
   connection identifiers such that it results in the same OSCORE
   Recipient ID.  Since the connection identifier is a byte string, it
   is converted to an OSCORE Recipient ID equal to the byte string.

   Examples:

   *  A connection identifier 0xFF (represented in the EDHOC message as
      0x41FF; see Section 3.3.2) is converted to the OSCORE Recipient ID
      0xFF.

   *  A connection identifier 0x21 (represented in the EDHOC message as
      0x21; see Section 3.3.2) is converted to the OSCORE Recipient ID
      0x21.

3.4.  Transport

   Cryptographically, EDHOC does not put requirements on the underlying
   layers.  Received messages are processed as the expected next message
   according to the protocol state; see Section 5.  If processing fails
   for any reason, then typically an error message is attempted to be
   sent and the EDHOC session is aborted.

   EDHOC is not bound to a particular transport layer and can even be
   used in environments without IP.  Ultimately, the application is free
   to choose how to transport EDHOC messages including errors.  In order
   to avoid unnecessary message processing or protocol termination, it
   is RECOMMENDED to use reliable transport, such as CoAP in reliable
   mode, which is the default transport; see Appendix A.2.  In general,
   the transport SHOULD handle:

   *  message loss,

   *  message duplication (see Section 7 for an alternative),

   *  flow control,

   *  congestion control,

   *  fragmentation and reassembly,

   *  demultiplexing EDHOC messages from other types of messages,

   *  denial-of-service mitigation, and

   *  message correlation (see Section 3.4.1).

   EDHOC does not require error-free transport since a change in message
   content is detected through the transcript hashes in a subsequent
   integrity verification; see Section 5.  The transport does not
   require additional means to handle message reordering because of the
   lockstep processing of EDHOC.

   EDHOC is designed to enable an authenticated key exchange with small
   messages, where the minimum message sizes are of the order
   illustrated in the first column of Table 1.  There is no maximum
   message size specified by the protocol; for example, this is
   dependent on the size of the authentication credentials (if they are
   transported, see Section 3.5).  The encryption of very large content
   in message_2 when using certain hash algorithms is described in
   Appendix G.

   The use of transport is specified in the application profile, which
   in particular, may specify limitations in message sizes; see
   Section 3.9.

3.4.1.  EDHOC Message Correlation

   Correlation between EDHOC messages is needed to facilitate the
   retrieval of the protocol state and security context during an EDHOC
   session.  It is also helpful for the Responder to get an indication
   that a received EDHOC message is the beginning of a new EDHOC
   session, such that no existing protocol state or security context
   needs to be retrieved.

   Correlation may be based on existing mechanisms in the transport
   protocol; for example, the CoAP Token may be used to correlate EDHOC
   messages in a CoAP response and in an associated CoAP request.  The
   connection identifiers may also be used to correlate EDHOC messages.

   If correlation between consecutive messages is not provided by other
   means, then the transport binding SHOULD mandate prepending of an
   appropriate connection identifier (when available from the EDHOC
   protocol) to the EDHOC message.  If message_1 indication is not
   provided by other means, then the transport binding SHOULD mandate
   prepending of message_1 with the CBOR simple value true (0xf5).

   Transport of EDHOC in CoAP payloads is described in Appendix A.2,
   including how to use connection identifiers and message_1 indication
   with CoAP.  A similar construction is possible for other client-
   server protocols.  Protocols that do not provide any correlation at
   all can prescribe prepending of the peer's connection identifier to
   all messages.

   Note that correlation between EDHOC messages may be obtained without
   transport support or connection identifiers, for example, if the
   endpoints only accept a single instance of the protocol at a time and
   execute conditionally on a correct sequence of messages.

3.5.  Authentication Parameters

   EDHOC supports various settings for how the other endpoint's public
   key for authentication may be transported, identified, and trusted.
   We shall use the term "authentication key" to mean key used for
   authentication in general, or specifically, the public key, when
   there is no risk for confusion.

   EDHOC performs the following authentication-related operations:

   *  EDHOC transports information about credentials in ID_CRED_I and
      ID_CRED_R (described in Section 3.5.3).  Based on this
      information, the authentication credentials CRED_I and CRED_R
      (described in Section 3.5.2) can be obtained.  EDHOC may also
      transport certain authentication-related information as external
      authorization data (see Section 3.8).

   *  EDHOC uses the authentication credentials in two ways (see
      Sections 5.3.2 and 5.4.2):

      -  The authentication credential is input to the integrity
         verification using the MAC fields.

      -  The authentication key of the authentication credential is used
         with the Signature_or_MAC field to verify proof-of-possession
         of the private key.

   Other authentication-related verifications are out of scope for EDHOC
   and are the responsibility of the application.  In particular, the
   authentication credential needs to be validated in the context of the
   connection for which EDHOC is used; see Appendix D.  EDHOC MUST allow
   the application to read received information about credentials in
   ID_CRED_R and ID_CRED_I.  EDHOC MUST have access to the
   authentication key and the authentication credential.

   Note that the type of authentication key, the type of authentication
   credential, and the identification of the credential have a large
   impact on the message size.  For example, the Signature_or_MAC field
   is much smaller with a static DH key than with a signature key.  A
   CWT Claims Set (CCS) is much smaller than a self-signed certificate /
   CWT, but if it is possible to reference the credential with a COSE
   header like 'kid', then that is in turn much smaller than a CCS.

3.5.1.  Authentication Keys

   The authentication key MUST be a signature key or a static Diffie-
   Hellman key.  The Initiator and Responder MAY use different types of
   authentication keys, e.g., one uses a signature key and the other
   uses a static Diffie-Hellman key.

   The authentication key algorithm needs to be compatible with the
   method and the selected cipher suite (see Section 3.6).  The
   authentication key algorithm needs to be compatible with the EDHOC
   key exchange algorithm when static Diffie-Hellman authentication is
   used and compatible with the EDHOC signature algorithm when signature
   authentication is used.

   Note that for most signature algorithms, the signature is determined
   jointly by the signature algorithm and the authentication key
   algorithm.  When using static Diffie-Hellman keys, the Initiator's
   and the Responder's private authentication keys are denoted as I and
   R, respectively, and the public authentication keys are denoted G_I
   and G_R, respectively.

   For X.509 certificates, the authentication key is represented by a
   SubjectPublicKeyInfo field, which also contains information about
   authentication key algorithm.  For CWT and CCS (see Section 3.5.2),
   the authentication key is represented by a 'cnf' claim [RFC8747]
   containing a COSE_Key [RFC9052], which contains information about
   authentication key algorithm.  In EDHOC, a raw public key (RPK) is an
   authentication key encoded as a COSE_Key wrapped in a CCS, an example
   is given in Figure 4.

3.5.2.  Authentication Credentials

   The authentication credentials, CRED_I and CRED_R, contain the public
   authentication key of the Initiator and Responder, respectively.  We
   use the notation CRED_x to refer to CRED_I or CRED_R.  Requirements
   on CRED_x applies both to CRED_I and to CRED_R.  The authentication
   credential typically also contains other parameters that needs to be
   verified by the application (see Appendix D) and in particular
   information about the identity ("subject") of the endpoint to prevent
   misbinding attacks (see Appendix D.2).

   EDHOC relies on COSE for identification of credentials (see
   Section 3.5.3), for example, X.509 certificates [RFC9360], C509
   certificates [C509-CERTS], CWTs [RFC8392], and CCSs [RFC8392].  When
   the identified credential is a chain or a bag, the authentication
   credential CRED_x is just the end entity X.509 or C509 certificate /
   CWT.  In the choice between a chain or a bag, it is RECOMMENDED to
   use a chain, since the certificates in a bag are unordered and may
   contain self-signed and extraneous certificates, which can add
   complexity to the process of extracting the end entity certificate.
   The Initiator and Responder MAY use different types of authentication
   credentials, e.g., one uses an RPK and the other uses a public key
   certificate.

   Since CRED_R is used in the integrity verification (see
   Section 5.3.2), it needs to be specified such that it is identical
   when used by the Initiator or Responder.  Similarly for CRED_I, see
   Section 5.4.2.  The Initiator and Responder are expected to agree on
   the specific encoding of the authentication credentials; see
   Section 3.9.  It is RECOMMENDED that the COSE 'kid' parameter, when
   used to identify the authentication credential, refers to such a
   specific encoding of the authentication credential.  The Initiator
   and Responder SHOULD use an available authentication credential
   without re-encoding, i.e. an authentication credential transported in
   EDHOC by value, or otherwise provisioned, SHOULD be used as is.  If
   for some reason re-encoding of an authentication credential passed by
   reference may occur, then a potential common encoding for CBOR-based
   credentials is deterministically encoded CBOR, as specified in
   Sections 4.2.1 and 4.2.2 of [RFC8949].

   *  When the authentication credential is an X.509 certificate, CRED_x
      SHALL be the DER-encoded certificate, encoded as a bstr [RFC9360].

   *  When the authentication credential is a C509 certificate, CRED_x
      SHALL be the C509 certificate [C509-CERTS].

   *  When the authentication credential is a CWT including a COSE_Key,
      CRED_x SHALL be the untagged CWT.

   *  When the authentication credential includes a COSE_Key but is not
      in a CWT, CRED_x SHALL be an untagged CCS.  This is how RPKs are
      encoded, see Figure 4 for an example.

      -  Naked COSE_Keys are thus dressed as CCS when used in EDHOC, in
         its simplest form by prefixing the COSE_Key with 0xA108A101 (a
         map with a 'cnf' claim).  In that case, the resulting
         authentication credential contains no other identity than the
         public key itself; see Appendix D.2.

   An example of CRED_x is shown below:

   {                                              /CCS/
     2 : "42-50-31-FF-EF-37-32-39",               /sub/
     8 : {                                        /cnf/
       1 : {                                      /COSE_Key/
         1 : 1,                                   /kty/
         2 : h'00',                               /kid/
        -1 : 4,                                   /crv/
        -2 : h'b1a3e89460e88d3a8d54211dc95f0b90   /x/
               3ff205eb71912d6db8f4af980d2db83a'
       }
     }
   }

      Figure 4: CCS Containing an X25519 Static Diffie-Hellman Key and
                             an EUI-64 Identity

3.5.3.  Identification of Credentials

   The ID_CRED fields, ID_CRED_R and ID_CRED_I, are transported in
   message_2 and message_3, respectively; see Sections 5.3.2 and 5.4.2.
   We use the notation ID_CRED_x to refer to ID_CRED_I or ID_CRED_R.
   Requirements on ID_CRED_x applies both to ID_CRED_I and to ID_CRED_R.
   The ID_CRED fields are used to identify and optionally transport
   credentials:

   *  ID_CRED_R is intended to facilitate for the Initiator retrieving
      the authentication credential CRED_R and the authentication key of
      R.

   *  ID_CRED_I is intended to facilitate for the Responder retrieving
      the authentication credential CRED_I and the authentication key of
      I.

   ID_CRED_x may contain the authentication credential CRED_x, for x = I
   or R, but for many settings, it is not necessary to transport the
   authentication credential within EDHOC.  For example, it may be pre-
   provisioned or acquired out-of-band over less constrained links.
   ID_CRED_I and ID_CRED_R do not have any cryptographic purpose in
   EDHOC since the authentication credentials are integrity protected by
   the Signature_or_MAC field.

   EDHOC relies on COSE for identification of credentials and supports
   all credential types for which COSE header parameters are defined,
   including X.509 certificates [RFC9360], C509 certificates
   [C509-CERTS], CWTs (Section 3.5.3.1) and CCSs (Section 3.5.3.1).

   ID_CRED_I and ID_CRED_R are of type COSE header_map, as defined in
   Section 3 of [RFC9052], and contain one or more COSE header
   parameters.  If a map contains several header parameters, the labels
   do not need to be sorted in bytewise lexicographic order.  ID_CRED_I
   and ID_CRED_R MAY contain different header parameters.  The header
   parameters typically provide some information about the format of the
   credential.

   Example: X.509 certificates can be identified by a hash value using
   the 'x5t' parameter; see Section 2 of [RFC9360]:

   *  ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R

   Example: CWT or CCS can be identified by a key identifier using the
   'kid' parameter; see Section 3.1 of [RFC9052]:

   *  ID_CRED_x = { 4 : kid_x }, where kid_x : kid, for x = I or R

   Note that COSE header parameters in ID_CRED_x are used to identify
   the message sender's credential.  Therefore, there is no reason to
   use the "-sender" header parameters, such as x5t-sender, defined in
   Section 3 of [RFC9360].  Instead, the corresponding parameter without
   "-sender", such as x5t, SHOULD be used.

   As stated in Section 3.1 of [RFC9052], applications MUST NOT assume
   that 'kid' values are unique and several keys associated with a 'kid'
   may need to be checked before the correct one is found.  Applications
   might use additional information such as 'kid context' or lower
   layers to determine which key to try first.  Applications should
   strive to make ID_CRED_x as unique as possible, since the recipient
   may otherwise have to try several keys.

   See Appendix C.3 for more examples.

3.5.3.1.  COSE Header Parameters for CWT and CWT Claims Set

   This document registers two new COSE header parameters, 'kcwt' and
   'kccs', for use with CBOR Web Token (CWT) [RFC8392] and CWT Claims
   Set (CCS) [RFC8392], respectively.  The CWT/CCS MUST contain a
   COSE_Key in a 'cnf' claim [RFC8747].  There may be any number of
   additional claims present in the CWT/CCS.

   CWTs sent in 'kcwt' are protected using a MAC or a signature and are
   similar to a certificate (when used with public key cryptography) or
   a Kerberos ticket (when used with symmetric key cryptography).  CCSs
   sent in 'kccs' are not protected and are therefore similar to raw
   public keys or self-signed certificates.

   Security considerations for 'kcwt' and 'kccs' are made in
   Section 9.8.

3.5.3.2.  Compact Encoding of ID_CRED Fields for 'kid'

   To comply with the Lightweight Authenticated Key Exchange (LAKE)
   message size requirements (see [LAKE-REQS]), two optimizations are
   made for the case when ID_CRED_x, for x = I or R, contains a single
   'kid' parameter.

   1.  The CBOR map { 4 : kid_x } is replaced by the byte string kid_x.

   2.  The representation of identifiers specified in Section 3.3.2 is
       applied to kid_x.

   These optimizations MUST be applied if and only if ID_CRED_x = { 4 :
   kid_x } and ID_CRED_x in PLAINTEXT_y of message_y, y = 2 or 3; see
   Sections 5.3.2 and 5.4.2.  Note that these optimizations are not
   applied to instances of ID_CRED_x that have no impact on message
   size, e.g., context_y, or the COSE protected header.  For example:

   *  For ID_CRED_x = { 4 : h'FF' }, the encoding in PLAINTEXT_y is not
      the CBOR map 0xA10441FF but the CBOR byte string h'FF', i.e.,
      0x41FF.

   *  For ID_CRED_x = { 4 : h'21' }, the encoding in PLAINTEXT_y is
      neither the CBOR map 0xA1044121 nor the CBOR byte string h'21',
      i.e., 0x4121, but the CBOR integer 0x21.

3.6.  Cipher Suites

   An EDHOC cipher suite consists of an ordered set of algorithms from
   the "COSE Algorithms" and "COSE Elliptic Curves" registries as well
   as the EDHOC MAC length.  All algorithm names and definitions follow
   COSE Algorithms [RFC9053].  Note that COSE sometimes uses peculiar
   names such as ES256 for Elliptic Curve Digital Signature Algorithm
   (ECDSA) with SHA-256, A128 for AES-128, and Ed25519 for the curve
   edwards25519.  Algorithms need to be specified with enough parameters
   to make them completely determined.  The EDHOC MAC length MUST be at
   least 8 bytes.  Any cryptographic algorithm used in the COSE header
   parameters in ID_CRED fields is selected independently of the
   selected cipher suite.  EDHOC is currently only specified for use
   with key exchange algorithms of type ECDH curves, but any Key
   Encapsulation Mechanism (KEM), including Post-Quantum Cryptography
   (PQC) KEMs, can be used in method 0; see Section 9.4.  Use of other
   types of key exchange algorithms to replace static DH authentication
   (methods 1, 2, and 3) would likely require a specification updating
   EDHOC with new methods.

   EDHOC supports all signature algorithms defined by COSE.  Just like
   in (D)TLS 1.3 [RFC8446] [RFC9147] and IKEv2 [RFC7296], a signature in
   COSE is determined jointly by the signature algorithm and the
   authentication key algorithm; see Section 3.5.1.  The exact details
   of the authentication key algorithm depend on the type of
   authentication credential.  COSE supports different formats for
   storing the public authentication keys including COSE_Key and X.509,
   which use different names and ways to represent the authentication
   key and the authentication key algorithm.

   An EDHOC cipher suite consists of the following parameters:

   *  EDHOC AEAD algorithm,

   *  EDHOC hash algorithm,

   *  EDHOC MAC length in bytes (Static DH),

   *  EDHOC key exchange algorithm (ECDH curve),

   *  EDHOC signature algorithm,

   *  application AEAD algorithm, and

   *  application hash algorithm.

   Each cipher suite is identified with a predefined integer label.

   EDHOC can be used with all algorithms and curves defined for COSE.
   Implementations can either use any combination of COSE algorithms and
   parameters to define their own private cipher suite or use one of the
   predefined cipher suites.  Private cipher suites can be identified
   with any of the four values: -24, -23, -22, and -21.  The predefined
   cipher suites are listed in the IANA registry (Section 10.2) with the
   initial content outlined here:

   *  Cipher suites 0-3, based on AES-CCM, are intended for constrained
      IoT where message overhead is a very important factor.  Note that
      AES-CCM-16-64-128 and AES-CCM-16-128-128 are compatible with the
      IEEE AES-CCM* mode of operation defined in Annex B of
      [IEEE.802.15.4-2015].

      -  Cipher suites 1 and 3 use a larger tag length (128 bits) in
         EDHOC than in the application AEAD algorithm (64 bits).

   *  Cipher suites 4 and 5, based on ChaCha20, are intended for less
      constrained applications and only use 128-bit tag lengths.

   *  Cipher suite 6, based on AES-GCM, is for general non-constrained
      applications.  It consists of high-performance algorithms that are
      widely used in non-constrained applications.

   *  Cipher suites 24 and 25 are intended for high security
      applications such as government use and financial applications.
      These cipher suites do not share any algorithms.  Cipher suite 24
      consists of algorithms from the Commercial National Security
      Algorithm (CNSA) 1.0 suite [CNSA].

   The different methods (Section 3.2) use the same cipher suites, but
   some algorithms are not used in some methods.  The EDHOC signature
   algorithm is not used in methods without signature authentication.

   The Initiator needs to have a list of cipher suites it supports in
   order of preference.  The Responder needs to have a list of cipher
   suites it supports.  SUITES_I contains cipher suites supported by the
   Initiator and formatted and processed as detailed in Section 5.2.1 to
   secure the cipher suite negotiation.  Examples of cipher suite
   negotiation are given in Section 6.3.2.

3.7.  Ephemeral Public Keys

   The ephemeral public keys in EDHOC (G_X and G_Y) use compact
   representation of elliptic curve points; see Appendix B.  In COSE,
   compact representation is achieved by formatting the ECDH ephemeral
   public keys as COSE_Keys of type EC2 or Octet Key Pair (OKP)
   according to Sections 7.1 and 7.2 of [RFC9053] but only including the
   'x' parameter in G_X and G_Y.  For Elliptic Curve Keys of type EC2,
   compact representation MAY be used also in the COSE_Key.  COSE always
   uses compact output for Elliptic Curve Keys of type EC2.  If the COSE
   implementation requires a 'y' parameter, the value y = false or a
   calculated y-coordinate can be used; see Appendix B.

3.8.  External Authorization Data (EAD)

   In order to reduce round trips and the number of messages or to
   simplify processing, external security applications may be integrated
   into EDHOC by transporting authorization-related data in the
   messages.

   EDHOC allows processing of external authorization data (EAD) to be
   defined in a separate specification and sent in dedicated fields of
   the four EDHOC messages: EAD_1, EAD_2, EAD_3, and EAD_4.  EAD is
   opaque data to EDHOC.

   Each EAD field, EAD_x, for x = 1, 2, 3, or 4, is a CBOR sequence (see
   Appendix C.1) consisting of one or more EAD items.  EAD item ead is a
   CBOR sequence of an ead_label and an optional ead_value; see Figure 5
   and Appendix C.2 for the CDDL definitions.

   ead = (
     ead_label : int,
     ? ead_value : bstr,
   )

                             Figure 5: EAD Item

   A security application may register one or more EAD labels (see
   Section 10.5) and specify the associated processing and security
   considerations.  The IANA registry contains the absolute value of the
   ead_label, |ead_label|; the same ead_value applies independently of
   the sign of the ead_label.

   An EAD item can be either critical or non-critical, determined by the
   sign of the ead_label in the EAD item transported in the EAD field.
   A negative value indicates that the EAD item is critical, and a
   nonnegative value indicates that the EAD item is non-critical.

   If an endpoint receives a critical EAD item it does not recognize or
   a critical EAD item that contains information that it cannot process,
   then the endpoint MUST send an EDHOC error message back as defined in
   Section 6, and the EDHOC session MUST be aborted.  The EAD item
   specification defines the error processing.  A non-critical EAD item
   can be ignored.

   The security application registering a new EAD item needs to describe
   under what conditions the EAD item is critical or non-critical, and
   thus whether the ead_label is used with a negative or positive sign.
   ead_label = 0 is used for padding; see Section 3.8.1.

   The security application may define multiple uses of certain EAD
   items, e.g., the same EAD item may be used in different EDHOC
   messages.  Multiple occurrences of an EAD item in one EAD field may
   also be specified, but the criticality of the repeated EAD item is
   expected to be the same.

   The EAD fields of EDHOC MUST only be used with registered EAD items;
   see Section 10.5.  Examples of the use of EAD are provided in
   Appendix E.

3.8.1.  Padding

   EDHOC message_1 and the plaintext of message_2, message_3, and
   message_4 can be padded with the use of the corresponding EAD_x
   field, for x = 1, 2, 3, or 4.  Padding in EAD_1 mitigates
   amplification attacks (see Section 9.7), and padding in EAD_2, EAD_3,
   and EAD_4 hides the true length of the plaintext (see Section 9.6).
   Padding MUST be ignored and discarded by the receiving application.

   Padding is obtained by using an EAD item with ead_label = 0 and a
   (pseudo)randomly generated byte string of appropriate length as
   ead_value, noting that the ead_label and the CBOR encoding of
   ead_value also add bytes.  For example:

   *  One-byte padding (optional ead_value omitted):

      EAD_x = 0x00

   *  Two-byte padding, using the empty byte string (0x40) as ead_value:

      EAD_x = 0x0040

   *  Three-byte padding, constructed from the pseudorandomly generated
      ead_value 0xe9 encoded as byte string:

      EAD_x = 0x0041e9

   Multiple occurrences of EAD items with ead_label = 0 are allowed.
   Certain padding lengths require the use of at least two such EAD
   items.

   Note that padding is non-critical because the intended behavior when
   receiving is to ignore it.

3.9.  Application Profile

   EDHOC requires certain parameters to be agreed upon between the
   Initiator and Responder.  Some parameters can be negotiated through
   the protocol execution (specifically, cipher suite; see Section 3.6),
   but other parameters are only communicated and may not be negotiated
   (e.g., which authentication method is used; see Section 3.2).  Yet,
   other parameters need to be known out-of-band to ensure successful
   completion, e.g., whether message_4 is used or not.  The application
   decides which endpoint is the Initiator and which is the Responder.

   The purpose of an application profile is to describe the intended use
   of EDHOC to allow for the relevant processing and verifications to be
   made, including things like the following:

   1.  How the endpoint detects that an EDHOC message is received.  This
       includes how EDHOC messages are transported, for example, in the
       payload of a CoAP message with a certain Uri-Path or Content-
       Format; see Appendix A.2.

       The method of transporting EDHOC messages may also describe data
       carried along with the messages that are needed for the transport
       to satisfy the requirements of Section 3.4, e.g., connection
       identifiers used with certain messages; see Appendix A.2.

   2.  Authentication method (METHOD; see Section 3.2).

   3.  Profile for authentication credentials (CRED_I and CRED_R; see
       Section 3.5.2), e.g., profile for certificate or CCS, including
       supported authentication key algorithms (subject public key
       algorithm in X.509 or C509 certificate).

   4.  Type used to identify credentials (ID_CRED_I and ID_CRED_R; see
       Section 3.5.3).

   5.  Use and type of external authorization data (EAD_1, EAD_2, EAD_3,
       and EAD_4; see Section 3.8).

   6.  Identifier used as the identity of the endpoint; see
       Appendix D.2.

   7.  If message_4 shall be sent/expected, and if not, how to ensure a
       protected application message is sent from the Responder to the
       Initiator; see Section 5.5.

   The application profile may also contain information about supported
   cipher suites.  The procedure for selecting and verifying a cipher
   suite is still performed as described in Sections 5.2.1 and 6.3, but
   it may become simplified by this knowledge.  EDHOC messages can be
   processed without the application profile, i.e., the EDHOC messages
   include information about the type and length of all fields.

   An example of an application profile is shown in Appendix F.

   For some parameters, like METHOD, the type of the ID_CRED field, or
   EAD, the receiver of an EDHOC message is able to verify compliance
   with the application profile and, if it needs to fail because of the
   lack of compliance, to infer the reason why the EDHOC session failed.

   For other encodings, like the profiling of CRED_x in the case that it
   is not transported, it may not be possible to verify that the lack of
   compliance with the application profile was the reason for failure.
   For example, integrity verification in message_2 or message_3 may
   fail not only because of a wrong credential.  For example, in case
   the Initiator uses a public key certificate by reference (i.e., not
   transported within the protocol), then both endpoints need to use an
   identical data structure as CRED_I or else the integrity verification
   will fail.

   Note that it is not necessary for the endpoints to specify a single
   transport for the EDHOC messages.  For example, a mix of CoAP and
   HTTP may be used along the path, and this may still allow correlation
   between messages.

   The application profile may be dependent on the identity of the other
   endpoint or other information carried in an EDHOC message, but it
   then applies only to the later phases of the protocol when such
   information is known.  (The Initiator does not know the identity of
   the Responder before having verified message_2, and the Responder
   does not know the identity of the Initiator before having verified
   message_3.)

   Other conditions may be part of the application profile, such as what
   is the target application or use (if there is more than one
   application/use) to the extent that EDHOC can distinguish between
   them.  In case multiple application profiles are used, the receiver
   needs to be able to determine which is applicable for a given EDHOC
   session, for example, based on the URI to which the EDHOC message is
   sent, or external authorization data type.

4.  Key Derivation

4.1.  Keys for EDHOC Message Processing

   EDHOC uses Extract-and-Expand [RFC5869] with the EDHOC hash algorithm
   in the selected cipher suite to derive keys used in message
   processing.  This section defines EDHOC_Extract (Section 4.1.1) and
   EDHOC_Expand (Section 4.1.2) and how to use them to derive PRK_out
   (Section 4.1.3), which is the shared secret session key resulting
   from a completed EDHOC session.

   EDHOC_Extract is used to derive fixed-length uniformly pseudorandom
   keys (PRKs) from ECDH shared secrets.  EDHOC_Expand is used to define
   EDHOC_KDF for generating MACs and for deriving output keying material
   (OKM) from PRKs.

   In EDHOC, a specific message is protected with a certain PRK, but how
   the key is derived depends on the authentication method
   (Section 3.2), as detailed in Section 5.

4.1.1.  EDHOC_Extract

   The pseudorandom keys (PRKs) used for EDHOC message processing are
   derived using EDHOC_Extract:

      PRK = EDHOC_Extract( salt, IKM )

   where the input keying material (IKM) and salt are defined for each
   PRK below.

   The definition of EDHOC_Extract depends on the EDHOC hash algorithm
   of the selected cipher suite:

   *  If the EDHOC hash algorithm is SHA-2, then EDHOC_Extract( salt,
      IKM ) = HKDF-Extract( salt, IKM ) [RFC5869].

   *  If the EDHOC hash algorithm is SHAKE128, then EDHOC_Extract( salt,
      IKM ) = KMAC128( salt, IKM, 256, "" ).

   *  If the EDHOC hash algorithm is SHAKE256, then EDHOC_Extract( salt,
      IKM ) = KMAC256( salt, IKM, 512, "" ).

   where the Keccak Message Authentication Code (KMAC) is specified in
   [SP800-185].

   The rest of the section defines the pseudorandom keys PRK_2e,
   PRK_3e2m, and PRK_4e3m; their use is shown in Figure 6.  The index of
   a PRK indicates its use or in what message protection operation it is
   involved.  For example, PRK_3e2m is involved in the encryption of
   message 3 and in calculating the MAC of message 2.

4.1.1.1.  PRK_2e

   The pseudorandom key PRK_2e is derived with the following input:

   *  The salt SHALL be TH_2.

   *  The IKM SHALL be the ephemeral-ephemeral ECDH shared secret G_XY
      (calculated from G_X and Y or G_Y and X) as defined in
      Section 6.3.1 of [RFC9053].  The use of G_XY gives forward secrecy
      in the sense that compromise of the private authentication keys
      does not compromise past session keys.

   Example: Assuming the use of curve25519, the ECDH shared secret G_XY
   is the output of the X25519 function [RFC7748]:

      G_XY = X25519( Y, G_X ) = X25519( X, G_Y )

   Example: Assuming the use of SHA-256, the extract phase of the key
   derivation function is HKDF-Extract, which produces PRK_2e as
   follows:

      PRK_2e = HMAC-SHA-256( TH_2, G_XY )

4.1.1.2.  PRK_3e2m

   The pseudorandom key PRK_3e2m is derived as follows:

   If the Responder authenticates with a static Diffie-Hellman key, then
   PRK_3e2m = EDHOC_Extract( SALT_3e2m, G_RX ), where

   *  SALT_3e2m is derived from PRK_2e (see Section 4.1.2) and

   *  G_RX is the ECDH shared secret calculated from G_R and X, or G_X
      and R (the Responder's private authentication key; see
      Section 3.5.1),

   else PRK_3e2m = PRK_2e.

4.1.1.3.  PRK_4e3m

   The pseudorandom key PRK_4e3m is derived as follows:

   If the Initiator authenticates with a static Diffie-Hellman key, then
   PRK_4e3m = EDHOC_Extract( SALT_4e3m, G_IY ), where

   *  SALT_4e3m is derived from PRK_3e2m (see Section 4.1.2) and

   *  G_IY is the ECDH shared secret calculated from G_I and Y, or G_Y
      and I (the Initiator's private authentication key; see
      Section 3.5.1),

   else PRK_4e3m = PRK_3e2m.

4.1.2.  EDHOC_Expand and EDHOC_KDF

   The output keying material (OKM) -- including keys, initialization
   vectors (IVs), and salts -- are derived from the PRKs using the
   EDHOC_KDF, which is defined through EDHOC_Expand:

      OKM = EDHOC_KDF( PRK, info_label, context, length )
          = EDHOC_Expand( PRK, info, length )

   where info is encoded as the CBOR sequence:

   info = (
     info_label : int,
     context : bstr,
     length : uint,
   )

   where:

   *  info_label is an int,

   *  context is a bstr, and

   *  length is the length of OKM in bytes.

   When EDHOC_KDF is used to derive OKM for EDHOC message processing,
   then the context includes one of the transcript hashes, TH_2, TH_3,
   or TH_4, defined in Sections 5.3.2 and 5.4.2.

   The definition of EDHOC_Expand depends on the EDHOC hash algorithm of
   the selected cipher suite:

   *  If the EDHOC hash algorithm is SHA-2, then EDHOC_Expand( PRK,
      info, length ) = HKDF-Expand( PRK, info, length ) [RFC5869].

   *  If the EDHOC hash algorithm is SHAKE128, then EDHOC_Expand( PRK,
      info, length ) = KMAC128( PRK, info, L, "" ).

   *  If the EDHOC hash algorithm is SHAKE256, then EDHOC_Expand( PRK,
      info, length ) = KMAC256( PRK, info, L, "" ).

   where L = 8 ⋅ length, the output length in bits.

   Figure 6 lists derivations made with EDHOC_KDF, where:

   *  hash_length is the length of output size of the EDHOC hash
      algorithm of the selected cipher suite,

   *  key_length is the length of the encryption key of the EDHOC AEAD
      algorithm of the selected cipher suite, and

   *  iv_length is the length of the initialization vector of the EDHOC
      AEAD algorithm of the selected cipher suite

   Further details of the key derivation and how the output keying
   material is used are specified in Section 5.

   KEYSTREAM_2   = EDHOC_KDF( PRK_2e,   0, TH_2,      plaintext_length )
   SALT_3e2m     = EDHOC_KDF( PRK_2e,   1, TH_2,      hash_length )
   MAC_2         = EDHOC_KDF( PRK_3e2m, 2, context_2, mac_length_2 )
   K_3           = EDHOC_KDF( PRK_3e2m, 3, TH_3,      key_length )
   IV_3          = EDHOC_KDF( PRK_3e2m, 4, TH_3,      iv_length )
   SALT_4e3m     = EDHOC_KDF( PRK_3e2m, 5, TH_3,      hash_length )
   MAC_3         = EDHOC_KDF( PRK_4e3m, 6, context_3, mac_length_3 )
   PRK_out       = EDHOC_KDF( PRK_4e3m, 7, TH_4,      hash_length )
   K_4           = EDHOC_KDF( PRK_4e3m, 8, TH_4,      key_length )
   IV_4          = EDHOC_KDF( PRK_4e3m, 9, TH_4,      iv_length )
   PRK_exporter  = EDHOC_KDF( PRK_out, 10, h'',       hash_length )

                 Figure 6: Key Derivations Using EDHOC_KDF

   h'' is CBOR diagnostic notation for the empty byte string, 0x40.

4.1.3.  PRK_out

   The pseudorandom key PRK_out, derived as shown in Figure 6, is the
   output session key of a completed EDHOC session.

   Keys for applications are derived using EDHOC_Exporter (see
   Section 4.2.1) from PRK_exporter, which in turn is derived from
   PRK_out as shown in Figure 6.  For the purpose of generating
   application keys, it is sufficient to store PRK_out or PRK_exporter.
   (Note that the word "store" used here does not imply that the
   application has access to the plaintext PRK_out since that may be
   reserved for code within a Trusted Execution Environment (TEE); see
   Section 9.8.)

4.2.  Keys for EDHOC Applications

   This section defines EDHOC_Exporter in terms of EDHOC_KDF and
   PRK_exporter.  A key update function is defined in Appendix H.

4.2.1.  EDHOC_Exporter

   Keying material for the application can be derived using the
   EDHOC_Exporter interface defined as:

      EDHOC_Exporter(exporter_label, context, length)
        = EDHOC_KDF(PRK_exporter, exporter_label, context, length)

   where:

   *  exporter_label is a registered uint from the "EDHOC Exporter
      Labels" registry (Section 10.1),

   *  context is a bstr defined by the application, and

   *  length is a uint defined by the application.

   The (exporter_label, context) pair used in EDHOC_Exporter must be
   unique, i.e., an (exporter_label, context) MUST NOT be used for two
   different purposes.  However, an application can re-derive the same
   key several times as long as it is done securely.  For example, in
   most encryption algorithms, the same key can be reused with different
   nonces.  The context can, for example, be the empty CBOR byte string.

   Examples of use of the EDHOC_Exporter are given in Appendix A.

5.  Message Formatting and Processing

   This section specifies formatting of the messages and processing
   steps.  Error messages are specified in Section 6.  Annotated traces
   of EDHOC sessions are provided in [RFC9529].

   An EDHOC message is encoded as a sequence of CBOR data items (CBOR
   Sequence [RFC8742]).  Additional optimizations are made to reduce
   message overhead.

   While EDHOC uses the COSE_Key, COSE_Sign1, and COSE_Encrypt0
   structures, only a subset of the parameters is included in the EDHOC
   messages; see Appendix C.3.  In order to recreate the COSE object,
   the recipient endpoint may need to add parameters to the COSE headers
   not included in the EDHOC message, for example, the parameter 'alg'
   to COSE_Sign1 or COSE_Encrypt0.

5.1.  EDHOC Message Processing Outline

   For each new/ongoing EDHOC session, the endpoints are assumed to keep
   an associated protocol state containing identifiers, keying material,
   etc. used for subsequent processing of protocol-related data.  The
   protocol state is assumed to be associated with an application
   profile (Section 3.9) that provides the context for how messages are
   transported, identified, and processed.

   EDHOC messages SHALL be processed according to the current protocol
   state.  The following steps are expected to be performed at reception
   of an EDHOC message:

   1.  Detect that an EDHOC message has been received, for example, by
       means of a port number, URI, or media type (Section 3.9).

   2.  Retrieve the protocol state according to the message correlation;
       see Section 3.4.1.  If there is no protocol state, in the case of
       message_1, a new protocol state is created.  The Responder
       endpoint needs to make use of available denial-of-service
       mitigation (Section 9.7).

   3.  If the message received is an error message, then process it
       according to Section 6, else process it as the expected next
       message according to the protocol state.

   The message processing steps SHALL be processed in order, unless
   otherwise stated.  If the processing fails for some reason, then
   typically an error message is sent, the EDHOC session is aborted, and
   the protocol state is erased.  When the composition and sending of
   one message is completed and before the next message is received,
   error messages SHALL NOT be sent.

   After having successfully processed the last message (message_3 or
   message_4 depending on application profile), the EDHOC session is
   completed; after which, no error messages are sent and EDHOC session
   output MAY be maintained even if error messages are received.
   Further details are provided in the following subsections and in
   Section 6.

   Different instances of the same message MUST NOT be processed in one
   EDHOC session.  Note that processing will fail if the same message
   appears a second time for EDHOC processing in the same EDHOC session
   because the state of the protocol has moved on and now expects
   something else.  Message deduplication MUST be done by the transport
   protocol (see Section 3.4) or, if not supported by the transport, as
   described in Section 7.

5.2.  EDHOC Message 1

5.2.1.  Formatting of Message 1

   message_1 SHALL be a CBOR Sequence (see Appendix C.1), as defined
   below.

   message_1 = (
     METHOD : int,
     SUITES_I : suites,
     G_X : bstr,
     C_I : bstr / -24..23,
     ? EAD_1,
   )

   suites = [ 2* int ] / int
   EAD_1 = 1* ead

   where:

   *  METHOD is an authentication method; see Section 3.2,

   *  SUITES_I is an array of cipher suites that the Initiator supports
      constructed as specified in Section 5.2.2,

   *  G_X is the ephemeral public key of the Initiator, and

   *  C_I is a variable-length connection identifier (note that
      connection identifiers are byte strings but certain values are
      represented as integers in the message; see Section 3.3.2), and

   *  EAD_1 is external authorization data; see Section 3.8.

5.2.2.  Initiator Composition of Message 1

   The processing steps are detailed below and in Section 6.3.

   The Initiator SHALL compose message_1 as follows:

   *  Construct SUITES_I as an array of cipher suites supported by I in
      order of preference by I with the first cipher suite in the array
      being the most preferred by I and the last being the one selected
      by I for this EDHOC session.  If the cipher suite most preferred
      by I is selected, then SUITES_I contains only that cipher suite
      and is encoded as an int.  All cipher suites, if any, preferred by
      I over the selected one MUST be included.  (See also Section 6.3.)

      -  The selected suite is based on what the Initiator can assume to
         be supported by the Responder; if the Initiator previously
         received from the Responder an error message with error code 2
         containing SUITES_R (see Section 6.3) indicating cipher suites
         supported by the Responder, then the Initiator SHOULD select
         its most preferred supported cipher suite among those (bearing
         in mind that error messages may be forged).

      -  The Initiator MUST NOT change its order of preference for
         cipher suites and MUST NOT omit a cipher suite preferred to the
         selected one because of previous error messages received from
         the Responder.

   *  Generate an ephemeral ECDH key pair using the curve in the
      selected cipher suite and format it as a COSE_Key.  Let G_X be the
      'x' parameter of the COSE_Key.

   *  Choose a connection identifier C_I and store it during the EDHOC
      session.

   *  Encode message_1 as a sequence of CBOR-encoded data items as
      specified in Section 5.2.1

5.2.3.  Responder Processing of Message 1

   The Responder SHALL process message_1 in the following order:

   1.  Decode message_1 (see Appendix C.1).

   2.  Process message_1.  In particular, verify that the selected
       cipher suite is supported and that no prior cipher suite as
       ordered in SUITES_I is supported.

   3.  If all processing completed successfully, and if EAD_1 is
       present, then make it available to the application for EAD
       processing.

   If any processing step fails, then the Responder MUST send an EDHOC
   error message back as defined in Section 6, and the EDHOC session
   MUST be aborted.

5.3.  EDHOC Message 2

5.3.1.  Formatting of Message 2

   message_2 SHALL be a CBOR Sequence (see Appendix C.1), as defined
   below.

   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
   )

   where:

   *  G_Y_CIPHERTEXT_2 is the concatenation of G_Y (i.e., the ephemeral
      public key of the Responder) and CIPHERTEXT_2.

5.3.2.  Responder Composition of Message 2

   The Responder SHALL compose message_2 as follows:

   *  Generate an ephemeral ECDH key pair using the curve in the
      selected cipher suite and format it as a COSE_Key.  Let G_Y be the
      'x' parameter of the COSE_Key.

   *  Choose a connection identifier C_R and store it for the length of
      the EDHOC session.

   *  Compute the transcript hash TH_2 = H( G_Y, H(message_1) ), where
      H() is the EDHOC hash algorithm of the selected cipher suite.  The
      input to the hash function is a CBOR Sequence.  Note that
      H(message_1) can be computed and cached already in the processing
      of message_1.

   *  Compute MAC_2 as in Section 4.1.2 with context_2 = << C_R,
      ID_CRED_R, TH_2, CRED_R, ? EAD_2 >> (see Appendix C.1 for
      notation).

      -  If the Responder authenticates with a static Diffie-Hellman key
         (method equals 1 or 3), then mac_length_2 is the EDHOC MAC
         length of the selected cipher suite.  If the Responder
         authenticates with a signature key (method equals 0 or 2), then
         mac_length_2 is equal to hash_length.

      -  C_R is a variable-length connection identifier.  Note that
         connection identifiers are byte strings but certain values are
         represented as integers in the message; see Section 3.3.2.

      -  ID_CRED_R is an identifier to facilitate the retrieval of
         CRED_R; see Section 3.5.3.

      -  CRED_R is a CBOR item containing the authentication credential
         of the Responder; see Section 3.5.2.

      -  EAD_2 is external authorization data; see Section 3.8.

   *  If the Responder authenticates with a static Diffie-Hellman key
      (method equals 1 or 3), then Signature_or_MAC_2 is MAC_2.  If the
      Responder authenticates with a signature key (method equals 0 or
      2), then Signature_or_MAC_2 is the 'signature' field of a
      COSE_Sign1 object, computed as specified in Section 4.4 of
      [RFC9052] using the signature algorithm of the selected cipher
      suite, the private authentication key of the Responder, and the
      following parameters as input (see Appendix C.3 for an overview of
      COSE and Appendix C.1 for notation):

      -  protected = << ID_CRED_R >>

      -  external_aad = << TH_2, CRED_R, ? EAD_2 >>

      -  payload = MAC_2

   *  CIPHERTEXT_2 is calculated with a binary additive stream cipher,
      using a keystream generated with EDHOC_Expand and the following
      plaintext:

      -  PLAINTEXT_2 = ( C_R, ID_CRED_R / bstr / -24..23,
         Signature_or_MAC_2, ? EAD_2 )

         o  If ID_CRED_R contains a single 'kid' parameter, i.e.,
            ID_CRED_R = { 4 : kid_R }, then the compact encoding is
            applied; see Section 3.5.3.2.

         o  C_R is the variable-length connection identifier.  Note that
            connection identifiers are byte strings, but certain values
            are represented as integers in the message; see
            Section 3.3.2.

      -  Compute KEYSTREAM_2 as in Section 4.1.2, where plaintext_length
         is the length of PLAINTEXT_2.  For the case of plaintext_length
         exceeding the EDHOC_KDF output size, see Appendix G.

      -  CIPHERTEXT_2 = PLAINTEXT_2 XOR KEYSTREAM_2

   *  Encode message_2 as a sequence of CBOR-encoded data items as
      specified in Section 5.3.1.

5.3.3.  Initiator Processing of Message 2

   The Initiator SHALL process message_2 in the following order:

   1.  Decode message_2 (see Appendix C.1).

   2.  Retrieve the protocol state using available message correlation
       (e.g., the CoAP Token, the 5-tuple, or the prepended C_I; see
       Section 3.4.1).

   3.  Decrypt CIPHERTEXT_2; see Section 5.3.2.

   4.  If all processing is completed successfully, then make ID_CRED_R
       and (if present) EAD_2 available to the application for
       authentication and EAD processing.  When and how to perform
       authentication is up to the application.

   5.  Obtain the authentication credential (CRED_R) and the
       authentication key of R from the application (or by other means).

   6.  Verify Signature_or_MAC_2 using the algorithm in the selected
       cipher suite.  The verification process depends on the method;
       see Section 5.3.2.  Make the result of the verification available
       to the application.

   If any processing step fails, then the Initiator MUST send an EDHOC
   error message back as defined in Section 6, and the EDHOC session
   MUST be aborted.

5.4.  EDHOC Message 3

5.4.1.  Formatting of Message 3

   message_3 SHALL be a CBOR Sequence (see Appendix C.1), as defined
   below.

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

5.4.2.  Initiator Composition of Message 3

   The Initiator SHALL compose message_3 as follows:

   *  Compute the transcript hash TH_3 = H(TH_2, PLAINTEXT_2, CRED_R),
      where H() is the EDHOC hash algorithm of the selected cipher
      suite.  The input to the hash function is a CBOR Sequence.  Note
      that TH_3 can be computed and cached already in the processing of
      message_2.

   *  Compute MAC_3 as in Section 4.1.2, with context_3 = << ID_CRED_I,
      TH_3, CRED_I, ? EAD_3 >>

      -  If the Initiator authenticates with a static Diffie-Hellman key
         (method equals 2 or 3), then mac_length_3 is the EDHOC MAC
         length of the selected cipher suite.  If the Initiator
         authenticates with a signature key (method equals 0 or 1), then
         mac_length_3 is equal to hash_length.

      -  ID_CRED_I is an identifier to facilitate the retrieval of
         CRED_I; see Section 3.5.3.

      -  CRED_I is a CBOR item containing the authentication credential
         of the Initiator; see Section 3.5.2.

      -  EAD_3 is external authorization data; see Section 3.8.

   *  If the Initiator authenticates with a static Diffie-Hellman key
      (method equals 2 or 3), then Signature_or_MAC_3 is MAC_3.  If the
      Initiator authenticates with a signature key (method equals 0 or
      1), then Signature_or_MAC_3 is the 'signature' field of a
      COSE_Sign1 object, computed as specified in Section 4.4 of
      [RFC9052] using the signature algorithm of the selected cipher
      suite, the private authentication key of the Initiator, and the
      following parameters as input (see Appendix C.3):

      -  protected = << ID_CRED_I >>

      -  external_aad = << TH_3, CRED_I, ? EAD_3 >>

      -  payload = MAC_3

   *  Compute a COSE_Encrypt0 object as defined in Sections 5.2 and 5.3
      of [RFC9052], with the EDHOC AEAD algorithm of the selected cipher
      suite, using the encryption key K_3, the initialization vector
      IV_3 (if used by the AEAD algorithm), the plaintext PLAINTEXT_3,
      and the following parameters as input (see Appendix C.3):

      -  protected = h''

      -  external_aad = TH_3

      -  K_3 and IV_3 are defined in Section 4.1.2

      -  PLAINTEXT_3 = ( ID_CRED_I / bstr / -24..23, Signature_or_MAC_3,
         ? EAD_3 )

         o  If ID_CRED_I contains a single 'kid' parameter, i.e.,
            ID_CRED_I = { 4 : kid_I }, then the compact encoding is
            applied; see Section 3.5.3.2.

      CIPHERTEXT_3 is the 'ciphertext' of COSE_Encrypt0.

   *  Compute the transcript hash TH_4 = H(TH_3, PLAINTEXT_3, CRED_I),
      where H() is the EDHOC hash algorithm of the selected cipher
      suite.  The input to the hash function is a CBOR Sequence.

   *  Calculate PRK_out as defined in Figure 6.  The Initiator can now
      derive application keys using the EDHOC_Exporter interface; see
      Section 4.2.1.

   *  Encode message_3 as a CBOR data item as specified in
      Section 5.4.1.

   *  Make the connection identifiers (C_I and C_R) and the application
      algorithms in the selected cipher suite available to the
      application.

   After creating message_3, the Initiator can compute PRK_out (see
   Section 4.1.3) and derive application keys using the EDHOC_Exporter
   interface (see Section 4.2.1).  The Initiator SHOULD NOT persistently
   store PRK_out or application keys until the Initiator has verified
   message_4 or a message protected with a derived application key, such
   as an OSCORE message, from the Responder and the application has
   authenticated the Responder.  This is similar to waiting for an
   acknowledgment (ACK) in a transport protocol.  The Initiator SHOULD
   NOT send protected application data until the application has
   authenticated the Responder.

5.4.3.  Responder Processing of Message 3

   The Responder SHALL process message_3 in the following order:

   1.  Decode message_3 (see Appendix C.1).

   2.  Retrieve the protocol state using available message correlation
       (e.g., the CoAP Token, the 5-tuple, or the prepended C_R; see
       Section 3.4.1).

   3.  Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
       and 5.3 of [RFC9052], with the EDHOC AEAD algorithm in the
       selected cipher suite and the parameters defined in
       Section 5.4.2.

   4.  If all processing completed successfully, then make ID_CRED_I and
       (if present) EAD_3 available to the application for
       authentication and EAD processing.  When and how to perform
       authentication is up to the application.

   5.  Obtain the authentication credential (CRED_I) and the
       authentication key of I from the application (or by other means).

   6.  Verify Signature_or_MAC_3 using the algorithm in the selected
       cipher suite.  The verification process depends on the method;
       see Section 5.4.2.  Make the result of the verification available
       to the application.

   7.  Make the connection identifiers (C_I and C_R) and the application
       algorithms in the selected cipher suite available to the
       application.

   After processing message_3, the Responder can compute PRK_out (see
   Section 4.1.3) and derive application keys using the EDHOC_Exporter
   interface (see Section 4.2.1).  The Responder SHOULD NOT persistently
   store PRK_out or application keys until the application has
   authenticated the Initiator.  The Responder SHOULD NOT send protected
   application data until the application has authenticated the
   Initiator.

   If any processing step fails, then the Responder MUST send an EDHOC
   error message back as defined in Section 6, and the EDHOC session
   MUST be aborted.

5.5.  EDHOC Message 4

   This section specifies message_4, which is OPTIONAL to support.  Key
   confirmation is normally provided by sending an application message
   from the Responder to the Initiator protected with a key derived with
   the EDHOC_Exporter, e.g., using OSCORE (see Appendix A).  In
   deployments where no protected application message is sent from the
   Responder to the Initiator, message_4 MUST be supported and MUST be
   used.  Two examples of such deployments are:

   1.  when EDHOC is only used for authentication and no application
       data is sent and

   2.  when application data is only sent from the Initiator to the
       Responder.

   Further considerations about when to use message_4 are provided in
   Sections 3.9 and 9.1.

5.5.1.  Formatting of Message 4

   message_4 SHALL be a CBOR Sequence (see Appendix C.1), as defined
   below.

   message_4 = (
     CIPHERTEXT_4 : bstr,

   )

5.5.2.  Responder Composition of Message 4

   The Responder SHALL compose message_4 as follows:

   *  Compute a COSE_Encrypt0 as defined in Sections 5.2 and 5.3 of
      [RFC9052], with the EDHOC AEAD algorithm of the selected cipher
      suite, using the encryption key K_4, the initialization vector
      IV_4 (if used by the AEAD algorithm), the plaintext PLAINTEXT_4,
      and the following parameters as input (see Appendix C.3):

      -  protected = h''

      -  external_aad = TH_4

      -  K_4 and IV_4 are defined in Section 4.1.2

      -  PLAINTEXT_4 = ( ? EAD_4 )

         o  EAD_4 is external authorization data; see Section 3.8.

      CIPHERTEXT_4 is the 'ciphertext' of COSE_Encrypt0.

   *  Encode message_4 as a CBOR data item as specified in
      Section 5.5.1.

5.5.3.  Initiator Processing of Message 4

   The Initiator SHALL process message_4 as follows:

   *  Decode message_4 (see Appendix C.1).

   *  Retrieve the protocol state using available message correlation
      (e.g., the CoAP Token, the 5-tuple, or the prepended C_I; see
      Section 3.4.1).

   *  Decrypt and verify the COSE_Encrypt0 as defined in Sections 5.2
      and 5.3 of [RFC9052], with the EDHOC AEAD algorithm in the
      selected cipher suite and the parameters defined in Section 5.5.2.

   *  Make (if present) EAD_4 available to the application for EAD
      processing.

   If any processing step fails, then the Initiator MUST send an EDHOC
   error message back as defined in Section 6, and the EDHOC session
   MUST be aborted.

   After verifying message_4, the Initiator is assured that the
   Responder has calculated the key PRK_out (key confirmation) and that
   no other party can derive the key.

6.  Error Handling

   This section defines the format for error messages and the processing
   associated with the currently defined error codes.  Additional error
   codes may be registered; see Section 10.4.

   Many kinds of errors can occur during EDHOC processing.  As in CoAP,
   an error can be triggered by errors in the received message or
   internal errors in the receiving endpoint.  Except for processing and
   formatting errors, it is up to the application when to send an error
   message.  Sending error messages is essential for debugging but MAY
   be skipped if, for example, an EDHOC session cannot be found or due
   to denial-of-service reasons; see Section 9.7.  Error messages in
   EDHOC are always fatal.  After sending an error message, the sender
   MUST abort the EDHOC session.  The receiver SHOULD treat an error
   message as an indication that the other party likely has aborted the
   EDHOC session.  But since error messages might be forged, the
   receiver MAY try to continue the EDHOC session.

   An EDHOC error message can be sent by either endpoint as a reply to
   any non-error EDHOC message.  How errors at the EDHOC layer are
   transported depends on lower layers, which need to enable error
   messages to be sent and processed as intended.

   error SHALL be a CBOR Sequence (see Appendix C.1), as defined below.

   error = (
     ERR_CODE : int,
     ERR_INFO : any,
   )

                       Figure 7: EDHOC Error Message

   where:

   *  ERR_CODE is an error code encoded as an integer.  The value 0 is
      reserved for success and can only be used internally; all other
      values (negative or positive) indicate errors.

   *  ERR_INFO is error information.  Content and encoding depend on the
      error code.

   The remainder of this section specifies the currently defined error
   codes; see Table 3.  Additional error codes and corresponding error
   information may be specified.

       +==========+===============+===============================+
       | ERR_CODE | ERR_INFO Type | Description                   |
       +==========+===============+===============================+
       |        0 |               | Reserved for success          |
       +----------+---------------+-------------------------------+
       |        1 | tstr          | Unspecified error             |
       +----------+---------------+-------------------------------+
       |        2 | suites        | Wrong selected cipher suite   |
       +----------+---------------+-------------------------------+
       |        3 | true          | Unknown credential referenced |
       +----------+---------------+-------------------------------+
       |       23 |               | Reserved                      |
       +----------+---------------+-------------------------------+

             Table 3: EDHOC Error Codes and Error Information

6.1.  Success

   Error code 0 MAY be used internally in an application to indicate
   success, i.e., as a standard value in case of no error, e.g., in
   status reporting or log files.  Error code 0 MUST NOT be used as part
   of the EDHOC message exchange.  If an endpoint receives an error
   message with error code 0, then it MUST abort the EDHOC session and
   MUST NOT send an error message.

6.2.  Unspecified Error

   Error code 1 is used for errors that do not have a specific error
   code defined.  ERR_INFO MUST be a text string containing a human-
   readable diagnostic message that SHOULD be written in English, for
   example, "Method not supported".  The diagnostic text message is
   mainly intended for software engineers who during debugging need to
   interpret it in the context of the EDHOC specification.  The
   diagnostic message SHOULD be provided to the calling application
   where it SHOULD be logged.

6.3.  Wrong Selected Cipher Suite

   Error code 2 MUST only be used when replying to message_1 in case the
   cipher suite selected by the Initiator is not supported by the
   Responder or if the Responder supports a cipher suite more preferred
   by the Initiator than the selected cipher suite; see Section 5.2.3.
   In this case, ERR_INFO = SUITES_R and is of type suites; see
   Section 5.2.1.  If the Responder does not support the selected cipher
   suite, then SUITES_R MUST include one or more supported cipher
   suites.  If the Responder supports a cipher suite in SUITES_I other
   than the selected cipher suite (independently of if the selected
   cipher suite is supported or not), then SUITES_R MUST include the
   supported cipher suite in SUITES_I, which is most preferred by the
   Initiator.  SUITES_R MAY include a single cipher suite; in which
   case, it is encoded as an int.  If the Responder does not support any
   cipher suite in SUITES_I, then it SHOULD include all its supported
   cipher suites in SUITES_R.

   In contrast to SUITES_I, the order of the cipher suites in SUITES_R
   has no significance.

6.3.1.  Cipher Suite Negotiation

   After receiving SUITES_R, the Initiator can determine which cipher
   suite to select (if any) for the next EDHOC run with the Responder.
   The Initiator SHOULD remember which selected cipher suite to use
   until the next message_1 has been sent; otherwise, the Initiator and
   Responder will run into an infinite loop where the Initiator selects
   its most preferred cipher suite and the Responder sends an error with
   supported cipher suites.

   After a completed EDHOC session, the Initiator MAY remember the
   selected cipher suite to use in future EDHOC sessions with this
   Responder.  Note that if the Initiator or Responder is updated with
   new cipher suite policies, any cached information may be outdated.

   Note that the Initiator's list of supported cipher suites and order
   of preference is fixed (see Sections 5.2.1 and 5.2.2).  Furthermore,
   the Responder SHALL only accept message_1 if the selected cipher
   suite is the first cipher suite in SUITES_I that the Responder also
   supports (see Section 5.2.3).  Following this procedure ensures that
   the selected cipher suite is the most preferred (by the Initiator)
   cipher suite supported by both parties.  For examples, see
   Section 6.3.2.

   If the selected cipher suite is not the first cipher suite that the
   Responder supports in SUITES_I received in message_1, then the
   Responder MUST abort the EDHOC session; see Section 5.2.3.  If
   SUITES_I in message_1 is manipulated, then the integrity verification
   of message_2 containing the transcript hash TH_2 will fail and the
   Initiator will abort the EDHOC session.

6.3.2.  Examples

   Assume that the Initiator supports the five cipher suites, 5, 6, 7,
   8, and 9, in decreasing order of preference.  Figures 8 and 9 show
   two examples of how the Initiator can format SUITES_I and how
   SUITES_R is used by Responders to give the Initiator information
   about the cipher suites that the Responder supports.

   In Example 1 (Figure 8), the Responder supports cipher suite 6 but
   not the initially selected cipher suite 5.  The Responder rejects the
   first message_1 with an error indicating support for suite 6 in
   SUITES_R.  The Initiator also supports suite 6 and therefore selects
   suite 6 in the second message_1.  The Initiator prepends in SUITES_I
   the selected suite 6 with the more preferred suites, in this case
   suite 5, to mitigate a potential attack on the cipher suite
   negotiation.

   Initiator                                                   Responder
   |              METHOD, SUITES_I = 5, G_X, C_I, EAD_1                |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                   ERR_CODE = 2, SUITES_R = 6                      |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |             METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1            |
   +------------------------------------------------------------------>|
   |                             message_1                             |

                Figure 8: Cipher Suite Negotiation Example 1

   In Example 2 (Figure 9), the Responder supports cipher suites 8 and 9
   but not the more preferred (by the Initiator) cipher suites 5, 6 or
   7.  To illustrate the negotiation mechanics, we let the Initiator
   first make a guess that the Responder supports suite 6 but not suite
   5.  Since the Responder supports neither 5 nor 6, it rejects the
   first message_1 with an error indicating support for suites 8 and 9
   in SUITES_R (in any order).  The Initiator also supports suites 8 and
   9, and prefers suite 8, so it selects suite 8 in the second
   message_1.  The Initiator prepends in SUITES_I the selected suite 8
   with the more preferred suites in order of preference, in this case,
   suites 5, 6 and 7, to mitigate a potential attack on the cipher suite
   negotiation.

   Note 1.  If the Responder had supported suite 5, then the first
            message_1 would not have been accepted either, since the
            Responder observes that suite 5 is more preferred by the
            Initiator than the selected suite 6.  In that case, the
            Responder would have included suite 5 in SUITES_R of the
            response, and it would then have become the selected and
            only suite in the second message_1.

   Note 2.  For each message_1, the Initiator MUST generate a new
            ephemeral ECDH key pair matching the selected cipher suite.

   Initiator                                                   Responder
   |            METHOD, SUITES_I = [5, 6], G_X, C_I, EAD_1             |
   +------------------------------------------------------------------>|
   |                             message_1                             |
   |                                                                   |
   |                  ERR_CODE = 2, SUITES_R = [9, 8]                  |
   |<------------------------------------------------------------------+
   |                               error                               |
   |                                                                   |
   |           METHOD, SUITES_I = [5, 6, 7, 8], G_X, C_I, EAD_1        |
   +------------------------------------------------------------------>|
   |                             message_1                             |

                Figure 9: Cipher Suite Negotiation Example 2

6.4.  Unknown Credential Referenced

   Error code 3 is used for errors due to a received credential
   identifier (ID_CRED_R in message_2 or ID_CRED_I message_3) containing
   a reference to a credential that the receiving endpoint does not have
   access to.  The intent with this error code is that the endpoint who
   sent the credential identifier should, for the next EDHOC session,
   try another credential identifier supported according to the
   application profile.

   For example, an application profile could list x5t and x5chain as
   supported credential identifiers and state that x5t should be used if
   it can be assumed that the X.509 certificate is available at the
   receiving side.  This error code thus enables the certificate chain
   to be sent only when needed, bearing in mind that error messages are
   not protected so an adversary can try to cause unnecessary, large
   credential identifiers.

   For the error code 3, the error information SHALL be the CBOR simple
   value true (0xf5).  Error code 3 MUST NOT be used when the received
   credential identifier type is not supported.

7.  EDHOC Message Deduplication

   By default, EDHOC assumes that message duplication is handled by the
   transport (which is exemplified by CoAP in this section); see
   Appendix A.2.

   Deduplication of CoAP messages is described in Section 4.5 of
   [RFC7252].  This handles the case when the same Confirmable (CON)
   message is received multiple times due to missing acknowledgment on
   the CoAP messaging layer.  The recommended processing in [RFC7252] is
   that the duplicate message is acknowledged, but the received message
   is only processed once by the CoAP stack.

   Message deduplication is resource demanding and therefore not
   supported in all CoAP implementations.  Since EDHOC is targeting
   constrained environments, it is desirable that EDHOC can optionally
   support transport layers that do not handle message duplication.
   Special care is needed to avoid issues with duplicate messages; see
   Section 5.1.

   The guiding principle here is similar to the deduplication processing
   on the CoAP messaging layer, i.e., a received duplicate EDHOC message
   SHALL NOT result in another instance of the next EDHOC message.  The
   result MAY be that a duplicate next EDHOC message is sent, provided
   it is still relevant with respect to the current protocol state.  In
   any case, the received message MUST NOT be processed more than once
   in the same EDHOC session.  This is called "EDHOC message
   deduplication".

   An EDHOC implementation MAY store the previously sent EDHOC message
   to be able to resend it.

   In principle, if the EDHOC implementation would deterministically
   regenerate the identical EDHOC message previously sent, it would be
   possible to instead store the protocol state to be able to recreate
   and resend the previously sent EDHOC message.  However, even if the
   protocol state is fixed, the message generation may introduce
   differences that compromise security.  For example, in the generation
   of message_3, if I is performing a (non-deterministic) ECDSA
   signature (say, method 0 or 1 and cipher suite 2 or 3), then
   PLAINTEXT_3 is randomized, but K_3 and IV_3 are the same, leading to
   a key and nonce reuse.

   The EDHOC implementation MUST NOT store the previous protocol state
   and regenerate an EDHOC message if there is a risk that the same key
   and IV are used for two (or more) distinct messages.

   The previous message or protocol state MUST NOT be kept longer than
   what is required for retransmission, for example, in the case of CoAP
   transport, no longer than the EXCHANGE_LIFETIME (see Section 4.8.2 of
   [RFC7252]).

8.  Compliance Requirements

   In the absence of an application profile specifying otherwise:

   *  An implementation MAY support only an Initiator or only a
      Responder.

   *  An implementation MAY support only a single method.  None of the
      methods are mandatory to implement.

   *  Implementations MUST support 'kid' parameters.  None of the other
      COSE header parameters are mandatory to implement.

   *  An implementation MAY support only a single credential type (CCS,
      CWT, X.509, or C509).  None of the credential types are mandatory
      to implement.

   *  Implementations MUST support the EDHOC_Exporter.

   *  Implementations MAY support message_4.  Error codes (ERR_CODE) 1
      and 2 MUST be supported.

   *  Implementations MUST support EAD.

   *  Implementations MUST support cipher suites 2 and 3.  Cipher suites
      2 (AES-CCM-16-64-128, SHA-256, 8, P-256, ES256, AES-CCM-16-64-128,
      SHA-256) and 3 (AES-CCM-16-128-128, SHA-256, 16, P-256, ES256,
      AES-CCM-16-64-128, SHA-256) only differ in the size of the MAC
      length, so supporting one or both of these is not significantly
      different.  Implementations only need to implement the algorithms
      needed for their supported methods.

9.  Security Considerations

9.1.  Security Properties

   EDHOC has similar security properties as can be expected from the
   theoretical SIGMA-I protocol [SIGMA] and the Noise XX pattern
   [Noise], which are similar to methods 0 and 3, respectively.  Proven
   security properties are detailed in the security analysis
   publications referenced at the end of this section.

   Using the terminology from [SIGMA], EDHOC provides forward secrecy,
   mutual authentication with aliveness, consistency, and peer
   awareness.  As described in [SIGMA], message_3 provides peer
   awareness to the Responder, while message_4 provides peer awareness
   to the Initiator.  By including the authentication credentials in the
   transcript hash, EDHOC protects against an identity misbinding attack
   like the Duplicate Signature Key Selection (DSKS) that the MAC-then-
   Sign variant of SIGMA-I is otherwise vulnerable to.

   As described in [SIGMA], different levels of identity protection are
   provided to the Initiator and Responder.  EDHOC provides identity
   protection of the Initiator against active attacks and identity
   protection of the Responder against passive attacks.  An active
   attacker can get the credential identifier of the Responder by
   eavesdropping on the destination address used for transporting
   message_1 and then sending its own message_1 to the same address.
   The roles should be assigned to protect the most sensitive identity/
   identifier, typically that which is not possible to infer from
   routing information in the lower layers.

   EDHOC messages might change in transit due to a noisy channel or
   through modification by an attacker.  Changes in message_1 and
   message_2 (except Signature_or_MAC_2 when the signature scheme is not
   strongly unforgeable) are detected when verifying Signature_or_MAC_2.
   Changes to not strongly unforgeable Signature_or_MAC_2 and message_3
   are detected when verifying CIPHERTEXT_3.  Changes to message_4 are
   detected when verifying CIPHERTEXT_4.

   Compared to [SIGMA], EDHOC adds an explicit method type and expands
   the message authentication coverage to additional elements such as
   algorithms, external authorization data, and previous plaintext
   messages.  This protects against an attacker replaying messages or
   injecting messages from another EDHOC session.

   EDHOC also adds the selection of connection identifiers and
   downgrade-protected negotiation of cryptographic parameters, i.e., an
   attacker cannot affect the negotiated parameters.  A single session
   of EDHOC does not include negotiation of cipher suites, but it
   enables the Responder to verify that the selected cipher suite is the
   most preferred cipher suite by the Initiator that is supported by
   both the Initiator and Responder and to abort the EDHOC session if
   not.

   As required by [RFC7258], IETF protocols need to mitigate pervasive
   monitoring when possible.  Therefore, EDHOC only supports methods
   with ephemeral Diffie-Hellman and provides a key update function (see
   Appendix H) for lightweight application protocol rekeying.  Either of
   these provides forward secrecy, in the sense that compromise of the
   private authentication keys does not compromise past session keys
   (PRK_out) and compromise of a session key does not compromise past
   session keys.  Frequently re-running EDHOC with ephemeral Diffie-
   Hellman forces attackers to perform dynamic key exfiltration where
   the attacker must have continuous interactions with the collaborator,
   which is a significant sustained attack.

   To limit the effect of breaches, it is important to limit the use of
   symmetric group keys for bootstrapping.  Therefore, EDHOC strives to
   make the additional cost of using raw public keys and self-signed
   certificates as small as possible.  Raw public keys and self-signed
   certificates are not a replacement for a public key infrastructure
   but SHOULD be used instead of symmetric group keys for bootstrapping.

   Compromise of the long-term keys (private signature or static DH
   keys) does not compromise the security of completed EDHOC sessions.
   Compromising the private authentication keys of one party lets an
   active attacker impersonate that compromised party in EDHOC sessions
   with other parties but does not let the attacker impersonate other
   parties in EDHOC sessions with the compromised party.  Compromise of
   the long-term keys does not enable a passive attacker to compromise
   future session keys (PRK_out).  Compromise of the HKDF input
   parameters (ECDH shared secret) leads to compromise of all session
   keys derived from that compromised shared secret.  Compromise of one
   session key does not compromise other session keys.  Compromise of
   PRK_out leads to compromise of all keying material derived with the
   EDHOC_Exporter.

   Based on the cryptographic algorithm requirements (Section 9.3),
   EDHOC provides a minimum of 64-bit security against online brute
   force attacks and a minimum of 128-bit security against offline brute
   force attacks.  To break 64-bit security against online brute force,
   an attacker would on average have to send 4.3 billion messages per
   second for 68 years, which is infeasible in constrained IoT radio
   technologies.  A forgery against a 64-bit MAC in EDHOC breaks the
   security of all future application data, while a forgery against a
   64-bit MAC in the subsequent application protocol (e.g., OSCORE
   [RFC8613]) typically only breaks the security of the data in the
   forged packet.

   As the EDHOC session is aborted when verification fails, the security
   against online attacks is given by the sum of the strength of the
   verified signatures and MACs (including MAC in AEAD).  As an example,
   if EDHOC is used with method 3, cipher suite 2, and message_4, the
   Responder is authenticated with 128-bit security against online
   attacks (the sum of the 64-bit MACs in message_2 and message_4).  The
   same principle applies for MACs in an application protocol keyed by
   EDHOC as long as EDHOC is re-run when verification of the first MACs
   in the application protocol fails.  As an example, if EDHOC with
   method 3 and cipher suite 2 is used as in Figure 2 of
   [EDHOC-CoAP-OSCORE], 128-bit mutual authentication against online
   attacks can be achieved after completion of the first OSCORE request
   and response.

   After sending message_3, the Initiator is assured that no other party
   than the Responder can compute the key PRK_out.  While the Initiator
   can securely send protected application data, the Initiator SHOULD
   NOT persistently store the keying material PRK_out until the
   Initiator has verified message_4 or a message protected with a
   derived application key, such as an OSCORE message, from the
   Responder.  After verifying message_3, the Responder is assured that
   an honest Initiator has computed the key PRK_out.  The Responder can
   securely derive and store the keying material PRK_out and send
   protected application data.

   External authorization data sent in message_1 (EAD_1) or message_2
   (EAD_2) should be considered unprotected by EDHOC; see Section 9.5.
   EAD_2 is encrypted, but the Responder has not yet authenticated the
   Initiator and the encryption does not provide confidentiality against
   active attacks.

   External authorization data sent in message_3 (EAD_3) or message_4
   (EAD_4) is protected between the Initiator and Responder by the
   protocol, but note that EAD fields may be used by the application
   before the message verification is completed; see Section 3.8.
   Designing a secure mechanism that uses EAD is not necessarily
   straightforward.  This document only provides the EAD transport
   mechanism, but the problem of agreeing on the surrounding context and
   the meaning of the information passed to and from the application
   remains.  Any new uses of EAD should be subject to careful review.

   Key Compromise Impersonation (KCI):  In EDHOC authenticated with
      signature keys, EDHOC provides KCI protection against an attacker
      having access to the long-term key or the ephemeral secret key.
      With static Diffie-Hellman key authentication, KCI protection
      would be provided against an attacker having access to the long-
      term Diffie-Hellman key but not to an attacker having access to
      the ephemeral secret key.  Note that the term KCI has typically
      been used for compromise of long-term keys and that an attacker
      with access to the ephemeral secret key can only attack that
      specific EDHOC session.

   Repudiation:  If an endpoint authenticates with a signature, the
      other endpoint can prove that the endpoint performed a run of the
      protocol by presenting the data being signed as well as the
      signature itself.  With static Diffie-Hellman key authentication,
      the authenticating endpoint can deny having participated in the
      protocol.

   Earlier versions of EDHOC have been formally analyzed [Bruni18]
   [Norrman20] [CottierPointcheval22] [Jacomme23] [GuentherIlunga22],
   and the specification has been updated based on the analysis.

9.2.  Cryptographic Considerations

   The SIGMA protocol requires that the encryption of message_3 provides
   confidentiality against active attackers and EDHOC message_4 relies
   on the use of authenticated encryption.  Hence, the message
   authenticating functionality of the authenticated encryption in EDHOC
   is critical, i.e., authenticated encryption MUST NOT be replaced by
   plain encryption only, even if authentication is provided at another
   level or through a different mechanism.

   To reduce message overhead, EDHOC does not use explicit nonces and
   instead relies on the ephemeral public keys to provide randomness to
   each EDHOC session.  A good amount of randomness is important for the
   key generation to provide liveness and to protect against
   interleaving attacks.  For this reason, the ephemeral keys MUST NOT
   be used in more than one EDHOC message, and both parties SHALL
   generate fresh, random ephemeral key pairs.  Note that an ephemeral
   key may be used to calculate several ECDH shared secrets.  When
   static Diffie-Hellman authentication is used, the same ephemeral key
   is used in both ephemeral-ephemeral and ephemeral-static ECDH.

   As discussed in [SIGMA], the encryption of message_2 only needs to
   protect against a passive attacker since active attackers can always
   get the Responder's identity by sending their own message_1.  EDHOC
   uses the EDHOC_Expand function (typically HKDF-Expand) as a binary
   additive stream cipher that is proven secure as long as the expand
   function is a Pseudorandom Function (PRF).  HKDF-Expand is not often
   used as a stream cipher as it is slow on long messages, and most
   applications require both confidentiality with indistinguishability
   under adaptive chosen ciphertext attack (IND-CCA2) as well as
   integrity protection.  For the encryption of message_2, any speed
   difference is negligible, IND-CCA2 does not increase security, and
   integrity is provided by the inner MAC (and signature depending on
   method).

   Requirements for how to securely generate, validate, and process the
   public keys depend on the elliptic curve.  For X25519 and X448, the
   requirements are defined in [RFC7748].  For X25519 and X448, the
   check for all-zero output as specified in Section 6 of [RFC7748] MUST
   be done.  For secp256r1, secp384r1, and secp521r1, the requirements
   are defined in Section 5 of [SP-800-56A].  For secp256r1, secp384r1,
   and secp521r1, at least partial public key validation MUST be done.

   The same authentication credential MAY be used for both the Initiator
   and Responder roles.  As noted in Section 12 of [RFC9052], the use of
   a single key for multiple algorithms is strongly discouraged unless
   proven secure by a dedicated cryptographic analysis.  In particular,
   this recommendation applies to using the same private key for static
   Diffie-Hellman authentication and digital signature authentication.
   A preliminary conjecture is that a minor change to EDHOC may be
   sufficient to fit the analysis of a secure shared signature and ECDH
   key usage in [Degabriele11] and [Thormarker21].  Note that
   Section 5.6.3.2 of [SP-800-56A] allows a key agreement key pair to be
   used with a signature algorithm in certificate requests.

   The property that a completed EDHOC session implies that another
   identity has been active is upheld as long as the Initiator does not
   have its own identity in the set of Responder identities it is
   allowed to communicate with.  In trust-on-first-use (TOFU) use cases
   (see Appendix D.5), the Initiator should verify that the Responder's
   identity is not equal to its own.  Any future EDHOC methods using,
   e.g., PSKs might need to mitigate this in other ways.  However, an
   active attacker can gain information about the set of identities an
   Initiator is willing to communicate with.  If the Initiator is
   willing to communicate with all identities except its own, an
   attacker can determine that a guessed Initiator identity is correct.
   To not leak any long-term identifiers, using a freshly generated
   authentication key as an identity in each initial TOFU session is
   RECOMMENDED.

   NIST SP 800-56A [SP-800-56A] forbids deriving secret and non-secret
   randomness from the same Key Derivation Function (KDF) instance, but
   this decision has been criticized by Krawczyk in [HKDFpaper] and
   doing so is common practice.  In addition to IVs, other examples are
   the challenge in Extensible Authentication Protocol Tunneled
   Transport Layer Security (EAP-TTLS), the RAND in 3GPP Authentication
   and Key Agreement (AKA), and the Session-Id in EAP-TLS 1.3.  Note
   that part of KEYSTREAM_2 is also non-secret randomness, as it is
   known or predictable to an attacker.  The more recent NIST SP 800-108
   [SP-800-108] aligns with [HKDFpaper] and states that, for a secure
   KDF, the revelation of one portion of the derived keying material
   must not degrade the security of any other portion of that keying
   material.

9.3.  Cipher Suites and Cryptographic Algorithms

   When using a private cipher suite or registering new cipher suites,
   the choice of the key length used in the different algorithms needs
   to be harmonized so that a sufficient security level is maintained
   for authentication credentials, the EDHOC session, and the protection
   of application data.  The Initiator and Responder should enforce a
   minimum security level.

   The output size of the EDHOC hash algorithm MUST be at least 256
   bits.  In particular, the hash algorithms SHA-1 and SHA-256/64
   (SHA-256 truncated to 64 bits) SHALL NOT be supported for use in
   EDHOC except for certificate identification with x5t and c5t.  For
   security considerations of SHA-1, see [RFC6194].  As EDHOC integrity
   protects all the authentication credentials, the choice of hash
   algorithm in x5t and c5t does not affect security and using the same
   hash algorithm as in the cipher suite, but with as much truncation as
   possible, is RECOMMENDED.  That is, when the EDHOC hash algorithm is
   SHA-256, using SHA-256/64 in x5t and c5t is RECOMMENDED.  The EDHOC
   MAC length MUST be at least 8 bytes and the tag length of the EDHOC
   AEAD algorithm MUST be at least 64 bits.  Note that secp256k1 is only
   defined for use with ECDSA and not for ECDH.  Note that some COSE
   algorithms are marked as not recommended in the COSE IANA registry.

9.4.  Post-Quantum Considerations

   As of the publication of this specification, it is unclear when or
   even if a quantum computer of sufficient size and power to exploit
   public key cryptography will exist.  Deployments that need to
   consider risks decades into the future should transition to Post-
   Quantum Cryptography (PQC) in the not-too-distant future.  Many other
   systems should take a slower wait-and-see approach where PQC is
   phased in when the quantum threat is more imminent.  Current PQC
   algorithms have limitations compared to Elliptic Curve Cryptography
   (ECC), and the data sizes would be problematic in many constrained
   IoT systems.

   Symmetric algorithms used in EDHOC, such as SHA-256 and AES-CCM-
   16-64-128, are practically secure against even large quantum
   computers.  Two of NIST's security levels for quantum-resistant
   public key cryptography are based on AES-128 and SHA-256.  A quantum
   computer will likely be expensive and slow due to heavy error
   correction.  Grover's algorithm, which is proven to be optimal,
   cannot effectively be parallelized.  It will provide little or no
   advantage in attacking AES, and AES-128 will remain secure for
   decades to come [NISTPQC].

   EDHOC supports all signature algorithms defined by COSE, including
   PQC signature algorithms such as HSS-LMS.  EDHOC is currently only
   specified for use with key exchange algorithms of type ECDH curves,
   but any Key Encapsulation Method (KEM), including PQC KEMs, can be
   used in method 0.  While the key exchange in method 0 is specified
   with the terms of the Diffie-Hellman protocol, the key exchange
   adheres to a KEM interface: G_X is then the public key of the
   Initiator, G_Y is the encapsulation, and G_XY is the shared secret.
   Use of PQC KEMs to replace static DH authentication would likely
   require a specification updating EDHOC with new methods.

9.5.  Unprotected Data and Privacy

   The Initiator and Responder must make sure that unprotected data and
   metadata do not reveal any sensitive information.  This also applies
   for encrypted data sent to an unauthenticated party.  In particular,
   it applies to EAD_1, ID_CRED_R, EAD_2, and error messages.  Using the
   same EAD_1 in several EDHOC sessions allows passive eavesdroppers to
   correlate the different sessions.  Note that even if ead_value is
   encrypted outside of EDHOC, the ead_labels in EAD_1 are revealed to
   passive attackers and the ead_labels in EAD_2 are revealed to active
   attackers.  Another consideration is that the list of supported
   cipher suites may potentially be used to identify the application.
   The Initiator and Responder must also make sure that unauthenticated
   data does not trigger any harmful actions.  In particular, this
   applies to EAD_1 and error messages.

   An attacker observing network traffic may use connection identifiers
   sent in clear in EDHOC or the subsequent application protocol to
   correlate packets sent on different paths or at different times.  The
   attacker may use this information for traffic flow analysis or to
   track an endpoint.  Application protocols using connection
   identifiers from EDHOC SHOULD provide mechanisms to update the
   connection identifiers and MAY provide mechanisms to issue several
   simultaneously active connection identifiers.  See [RFC9000] for a
   non-constrained example of such mechanisms.  Connection identifiers
   can, e.g., be chosen randomly among the set of unused 1-byte
   connection identifiers.  Connection identity privacy mechanisms are
   only useful when there are not fixed identifiers, such as IP address
   or MAC address in the lower layers.

9.6.  Updated Internet Threat Model Considerations

   Since the publication of [RFC3552], there has been an increased
   awareness of the need to protect against endpoints that are
   compromised or malicious or whose interests simply do not align with
   the interests of users [THREAT-MODEL-GUIDANCE].  [RFC7624] describes
   an updated threat model for Internet confidentiality; see
   Section 9.1.  [THREAT-MODEL-GUIDANCE] further expands the threat
   model.  Implementations and users should take these threat models
   into account and consider actions to reduce the risk of tracking by
   other endpoints.  In particular, even data sent protected to the
   other endpoint, such as ID_CRED fields and EAD fields, can be used
   for tracking; see Section 2.7 of [THREAT-MODEL-GUIDANCE].

   The fields ID_CRED_I, ID_CRED_R, EAD_2, EAD_3, and EAD_4 have
   variable length, and information regarding the length may leak to an
   attacker.  A passive attacker may, e.g., be able to differentiate
   endpoints using identifiers of different length.  To mitigate this
   information leakage, an implementation may ensure that the fields
   have a fixed length or use padding.  An implementation may, e.g.,
   only use fixed length identifiers like 'kid' of length 1.
   Alternatively, padding may be used (see Section 3.8.1) to hide the
   true length of, e.g., certificates by value in 'x5chain' or 'c5c'.

9.7.  Denial of Service

   EDHOC itself does not provide countermeasures against denial-of-
   service attacks.  In particular, by sending a number of new or
   replayed message_1, an attacker may cause the Responder to allocate
   the state, perform cryptographic operations, and amplify messages.
   To mitigate such attacks, an implementation SHOULD make use of
   available lower layer mechanisms.  For instance, when EDHOC is
   transferred as an exchange of CoAP messages, the CoAP server can use
   the Echo option defined in [RFC9175], which forces the CoAP client to
   demonstrate reachability at its apparent network address.  To avoid
   an additional round trip, the Initiator can reduce the amplification
   factor by padding message_1, i.e., using EAD_1; see Section 3.8.1.
   Note that while the Echo option mitigates some resource exhaustion
   aspects of spoofing, it does not protect against a distributed
   denial-of-service attack made by real, potentially compromised,
   clients.  Similarly, limiting amplification only reduces the impact,
   which still may be significant because of a large number of clients
   engaged in the attack.

   An attacker can also send a faked message_2, message_3, message_4, or
   error in an attempt to trick the receiving party to send an error
   message and abort the EDHOC session.  EDHOC implementations MAY
   evaluate if a received message is likely to have been forged by an
   attacker and ignore it without sending an error message or aborting
   the EDHOC session.

9.8.  Implementation Considerations

   The availability of a secure random number generator is essential for
   the security of EDHOC.  If no true random number generator is
   available, a random seed MUST be provided from an external source and
   used with a cryptographically secure pseudorandom number generator.
   As each pseudorandom number must only be used once, an implementation
   needs to get a unique input to the pseudorandom number generator
   after reboot or continuously store state in nonvolatile memory.
   Appendix B.1.1 of [RFC8613] describes issues and solution approaches
   for writing to nonvolatile memory.  Intentionally or unintentionally
   weak or predictable pseudorandom number generators can be abused or
   exploited for malicious purposes.  [RFC8937] describes a way for
   security protocol implementations to augment their (pseudo)random
   number generators using a long-term private key and a deterministic
   signature function.  This improves randomness from broken or
   otherwise subverted random number generators.  The same idea can be
   used with other secrets and functions, such as a Diffie-Hellman
   function or a symmetric secret, and a PRF like HMAC or KMAC.  It is
   RECOMMENDED to not trust a single source of randomness and to not put
   unaugmented random numbers on the wire.

   For many constrained IoT devices, it is problematic to support
   several crypto primitives.  Existing devices can be expected to
   support either ECDSA or Edwards-curve Digital Signature Algorithm
   (EdDSA).  If ECDSA is supported, "deterministic ECDSA", as specified
   in [RFC6979], MAY be used.  Pure deterministic elliptic-curve
   signatures, such as deterministic ECDSA and EdDSA, have gained
   popularity over randomized ECDSA as their security does not depend on
   a source of high-quality randomness.  Recent research has however
   found that implementations of these signature algorithms may be
   vulnerable to certain side-channel and fault injection attacks due to
   their determinism.  For example, see Section 1 of [HEDGED-ECC-SIGS]
   for a list of attack papers.  As suggested in Section 2.1.1 of
   [RFC9053], this can be addressed by combining randomness and
   determinism.

   Appendix D of [CURVE-REPR] describes how Montgomery curves, such as
   X25519 and X448, and (twisted) Edwards curves, such as Ed25519 and
   Ed448, can be mapped to and from short-Weierstrass form for
   implementations on platforms that accelerate elliptic curve group
   operations in short-Weierstrass form.

   All private keys, symmetric keys, and IVs MUST be secret.  Only the
   Responder SHALL have access to the Responder's private authentication
   key, and only the Initiator SHALL have access to the Initiator's
   private authentication key.  Implementations should provide
   countermeasures to side-channel attacks, such as timing attacks.
   Intermediate computed values, such as ephemeral ECDH keys and ECDH
   shared secrets, MUST be deleted after key derivation is completed.

   The Initiator and Responder are responsible for verifying the
   integrity and validity of certificates.  Verification of validity may
   require the use of a Real-Time Clock (RTC).  The selection of trusted
   certification authorities (CAs) should be done very carefully and
   certificate revocation should be supported.  The choice of revocation
   mechanism is left to the application.  For example, in case of X.509
   certificates, Certificate Revocation Lists [RFC5280] or the Online
   Certificate Status Protocol (OCSP) [RFC6960] may be used.

   Similar considerations as for certificates are needed for CWT/CCS.
   The endpoints are responsible for verifying the integrity and
   validity of CWT/CCS and to handle revocation.  The application needs
   to determine what trust anchors are relevant and have a well-defined
   trust-establishment process.  A self-signed certificate / CWT or CCS
   appearing in the protocol cannot be a trigger to modify the set of
   trust anchors.  One common way for a new trust anchor to be added to
   (or removed from) a device is by means firmware upgrade.  See
   [RFC9360] for a longer discussion on trust and validation in
   constrained devices.

   Just like for certificates, the contents of the COSE header
   parameters 'kcwt' and 'kccs' defined in Section 10.6 must be
   processed as untrusted inputs.  Endpoints that intend to rely on the
   assertions made by a CWT/CCS obtained from any of these methods need
   to validate the contents.  For 'kccs', which enables transport of raw
   public keys, the data structure used does not include any protection
   or verification data. 'kccs' may be used for unauthenticated
   operations, e.g., trust on first use, with the limitations and
   caveats entailed; see Appendix D.5.

   The Initiator and Responder are allowed to select connection
   identifiers C_I and C_R, respectively, for the other party to use in
   the ongoing EDHOC session as well as in a subsequent application
   protocol (e.g., OSCORE [RFC8613]).  The choice of the connection
   identifier is not security critical in EDHOC but intended to simplify
   the retrieval of the right security context in combination with using
   short identifiers.  If the wrong connection identifier of the other
   party is used in a protocol message, it will result in the receiving
   party not being able to retrieve a security context (which will abort
   the EDHOC session) or retrieve the wrong security context (which also
   aborts the EDHOC session as the message cannot be verified).

   If two nodes unintentionally initiate two simultaneous EDHOC sessions
   with each other, even if they only want to complete a single EDHOC
   session, they MAY abort the EDHOC session with the lexicographically
   smallest G_X.  Note that in cases where several EDHOC sessions with
   different parameter sets (method, COSE headers, etc.) are used, an
   attacker can affect which parameter set will be used by blocking some
   of the parameter sets.

   If supported by the device, it is RECOMMENDED that at least the long-
   term private keys are stored in a Trusted Execution Environment (TEE)
   (for example, see [RFC9397]) and that sensitive operations using
   these keys are performed inside the TEE.  To achieve even higher
   security, it is RECOMMENDED that additional operations such as
   ephemeral key generation, all computations of shared secrets, and
   storage of the PRK keys can be done inside the TEE.  The use of a TEE
   aims at preventing code within that environment to be tampered with
   and preventing data used by such code to be read or tampered with by
   code outside that environment.

   Note that HKDF-Expand has a relatively small maximum output length of
   255 ⋅ hash_length, where hash_length is the output size in bytes of
   the EDHOC hash algorithm of the selected cipher suite.  This means
   that when SHA-256 is used as a hash algorithm, PLAINTEXT_2 cannot be
   longer than 8160 bytes.  This is probably not a limitation for most
   intended applications, but to be able to support, for example, long
   certificate chains or large external authorization data, there is a
   backwards compatible method specified in Appendix G.

   The sequence of transcript hashes in EDHOC (TH_2, TH_3, and TH_4)
   does not make use of a so-called running hash.  This is a design
   choice, as running hashes are often not supported on constrained
   platforms.

   When parsing a received EDHOC message, implementations MUST abort the
   EDHOC session if the message does not comply with the CDDL for that
   message.  Implementations are not required to support non-
   deterministic encodings and MAY abort the EDHOC session if the
   received EDHOC message is not encoded using deterministic CBOR.
   Implementations MUST abort the EDHOC session if validation of a
   received public key fails or if any cryptographic field has the wrong
   length.  It is RECOMMENDED to abort the EDHOC session if the received
   EDHOC message is not encoded using deterministic CBOR.

10.  IANA Considerations

   This section gives IANA considerations and, unless otherwise noted,
   conforms with [RFC8126].

10.1.  EDHOC Exporter Label Registry

   IANA has created a new registry under the new registry group
   "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name:  EDHOC Exporter Labels

   Reference:  RFC 9528

        +=============+==============================+===========+
        | Label       | Description                  | Reference |
        +=============+==============================+===========+
        | 0           | Derived OSCORE Master Secret | RFC 9528  |
        +-------------+------------------------------+-----------+
        | 1           | Derived OSCORE Master Salt   | RFC 9528  |
        +-------------+------------------------------+-----------+
        | 2-22        | Unassigned                   |           |
        +-------------+------------------------------+-----------+
        | 23          | Reserved                     | RFC 9528  |
        +-------------+------------------------------+-----------+
        | 24-32767    | Unassigned                   |           |
        +-------------+------------------------------+-----------+
        | 32768-65535 | Reserved for Private Use     |           |
        +-------------+------------------------------+-----------+

                      Table 4: EDHOC Exporter Labels

   This registry also has a "Change Controller" field.  For
   registrations made by IETF documents, the IETF is listed.

                 +=============+=========================+
                 | Range       | Registration Procedures |
                 +=============+=========================+
                 | 0-23        | Standards Action        |
                 +-------------+-------------------------+
                 | 24-32767    | Expert Review           |
                 +-------------+-------------------------+
                 | 32768-65535 | Private Use             |
                 +-------------+-------------------------+

                    Table 5: Registration Procedures for
                           EDHOC Exporter Labels

10.2.  EDHOC Cipher Suites Registry

   IANA has created a new registry under the new registry group
   "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name:  EDHOC Cipher Suites

   Reference:  RFC 9528

   The columns of the registry are Value, Array, Description, and
   Reference, where Value is an integer and the other columns are text
   strings.  The initial contents of the registry are:

   +=======+================+=============================+===========+
   | Value | Array          | Description                 | Reference |
   +=======+================+=============================+===========+
   | -24   | N/A            | Private Use                 | RFC 9528  |
   +-------+----------------+-----------------------------+-----------+
   | -23   | N/A            | Private Use                 | RFC 9528  |
   +-------+----------------+-----------------------------+-----------+
   | -22   | N/A            | Private Use                 | RFC 9528  |
   +-------+----------------+-----------------------------+-----------+
   | -21   | N/A            | Private Use                 | RFC 9528  |
   +-------+----------------+-----------------------------+-----------+
   | 0     | 10, -16, 8, 4, | AES-CCM-16-64-128, SHA-256, | RFC 9528  |
   |       | -8, 10, -16    | 8, X25519, EdDSA,           |           |
   |       |                | AES-CCM-16-64-128, SHA-256  |           |
   +-------+----------------+-----------------------------+-----------+
   | 1     | 30, -16, 16,   | AES-CCM-16-128-128,         | RFC 9528  |
   |       | 4, -8, 10, -16 | SHA-256, 16, X25519, EdDSA, |           |
   |       |                | AES-CCM-16-64-128, SHA-256  |           |
   +-------+----------------+-----------------------------+-----------+
   | 2     | 10, -16, 8, 1, | AES-CCM-16-64-128, SHA-256, | RFC 9528  |
   |       | -7, 10, -16    | 8, P-256, ES256,            |           |
   |       |                | AES-CCM-16-64-128, SHA-256  |           |
   +-------+----------------+-----------------------------+-----------+
   | 3     | 30, -16, 16,   | AES-CCM-16-128-128,         | RFC 9528  |
   |       | 1, -7, 10, -16 | SHA-256, 16, P-256, ES256,  |           |
   |       |                | AES-CCM-16-64-128, SHA-256  |           |
   +-------+----------------+-----------------------------+-----------+
   | 4     | 24, -16, 16,   | ChaCha20/Poly1305, SHA-256, | RFC 9528  |
   |       | 4, -8, 24, -16 | 16, X25519, EdDSA,          |           |
   |       |                | ChaCha20/Poly1305, SHA-256  |           |
   +-------+----------------+-----------------------------+-----------+
   | 5     | 24, -16, 16,   | ChaCha20/Poly1305, SHA-256, | RFC 9528  |
   |       | 1, -7, 24, -16 | 16, P-256, ES256,           |           |
   |       |                | ChaCha20/Poly1305, SHA-256  |           |
   +-------+----------------+-----------------------------+-----------+
   | 6     | 1, -16, 16, 4, | A128GCM, SHA-256, 16,       | RFC 9528  |
   |       | -7, 1, -16     | X25519, ES256, A128GCM,     |           |
   |       |                | SHA-256                     |           |
   +-------+----------------+-----------------------------+-----------+
   | 23    |                | Reserved                    | RFC 9528  |
   +-------+----------------+-----------------------------+-----------+
   | 24    | 3, -43, 16, 2, | A256GCM, SHA-384, 16,       | RFC 9528  |
   |       | -35, 3, -43    | P-384, ES384, A256GCM,      |           |
   |       |                | SHA-384                     |           |
   +-------+----------------+-----------------------------+-----------+
   | 25    | 24, -45, 16,   | ChaCha20/Poly1305,          | RFC 9528  |
   |       | 5, -8, 24, -45 | SHAKE256, 16, X448, EdDSA,  |           |
   |       |                | ChaCha20/Poly1305, SHAKE256 |           |
   +-------+----------------+-----------------------------+-----------+

                       Table 6: EDHOC Cipher Suites

          +===============+=====================================+
          | Range         | Registration Procedures             |
          +===============+=====================================+
          | -65536 to -25 | Specification Required              |
          +---------------+-------------------------------------+
          | -24 to -21    | Private Use                         |
          +---------------+-------------------------------------+
          | -20 to 23     | Standards Action with Expert Review |
          +---------------+-------------------------------------+
          | 24 to 65535   | Specification Required              |
          +---------------+-------------------------------------+

             Table 7: Registration Procedures for EDHOC Cipher
                                   Suites

10.3.  EDHOC Method Type Registry

   IANA has created a new registry under the new registry group
   "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name:  EDHOC Method Types

   Reference:  RFC 9528

   The columns of the registry are Value, Initiator Authentication Key,
   Responder Authentication Key, and Reference, where Value is an
   integer and the key columns are text strings describing the
   authentication keys.

   The initial contents of the registry are shown in Table 2.  Method 23
   is Reserved.

          +===============+=====================================+
          | Range         | Registration Procedures             |
          +===============+=====================================+
          | -65536 to -25 | Specification Required              |
          +---------------+-------------------------------------+
          | -24 to 23     | Standards Action with Expert Review |
          +---------------+-------------------------------------+
          | 24 to 65535   | Specification Required              |
          +---------------+-------------------------------------+

          Table 8: Registration Procedures for EDHOC Method Types

10.4.  EDHOC Error Codes Registry

   IANA has created a new registry under the new registry group
   "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name:  EDHOC Error Codes

   Reference:  RFC 9528

   The columns of the registry are ERR_CODE, ERR_INFO Type, Description,
   Change Controller, and Reference, where ERR_CODE is an integer,
   ERR_INFO is a CDDL defined type, and Description is a text string.
   The initial contents of the registry are shown in Table 3.  Error
   code 23 is Reserved.  This registry also has a "Change Controller"
   field.  For registrations made by IETF documents, the IETF is listed.

                +===============+=========================+
                | Range         | Registration Procedures |
                +===============+=========================+
                | -65536 to -25 | Expert Review           |
                +---------------+-------------------------+
                | -24 to 23     | Standards Action        |
                +---------------+-------------------------+
                | 24 to 65535   | Expert Review           |
                +---------------+-------------------------+

                    Table 9: Registration Procedures for
                             EDHOC Error Codes

10.5.  EDHOC External Authorization Data Registry

   IANA has created a new registry under the new registry group
   "Ephemeral Diffie-Hellman Over COSE (EDHOC)" as follows:

   Registry Name:  EDHOC External Authorization Data

   Reference:  RFC 9528

   The columns of the registry are Name, Label, Description, and
   Reference, where Label is a nonnegative integer and the other columns
   are text strings.  The initial contents of the registry are shown in
   Table 10.  EAD label 23 is Reserved.

         +=========+=======+====================+===============+
         | Name    | Label | Description        | Reference     |
         +=========+=======+====================+===============+
         | Padding | 0     | Randomly generated | RFC 9528,     |
         |         |       | CBOR byte string   | Section 3.8.1 |
         +---------+-------+--------------------+---------------+
         |         | 23    | Reserved           | RFC 9528      |
         +---------+-------+--------------------+---------------+

                        Table 10: EDHOC EAD Labels

           +=============+=====================================+
           | Range       | Registration Procedures             |
           +=============+=====================================+
           | 0 to 23     | Standards Action with Expert Review |
           +-------------+-------------------------------------+
           | 24 to 65535 | Specification Required              |
           +-------------+-------------------------------------+

              Table 11: Registration Procedures for EDHOC EAD
                                   Labels

10.6.  COSE Header Parameters Registry

   IANA has registered the following entries in the "COSE Header
   Parameters" registry under the registry group "CBOR Object Signing
   and Encryption (COSE)" (see Table 12).  The value of the 'kcwt'
   header parameter is a COSE Web Token (CWT) [RFC8392], and the value
   of the 'kccs' header parameter is a CWT Claims Set (CCS); see
   Section 1.4.  The CWT/CCS must contain a COSE_Key in a 'cnf' claim
   [RFC8747].  The Value Registry column for this item is empty and
   omitted from the table below.

     +======+=======+===============+===============================+
     | Name | Label | Value Type    | Description                   |
     +======+=======+===============+===============================+
     | kcwt | 13    | COSE_Messages | A CBOR Web Token (CWT)        |
     |      |       |               | containing a COSE_Key in a    |
     |      |       |               | 'cnf' claim and possibly      |
     |      |       |               | other claims.  CWT is defined |
     |      |       |               | in RFC 8392.  COSE_Messages   |
     |      |       |               | is defined in RFC 9052.       |
     +------+-------+---------------+-------------------------------+
     | kccs | 14    | map           | A CWT Claims Set (CCS)        |
     |      |       |               | containing a COSE_Key in a    |
     |      |       |               | 'cnf' claim and possibly      |
     |      |       |               | other claims.  CCS is defined |
     |      |       |               | in RFC 8392.                  |
     +------+-------+---------------+-------------------------------+

                  Table 12: COSE Header Parameter Labels

10.7.  Well-Known URI Registry

   IANA has added the well-known URI "edhoc" to the "Well-Known URIs"
   registry.

   URI Suffix:  edhoc

   Change Controller:  IETF

   Reference:  RFC 9528

   Related Information:  None

10.8.  Media Types Registry

   IANA has added the media types "application/edhoc+cbor-seq" and
   "application/cid-edhoc+cbor-seq" to the "Media Types" registry.

10.8.1.  application/edhoc+cbor-seq Media Type Registration

   Type name:  application

   Subtype name:  edhoc+cbor-seq

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See Section 7 of RFC 9528.

   Interoperability considerations:  N/A

   Published specification:  RFC 9528

   Applications that use this media type:  To be identified

   Fragment identifier considerations:  N/A

   Additional information:

      Magic number(s):  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  See
      "Authors' Addresses" section in RFC 9528.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See "Authors' Addresses" section.

   Change Controller:  IETF

10.8.2.  application/cid-edhoc+cbor-seq Media Type Registration

   Type name:  application

   Subtype name:  cid-edhoc+cbor-seq

   Required parameters:  N/A

   Optional parameters:  N/A

   Encoding considerations:  binary

   Security considerations:  See Section 7 of RFC 9528.

   Interoperability considerations:  N/A

   Published specification:  RFC 9528

   Applications that use this media type:  To be identified

   Fragment identifier considerations:  N/A

   Additional information:

      Magic number(s):  N/A

      File extension(s):  N/A

      Macintosh file type code(s):  N/A

   Person & email address to contact for further information:  See
      "Authors' Addresses" section in RFC 9528.

   Intended usage:  COMMON

   Restrictions on usage:  N/A

   Author:  See "Authors' Addresses" section.

   Change Controller:  IETF

10.9.  CoAP Content-Formats Registry

   IANA has added the media types "application/edhoc+cbor-seq" and
   "application/cid-edhoc+cbor-seq" to the "CoAP Content-Formats"
   registry under the registry group "Constrained RESTful Environments
   (CoRE) Parameters".

   +================================+================+====+===========+
   | Content Type                   | Content Coding | ID | Reference |
   +================================+================+====+===========+
   | application/edhoc+cbor-seq     | -              | 64 | RFC 9528  |
   +--------------------------------+----------------+----+-----------+
   | application/cid-edhoc+cbor-seq | -              | 65 | RFC 9528  |
   +--------------------------------+----------------+----+-----------+

                    Table 13: CoAP Content-Format IDs

10.10.  Resource Type (rt=) Link Target Attribute Values Registry

   IANA has added the resource type "core.edhoc" to the "Resource Type
   (rt=) Link Target Attribute Values" registry under the registry group
   "Constrained RESTful Environments (CoRE) Parameters".

   Value:  core.edhoc

   Description:  EDHOC resource

   Reference:  RFC 9528

10.11.  Expert Review Instructions

   The IANA registries established in this document are defined as
   "Expert Review", "Specification Required", or "Standards Action with
   Expert Review".  This section gives some general guidelines for what
   the experts should be looking for, but they are being designated as
   experts for a reason so they should be given substantial latitude.

   Expert reviewers should take into consideration the following points:

   *  The clarity and correctness of registrations.  Experts are
      expected to check the clarity of purpose and use of the requested
      entries.  Expert needs to make sure the values of algorithms are
      taken from the right registry when that is required.  Experts
      should consider requesting an opinion on the correctness of
      registered parameters from relevant IETF working groups.
      Encodings that do not meet these objectives of clarity and
      completeness should not be registered.

   *  The expected usage of fields when approving code point assignment.
      The length of the encoded value should be weighed against how many
      code points of that length are left, the size of device it will be
      used on, and the number of code points left that encode to that
      size.

   *  It is recommended to have a specification even if the registration
      procedure is "Expert Review".  When specifications are not
      provided for a request where Expert Review is the assignment
      policy, the description provided needs to have sufficient
      information to verify the code points as above.

11.  References

11.1.  Normative References

   [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>.

   [RFC3279]  Bassham, L., Polk, W., and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, DOI 10.17487/RFC3279, April
              2002, <https://www.rfc-editor.org/info/rfc3279>.

   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/info/rfc5116>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/info/rfc5869>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <https://www.rfc-editor.org/info/rfc6090>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/info/rfc6960>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/info/rfc6979>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [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>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8410]  Josefsson, S. and J. Schaad, "Algorithm Identifiers for
              Ed25519, Ed448, X25519, and X448 for Use in the Internet
              X.509 Public Key Infrastructure", RFC 8410,
              DOI 10.17487/RFC8410, August 2018,
              <https://www.rfc-editor.org/info/rfc8410>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/info/rfc8610>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [RFC8724]  Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
              Zuniga, "SCHC: Generic Framework for Static Context Header
              Compression and Fragmentation", RFC 8724,
              DOI 10.17487/RFC8724, April 2020,
              <https://www.rfc-editor.org/info/rfc8724>.

   [RFC8742]  Bormann, C., "Concise Binary Object Representation (CBOR)
              Sequences", RFC 8742, DOI 10.17487/RFC8742, February 2020,
              <https://www.rfc-editor.org/info/rfc8742>.

   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/info/rfc8747>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/info/rfc9052>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/info/rfc9053>.

   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/info/rfc9175>.

   [RFC9360]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header Parameters for Carrying and Referencing X.509
              Certificates", RFC 9360, DOI 10.17487/RFC9360, February
              2023, <https://www.rfc-editor.org/info/rfc9360>.

11.2.  Informative References

   [Bruni18]  Bruni, A., Sahl Jørgensen, T., Grønbech Petersen, T., and
              C. Schürmann, "Formal Verification of Ephemeral Diffie-
              Hellman Over COSE (EDHOC)", November 2018,
              <https://www.springerprofessional.de/en/formal-
              verification-of-ephemeral-diffie-hellman-over-cose-
              edhoc/16284348>.

   [C509-CERTS]
              Preuß Mattsson, J., Selander, G., Raza, S., Höglund, J.,
              and M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-09, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              cbor-encoded-cert-09>.

   [CborMe]   Bormann, C., "CBOR Playground", <https://cbor.me/>.

   [CNSA]     Wikipedia, "Commercial National Security Algorithm Suite",
              October 2023, <https://en.wikipedia.org/w/index.php?title=
              Commercial_National_Security_Algorithm_Suite&oldid=1181333
              611>.

   [CoAP-SEC-PROT]
              Mattsson, J. P., Palombini, F., and M. Vučinić,
              "Comparison of CoAP Security Protocols", Work in Progress,
              Internet-Draft, draft-ietf-iotops-security-protocol-
              comparison-04, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-iotops-
              security-protocol-comparison-04>.

   [CottierPointcheval22]
              Cottier, B. and D. Pointcheval, "Security Analysis of the
              EDHOC protocol", September 2022,
              <https://arxiv.org/abs/2209.03599>.

   [CURVE-REPR]
              Struik, R., "Alternative Elliptic Curve Representations",
              Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
              representations-23, 21 January 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lwig-
              curve-representations-23>.

   [Degabriele11]
              Degabriele, J., Lehmann, A., Paterson, K., Smart, N., and
              M. Strefler, "On the Joint Security of Encryption and
              Signature in EMV", December 2011,
              <https://eprint.iacr.org/2011/615>.

   [EAT]      Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
              Wallace, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-25, 15
              January 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-rats-eat-25>.

   [EDHOC-CoAP-OSCORE]
              Palombini, F., Tiloca, M., Höglund, R., Hristozov, S., and
              G. Selander, "Using Ephemeral Diffie-Hellman Over COSE
              (EDHOC) with the Constrained Application Protocol (CoAP)
              and Object Security for Constrained RESTful Environments
              (OSCORE)", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-edhoc-10, 29 November 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              oscore-edhoc-10>.

   [GuentherIlunga22]
              Günther, F. and M. Mukendi, "Careful with MAc-then-SIGn: A
              Computational Analysis of the EDHOC Lightweight
              Authenticated Key Exchange Protocol", December 2022,
              <https://eprint.iacr.org/2022/1705>.

   [HEDGED-ECC-SIGS]
              Preuß Mattsson, J., Thormarker, E., and S. Ruohomaa,
              "Hedged ECDSA and EdDSA Signatures", Work in Progress,
              Internet-Draft, draft-irtf-cfrg-det-sigs-with-noise-02, 1
              March 2024, <https://datatracker.ietf.org/doc/html/draft-
              irtf-cfrg-det-sigs-with-noise-02>.

   [HKDFpaper]
              Krawczyk, H., "Cryptographic Extraction and Key
              Derivation: The HKDF Scheme", May 2010,
              <https://eprint.iacr.org/2010/264.pdf>.

   [IEEE.802.15.4-2015]
              IEEE, "IEEE Standard for Low-Rate Wireless Networks",
              DOI 10.1109/IEEESTD.2016.7460875, April 2016,
              <https://ieeexplore.ieee.org/document/7460875>.

   [Jacomme23]
              Jacomme, C., Klein, E., Kremer, S., and M. Racouchot, "A
              comprehensive, formal and automated analysis of the EDHOC
              protocol", October 2022,
              <https://hal.inria.fr/hal-03810102/>.

   [KUDOS]    Höglund, R. and M. Tiloca, "Key Update for OSCORE
              (KUDOS)", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-key-update-07, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              oscore-key-update-07>.

   [LAKE-AUTHZ]
              Selander, G., Mattsson, J. P., Vučinić, M., Fedrecheski,
              G., and M. Richardson, "Lightweight Authorization using
              Ephemeral Diffie-Hellman Over COSE", Work in Progress,
              Internet-Draft, draft-ietf-lake-authz-01, 4 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lake-
              authz-01>.

   [LAKE-REQS]
              Vučinić, M., Selander, G., Preuß Mattsson, J., and D.
              Garcia-Carillo, "Requirements for a Lightweight AKE for
              OSCORE", Work in Progress, Internet-Draft, draft-ietf-
              lake-reqs-04, 8 June 2020,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lake-
              reqs-04>.

   [NISTPQC]  National Institute Standards and Technology (NIST), "Post-
              Quantum Cryptography FAQs",
              <https://csrc.nist.gov/Projects/post-quantum-cryptography/
              faqs>.

   [Noise]    Perrin, T., "The Noise Protocol Framework", Revision 34,
              July 2018, <https://noiseprotocol.org/noise.html>.

   [Norrman20]
              Norrman, K., Sundararajan, V., and A. Bruni, "Formal
              Analysis of EDHOC Key Establishment for Constrained IoT
              Devices", September 2020,
              <https://arxiv.org/abs/2007.11427>.

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              DOI 10.17487/RFC2986, November 2000,
              <https://www.rfc-editor.org/info/rfc2986>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [RFC6194]  Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security
              Considerations for the SHA-0 and SHA-1 Message-Digest
              Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011,
              <https://www.rfc-editor.org/info/rfc6194>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <https://www.rfc-editor.org/info/rfc7258>.

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC7624]  Barnes, R., Schneier, B., Jennings, C., Hardie, T.,
              Trammell, B., Huitema, C., and D. Borkmann,
              "Confidentiality in the Face of Pervasive Surveillance: A
              Threat Model and Problem Statement", RFC 7624,
              DOI 10.17487/RFC7624, August 2015,
              <https://www.rfc-editor.org/info/rfc7624>.

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,
              <https://www.rfc-editor.org/info/rfc8366>.

   [RFC8376]  Farrell, S., Ed., "Low-Power Wide Area Network (LPWAN)
              Overview", RFC 8376, DOI 10.17487/RFC8376, May 2018,
              <https://www.rfc-editor.org/info/rfc8376>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

   [RFC8937]  Cremers, C., Garratt, L., Smyshlyaev, S., Sullivan, N.,
              and C. Wood, "Randomness Improvements for Security
              Protocols", RFC 8937, DOI 10.17487/RFC8937, October 2020,
              <https://www.rfc-editor.org/info/rfc8937>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/info/rfc9000>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/info/rfc9147>.

   [RFC9176]  Amsüss, C., Ed., Shelby, Z., Koster, M., Bormann, C., and
              P. van der Stok, "Constrained RESTful Environments (CoRE)
              Resource Directory", RFC 9176, DOI 10.17487/RFC9176, April
              2022, <https://www.rfc-editor.org/info/rfc9176>.

   [RFC9397]  Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", RFC 9397, DOI 10.17487/RFC9397, July 2023,
              <https://www.rfc-editor.org/info/rfc9397>.

   [RFC9529]  Selander, G., Preuß Mattsson, J., Serafin, M., Tiloca, M.,
              and M. Vučinić, "Traces of Ephemeral Diffie-Hellman Over
              COSE (EDHOC)", RFC 9529, DOI 10.17487/RFC9529, March 2024,
              <https://www.rfc-editor.org/info/rfc9529>.

   [SECG]     Certicom Research, "SEC 1: Elliptic Curve Cryptography",
              Standards for Efficient Cryptography, May 2009,
              <https://www.secg.org/sec1-v2.pdf>.

   [SIGMA]    Krawczyk, H., "SIGMA: the 'SIGn-and-MAc' Approach to
              Authenticated Diffie-Hellman and Its Use in the IKE-
              Protocols", June 2003,
              <https://www.iacr.org/cryptodb/archive/2003/
              CRYPTO/1495/1495.pdf>.

   [SP-800-108]
              Chen, L., "Recommendation for Key Derivation Using
              Pseudorandom Functions", NIST Special Publication 800-108
              Revision 1, DOI 10.6028/NIST.SP.800-108r1-upd1, August
              2022, <https://doi.org/10.6028/NIST.SP.800-108r1-upd1>.

   [SP-800-56A]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key-Establishment
              Schemes Using Discrete Logarithm Cryptography",
              NIST Special Publication 800-56A Revision 3,
              DOI 10.6028/NIST.SP.800-56Ar3, April 2018,
              <https://doi.org/10.6028/NIST.SP.800-56Ar3>.

   [SP800-185]
              Kelsey, J., Chang, S., and R. Perlner, "SHA-3 Derived
              Functions cSHAKE, KMAC, TupleHash and ParallelHash",
              NIST Special Publication 800-185,
              DOI 10.6028/NIST.SP.800-185, December 2016,
              <https://doi.org/10.6028/NIST.SP.800-185>.

   [Thormarker21]
              Thormarker, E., "On using the same key pair for Ed25519
              and an X25519 based KEM", April 2021,
              <https://eprint.iacr.org/2021/509.pdf>.

   [THREAT-MODEL-GUIDANCE]
              Arkko, J. and S. Farrell, "Internet Threat Model
              Guidance", Work in Progress, Internet-Draft, draft-arkko-
              arch-internet-threat-model-guidance-00, 12 July 2021,
              <https://datatracker.ietf.org/doc/html/draft-arkko-arch-
              internet-threat-model-guidance-00>.

Appendix A.  Use with OSCORE and Transfer over CoAP

   This appendix describes how to derive an OSCORE security context when
   EDHOC is used to key OSCORE and how to transfer EDHOC messages over
   CoAP.  The use of CoAP or OSCORE with EDHOC is optional, but if you
   are using CoAP or OSCORE, then certain normative requirements apply
   as detailed in the subsections.

A.1.  Deriving the OSCORE Security Context

   This section specifies how to use EDHOC output to derive the OSCORE
   security context.

   After successful processing of EDHOC message_3, the Client and Server
   derive Security Context parameters for OSCORE as follows (see
   Section 3.2 of [RFC8613]):

   *  The Master Secret and Master Salt SHALL be derived by using the
      EDHOC_Exporter interface (see Section 4.2.1):

      -  The EDHOC Exporter Labels for deriving the OSCORE Master Secret
         and OSCORE Master Salt are the uints 0 and 1, respectively.

      -  The context parameter is h'' (0x40), the empty CBOR byte
         string.

      -  By default, oscore_key_length is the key length (in bytes) of
         the application AEAD Algorithm of the selected cipher suite for
         the EDHOC session.  Also by default, oscore_salt_length has
         value 8.  The Initiator and Responder MAY agree out-of-band on
         a longer oscore_key_length than the default and on shorter or
         longer than the default oscore_salt_length.

      Master Secret = EDHOC_Exporter( 0, h'', oscore_key_length )
      Master Salt   = EDHOC_Exporter( 1, h'', oscore_salt_length )

   *  The AEAD Algorithm SHALL be the application AEAD algorithm of the
      selected cipher suite for the EDHOC session.

   *  The HKDF Algorithm SHALL be the one based on the application hash
      algorithm of the selected cipher suite for the EDHOC session.  For
      example, if SHA-256 is the application hash algorithm of the
      selected cipher suite, HKDF SHA-256 is used as the HKDF Algorithm
      in the OSCORE Security Context.

   *  The relationship between identifiers in OSCORE and EDHOC is
      specified in Section 3.3.3.  The OSCORE Sender ID and Recipient ID
      SHALL be determined by EDHOC connection identifiers C_R and C_I
      for the EDHOC session as shown in Table 14.

       +=================+==================+=====================+
       |                 | OSCORE Sender ID | OSCORE Recipient ID |
       +=================+==================+=====================+
       | EDHOC Initiator |       C_R        |         C_I         |
       +-----------------+------------------+---------------------+
       | EDHOC Responder |       C_I        |         C_R         |
       +-----------------+------------------+---------------------+

           Table 14: Usage of Connection Identifiers in OSCORE

   The Client and Server SHALL use the parameters above to establish an
   OSCORE Security Context, as per Section 3.2.1 of [RFC8613].

   From then on, the Client and Server retrieve the OSCORE protocol
   state using the Recipient ID and optionally other transport
   information such as the 5-tuple.

A.2.  Transferring EDHOC over CoAP

   This section specifies how EDHOC can be transferred as an exchange of
   CoAP [RFC7252] messages.  CoAP provides a reliable transport that can
   preserve packet ordering, provides flow and congestion control, and
   handles message duplication.  CoAP can also perform fragmentation and
   mitigate certain denial-of-service attacks.  The underlying CoAP
   transport should be used in reliable mode, in particular, when
   fragmentation is used, to avoid, e.g., situations with hanging
   endpoints waiting for each other.

   EDHOC may run with the Initiator either being a CoAP client or CoAP
   server.  We denote the former by the "forward message flow" (see
   Appendix A.2.1) and the latter by the "reverse message flow" (see
   Appendix A.2.2).  By default, we assume the forward message flow, but
   the roles SHOULD be chosen to protect the most sensitive identity;
   see Section 9.

   According to this specification, EDHOC is transferred in POST
   requests to the Uri-Path: "/.well-known/edhoc" (see Section 10.7) and
   2.04 (Changed) responses.  An application may define its own path
   that can be discovered, e.g., using a resource directory [RFC9176].
   Client applications can use the resource type "core.edhoc" to
   discover a server's EDHOC resource, i.e., where to send a request for
   executing the EDHOC protocol; see Section 10.10.  An alternative
   transfer of the forward message flow is specified in
   [EDHOC-CoAP-OSCORE].

   In order for the server to correlate a message received from a client
   to a message previously sent in the same EDHOC session over CoAP,
   messages sent by the client SHALL be prepended with the CBOR
   serialization of the connection identifier that the server has
   selected; see Section 3.4.1.  This applies both to the forward and
   the reverse message flows.  To indicate a new EDHOC session in the
   forward message flow, message_1 SHALL be prepended with the CBOR
   simple value true (0xf5).  Even if CoAP is carried over a reliable
   transport protocol, such as TCP, the prepending of identifiers
   specified here SHALL be practiced to enable interoperability
   independent of how CoAP is transported.

   The prepended identifiers are encoded in CBOR and thus self-
   delimiting.  The representation of identifiers described in
   Section 3.3.2 SHALL be used.  They are sent in front of the actual
   EDHOC message to keep track of messages in an EDHOC session, and only
   the part of the body following the identifier is used for EDHOC
   processing.  In particular, the connection identifiers within the
   EDHOC messages are not impacted by the prepended identifiers.

   An EDHOC message has media type "application/edhoc+cbor-seq", whereas
   an EDHOC message prepended by a connection identifier has media type
   "application/cid-edhoc+cbor-seq"; see Section 10.9.

   To mitigate certain denial-of-service attacks, the CoAP server MAY
   respond to the first POST request with a 4.01 (Unauthorized)
   containing an Echo option [RFC9175].  This forces the Initiator to
   demonstrate reachability at its apparent network address.  If message
   fragmentation is needed, the EDHOC messages may be fragmented using
   the CoAP Block-Wise Transfer mechanism [RFC7959].

   EDHOC error messages need to be transported in response to a message
   that failed (see Section 6).  EDHOC error messages transported with
   CoAP are carried in the payload.

   Note that the transport over CoAP can serve as a blueprint for other
   client-server protocols:

   *  The client prepends the connection identifier selected by the
      server (or, for message_1, the CBOR simple value true) to any
      request message it sends.

   *  The server does not send any such indicator, as responses are
      matched to request by the client-server protocol design.

A.2.1.  The Forward Message Flow

   In the forward message flow, the CoAP client is the Initiator and the
   CoAP server is the Responder.  This flow protects the client identity
   against active attackers and the server identity against passive
   attackers.

   In the forward message flow, the CoAP Token enables correlation on
   the Initiator (client) side, and the prepended C_R enables
   correlation on the Responder (server) side.

   *  EDHOC message_1 is sent in the payload of a POST request from the
      client to the server's resource for EDHOC, prepended with the
      identifier true (0xf5), indicating a new EDHOC session.

   *  EDHOC message_2 or the EDHOC error message is sent from the server
      to the client in the payload of the response, in the former case
      with response code 2.04 (Changed) and in the latter with response
      code as specified in Appendix A.2.3.

   *  EDHOC message_3 or the EDHOC error message is sent from the client
      to the server's resource in the payload of a POST request,
      prepended with connection identifier C_R.

   *  If EDHOC message_4 is used, or in case of an error message, it is
      sent from the server to the client in the payload of the response,
      with response codes analogously to message_2.  In case of an error
      message sent in response to message_4, it is sent analogously to
      the error message sent in response to message_2.

   An example of a completed EDHOC session over CoAP in the forward
   message flow is shown in Figure 10.

       Client    Server
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          | Content-Format: application/cid-edhoc+cbor-seq
         |          | Payload: true, EDHOC message_1
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_2
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          | Content-Format: application/cid-edhoc+cbor-seq
         |          | Payload: C_R, EDHOC message_3
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_4
         |          |

               Figure 10: Example of the Forward Message Flow

   The forward message flow of EDHOC can be combined with an OSCORE
   exchange in a total of two round trips; see [EDHOC-CoAP-OSCORE].

A.2.2.  The Reverse Message Flow

   In the reverse message flow, the CoAP client is the Responder and the
   CoAP server is the Initiator.  This flow protects the server identity
   against active attackers and the client identity against passive
   attackers.

   In the reverse message flow, the CoAP Token enables correlation on
   the Responder (client) side, and the prepended C_I enables
   correlation on the Initiator (server) side.

   *  To trigger a new EDHOC session, the client makes an empty POST
      request to the server's resource for EDHOC.

   *  EDHOC message_1 is sent from the server to the client in the
      payload of the response with response code 2.04 (Changed).

   *  EDHOC message_2 or the EDHOC error message is sent from the client
      to the server's resource in the payload of a POST request,
      prepended with connection identifier C_I.

   *  EDHOC message_3 or the EDHOC error message is sent from the server
      to the client in the payload of the response, in the former case
      with response code 2.04 (Changed) and in the latter with response
      code as specified in Appendix A.2.3.

   *  If EDHOC message_4 is used, or in case of an error message, it is
      sent from the client to the server's resource in the payload of a
      POST request, prepended with connection identifier C_I.  In case
      of an error message sent in response to message_4, it is sent
      analogously to an error message sent in response to message_2.

   An example of a completed EDHOC session over CoAP in the reverse
   message flow is shown in Figure 11.

       Client    Server
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_1
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/edhoc"
         |          | Content-Format: application/cid-edhoc+cbor-seq
         |          | Payload: C_I, EDHOC message_2
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/edhoc+cbor-seq
         |          | Payload: EDHOC message_3
         |          |

               Figure 11: Example of the Reverse Message Flow

A.2.3.  Errors in EDHOC over CoAP

   When using EDHOC over CoAP, EDHOC error messages sent as CoAP
   responses MUST be sent in the payload of error responses, i.e., they
   MUST specify a CoAP error response code.  In particular, it is
   RECOMMENDED that such error responses have response code either 4.00
   (Bad Request) in case of client error (e.g., due to a malformed EDHOC
   message) or 5.00 (Internal Server Error) in case of server error
   (e.g., due to failure in deriving EDHOC keying material).  The
   Content-Format of the error response MUST be set to "application/
   edhoc+cbor-seq"; see Section 10.9.

Appendix B.  Compact Representation

   This section defines a format for compact representation based on the
   Elliptic-Curve-Point-to-Octet-String Conversion defined in
   Section 2.3.3 of [SECG].

   As described in Section 4.2 of [RFC6090], the x-coordinate of an
   elliptic curve public key is a suitable representative for the entire
   point whenever scalar multiplication is used as a one-way function.
   One example is ECDH with compact output, where only the x-coordinate
   of the computed value is used as the shared secret.

   In EDHOC, compact representation is used for the ephemeral public
   keys (G_X and G_Y); see Section 3.7.  Using the notation from [SECG],
   the output is an octet string of length ceil( (log2 q) / 8 ), where
   ceil(x) is the smallest integer not less than x.  See [SECG] for a
   definition of q, M, X, xp, and ~yp.  The steps in Section 2.3.3 of
   [SECG] are replaced with the following steps:

   1.  Convert the field element xp to an octet string X of length ceil(
       (log2 q) / 8 ) octets using the conversion routine specified in
       Section 2.3.5 of [SECG].

   2.  Output M = X.

   The encoding of the point at infinity is not supported.

   Compact representation does not change any requirements on
   validation; see Section 9.2.  Using compact representation has some
   security benefits.  An implementation does not need to check that the
   point is not the point at infinity (the identity element).
   Similarly, as not even the sign of the y-coordinate is encoded,
   compact representation trivially avoids so-called "benign
   malleability" attacks where an attacker changes the sign; see [SECG].

   The following may be needed for validation or compatibility with APIs
   that do not support compact representation or do not support the full
   [SECG] format:

   *  If a compressed y-coordinate is required, then the value ~yp set
      to zero can be used.  In such a case, the compact representation
      described above can be transformed into the Standards for
      Efficient Cryptography Group (SECG) point-compressed format by
      prepending it with the single byte 0x02 (i.e., M = 0x02 || X).

   *  If an uncompressed y-coordinate is required, then a y-coordinate
      has to be calculated following Section 2.3.4 of [SECG] or
      Appendix C of [RFC6090].  Any of the square roots (see [SECG] or
      [RFC6090]) can be used.  The uncompressed SECG format is M =
      0x04 || X || Y.

   For example: The curve P-256 has the parameters (using the notation
   in [RFC6090]):

   *  p = 2^256 - 2^224 + 2^192 + 2^96 - 1

   *  a = -3

   *  b = 410583637251521421293261297800472684091144410159937255
      54835256314039467401291

   Given an example x:

   *  x = 115792089183396302095546807154740558443406795108653336
      398970697772788799766525

   We can calculate y as the square root w = (x^3 + a ⋅ x + b)^((p +
   1)/4) (mod p).

   *  y = 834387180070192806820075864918626005281451259964015754
      16632522940595860276856

   Note that this does not guarantee that (x, y) is on the correct
   elliptic curve.  A full validation according to Section 5.6.2.3.3 of
   [SP-800-56A] is done by also checking that 0 ≤ x < p and that y^2 ≡
   x^3 + a ⋅ x + b (mod p).

Appendix C.  Use of CBOR, CDDL, and COSE in EDHOC

   This appendix is intended to help implementors not familiar with CBOR
   [RFC8949], CDDL [RFC8610], COSE [RFC9052], and HKDF [RFC5869].

C.1.  CBOR and CDDL

   The Concise Binary Object Representation (CBOR) [RFC8949] is a data
   format designed for small code size and small message size.  CBOR
   builds on the JSON data model but extends it by, e.g., encoding
   binary data directly without base64 conversion.  In addition to the
   binary CBOR encoding, CBOR also has a diagnostic notation that is
   readable and editable by humans.  The Concise Data Definition
   Language (CDDL) [RFC8610] provides a way to express structures for
   protocol messages and APIs that use CBOR.  [RFC8610] also extends the
   diagnostic notation.

   CBOR data items are encoded to or decoded from byte strings using a
   type-length-value encoding scheme, where the three highest order bits
   of the initial byte contain information about the major type.  CBOR
   supports several types of data items, integers (int, uint), simple
   values, byte strings (bstr), and text strings (tstr).  CBOR also
   supports arrays [] of data items, maps {} of pairs of data items, and
   sequences [RFC8742] of data items.  Some examples are given below.

   The EDHOC specification sometimes use CDDL names in CBOR diagnostic
   notation as in, e.g., << ID_CRED_R, ? EAD_2 >>.  This means that
   EAD_2 is optional and that ID_CRED_R and EAD_2 should be substituted
   with their values before evaluation.  That is, if ID_CRED_R = { 4 :
   h'' } and EAD_2 is omitted, then << ID_CRED_R, ? EAD_2 >> = << { 4 :
   h'' } >>, which encodes to 0x43a10440.  We also make use of the
   occurrence symbol "*", like in, e.g., 2* int, meaning two or more
   CBOR integers.

   For a complete specification and more examples, see [RFC8949] and
   [RFC8610].  We recommend implementors get used to CBOR by using the
   CBOR playground [CborMe].

          +==================+==============+==================+
          | Diagnostic       | Encoded      | Type             |
          +==================+==============+==================+
          | 1                | 0x01         | unsigned integer |
          +------------------+--------------+------------------+
          | 24               | 0x1818       | unsigned integer |
          +------------------+--------------+------------------+
          | -24              | 0x37         | negative integer |
          +------------------+--------------+------------------+
          | -25              | 0x3818       | negative integer |
          +------------------+--------------+------------------+
          | true             | 0xf5         | simple value     |
          +------------------+--------------+------------------+
          | h''              | 0x40         | byte string      |
          +------------------+--------------+------------------+
          | h'12cd'          | 0x4212cd     | byte string      |
          +------------------+--------------+------------------+
          | '12cd'           | 0x4431326364 | byte string      |
          +------------------+--------------+------------------+
          | "12cd"           | 0x6431326364 | text string      |
          +------------------+--------------+------------------+
          | { 4 : h'cd' }    | 0xa10441cd   | map              |
          +------------------+--------------+------------------+
          | << 1, 2, true >> | 0x430102f5   | byte string      |
          +------------------+--------------+------------------+
          | [ 1, 2, true ]   | 0x830102f5   | array            |
          +------------------+--------------+------------------+
          | ( 1, 2, true )   | 0x0102f5     | sequence         |
          +------------------+--------------+------------------+
          | 1, 2, true       | 0x0102f5     | sequence         |
          +------------------+--------------+------------------+

                Table 15: Examples of Use of CBOR and CDDL

C.2.  CDDL Definitions

   This section compiles the CDDL definitions for ease of reference.

   suites = [ 2* int ] / int

   ead = (
     ead_label : int,
     ? ead_value : bstr,
   )

   EAD_1 = 1* ead
   EAD_2 = 1* ead
   EAD_3 = 1* ead
   EAD_4 = 1* ead

   message_1 = (
     METHOD : int,
     SUITES_I : suites,
     G_X : bstr,
     C_I : bstr / -24..23,
     ? EAD_1,
   )

   message_2 = (
     G_Y_CIPHERTEXT_2 : bstr,
   )

   PLAINTEXT_2 = (
     C_R,
     ID_CRED_R : map / bstr / -24..23,
     Signature_or_MAC_2 : bstr,
     ? EAD_2,
   )

   message_3 = (
     CIPHERTEXT_3 : bstr,
   )

   PLAINTEXT_3 = (
     ID_CRED_I : map / bstr / -24..23,
     Signature_or_MAC_3 : bstr,
     ? EAD_3,
   )

   message_4 = (
     CIPHERTEXT_4 : bstr,
   )

   PLAINTEXT_4 = (
     ? EAD_4,
   )

   error = (
     ERR_CODE : int,
     ERR_INFO : any,
   )

   info = (
     info_label : int,
     context : bstr,
     length : uint,
   )

C.3.  COSE

   CBOR Object Signing and Encryption (COSE) [RFC9052] describes how to
   create and process signatures, MACs, and encryptions using CBOR.
   COSE builds on JSON Object Signing and Encryption (JOSE) but is
   adapted to allow more efficient processing in constrained devices.
   EDHOC makes use of COSE_Key, COSE_Encrypt0, and COSE_Sign1 objects in
   the message processing:

   *  ECDH ephemeral public keys of type EC2 or OKP in message_1 and
      message_2 consist of the COSE_Key parameter named 'x'; see
      Sections 7.1 and 7.2 of [RFC9053].

   *  The ciphertexts in message_3 and message_4 consist of a subset of
      the single recipient encrypted data object COSE_Encrypt0, which is
      described in Sections 5.2 and 5.3 of [RFC9052].  The ciphertext is
      computed over the plaintext and associated data, using an
      encryption key and an initialization vector.  The associated data
      is an Enc_structure consisting of protected headers and externally
      supplied data (external_aad).  COSE constructs the input to the
      AEAD [RFC5116] for message_i (i = 3 or 4; see Sections 5.4 and
      5.5, respectively) as follows:

      -  Secret key K = K_i

      -  Nonce N = IV_i

      -  Plaintext P for message_i

      -  Associated Data A = [ "Encrypt0", h'', TH_i ]

   *  Signatures in message_2 of method 0 and 2, and in message_3 of
      method 0 and 1, consist of a subset of the single signer data
      object COSE_Sign1, which is described in Sections 4.2 and 4.4 of
      [RFC9052].  The signature is computed over a Sig_structure
      containing payload, protected headers and externally supplied data
      (external_aad) using a private signature key, and verified using
      the corresponding public signature key.  For COSE_Sign1, the
      message to be signed is:

       [ "Signature1", protected, external_aad, payload ]

      where protected, external_aad, and payload are specified in
      Sections 5.3 and 5.4.

   Different header parameters to identify X.509 or C509 certificates by
   reference are defined in [RFC9360] and [C509-CERTS]:

   *  by a hash value with the 'x5t' or 'c5t' parameters, respectively:

      -  ID_CRED_x = { 34 : COSE_CertHash }, for x = I or R and

      -  ID_CRED_x = { 22 : COSE_CertHash }, for x = I or R,

   *  or by a URI with the 'x5u' or 'c5u' parameters, respectively:

      -  ID_CRED_x = { 35 : uri }, for x = I or R, and

      -  ID_CRED_x = { 23 : uri }, for x = I or R.

   When ID_CRED_x does not contain the actual credential, it may be very
   short, e.g., if the endpoints have agreed to use a key identifier
   parameter 'kid':

   *  ID_CRED_x = { 4 : kid_x }, where kid_x : kid, for x = I or R.  For
      further optimization, see Section 3.5.3.

   Note that ID_CRED_x can contain several header parameters, for
   example, { x5u, x5t } or { kid, kid_context }.

   ID_CRED_x MAY also identify the credential by value.  For example, a
   certificate chain can be transported in an ID_CRED field with COSE
   header parameter c5c or x5chain, as defined in [C509-CERTS] and
   [RFC9360].  Credentials of type CWT and CCS can be transported with
   the COSE header parameters registered in Section 10.6.

Appendix D.  Authentication-Related Verifications

   EDHOC performs certain authentication-related operations (see
   Section 3.5), but in general, it is necessary to make additional
   verifications beyond EDHOC message processing.  Which verifications
   that are needed depend on the deployment, in particular, the trust
   model and the security policies, but most commonly, it can be
   expressed in terms of verifications of credential content.

   EDHOC assumes the existence of mechanisms (certification authority or
   other trusted third party, pre-provisioning, etc.) for generating and
   distributing authentication credentials and other credentials, as
   well as the existence of trust anchors (CA certificates, trusted
   public keys, etc.).  For example, a public key certificate or CWT may
   rely on a trusted third party whose public key is pre-provisioned,
   whereas a CCS or a self-signed certificate / CWT may be used when
   trust in the public key can be achieved by other means, or in the
   case of trust on first use, see Appendix D.5.

   In this section, we provide some examples of such verifications.
   These verifications are the responsibility of the application but may
   be implemented as part of an EDHOC library.

D.1.  Validating the Authentication Credential

   In addition to the authentication key, the authentication credential
   may contain other parameters that need to be verified.  For example:

   *  In X.509 and C509 certificates, signature keys typically have key
      usage "digitalSignature", and Diffie-Hellman public keys typically
      have key usage "keyAgreement" [RFC3279] [RFC8410].

   *  In X.509 and C509 certificates, validity is expressed using Not
      After and Not Before.  In CWT and CCS, the "exp" and "nbf" claims
      have similar meanings.

D.2.  Identities

   The application must decide on allowing a connection or not,
   depending on the intended endpoint, and in particular whether it is a
   specific identity or in a set of identities.  To prevent misbinding
   attacks, the identity of the endpoint is included in a MAC verified
   through the protocol.  More details and examples are provided in this
   section.

   Policies for what connections to allow are typically set based on the
   identity of the other endpoint, and endpoints typically only allow
   connections from a specific identity or a small restricted set of
   identities.  For example, in the case of a device connecting to a
   network, the network may only allow connections from devices that
   authenticate with certificates having a particular range of serial
   numbers and signed by a particular CA.  Conversely, a device may only
   be allowed to connect to a network that authenticates with a
   particular public key.

   *  When a Public Key Infrastructure (PKI) is used with certificates,
      the identity is the subject whose unique name, e.g., a domain
      name, a Network Access Identifier (NAI), or an Extended Unique
      Identifier (EUI), is included in the endpoint's certificate.

   *  Similarly, when a PKI is used with CWTs, the identity is the
      subject identified by the relevant claim(s), such as 'sub'
      (subject).

   *  When PKI is not used (e.g., CCS, self-signed certificate / CWT),
      the identity is typically directly associated with the
      authentication key of the other party.  For example, if identities
      can be expressed in the form of unique subject names assigned to
      public keys, then a binding to identity is achieved by including
      both the public key and associated subject name in the
      authentication credential.  CRED_I or CRED_R may be a self-signed
      certificate / CWT or CCS containing the authentication key and the
      subject name; see Section 3.5.2.  Thus, each endpoint needs to
      know the specific authentication key / unique associated subject
      name or set of public authentication keys / unique associated
      subject names, which it is allowed to communicate with.

   To prevent misbinding attacks in systems where an attacker can
   register public keys without proving knowledge of the private key,
   SIGMA [SIGMA] enforces a MAC to be calculated over the "identity".
   EDHOC follows SIGMA by calculating a MAC over the whole
   authentication credential, which in case of an X.509 or C509
   certificate, includes the "subject" and "subjectAltName" fields and,
   in the case of CWT or CCS, includes the "sub" claim.

   (While the SIGMA paper only focuses on the identity, the same
   principle is true for other information such as policies associated
   with the public key.)

D.3.  Certification Path and Trust Anchors

   When a Public Key Infrastructure (PKI) is used with certificates, the
   trust anchor is a certification authority (CA) certificate.  Each
   party needs at least one CA public key certificate or just the CA
   public key.  The certification path contains proof that the subject
   of the certificate owns the public key in the certificate.  Only
   validated public key certificates are to be accepted.

   Similarly, when a PKI is used with CWTs, each party needs to have at
   least one trusted third-party public key as a trust anchor to verify
   the end entity CWTs.  The trusted third-party public key can, e.g.,
   be stored in a self-signed CWT or in a CCS.

   The signature of the authentication credential needs to be verified
   with the public key of the issuer.  X.509 and C509 certificates
   includes the "Issuer" field.  In CWT and CCS, the "iss" claim has a
   similar meaning.  The public key is either a trust anchor or the
   public key in another valid and trusted credential in a certification
   path from the trust anchor to the authentication credential.

   Similar verifications as made with the authentication credential (see
   Appendix D.1) are also needed for the other credentials in the
   certification path.

   When PKI is not used (CCS and self-signed certificate / CWT), the
   trust anchor is the authentication key of the other party; in which
   case, there is no certification path.

D.4.  Revocation Status

   The application may need to verify that the credentials are not
   revoked; see Section 9.8.  Some use cases may be served by short-
   lived credentials, for example, where the validity of the credential
   is on par with the interval between revocation checks.  But, in
   general, credential lifetime and revocation checking are
   complementary measures to control credential status.  Revocation
   information may be transported as External Authorization Data (EAD);
   see Appendix E.

D.5.  Unauthenticated Operation

   EDHOC might be used without authentication by allowing the Initiator
   or Responder to communicate with any identity except its own.  Note
   that EDHOC without mutual authentication is vulnerable to active on-
   path attacks and therefore unsafe for general use.  However, it is
   possible to later establish a trust relationship with an unknown or
   not-yet-trusted endpoint.  Some examples are listed below:

   *  The EDHOC authentication credential can be verified out-of-band at
      a later stage.

   *  The EDHOC session key can be bound to an identity out-of-band at a
      later stage.

   *  Trust on first use (TOFU) can be used to verify that several EDHOC
      connections are made to the same identity.  TOFU combined with
      proximity is a common IoT deployment model that provides good
      security if done correctly.  Note that secure proximity based on
      short range wireless technology requires very low signal strength
      or very low latency.

Appendix E.  Use of External Authorization Data

   In order to reduce the number of messages and round trips, or to
   simplify processing, external security applications may be integrated
   into EDHOC by transporting related external authorization data (EAD)
   in the messages.

   The EAD format is specified in Section 3.8.  This section contains
   examples and further details of how EAD may be used with an
   appropriate accompanying specification.

   *  One example is third-party-assisted authorization, requested with
      EAD_1, and an authorization artifact ("voucher", cf. [RFC8366])
      returned in EAD_2; see [LAKE-AUTHZ].

   *  Another example is remote attestation, requested in EAD_2, and an
      Entity Attestation Token (EAT) [EAT] returned in EAD_3.

   *  A third example is certificate enrollment, where a Certificate
      Signing Request (CSR) [RFC2986] is included in EAD_3, and the
      issued public key certificate (X.509 [RFC5280] and C509
      [C509-CERTS]) or a reference thereof is returned in EAD_4.

   External authorization data should be considered unprotected by
   EDHOC, and the protection of EAD is the responsibility of the
   security application (third-party authorization, remote attestation,
   certificate enrollment, etc.).  The security properties of the EAD
   fields (after EDHOC processing) are discussed in Section 9.1.

   The content of the EAD field may be used in the EDHOC processing of
   the message in which they are contained.  For example,
   authentication-related information, like assertions and revocation
   information, transported in EAD fields may provide input about trust
   anchors or validity of credentials relevant to the authentication
   processing.  The EAD fields (like ID_CRED fields) are therefore made
   available to the application before the message is verified; see
   details of message processing in Section 5.  In the first example
   above, a voucher in EAD_2 made available to the application can
   enable the Initiator to verify the identity or the public key of the
   Responder before verifying the signature.  An application allowing
   EAD fields containing authentication information thus may need to
   handle authentication-related verifications associated with EAD
   processing.

   Conversely, the security application may need to wait for EDHOC
   message verification to complete.  In the third example above, the
   validation of a CSR carried in EAD_3 is not started by the Responder
   before EDHOC has successfully verified message_3 and proven the
   possession of the private key of the Initiator.

   The security application may reuse EDHOC protocol fields that
   therefore need to be available to the application.  For example, the
   security application may use the same crypto algorithms as in the
   EDHOC session and therefore needs access to the selected cipher suite
   (or the whole SUITES_I).  The application may use the ephemeral
   public keys G_X and G_Y as ephemeral keys or as nonces; see
   [LAKE-AUTHZ].

   The processing of the EAD item (ead_label, ? ead_value) by the
   security application needs to be described in the specification where
   the ead_label is registered (see Section 10.5), including the
   optional ead_value for each message and actions in case of errors.
   An application may support multiple security applications that make
   use of EAD, which may result in multiple EAD items in one EAD field;
   see Section 3.8.  Any dependencies on security applications with
   previously registered EAD items need to be documented, and the
   processing needs to consider their simultaneous use.

   Since data carried in EAD may not be protected, or processed by the
   application before the EDHOC message is verified, special
   considerations need to be made such that it does not violate security
   and privacy requirements of the service that uses this data; see
   Section 9.5.  The content in an EAD item may impact the security
   properties provided by EDHOC.  Security applications making use of
   the EAD items must perform the necessary security analysis.

Appendix F.  Application Profile Example

   This appendix contains a rudimentary example of an application
   profile; see Section 3.9.

   For use of EDHOC with application X, the following assumptions are
   made:

   1.  Transfer in CoAP as specified in Appendix A.2 with requests
       expected by the CoAP server (= Responder) at /app1-edh, no
       Content-Format needed.

   2.  METHOD = 1 (I uses signature key; R uses static DH key.)

   3.  CRED_I is an IEEE 802.1AR Initial Device Identifier (IDevID)
       encoded as a C509 certificate of type 0 [C509-CERTS].

       *  R acquires CRED_I out-of-band, indicated in EAD_1.

       *  ID_CRED_I = {4: h''} is a 'kid' with the value of the empty
          CBOR byte string.

   4.  CRED_R is a CCS of type OKP as specified in Section 3.5.2.

       *  The CBOR map has parameters 1 (kty), -1 (crv), and -2
          (x-coordinate).

       *  ID_CRED_R is {14 : CCS}.

   5.  External authorization data is defined and processed as specified
       in [LAKE-AUTHZ].

   6.  EUI-64 is used as the identity of the endpoint (see an example in
       Section 3.5.2).

   7.  No use of message_4.  The application sends protected messages
       from R to I.

Appendix G.  Long PLAINTEXT_2

   By the definition of encryption of PLAINTEXT_2 with KEYSTREAM_2, it
   is limited to lengths of PLAINTEXT_2 not exceeding the output of
   EDHOC_KDF; see Section 4.1.2.  If the EDHOC hash algorithm is SHA-2,
   then HKDF-Expand is used, which limits the length of the EDHOC_KDF
   output to 255 ⋅ hash_length, where hash_length is the length of the
   output of the EDHOC hash algorithm given by the cipher suite.  For
   example, with SHA-256 as the EDHOC hash algorithm, the length of the
   hash output is 32 bytes and the maximum length of PLAINTEXT_2 is 255
   ⋅ 32 = 8160 bytes.

   While PLAINTEXT_2 is expected to be much shorter than 8 kB for the
   intended use cases, it seems nevertheless prudent to specify a
   solution for the event that this should turn out to be a limitation.

   A potential work-around is to use a cipher suite with a different
   hash function.  In particular, the use of KMAC removes all practical
   limitations in this respect.

   This section specifies a solution that works with any hash function
   by making use of multiple invocations of HKDF-Expand and negative
   values of info_label.

   Consider the PLAINTEXT_2 partitioned in parts P(i) of length equal to
   M = 255 ⋅ hash_length, except possibly the last part P(last), which
   has 0 < length ≤ M.

   PLAINTEXT_2 = P(0) | P(1) | ... | P(last)

   where "|" indicates concatenation.

   The object is to define a matching KEYSTREAM_2 of the same length and
   perform the encryption in the same way as defined in Section 5.3.2:

   CIPHERTEXT_2 = PLAINTEXT_2 XOR KEYSTREAM_2

   Define the keystream as:

   KEYSTREAM_2 = OKM(0) | OKM(1)  | ... | OKM(last)

   where:

   OKM(i) = EDHOC_KDF( PRK_2e, -i, TH_2, length(P(i)) )

   Note that if length(PLAINTEXT_2) ≤ M, then P(0) = PLAINTEXT_2 and the
   definition of KEYSTREAM_2 = OKM(0) coincides with Figure 6.

   This describes the processing of the Responder when sending
   message_2.  The Initiator makes the same calculations when receiving
   message_2 but interchanging PLAINTEXT_2 and CIPHERTEXT_2.

   An application profile may specify if it supports or does not support
   the method described in this appendix.

Appendix H.  EDHOC_KeyUpdate

   To provide forward secrecy in an even more efficient way than re-
   running EDHOC, this section specifies the optional function
   EDHOC_KeyUpdate in terms of EDHOC_KDF and PRK_out.

   When EDHOC_KeyUpdate is called, a new PRK_out is calculated as the
   output of the EDHOC_Expand function with the old PRK_out as input.
   The change of PRK_out causes a change to PRK_exporter, which enables
   the derivation of new application keys superseding the old ones,
   using EDHOC_Exporter; see Section 4.2.1.  The process is illustrated
   by the following pseudocode.

   EDHOC_KeyUpdate( context ):
      new PRK_out = EDHOC_KDF( old PRK_out, 11, context, hash_length )
      new PRK_exporter = EDHOC_KDF( new PRK_out, 10, h'', hash_length )

   where hash_length denotes the output size in bytes of the EDHOC hash
   algorithm of the selected cipher suite.

   The EDHOC_KeyUpdate takes a context as input to enable binding of the
   updated PRK_out to some event that triggered the key update.  The
   Initiator and Responder need to agree on the context, which can,
   e.g., be a counter, a pseudorandom number, or a hash.  To provide
   forward secrecy, the old PRK_out and keys derived from it (old
   PRK_exporter and old application keys) must be deleted as soon as
   they are not needed.  When to delete the old keys and how to verify
   that they are not needed is up to the application.  Note that the
   security properties depend on the type of context and the number of
   KeyUpdate iterations.

   An application using EDHOC_KeyUpdate needs to store PRK_out.
   Compromise of PRK_out leads to compromise of all keying material
   derived with the EDHOC_Exporter since the last invocation of the
   EDHOC_KeyUpdate function.

   While this key update method provides forward secrecy, it does not
   give as strong security properties as re-running EDHOC.
   EDHOC_KeyUpdate can be used to meet cryptographic limits and provide
   partial protection against key leakage, but it provides significantly
   weaker security properties than re-running EDHOC with ephemeral
   Diffie-Hellman.  Even with frequent use of EDHOC_KeyUpdate,
   compromise of one session key compromises all future session keys,
   and an attacker therefore only needs to perform static key
   exfiltration [RFC7624], which is less complicated and has a lower
   risk profile than the dynamic case; see Section 9.1.

   A similar method to do a key update for OSCORE is KUDOS; see [KUDOS].

Appendix I.  Example Protocol State Machine

   This appendix describes an example protocol state machine for the
   Initiator and Responder.  States are denoted in all capitals, and
   parentheses denote actions taken only in some circumstances.

   Note that this state machine is just an example, and that details of
   processing are omitted.  For example:

   *  when error messages are being sent (with one exception);

   *  how credentials and EAD are processed by EDHOC and the application
      in the RCVD state; and

   *  what verifications are made, which includes not only MACs and
      signatures.

I.1.  Initiator State Machine

   The Initiator sends message_1, triggering the state machine to
   transition from START to WAIT_M2, and waits for message_2.

   If the incoming message is an error message, then the Initiator
   transitions from WAIT_M2 to ABORTED.  In case of error code 2 (Wrong
   Selected Cipher Suite), the Initiator remembers the supported cipher
   suites for this particular Responder and transitions from ABORTED to
   START.  The message_1 that the Initiator subsequently sends takes
   into account the cipher suites supported by the Responder.

   Upon receiving a non-error message, the Initiator transitions from
   WAIT_M2 to RCVD_M2 and processes the message.  If a processing error
   occurs on message_2, then the Initiator transitions from RCVD_M2 to
   ABORTED.  In case of successful processing of message_2, the
   Initiator transitions from RCVD_M2 to VRFD_M2.

   The Initiator prepares and processes message_3 for sending.  If any
   processing error is encountered, the Initiator transitions from
   VRFD_M2 to ABORTED.  If message_3 is successfully sent, the Initiator
   transitions from VRFD_M2 to COMPLETED.

   If the application profile includes message_4, then the Initiator
   waits for message_4.  If the incoming message is an error message,
   then the Initiator transitions from COMPLETED to ABORTED.  Upon
   receiving a non-error message, the Initiator transitions from
   COMPLETED (="WAIT_M4") to RCVD_M4 and processes the message.  If a
   processing error occurs on message_4, then the Initiator transitions
   from RCVD_M4 to ABORTED.  In case of successful processing of
   message_4, the Initiator transitions from RCVD_M4 to PERSISTED
   (="VRFD_M4").

   If the application profile does not include message_4, then the
   Initiator waits for an incoming application message.  If the
   decryption and verification of the application message is successful,
   then the Initiator transitions from COMPLETED to PERSISTED.

        +- - - - - - - - - -> START
        |                       |
                                | Send message_1
        |                       |
              Receive error     v
    ABORTED <---------------- WAIT_M2
        ^                       |
        |                       | Receive message_2
        |                       |
        |    Processing error   v
        +-------------------- RCVD_M2
        ^                       |
        |                       | Verify message_2
        |                       |
        |    Processing error   v
        +-------------------- VRFD_M2
        ^                       |
        |                       | Send message_3
        |                       |
        |    (Receive error)    v
        +-------------------- COMPLETED ----------------+
        ^                       |                       |
        |                       | (Receive message_4)   |
        |                       |                       |
        |   (Processing error)  v                       | (Verify
        +------------------- (RCVD_M4)                  |  application
                                |                       |  message)
                                | (Verify message_4)    |
                                |                       |
                                v                       |
                              PERSISTED <---------------+

                     Figure 12: Initiator State Machine

I.2.  Responder State Machine

   Upon receiving message_1, the Responder transitions from START to
   RCVD_M1.

   If a processing error occurs on message_1, the Responder transitions
   from RCVD_M1 to ABORTED.  This includes sending an error message with
   error code 2 (Wrong Selected Cipher Suite) if the selected cipher
   suite in message_1 is not supported.  In case of successful
   processing of message_1, the Responder transitions from RCVD_M1 to
   VRFD_M1.

   The Responder prepares and processes message_2 for sending.  If any
   processing error is encountered, the Responder transitions from
   VRFD_M1 to ABORTED.  If message_2 is successfully sent, the Initiator
   transitions from VRFD_M2 to WAIT_M3 and waits for message_3.

   If the incoming message is an error message, then the Responder
   transitions from WAIT_M3 to ABORTED.

   Upon receiving message_3, the Responder transitions from WAIT_M3 to
   RCVD_M3.  If a processing error occurs on message_3, the Responder
   transitions from RCVD_M3 to ABORTED.  In case of successful
   processing of message_3, the Responder transitions from RCVD_M3 to
   COMPLETED (="VRFD_M3").

   If the application profile includes message_4, the Responder prepares
   and processes message_4 for sending.  If any processing error is
   encountered, the Responder transitions from COMPLETED to ABORTED.

   If message_4 is successfully sent, or if the application profile does
   not include message_4, the Responder transitions from COMPLETED to
   PERSISTED.

                                        START
                                          |
                                          | Receive message_1
                                          |
                       Processing error   v
              ABORTED <---------------- RCVD_M1
                  ^                       |
                  |                       | Verify message_1
                  |                       |
                  |    Processing error   v
                  +-------------------- VRFD_M1
                  ^                       |
                  |                       | Send message_2
                  |                       |
                  |     Receive error     v
                  +-------------------- WAIT_M3
                  ^                       |
                  |                       | Receive message_3
                  |                       |
                  |    Processing error   v
                  +-------------------- RCVD_M3
                  ^                       |
                  |                       | Verify message_3
                  |                       |
                  |   (Processing error)  v
                  +------------------- COMPLETED
                                          |
                                          | (Send message_4)
                                          |
                                          v
                                       PERSISTED

                     Figure 13: Responder State Machine

Acknowledgments

   The authors want to thank Christian Amsüss, Karthikeyan Bhargavan,
   Carsten Bormann, Alessandro Bruni, Timothy Claeys, Baptiste Cottier,
   Roman Danyliw, Martin Disch, Martin Duke, Donald Eastlake 3rd, Lars
   Eggert, Stephen Farrell, Loïc Ferreira, Theis Grønbech Petersen,
   Felix Günther, Dan Harkins, Klaus Hartke, Russ Housley, Stefan
   Hristozov, Marc Ilunga, Charlie Jacomme, Elise Klein, Erik Kline,
   Steve Kremer, Alexandros Krontiris, Ilari Liusvaara, Rafa Marín-
   López, Kathleen Moriarty, David Navarro, Karl Norrman, Salvador
   Pérez, Radia Perlman, David Pointcheval, Maïwenn Racouchot, Eric
   Rescorla, Michael Richardson, Thorvald Sahl Jørgensen, Zaheduzzaman
   Sarker, Jim Schaad, Michael Scharf, Carsten Schürmann, John Scudder,
   Ludwig Seitz, Brian Sipos, Stanislav Smyshlyaev, Valery Smyslov,
   Peter van der Stok, Rene Struik, Vaishnavi Sundararajan, Erik
   Thormarker, Marco Tiloca, Sean Turner, Michel Veillette, Mališa
   Vučinić, Paul Wouters, and Lei Yan for reviewing and commenting on
   intermediate draft versions of this document.

   We are especially indebted to the late Jim Schaad for his continuous
   review and implementation of draft versions of this document, as well
   as his work on other technologies such as COSE and OSCORE without
   which EDHOC would not have been.

   Work on this document has in part been supported by the H2020 project
   SIFIS-Home (grant agreement 952652).

Authors' Addresses

   Göran Selander
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: goran.selander@ericsson.com


   John Preuß Mattsson
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: john.mattsson@ericsson.com


   Francesca Palombini
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: francesca.palombini@ericsson.com
