



Internet Engineering Task Force (IETF)                           R. Mahy
Request for Comments: 9734                Rohan Mahy Consulting Services
Category: Standards Track                                  February 2025
ISSN: 2070-1721


 X.509 Certificate Extended Key Usage (EKU) for Instant Messaging URIs

Abstract

   RFC 5280 specifies several extended key purpose identifiers
   (KeyPurposeIds) for X.509 certificates.  This document defines an
   Instant Messaging (IM) identity KeyPurposeId for inclusion in the
   Extended Key Usage (EKU) extension of X.509 v3 public key
   certificates

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/rfc9734.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
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   in the Revised BSD License.

Table of Contents

   1.  Introduction
   2.  Conventions and Definitions
   3.  The IM URI EKU
   4.  Security Considerations
   5.  IANA Considerations
   6.  References
     6.1.  Normative References
     6.2.  Informative References
   Appendix A.  ASN.1 Module
   Acknowledgments
   Author's Address

1.  Introduction

   Instant Messaging (IM) systems using the Messaging Layer Security
   (MLS) [RFC9420] protocol can incorporate per-client identity
   certificate credentials.  A subjectAltName in these certificates can
   be an IM URI [RFC3860] or Extensible Messaging and Presence Protocol
   (XMPP) URI [RFC6121], for example.

   Organizations may be unwilling to issue certificates for an IM client
   using a general KeyPurposeId, such as id-kp-serverAuth or id-kp-
   clientAuth, because of the risk that such certificates could be
   abused in a cross-protocol attack.

   An explanation of MLS credentials as they apply to IM is described in
   [E2E-IDENTITY].  These credentials are expected to be heavily used in
   the More Instant Messaging Interoperability (MIMI) Working Group.

2.  Conventions and Definitions

   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.

3.  The IM URI EKU

   This specification defines the KeyPurposeId id-kp-imUri, which may be
   included in certificates used to prove the identity of an IM client.
   This EKU extension MAY, at the option of the certificate issuer, be
   either critical or non-critical.

   id-kp  OBJECT IDENTIFIER  ::= {
     iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) kp(3) }

   id-kp-imUri OBJECT IDENTIFIER ::= { id-kp 40 }

4.  Security Considerations

   The security considerations of [RFC5280] are applicable to this
   document.  The id-kp-imUri extended key purpose does not introduce
   new security risks but instead reduces existing security risks by
   providing means to identify if the certificate is generated to sign
   IM identity credentials.  Issuers SHOULD NOT set the id-kp-imUri
   extended key purpose and an id-kp-clientAuth or id-kp-serverAuth
   extended key purpose: that would defeat the improved specificity
   offered by having an id-kp-imUri extended key purpose.

5.  IANA Considerations

   IANA has registered the following OID in the "SMI Security for PKIX
   Extended Key Purpose" registry (1.3.6.1.5.5.7.3).  This OID is
   defined in Section 3.

                  +=========+=============+============+
                  | Decimal | Description | References |
                  +=========+=============+============+
                  | 40      | id-kp-imUri | RFC 9734   |
                  +---------+-------------+------------+

                                 Table 1

   IANA has also registered the following ASN.1 [ITU.X690.2021] module
   OID in the "SMI Security for PKIX Module Identifier" registry
   (1.3.6.1.5.5.7.0).  This OID is defined in Appendix A.

                 +=========+===============+============+
                 | Decimal | Description   | References |
                 +=========+===============+============+
                 | 113     | id-mod-im-eku | RFC 9734   |
                 +---------+---------------+------------+

                                 Table 2

6.  References

6.1.  Normative References

   [ITU.X680.2021]
              ITU-T, "Information Technology - Abstract Syntax Notation
              One (ASN.1): Specification of basic notation", ITU-T
              Recommendation X.680, ISO/IEC 8824-1:2021, February 2021,
              <https://www.itu.int/rec/T-REC-X.680>.

   [ITU.X690.2021]
              ITU-T, "Information Technology - ASN.1 encoding rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER)", ITU-T Recommendation X.690, ISO/IEC 8825-1-2021,
              February 2021, <https://www.itu.int/rec/T-REC-X.690>.

   [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>.

   [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>.

   [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>.

6.2.  Informative References

   [E2E-IDENTITY]
              Barnes, R. and R. Mahy, "Identity for E2E-Secure
              Communications", Work in Progress, Internet-Draft, draft-
              barnes-mimi-identity-arch-02, 4 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-barnes-mimi-
              identity-arch-02>.

   [RFC3860]  Peterson, J., "Common Profile for Instant Messaging
              (CPIM)", RFC 3860, DOI 10.17487/RFC3860, August 2004,
              <https://www.rfc-editor.org/info/rfc3860>.

   [RFC6121]  Saint-Andre, P., "Extensible Messaging and Presence
              Protocol (XMPP): Instant Messaging and Presence",
              RFC 6121, DOI 10.17487/RFC6121, March 2011,
              <https://www.rfc-editor.org/info/rfc6121>.

   [RFC9420]  Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
              July 2023, <https://www.rfc-editor.org/info/rfc9420>.

Appendix A.  ASN.1 Module

   The following module adheres to ASN.1 specifications [ITU.X680.2021]
   and [ITU.X690.2021].

   <CODE BEGINS>

   IM-EKU
     { iso(1) identified-organization(3) dod(6) internet(1)
     security(5) mechanisms(5) pkix(7) id-mod(0)
     id-mod-im-eku (113) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- OID Arc

   id-kp OBJECT IDENTIFIER ::=
     { iso(1) identified-organization(3) dod(6) internet(1)
       security(5) mechanisms(5) pkix(7) kp(3) }

   -- Extended Key Usage Values

   id-kp-imUri OBJECT IDENTIFIER ::= { id-kp 40 }

   END

   <CODE ENDS>

Acknowledgments

   Thanks to Sean Turner and Russ Housley for reviews, suggestions,
   corrections, and encouragement.

Author's Address

   Rohan Mahy
   Rohan Mahy Consulting Services
   Email: rohan.ietf@gmail.com
