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
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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. This 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 4. 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-01, 23 October 2023,
<https://datatracker.ietf.org/doc/html/draft-barnes-mimi-
identity-arch-01>.
[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