Status of This Memo

This is an Internet Standards Track document.¶

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Table of Contents

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.¶

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.¶

6. References

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