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 athttps://www.rfc-editor.org/info/rfc9734.¶

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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 certificatecredentials. 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 IMclient using a general KeyPurposeId, such asid-kp-serverAuth orid-kp-clientAuth, because of the risk that such certificates could beabused in a cross-protocol attack.¶

An explanation of MLS credentials as they apply to IM isdescribed in[E2E-IDENTITY]. These credentials areexpected 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 KeyPurposeIdid-kp-imUri, which may be included in certificates used to prove the identity of an IM client. This EKU extensionMAY, 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 thisdocument. Theid-kp-imUri extended key purpose does not introduce new securityrisks but instead reduces existing security risks by providing meansto identify if the certificate is generated to sign IM identity credentials.IssuersSHOULD NOT set theid-kp-imUri extended key purpose and anid-kp-clientAuth orid-kp-serverAuth extended key purpose: that woulddefeat the improved specificity offered by having anid-kp-imUri extended keypurpose.¶

5.IANA Considerations

IANA has registered the following OID in the "SMI Securityfor PKIX Extended Key Purpose" registry (1.3.6.1.5.5.7.3). ThisOID is defined inSection 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 inAppendix 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 Arcid-kp OBJECT IDENTIFIER ::=  { iso(1) identified-organization(3) dod(6) internet(1)    security(5) mechanisms(5) pkix(7) kp(3) }-- Extended Key Usage Valuesid-kp-imUri OBJECT IDENTIFIER ::= { id-kp 40 }END<CODE ENDS>

Acknowledgments

Thanks toSean Turner andRuss Housley for reviews, suggestions, corrections, and encouragement.¶

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