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

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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. The subjectAltName of these certificates can be an IM URI, for example. Since IM clients could be very numerous, operators are reticent to issue certificates for these users that might accidentally be used to validate a TLS connection because it has the KeyPurposeId id-kp-serverAuth or id-kp-clientAuth.¶

An explanation of MLS credentials as they apply to Instant Messaging is described in [I-D.barnes-mimi-identity-arch]. 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 Extended Key Usage

This specification defines the KeyPurposeId id-kp-imUri, which is used for signing messages to prove the identity of an Instant Messaging client. This Extended Key Usage is optionally 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 TBD1 }

4. Security Considerations

The Security Considerations of [RFC5280] are applicable to this document. This 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.¶

5. IANA Considerations

IANA is requested to register the following OIDs in the "SMI Security for PKIX Extended Key Purpose" registry (1.3.6.1.5.5.7.3). These OIDs are defined in Section 4.¶

IANA is also requested to register 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.¶

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 (TBD2) }

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 TBD1 }

END


<CODE ENDS>

Appendix B. Change log

• added ASN.1 module¶

• specified that eku is optionally critical¶

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