KeyPackage Context Extension for Message Layer Security (MLS)

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Table of Contents

1. Terminology

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.¶

The terms MLS client, MLS group, LeafNode, GroupContext, KeyPackage, GroupContextExtensions Proposal, Credential, CredentialType, and RequiredCapabilities have the same meanings as in the MLS protocol [I-D.ietf-mls-protocol].¶

2. Introduction

In some use cases of MLS, a client might wish to provide a KeyPackage to another client, but communicate that the specific KeyPackage is only to be used in a specific context, for example to join a specific MLS group. This document describes a KeyPackage extension that can convey that context.¶

3. Extension Description

This document specifies a KeyPackage MLS extension kp_context of type ContextPair. The syntax is described using the TLS Presentation Language [RFC8446]¶

Each PerDomainTrustAnchor represents a specific identity domain which is expected and authorized to participate in the MLS group. It contains the domain name and the specific trust anchor used to validate identities for members in that domain.¶

enum {
  reserved(0),
  groupid(1),
  uri(2),
  domain(3),
  jwk_thumbprint(4)
  (255)
} ContextType;

struct {
    ContextType context_type;
    opaque context_value<V>;
} ContextPair;

ContextPair kp_context;

4. IANA Considerations

This document proposes registration of a new MLS Extension Type.¶

RFC EDITOR: Please replace XXXX throughout with the RFC number assigned to this document¶

  Template:
  Value: 0x000B
  Name: kp_context
  Message(s): This extension may appear in KeyPackage objects
  Recommended: Y
  Reference: RFC XXXX

   Namespace Identifier:  Requested of IANA (formal) or assigned by IANA
      (informal).

   Version:  1

   Date:  2023-08-01

   Registrant:
     Rohan Mahy
     rohan.ietf@gmail.com

   Purpose:  Described in Section 3 of RFCXXXX.

   Syntax:  Described in Section 3 of RFCXXXX.

   Assignment:  Described in Section 4.1 of RFCXXXX.

   Security and Privacy:  Described in Section 5 of RFCXXXX.

   Interoperability:  Described in Section 3 of this document.

   Resolution:  Described in Section 3 of this document.

   Documentation:  RFCXXXX

   Additional Information:  none

   Revision Information:  n/a

5. Security Considerations

The Security Considerations of MLS apply.¶

The use of this extension may reveal the client's intentions or wishes in an out-of-band protocol, which may have weaker privacy protections than MLS handshake messages.¶

Author's Address