Internet-Draft MLS KeyPackage Context October 2023
Mahy Expires 24 April 2024 [Page]
Intended Status:
R. Mahy

KeyPackage Context Extension for Message Layer Security (MLS)


This document describes a Message Layer Security (MLS) KeyPackage extension to convey a specific context or anticipated use for the KeyPackage. It is useful when a client provides the KeyPackage out-of-band to another client, and wants the specific KeyPackage used only in the anticipated context, for example a specific MLS group.

About This Document

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This Internet-Draft will expire on 24 April 2024.

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 {
} 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

4.1. kp_context MLS Extension Type

The kp_context MLS Extension Type is used inside KeyPackage objects. It contains a URN Anchors object representing the trust anchors which are expected for identity validation inside the MLS group.

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

4.2. urn:ietf:mls:kp_context:group_id URN registration

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

   Version:  1

   Date:  2023-08-01

     Rohan Mahy

   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.

6. Normative References

Barnes, R., Beurdouche, B., Robert, R., Millican, J., Omara, E., and K. Cohn-Gordon, "The Messaging Layer Security (MLS) Protocol", Work in Progress, Internet-Draft, draft-ietf-mls-protocol-20, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Rescorla, E., "The Transport Layer Security (TLS) Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446, , <>.

Author's Address

Rohan Mahy