A more efficient FramedContentTBS structure in Messsaging Layer Security (MLS)
draft-mahy-mls-new-framed-content-tbs-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Rohan Mahy | ||
| Last updated | 2025-10-16 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
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draft-mahy-mls-new-framed-content-tbs-00
Messaging Layer Security R. Mahy
Internet-Draft 16 October 2025
Intended status: Informational
Expires: 19 April 2026
A more efficient FramedContentTBS structure in Messsaging Layer Security
(MLS)
draft-mahy-mls-new-framed-content-tbs-00
Abstract
Most MLS signatures are signed over the relatively large GroupContext
structure. This document defines a way to safely sign using a pre-
hashed version of the GroupContext structure for better efficiency.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at
https://rohanmahy.github.io/mls-new-framed-content-tbs/draft-mahy-
mls-new-framed-content-tbs.html. Status information for this
document may be found at https://datatracker.ietf.org/doc/draft-mahy-
mls-new-framed-content-tbs/.
Discussion of this document takes place on the Messaging Layer
Security Working Group mailing list (mailto:mls@ietf.org), which is
archived at https://mailarchive.ietf.org/arch/browse/mls/. Subscribe
at https://www.ietf.org/mailman/listinfo/mls/.
Source for this draft and an issue tracker can be found at
https://github.com/rohanmahy/mls-new-framed-content-tbs.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 19 April 2026.
Copyright Notice
Copyright (c) 2025 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 3
3. Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Security Considerations . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1. Normative References . . . . . . . . . . . . . . . . . . 4
6.2. Informative References . . . . . . . . . . . . . . . . . 4
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
In MLS [RFC9420] all in-member application, commit, and proposal
messages (whether sent via a PublicMessage, PrivateMessage or
SemiPrivateMessage [I-D.mahy-mls-semiprivatemessage]); and all
external commits contain a signature of the FramedContentTBS
structure. This structure contains the full GroupContext of the
group, which can store a large amount of group state. (For example,
in the MIMI protocol [I-D.ietf-mimi-protocol] it contains the
participant list for the group, which could contain thousands of
URIs.)
The GroupContext only changes once per epoch, therefore it is an
excellent candidate to pre-hash and cache for efficiency reasons.
This document defines an a replacement for the FramedContentTBS
structure that uses a pre-hashed GroupContext, and an MLS extension
for safely negotiating the use of the new struct.
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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. Mechanism
This document defines the new_framed_content_tbs extension. When
present in a LeafNode.capabilities.extensions list, it indicates that
the client supports this extension. When present in the
required_capabilities.extension_types list in GroupContext.extensions
it indicates that every member of the group MUST use the new
FramedContentTBS structure.
The FramedContentTBS structure is replaced with the new structure
below. The only change is that the GroupContext is replaced with a
hash (from the group's current MLS cipher suite) of the GroupContext.
context_hash = RefHash(GroupContext);
struct {
ProtocolVersion version = mls10;
WireFormat wire_format;
FramedContent content;
select (FramedContentTBS.content.sender.sender_type) {
case member:
case new_member_commit:
opaque context_hash<V>;
case external:
case new_member_proposal:
struct{};
};
} FramedContentTBS;
4. Security Considerations
TODO Security
5. IANA Considerations
This document requests the addition of a new MLS Extension Type value
under the heading of "Messaging Layer Security".
RFC EDITOR: Please replace XXXX throughout with the RFC number
assigned to this document.
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The new_framed_content_tbs MLS Extension Type is used inside LeafNode
and GroupContext objects. In a LeafNode it indicates support the the
extension. In the required_capabilities of the GroupContext it
indicates the extension is being used.
Value: 0x000b (suggested)
Name: new_framed_content_tbs
Message(s): GC,LN: This extension may appear in GroupContext and
LeafNode objects
Recommended: Y
Reference: RFC XXXX
6. References
6.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
[RFC9420] Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.
6.2. Informative References
[I-D.ietf-mimi-protocol]
Barnes, R., Hodgson, M., Kohbrok, K., Mahy, R., Ralston,
T., and R. Robert, "More Instant Messaging
Interoperability (MIMI) using HTTPS and MLS", Work in
Progress, Internet-Draft, draft-ietf-mimi-protocol-04, 7
July 2025, <https://datatracker.ietf.org/doc/html/draft-
ietf-mimi-protocol-04>.
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[I-D.mahy-mls-semiprivatemessage]
Mahy, R., "Semi-Private Messages in the Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-mahy-mls-semiprivatemessage-05, 21 April
2025, <https://datatracker.ietf.org/doc/html/draft-mahy-
mls-semiprivatemessage-05>.
Acknowledgments
TODO acknowledge.
Author's Address
Rohan Mahy
Email: rohan.ietf@gmail.com
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