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Messaging Layer Security Ciphersuite using XWing Key Exchange Mechanism

Document Type Active Internet-Draft (individual)
Author Rohan Mahy
Last updated 2024-03-05 (Latest revision 2024-03-04)
Replaces draft-mahy-mls-x25519kyber768draft00
RFC stream (None)
Intended RFC status (None)
Stream Stream state (No stream defined)
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IESG IESG state I-D Exists
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MLS                                                              R. Mahy
Internet-Draft                                              Unaffiliated
Intended status: Informational                              4 March 2024
Expires: 5 September 2024

Messaging Layer Security Ciphersuite using XWing Key Exchange Mechanism


   This document registers a new Messaging Layer Security (MLS)
   ciphersuite using the X-Wing hybrid post-quantum resistant /
   traditional (PQ/T) Key Exchange Mechanism (KEM).

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at

   Discussion of this document takes place on the MLS Working Group
   mailing list (, which is archived at  Subscribe at

   Source for this draft and an issue tracker can be found at

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
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   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 5 September 2024.

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Copyright Notice

   Copyright (c) 2024 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
   Provisions Relating to IETF Documents (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   The potential availability of a cryptographically-relevant quantum
   computer has caused concern that well-funded adversaries could
   overturn long-held assumptions about the security assurances of
   classical Key Exchange Mechanisms (KEMs) and classical cryptographic
   signatures, which are fundamental to modern security protocols,
   including the MLS protocol [RFC9420].

   The MLS Working Group has expressed strong desire to have a handful
   of complimentary post-quantum security extensions for use with the
   MLS protocol to address the related threats:

   1.  A straightforward MLS cipher suite that replaces a classical KEM
       with a hybrid post-quantum/traditional KEM.  Such a cipher suite
       could be implemented as a drop-in replacement in many MLS
       libraries without changes to any other part of the MLS stack.
       The aim is for implementations to have a single KEM which would
       be performant and work for the vast majority of implementations.
       It addresses the the harvest-now / decrypt-later threat model
       using the simplest, and most practicable solution available.

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   2.  Versions of existing cipher suites that use post-quantum
       signatures; and specific guidelines on the construction, use, and
       validation of hybrid signatures.

   3.  One or more mechanisms which reduce the bandwidth or storage
       requirements, or improve performance when using post-quantum
       algorithms (for example by updating post-quantum keys less
       frequently than classical keys, or by sharing portions of post-
       quantum keys across a large number of clients or groups.)

   This document addresses the first of these work items.  It reserves
   an MLS cipher suite value based on the MLS default cipher suite, but
   replacing the KEM with the X-Wing [I-D.connolly-cfrg-xwing-kem]
   hybrid post-quantum / traditional KEM.  The IANA Hybrid Public Key
   Encryption (HPKE) [RFC9180] Key Exchange Mechanism (KEM) Identifier
   value for X-Wing is pending at the time of this writing.

   X-Wing is a "concrete, simple choice for [a] post-quantum hybrid KEM,
   that should be suitable for the vast majority of use cases".  X-Wing
   combines the ML-KEM [MLKEM] post-quantum KEM and the X25519 [RFC7748]
   traditional KEM.  The MLS cipher suite uses the other components of
   the default MLS cipher suite

   This document replaces a previous, similar proposal based on the KEM
   in [I-D.draft-westerbaan-cfrg-hpke-xyber768d00].

2.  Security Considerations

   This ciphersuite uses a hybrid post-quantum/traditional KEM and a
   traditional signature algorithm.  As such, it is designed to provide
   confidentiality against quantum and classical attacks, but provides
   authenticity against classical attacks only.  This is actually very
   useful, because an attacker could store MLS-encrypted traffic that
   uses any classical KEM today.  If years or decades in the future a
   quantum attack on classical KEMs becomes feasible, the traffic sent
   today (some of which could still be sensitive in the future) will
   then be readable.  By contrast, an attack on a signature algorithm in
   MLS would require an active attack which can extract the private key
   during the signature key's lifetime.

   The security properties of [I-D.connolly-cfrg-xwing-kem] apply.

3.  IANA Considerations

   This document registers a new MLS Ciphersuite value.

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   Value:        new assignment
   Name:         MLS_128_XWING_AES128GCM_SHA256_Ed25519
   Recommended:  N
   Reference:    This document

4.  References

4.1.  Normative References

              Connolly, D., Schwabe, P., and B. Westerbaan, "X-Wing:
              general-purpose hybrid post-quantum KEM", Work in
              Progress, Internet-Draft, draft-connolly-cfrg-xwing-kem-
              01, 23 January 2024,

              Westerbaan, B. and C. A. Wood, "X25519Kyber768Draft00
              hybrid post-quantum KEM for HPKE", Work in Progress,
              Internet-Draft, draft-westerbaan-cfrg-hpke-xyber768d00-02,
              4 May 2023, <

   [RFC9180]  Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <>.

   [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, <>.

4.2.  Informative References

   [MLKEM]    National Institute of Standards and Technology, "FIPS 203
              (Initial Draft): Module-Lattice-Based Key-Encapsulation
              Mechanism Standard", n.d.,

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <>.


   Thanks to Joël Alwen, Marta Mularczyk, Britta Hale, and Richard

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Author's Address

   Rohan Mahy

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