ML-KEM and Hybrid Cipher Suites for Messaging Layer Security
draft-ietf-mls-pq-ciphersuites-01

MLS                                                              R. Mahy
Internet-Draft                                                          
Intended status: Informational                              R. L. Barnes
Expires: 8 May 2026                                                Cisco
                                                         4 November 2025

      ML-KEM and Hybrid Cipher Suites for Messaging Layer Security
                   draft-ietf-mls-pq-ciphersuites-01

Abstract

   This document registers new cipher suites for Messaging Layer
   Security (MLS) based on "post-quantum" algorithms, which are intended
   to be resilient to attack by quantum computers.  These cipher suites
   are constructed using the new Module-Lattice Key Encapsulation
   Mechanism (ML-KEM), optionally in combination with traditional
   elliptic curve KEMs, together with appropriate authenticated
   encryption, hash, and signature algorithms.

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://mlswg.github.io/mls-pq-ciphersuites/#go.draft-ietf-mls-pq-
   ciphersuites.html.  Status information for this document may be found
   at https://datatracker.ietf.org/doc/draft-ietf-mls-pq-ciphersuites/.

   Discussion of this document takes place on the MLS 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/mlswg/mls-pq-ciphersuites/.

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
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  MLS Cipher Suites . . . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   5
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   7
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   7
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   7

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].

   Of particular concern are "harvest now, decrypt later" attacks, by
   which an attacker could collect encrypted traffic now, before a
   quantum computer exists, and later use a quantum computer to break
   the confidentiality protections on the collected traffic.

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   In response to these concerns, the cryptographic community has
   defined "post-quantum" algorithms, which are designed to be resilient
   to attacks by quantum computers.  Symmetric algorithms can be made
   post-quantum secure simply by using longer keys and hashes.  For
   asymmetric operations such as KEMs and signatures, entirely new
   algorithms are needed.

   In this document, we define ciphersuites that use the post-quantum
   secure Module-Lattice-Based KEM (ML-KEM) [MLKEM] together with
   appropriate symmetric algorithms, and either traditional or Module-
   Lattice-Based Digital Signature Algorithm (ML-DSA) [MLDSA] post-
   quantum signature algorithms.  The traditional signature cipher
   suites address the risk of "harvest now, decrypt later" attacks,
   while not taking on the additional cost of post-quantum signatures.
   The cipher suites with post-quantum signatures use only post-quantum
   KEMs.

   Following the pattern of base MLS, we define several variations, to
   allow for users that prefer to only use NIST-approved cryptography,
   users that prefer a higher security level, and users that prefer a
   PQ/traditional hybrid KEM over pure ML-KEM:

   *  ML-KEM-768 + X25519 (128-bit security, Non-NIST, PQ/T hybrid)

   *  ML-KEM-768 + P-256 (128-bit security, NIST, PQ/T hybrid)

   *  ML-KEM-1024 + P-384 (192-bit security, NIST, PQ/T hybrid)

   *  ML-KEM-768 (128-bit security, NIST, pure PQ KEM)

   *  ML-KEM-1024 (192-bit security, NIST, pure PQ KEM)

   *  ML-KEM-768 (192-bit security, NIST, pure PQ)

   *  ML-KEM-1024 (256-bit security, NIST, pure PQ)

   For all the cipher suites defined in this document, we use AES256 GCM
   [GCM] as the Authenticated Encryption with Authenticated Data (AEAD)
   [RFC5116] algorithm; HMAC [RFC2104] with SHA-384 [SHS] as the hash
   function; and SHAKE256 (Section 3.2 of [FIPS202]) as the Key
   Derivation Function (KDF).

   For the PQ/T hybrid KEMs and the pure ML-KEM HPKE integration, we use
   the KEMs defined in [I-D.ietf-hpke-pq].  The signature schemes for
   ML-DSA-65 and ML-DSA-87 [MLDSA] are defined in [I-D.ietf-tls-mldsa].

2.  IANA Considerations

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2.1.  MLS Cipher Suites

   This document requests that IANA add the following entries to the
   "MLS Cipher Suites" registry, replacing "XXXX" with the RFC number
   assigned to this document:

   +=====+===============================================+===+=========+
   |Value|Name                                           |Rec|Reference|
   +=====+===============================================+===+=========+
   |TBD1 |MLS_128_MLKEM768X25519_AES256GCM_SHA384_Ed25519|Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+
   |TBD2 |MLS_128_MLKEM768P256_AES256GCM_SHA384_P256     |Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+
   |TBD3 |MLS_192_MLKEM1024P384_AES256GCM_SHA384_P384    |Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+
   |TBD4 |MLS_128_MLKEM768_AES256GCM_SHA384_P256         |Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+
   |TBD5 |MLS_192_MLKEM1024_AES256GCM_SHA384_P384        |Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+
   |TBD6 |MLS_192_MLKEM768_AES256GCM_SHA384_MLDSA65      |Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+
   |TBD7 |MLS_256_MLKEM1024_AES256GCM_SHA512_MLDSA87     |Y  |RFCXXXX  |
   +-----+-----------------------------------------------+---+---------+

                                  Table 1

   The mapping of cipher suites to HPKE primitives [I-D.ietf-hpke-hpke],
   HMAC hash functions, and TLS signature schemes [RFC8446] is as
   follows:

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    +======+======+========+========+========+========================+
    |Value |KEM   | KDF    | AEAD   | Hash   | Signature              |
    +======+======+========+========+========+========================+
    |0xTBD1|0x647a| 0x0011 | 0x0002 | SHA384 | ed25519                |
    +------+------+--------+--------+--------+------------------------+
    |0xTBD2|0x0050| 0x0011 | 0x0002 | SHA384 | ecdsa_secp256r1_sha256 |
    +------+------+--------+--------+--------+------------------------+
    |0xTBD3|0x0051| 0x0011 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
    +------+------+--------+--------+--------+------------------------+
    |0xTBD4|0x0041| 0x0011 | 0x0002 | SHA384 | ecdsa_secp256r1_sha256 |
    +------+------+--------+--------+--------+------------------------+
    |0xTBD5|0x0042| 0x0011 | 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
    +------+------+--------+--------+--------+------------------------+
    |0xTBD6|0x0041| 0x0011 | 0x0002 | SHA384 | mldsa65                |
    +------+------+--------+--------+--------+------------------------+
    |0xTBD7|0x0042| 0x0011 | 0x0002 | SHA384 | mldsa87                |
    +------+------+--------+--------+--------+------------------------+

                                  Table 2

   The hash used for the MLS transcript hash is the one referenced in
   the cipher suite name.  "SHA384" refers to the SHA-384 functions
   defined in [SHS].

3.  Security Considerations

   The first five ciphersuites defined in this document combine a post-
   quantum (or PQ/T hybrid) KEM with a traditional signature algorithm.
   As such, they are designed to provide confidentiality against quantum
   and classical attacks, but provide authenticity against classical
   attacks only.  Thus, these cipher suites do not provide full post-
   quantum security, only post-quantum confidentiality.

   The last two cipher suites also use post-quantum signature
   algorithms.

   For security considerations related to the KEMs used in this
   document, please see the documents that define those KEMs
   [I-D.ietf-hpke-pq] and [I-D.irtf-cfrg-hybrid-kems].  For security
   considerations related to the post-quantum signature algorithms used
   in this document, please see [I-D.ietf-tls-mldsa] and
   [I-D.ietf-lamps-dilithium-certificates].

4.  References

4.1.  Normative References

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   [FIPS202]  "SHA-3 standard :: permutation-based hash and extendable-
              output functions", National Institute of Standards and
              Technology (U.S.), DOI 10.6028/nist.fips.202, 2015,
              <https://doi.org/10.6028/nist.fips.202>.

   [GCM]      Dworkin, M., "Recommendation for block cipher modes of
              operation :: GaloisCounter Mode (GCM) and GMAC", National
              Institute of Standards and Technology,
              DOI 10.6028/nist.sp.800-38d, 2007,
              <https://doi.org/10.6028/nist.sp.800-38d>.

   [I-D.ietf-hpke-hpke]
              Barnes, R., Bhargavan, K., Lipp, B., and C. A. Wood,
              "Hybrid Public Key Encryption", Work in Progress,
              Internet-Draft, draft-ietf-hpke-hpke-01, 24 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-hpke-
              hpke-01>.

   [I-D.ietf-hpke-pq]
              Barnes, R., "Post-Quantum and Post-Quantum/Traditional
              Hybrid Algorithms for HPKE", Work in Progress, Internet-
              Draft, draft-ietf-hpke-pq-01, 30 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-hpke-pq-
              01>.

   [I-D.ietf-tls-mldsa]
              Hollebeek, T., Schmieg, S., and B. Westerbaan, "Use of ML-
              DSA in TLS 1.3", Work in Progress, Internet-Draft, draft-
              ietf-tls-mldsa-01, 26 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tls-
              mldsa-01>.

   [MLDSA]    "Module-lattice-based digital signature standard",
              National Institute of Standards and Technology (U.S.),
              DOI 10.6028/nist.fips.204, August 2024,
              <https://doi.org/10.6028/nist.fips.204>.

   [MLKEM]    "Module-lattice-based key-encapsulation mechanism
              standard", National Institute of Standards and Technology
              (U.S.), DOI 10.6028/nist.fips.203, August 2024,
              <https://doi.org/10.6028/nist.fips.203>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <https://www.rfc-editor.org/rfc/rfc2104>.

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   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <https://www.rfc-editor.org/rfc/rfc5116>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [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>.

   [SHS]      "Secure hash standard", National Institute of Standards
              and Technology (U.S.), DOI 10.6028/nist.fips.180-4, 2015,
              <https://doi.org/10.6028/nist.fips.180-4>.

4.2.  Informative References

   [I-D.ietf-lamps-dilithium-certificates]
              Massimo, J., Kampanakis, P., Turner, S., and B.
              Westerbaan, "Internet X.509 Public Key Infrastructure -
              Algorithm Identifiers for the Module-Lattice-Based Digital
              Signature Algorithm (ML-DSA)", Work in Progress, Internet-
              Draft, draft-ietf-lamps-dilithium-certificates-13, 30
              September 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-lamps-dilithium-certificates-13>.

   [I-D.irtf-cfrg-hybrid-kems]
              Connolly, D., Barnes, R., and P. Grubbs, "Hybrid PQ/T Key
              Encapsulation Mechanisms", Work in Progress, Internet-
              Draft, draft-irtf-cfrg-hybrid-kems-07, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              hybrid-kems-07>.

Acknowledgments

   This work would not be possible without the hard work of the CFRG
   Hybrid KEM design team: Aron Wussler, Bas Westerbaan, Deirdre
   Connolly, Mike Ounsworth, Nick Sullivan, and Stephen Farrell.  Thanks
   also to Joël Alwen, Marta Mularczyk, and Britta Hale.

Authors' Addresses

   Rohan Mahy
   Email: rohan.ietf@gmail.com

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   Richard L. Barnes
   Cisco
   Email: rlb@ipv.sx

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