ML-KEM and Hybrid Cipher Suites for Messaging Layer Security
draft-mahy-mls-pq-00

MLS                                                              R. Mahy
Internet-Draft                            Rohan Mahy Consulting Services
Intended status: Informational                              R. L. Barnes
Expires: 3 September 2025                                          Cisco
                                                            2 March 2025

      ML-KEM and Hybrid Cipher Suites for Messaging Layer Security
                          draft-mahy-mls-pq-00

Abstract

   This document reigsters 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.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-mahy-mls-pq/.

   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/rohanmahy/mahy-mls-xwing/.

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.  HPKE KDF Identifiers  . . . . . . . . . . . . . . . . . .   3
     2.2.  MLS Cipher Suites . . . . . . . . . . . . . . . . . . . .   4
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   4.  Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .   6
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   6

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 KEM and signature, 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 traditional signature
   algorithms.  These cipher suites address the risk of "harvest now,
   decrypt later" attacks, while not taking on the additional cost of
   post-quantum signatures.

   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 (Medium security, Non-NIST, PQ/T hybrid)

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

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

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

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

   For the PQ/T hybrid cipher suites, we use the KEM combinators defined
   in [I-D.connolly-cfrg-xwing-kem] and [I-D.irtf-cfrg-hybrid-kems].
   For the pure-PQ cipher suites, we use the HPKE integration for ML-KEM
   defined in [I-D.connolly-cfrg-hpke-mlkem].

2.  IANA Considerations

2.1.  HPKE KDF Identifiers

   This document requests that IANA add the following entry to the HPKE
   KDF Identifiers registry.

   *  Value: TBD0

   *  KDF: XOF(SHAKE256)

   *  Nh: 32

   *  Reference: Section 3.2 of [FIPS202]

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2.2.  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_X_Wing_AES256GCM_SHA384_Ed25519   |Y  | RFCXXXX   |
    +-----+-------------------------------------------+---+-----------+
    |TBD2 | MLS_128_QSF-KEM(ML-KEM-                   |Y  | RFCXXXX   |
    |     | 768,P-256)_AES256GCM_SHA384_P256          |   |           |
    +-----+-------------------------------------------+---+-----------+
    |TBD3 | MLS_192_QSF-KEM(ML-KEM-                   |Y  | RFCXXXX   |
    |     | 1024,P-384)_AES256GCM_SHA384_P384         |   |           |
    +-----+-------------------------------------------+---+-----------+
    |TBD4 | MLS_128_ML_KEM_768_AES256GCM_SHA384_P256  |Y  | RFCXXXX   |
    +-----+-------------------------------------------+---+-----------+
    |TBD5 | MLS_192_ML_KEM_1024_AES256GCM_SHA384_P384 |Y  | RFCXXXX   |
    +-----+-------------------------------------------+---+-----------+

                                  Table 1

   All of these cipher suites use HMAC [RFC2104] with SHA384 as their
   MAC function.  The mapping of cipher suites to HPKE primitives
   [RFC9180], HMAC hash functions, and TLS signature schemes [RFC8446]
   is as follows:

    +========+========+====+========+========+========================+
    | Value  | KEM    |KDF | AEAD   | Hash   | Signature              |
    +========+========+====+========+========+========================+
    | 0xTBD1 | 0x647a |TBD0| 0x0002 | SHA384 | ed25519                |
    +--------+--------+----+--------+--------+------------------------+
    | 0xTBD2 | TBDH0  |TBD0| 0x0002 | SHA384 | ecdsa_secp256r1_sha256 |
    +--------+--------+----+--------+--------+------------------------+
    | 0xTBD3 | TBDH1  |TBD0| 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
    +--------+--------+----+--------+--------+------------------------+
    | 0xTBD4 | 0x0041 |TBD0| 0x0002 | SHA384 | ecdsa_secp256r1_sha256 |
    +--------+--------+----+--------+--------+------------------------+
    | 0xTBD5 | 0x0042 |TBD0| 0x0002 | SHA384 | ecdsa_secp384r1_sha384 |
    +--------+--------+----+--------+--------+------------------------+

                                  Table 2

   The values TBDH0 and TBDH1 refer to the code points to be assigned by
   IANA for the following hybrid KEMs defined in
   [I-D.irtf-cfrg-hybrid-kems]:

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   *  TBDH0 = QSF-KEM(ML-KEM-768,P-256)-XOF(SHAKE256)-KDF(SHA3-256)

   *  TBDH1 = QSF-KEM(ML-KEM-1024,P-384)-XOF(SHAKE256)-KDF(SHA3-256)

   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

   This 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.  Cipher suites using
   post-quantum secure signature algorithms may be defined in the
   future.

   For security considerations related to the KEMs used in this
   document, please see the documents that define those KEMs
   [I-D.connolly-cfrg-xwing-kem] [I-D.irtf-cfrg-hybrid-kems]
   [I-D.connolly-cfrg-hpke-mlkem].

4.  Normative References

   [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>.

   [I-D.connolly-cfrg-hpke-mlkem]
              Connolly, D., "ML-KEM for HPKE", Work in Progress,
              Internet-Draft, draft-connolly-cfrg-hpke-mlkem-04, 18
              October 2024, <https://datatracker.ietf.org/doc/html/
              draft-connolly-cfrg-hpke-mlkem-04>.

   [I-D.connolly-cfrg-xwing-kem]
              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-
              06, 21 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-connolly-
              cfrg-xwing-kem-06>.

   [I-D.irtf-cfrg-hybrid-kems]
              Connolly, D., "Hybrid PQ/T Key Encapsulation Mechanisms",
              Work in Progress, Internet-Draft, draft-irtf-cfrg-hybrid-

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              kems-03, 25 February 2025,
              <https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-
              hybrid-kems-03>.

   [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>.

   [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>.

   [RFC9180]  Barnes, R., Bhargavan, K., Lipp, B., and C. Wood, "Hybrid
              Public Key Encryption", RFC 9180, DOI 10.17487/RFC9180,
              February 2022, <https://www.rfc-editor.org/rfc/rfc9180>.

   [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>.

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
   Rohan Mahy Consulting Services
   Email: rohan.ietf@gmail.com

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

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