| Internet-Draft | MLS Cipher Suites with ML-KEM | March 2025 |
| Mahy & Barnes | Expires 3 September 2025 | [Page] |
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.¶
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/.¶
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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.¶
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].¶
This document requests that IANA add the following entry to the HPKE KDF Identifiers registry.¶
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-768,P-256)_AES256GCM_SHA384_P256 | Y | RFCXXXX |
| TBD3 | MLS_192_QSF-KEM(ML-KEM-1024,P-384)_AES256GCM_SHA384_P384 | Y | RFCXXXX |
| TBD4 | MLS_128_ML_KEM_768_AES256GCM_SHA384_P256 | Y | RFCXXXX |
| TBD5 | MLS_192_ML_KEM_1024_AES256GCM_SHA384_P384 | Y | RFCXXXX |
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 |
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]:¶
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].¶
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].¶
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.¶