ML-KEM and Hybrid Cipher Suites for Messaging Layer Security

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Table of Contents

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.¶

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 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 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 XOF(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].¶

2. IANA Considerations

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.ietf-hpke-pq] and [I-D.irtf-cfrg-hybrid-kems].¶

4. References

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