Messaging Layer Security Ciphersuite using X25519Kyber768Draft00 Key Exchange Mechanism

This document registers a new Messaging Layer Security (MLS) ciphersuite using the hybrid post-quantum resistant / traditional (PQ/T) Key Exchange Mechanism X25519Kyber768Draft00.¶
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-x25519kyber768draft00/.¶
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/rohan-wire/mls-x25519kyber768draft00/.¶
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2. Security Considerations
3. IANA Considerations
4. Normative References
This document reserves a Messaging Layer Security (MLS) [I-D.ietf-mls-protocol] ciphersuite value based on the MLS default ciphersuite, but replacing the KEM with the hybrid post-quantum / traditional Key Exchange Mechanism X25519Kyber768Draft00 [I-D.draft-westerbaan-cfrg-hpke-xyber768d00] which was assigned the Hybrid Public Key Encryption (HPKE) Key Exchange Mechanism (KEM) Identifier value 0x0030.¶
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.draft-westerbaan-cfrg-hpke-xyber768d00] apply.¶
This document registers a new MLS Ciphersuite value.¶
Thanks to Joël Alwen, Marta Mularczyk, and Britta Hale.¶