Messaging Layer Security Ciphersuite using X25519Kyber768Draft00 Key Exchange Mechanism

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Table of Contents

1. Introduction

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.¶

2. Security Considerations

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.¶

3. IANA Considerations

This document registers a new MLS Ciphersuite value.¶

Value:     0x0030 (please)
Name:      MLS_128_X25519Kyber768Draft00_AES128GCM_SHA256_Ed25519
Required:  N
Reference: This document

Acknowledgments

Thanks to Joël Alwen, Marta Mularczyk, and Britta Hale.¶

Authors' Addresses