Messaging Layer Security Ciphersuite using XWing Key Exchange Mechanism

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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].¶

The MLS Working Group has expressed strong desire to have a handful of complimentary post-quantum security extensions for use with the MLS protocol to address the related threats:¶

1. A straightforward MLS cipher suite that replaces a classical KEM with a hybrid post-quantum/traditional KEM. Such a cipher suite could be implemented as a drop-in replacement in many MLS libraries without changes to any other part of the MLS stack. The aim is for implementations to have a single KEM which would be performant and work for the vast majority of implementations. It addresses the the harvest-now / decrypt-later threat model using the simplest, and most practicable solution available.¶

2. Versions of existing cipher suites that use post-quantum signatures; and specific guidelines on the construction, use, and validation of hybrid signatures.¶

3. One or more mechanisms which reduce the bandwidth or storage requirements, or improve performance when using post-quantum algorithms (for example by updating post-quantum keys less frequently than classical keys, or by sharing portions of post-quantum keys across a large number of clients or groups.)¶

This document addresses the first of these work items. It reserves an MLS cipher suite value based on the MLS default cipher suite, but replacing the KEM with the X-Wing [I-D.connolly-cfrg-xwing-kem] hybrid post-quantum / traditional KEM. The IANA Hybrid Public Key Encryption (HPKE) [RFC9180] Key Exchange Mechanism (KEM) Identifier value for X-Wing is pending at the time of this writing.¶

X-Wing is a "concrete, simple choice for [a] post-quantum hybrid KEM, that should be suitable for the vast majority of use cases". X-Wing combines the ML-KEM [MLKEM] post-quantum KEM and the X25519 [RFC7748] traditional KEM. The MLS cipher suite uses the other components of the default MLS cipher suite MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519.¶

This document replaces a previous, similar proposal based on the KEM in [I-D.draft-westerbaan-cfrg-hpke-xyber768d00].¶

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.connolly-cfrg-xwing-kem] apply.¶

3. IANA Considerations

This document registers a new MLS Ciphersuite value.¶

Value:        new assignment
Name:         MLS_128_XWING_AES128GCM_SHA256_Ed25519
Recommended:  N
Reference:    This document

Acknowledgments

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

Author's Address