[MLS] confirming cipher suites decisions

Sean Turner <sean@sn3rd.com> Thu, 06 February 2020 16:08 UTC

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From: Sean Turner <sean@sn3rd.com>
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Date: Thu, 6 Feb 2020 17:08:00 +0100
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Subject: [MLS] confirming cipher suites decisions
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Hi!

tl;dr: confirming MTI suite selections and rationale for avoiding proliferation

During the F2F Interim in January, the WG discussed cipher suites-related issues. Namely, whether a per-group signature scheme should be driven by the chosen cipher suite, what were the MTI (Mandatory To Implement) cipher suites, and what the actual algorithm should be.

There was rough agreement that there should be one signature scheme per group and that should be driven by the cipher suite. There are, at least, three things to consider: 1) if a potential group member does not support the algorithm, then they will not become a member or the group will need to downgrade; 2) when the group needs/wants to update, it is a flag day; and, 3) the cipher suites will have a similar combinatorial issues as the TLS cipher suites prior to TLS 1.3. The agreement was “rough” because 1) likely has some important implications.

The MLS cipher suites defined were as follows: 
- MLS10_128_HPKEX25519_AES128GCM_SHA256_Ed25519
- MLS10_128_HPKEP256_AES128GCM_SHA256_P256
- MLS10_128_HPKEX25519_CHACHA20POLY1305_SHA256_Ed25519
- MLS10_256_HPKEX448_AES256GCM_SHA384_Ed448
- MLS10_256_HPKEP521_AES256GCM_SHA384_P521
- MLS10_256_HPKEX448_CHACHA20POLY1305_SHA384_Ed448

At the interim, the consensus was to make the non-NIST suites the MTI.  The rationale was that those implementation that need to be NIST compliant will do so regardless of the choice made by the WG.

In looking at the actual cipher suites, it was noted that the 256-bit schemes the SHA should be SHA-512. The rationale agreed was that SHA-384 is SHA-512 cut in half, so just do SHA-512 because it is one less operation.

To avoid the proliferation of cipher suites, guidance will be provided to be conservative about allocating new code points. The consensus at the interim was that the suites provided were minimal and provided good coverage for the known use cases:
- (X25519, AES-GCM, Ed25519) - Good for desktop
- (P-256, AES-GCM, P-256) - Compliance
- (X25519, ChachaPoly, Ed25519) - Good for mobile

The chairs need to confirm the interim’s consensus on list, so please let the WG know by 2359 UTC 20 February whether you disagree with these choices and why.

NOTE: The final text will obviously be reviewed, but is being composed as part of the following PR:
https://github.com/mlswg/mls-protocol/pull/279

NOTE: We combined these cipher suite related consensus points, but if we only come to consensus on some of these we can still incorporate what we do agree on.

Cheers,

Nick and Sean