Re: [MLS] confirming cipher suites decisions

Dennis Jackson <dennis.jackson@cs.ox.ac.uk> Sat, 15 February 2020 12:27 UTC

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From: Dennis Jackson <dennis.jackson@cs.ox.ac.uk>
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Date: Sat, 15 Feb 2020 12:27:07 +0000
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Subject: Re: [MLS] confirming cipher suites decisions
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Hi Benjamin,

Signatures are not committing (in general) [1]. E.g. with Ed25519,
you can pick a public key such that you can produce a signature which is
valid for any message.

Best,
Dennis

[1] https://eprint.iacr.org/2019/779


On 15/02/2020 08:19, Benjamin Beurdouche wrote:
> Hi Paul,
> 
> Yes, thanks for reminding us of this. This is something we need to think
> about seriously indeed...
> 
> My intuition is that since we also have signatures we should be safe.
> But I agree that we have to evaluate carefully the differences between
> the committing and non-committing modes.
> 
> B.
> 
> 
>> On Feb 14, 2020, at 9:48 PM, Paul Grubbs <pag225@cornell.edu> wrote:
>>
>> 
>> Hey all, quick question - all the AE schemes used in these cipher
>> suites are non-committing (meaning they misbehave in weird ways when
>> keys may be adversarially known or chosen). Not including a cipher
>> suite which uses a committing AE (cAE) scheme may make reasoning about
>> the protocol's behavior difficult in some settings. Since the latency
>> increase of committing AE would not be terribly high (one such scheme
>> is AES128-CTR-then-HMAC-SHA384 with HKDF-derived encryption and
>> authentication keys, which is reasonably close to what Signal uses),
>> might the inclusion of a cAE cipher suite be worth discussing?
>>
>> On Thu, Feb 13, 2020 at 9:49 AM Sean Turner <sean@sn3rd.com
>> <mailto:sean@sn3rd.com>> wrote:
>>
>>
>>
>>     > On Feb 6, 2020, at 11:08, Sean Turner <sean@sn3rd.com
>>     <mailto:sean@sn3rd...com>> wrote:
>>     >
>>     > 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.
>>
>>     I finally got around to doing my homework related to PR279. My bit
>>     was the more procedural bits that instructs IANA to establish/use
>>     a Designated Expert pool:
>>     https://github.com/mlswg/mls-protocol/pull/307
>>
>>     spt
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