Re: [MLS] Proposals for handling concurrent messages

> On 20 Mar 2019, at 15:23, Richard Barnes <rlb@ipv.sx> wrote:
> 
> I would be interested to hear folks' thoughts on whether the benefits of reduced synchronization would be worth the costs of limiting DH group selection (either old ECDH or new stuff without a lot of implementation).

On the Matrix side we are still very much hoping that MLS will somehow support reduced synchronisation, as otherwise the sequencing of membership changes acts as a very undesirable centralising force when communicating in otherwise decentralised groups. The below approach seems quite promising and from our perspective could merit picking curves which support it.

Matthew

>> On Tue, Mar 19, 2019 at 6:33 PM M.A.L. Weidner <malw2@cam.ac.uk> wrote:
>> Dear MLS Working Group,
>> 
>> I've attached a document that details some proposals for how to better handle concurrent messages; see mainly Section 2.
>> 
>> The basic idea is to use some operator * which "combines" key pairs.  E.g., for Diffie-Hellman keys, (x, g^x) * (y, g^y) = (x + y (mod |g|), g^x g^y).
>> 
>> With this operator, we can combine concurrent updates.  E.g., if a updates with x and b updates with y, then the tree changes by: (embedded image)
>> <causal_fig2.png>
>> With some extra effort, I believe we can even support concurrent Adds (Section 3.3).
>> 
>> My apologies if there are mistakes or if this has been discussed before.
>> 
>> Best,
>> Matthew Weidner
>> MPhil Student
>> Computer Laboratory, Cambridge
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