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Re: [MLS] Message ordering

Karthikeyan Bhargavan <karthik.bhargavan@gmail.com> Tue, 29 May 2018 20:54 UTC

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From: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
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Date: Tue, 29 May 2018 22:53:23 +0200
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To: Alexey Ermishkin <scratch@virgilsecurity.com>
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Subject: Re: [MLS] Message ordering
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> All messages (not only state change messages) must have a counter field
> which must be unique among all the messages and server must reject messages
> that have this field duplicated.

Good catch. I think this may be too strong a requirement. 
With the right design adjustments, we will probably find that per-sender counters are enough.

-Karthik


> This might be ok for a group of 3. But as MLS targets groups up to 50k
> users, I believe  the percent of rejected messages will dramatically
> increase and affect user experience.
> I know that ART or TreeKem is, in the end, supposed to be bound to double
> ratchet's KDF chain which have one sequence of message numbers per "epoch"
> but that clearly won't work for large groups where participants will
> constantly have to work on some "consensus" during communication.
> I believe there's better solution to this problem. Maybe we should consider
> making a unique KDF chain (prefix?) for each group member and perform
> timestamp-based ordering, I'm not sure.
> 
> Regards, 
> Alex
> 
> 
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> MLS@ietf.org
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