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Re: [MLS] Message ordering

Eric Rescorla <ekr@rtfm.com> Tue, 29 May 2018 20:57 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Tue, 29 May 2018 13:56:40 -0700
Message-ID: <CABcZeBMeoCX2pUbqLgoZhsJ4A0_yhOmeYbPQYA2+VxVdNsv3yw@mail.gmail.com>
To: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
Cc: Alexey Ermishkin <scratch@virgilsecurity.com>, mls@ietf.org
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Subject: Re: [MLS] Message ordering
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The difficulty is that key management also depends on clear ordering.

-Ekr


On Tue, May 29, 2018 at 1:53 PM, Karthikeyan Bhargavan <
karthik.bhargavan@gmail.com> wrote:

> > 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
> >
> >
> > _______________________________________________
> > MLS mailing list
> > MLS@ietf.org
> > https://www.ietf.org/mailman/listinfo/mls
>
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