Re: [MLS] Message ordering

Key management or, let’s call it group sate change, usually happens less frequently than the messaging itself. It can also be performed as a background task, affecting user experience much less.

There could also be some quotas on key rotation depending on the group size. 

Suppose we have a policy that that every member can rotate his key only once per 24 hours.
In case of evenly distributed group It would allow a group of ~40k people have state update about every 2 seconds which will reduce the waiting time for newcomers and reduce the possible merge conflicts 

From: MLS <mls-bounces@ietf.org> On Behalf Of Eric Rescorla
Sent: Wednesday, May 30, 2018 1:57 AM
To: Karthikeyan Bhargavan <karthik.bhargavan@gmail.com>
Cc: Alexey Ermishkin <scratch@virgilsecurity.com>; mls@ietf.org
Subject: Re: [MLS] Message ordering

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 <mailto: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 <mailto:MLS@ietf.org> 
> https://www.ietf.org/mailman/listinfo/mls

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