Re: [MLS] Group ID

Richard Barnes <rlb@ipv.sx> Wed, 05 June 2019 20:30 UTC

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From: Richard Barnes <rlb@ipv.sx>
Date: Wed, 5 Jun 2019 13:29:58 -0700
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To: Raphael Robert <raphael=40wire.com@dmarc.ietf.org>
Cc: Messaging Layer Security WG <mls@ietf.org>, Marek Jankowski <mjankowski309@gmail.com>
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Subject: Re: [MLS] Group ID
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Sorry, hit "send" prematurely.

On Wed, Jun 5, 2019 at 1:29 PM Richard Barnes <rlb@ipv.sx>; wrote:

> Like Raphael, I'm not concerned about the ordering requirement, since we
> already have an ordering requirement.  Note, however, that whatever
> identifier is used here will probably have to be stable within an epoch, so
> the DS will still be able to see traffic volumes within an epoch.  The DS
> may also be able to use time correlations to infer which opaque groupID
> values represent the same group.
>
> That still seems like it makes it harder for the DS to be malicious, so it
> seems worthwhile to me.  Benjamin Beurdouche had raised a similar idea
> earlier, of replacing the (groupID, epoch) pair with a single ID that is
> opaque to the DS.  I'm increasingly thinking that that's probably the way
> to go.  Marek / Benjamin, if you'd like to update PR#152 to do this, that
> could help move this along.
>

https://github.com/mlswg/mls-protocol/pull/152

I wonder if we still want some constant group ID value, even if it's known
> only to group members.  No concrete
>

... need is occurring to me right now, but it seems like there might be
some utility in separating contexts.

--Richard


>
>
> On Wed, Jun 5, 2019 at 1:00 AM Raphael Robert <raphael=
> 40wire.com@dmarc.ietf.org>; wrote:
>
>> Hi Marek,
>>
>> > I am aware it does not support reordering, But I'm sure one can figure
>> an alternative that does.
>>
>> We looked into doing exactly that. Server-side ordering is unfortunately
>> a hard requirement for TreeKEM, so this is an open issue. We’d be happy to
>> hear about a proposal that covers both problems!
>>
>> Raphael
>>
>> > On 11 Jun 2019, at 16:33, Marek Jankowski <mjankowski309@gmail.com>;
>> wrote:
>> >
>> > Hi,
>> > Regarding some issues that were raised in the last couple of months, I
>> wonder if we should hide the group ID from the DS. I think the group ID is
>> valuable metadata -- the DS may keep track of it and by that identify
>> sudden changes in the traffic, which may leak information.
>> > Please consider the following alternatives:
>> > group_id[n] = HKDF-Expand-Label(confirmation_key[n-1],group_id[n-1],
>> "group_id", 4)
>> > I am aware it does not support reordering, But I'm sure one can figure
>> an alternative that does.
>> >
>> > Marek.
>> > _______________________________________________
>> > MLS mailing list
>> > MLS@ietf.org
>> > https://www.ietf.org/mailman/listinfo/mls
>>
>> _______________________________________________
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>>
>