Re: [MLS] Syntax and mechanics for external commit

Raphael Robert <> Wed, 07 October 2020 12:57 UTC

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From: Raphael Robert <>
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Date: Wed, 7 Oct 2020 14:57:26 +0200
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To: Joel Alwen <>
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Subject: Re: [MLS] Syntax and mechanics for external commit
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I think that’s exactly the motivation we need for inline proposals, this surplus of signatures is not specific to this use case!


> On 7 Oct 2020, at 13:27, Joel Alwen <> wrote:
> On 7 Oct 2020, at 03:20, Richard Barnes <> wrote:
>> Assume for the moment that we are not going to do the above asymmetric
>> calculation for every commit.  Then we need some extra, optional syntax  to
>> carry `enc`, either as an optional field on Commit or as a new Proposal,
>> which is the agreed mechanism for extending Commits.  (If we do it every
>> time, we can just make this part of Commit.)  In the below, I’ll assume a
>> new Proposal, say ExternalInitSecret.
> Isn't it a bit redundant to have the External party prepare a full
> (ExternInitSecret) proposal packet only to then immediately commit to it? That
> means a full extra frame of bandwidth, an extra signature for the external party
> and an extra sig verification for group members.
> I'm wondering what the motivation is for making this an explicit proposal
> instead of, say, a second mode for commits. (E.g. a commit uses init_secret[n]
> iff no kem_output field is included in the commit packet. Otherwise it uses the
> "external_init_secret" computed as Richard described.)
> ATM I can't think of a scenario where we wouldnt want the external committer to
> also be the one creating the ExternalInitSecret proposal and immediatly
> committing to it...
> - Joël
> On 07/10/2020 12:36, Raphael Robert wrote:
>> Thanks Richard for looking at all aspects in detail!
>> I thought about these things as well and will comment inline:
>>> On 7 Oct 2020, at 03:20, Richard Barnes <> wrote:
>>> Hey all,
>>> I wanted to send some thoughts on how to implement external commit, as a
>>> prelude to a PR.  This is a little bit of an essay, so tl;dr, the proposal
>>> is:
>>> - Rather than re-using Proposal/Commit, we should make a new ExternalCommit
>>> message, parallel to Proposal/Commit
>> I think we are better off with re-using the current Commit syntax and I’ll
>> explain why further below.
>>> - We should also define a syntax for telling the joiner the requisite
>>> information about the group
>>> # HPKE-based init secret
>>> The concept here is as follows:
>>> - An HPKE/KEM key pair `(skG, pkG)` is derived off of the key schedule for
>>> each epoch - The public key `pkG` of that key pair is published along with
>>> with other group metadata - The joiner calls SetupBaseS(pkG,
>>> some_public_group_context) to get an encapsulated key `enc` and an HPKE
>>> context `ctx` - The joiner sends the encapsulated key to the group with
>>> their external commit - The members of the group call SetupBaseS(enc, skG,
>>> some_public_group_context) to get an equivalent HPKE context `ctx` -
>>> Everyone calls `ctx.export(MLS_export_label, init_secret_size)` to derive
>>> the init secret
>>> So there are two syntactic requirements:
>>> 1. Publishing the group’s public key `pkG` 2. Sending the encapsulated key
>>> `enc` to the group
>> From the discussion at the interim, I think there is consensus about this
>> part, we just need to add that to the PR.
>>> # What Proposals?
>>> The current PR correctly requires that the external Commit MUST cover an
>>> Add proposal for the new member.   It does not forbid the Commit covering
>>> *other* proposals.  It seems like it might be useful in a couple of cases
>>> to keep that option open:
>> * Including PSK proposals for additional
>>> authentication when joining * Including a Remove proposal for your prior
>>> appearance when re-joining
>>> The only current proposal that would be nonsensical is an Update.
>>> Whether we do this has some impact on the syntax, as discussed below.
>> While that is not very explicit in the current PR, my approach was the
>> following:
>> All Proposals should be allowed in an external Commit:
>> - Adds: The joiner (new member) could right away add more members, or Commit
>> to already existing Add Proposals if those are accessible. - Removes: The
>> joiner can remove prior appearances of itself (like you pointed out) or
>> Commit to already existing Remove Proposals. - Updates: The joiner should of
>> course not issue own Updates and Commit to them, but that is already the case
>> for internal Commits. The joiner can Commit to already existing Update
>> Proposals from other members.
>> Whether all of the above is allowed should only be governed by the policy for
>> a group, nothing else. As a reminder: all clients MUST have the same policy
>> for a certain group in order ta validate/refute Proposals and Commits.
>> This pretty much motivates my idea for fully re-using the existing Commit
>> syntax and only introduce the new ExternalInitSecret Proposal.
>>> # External commit syntax: Separate or Together
>>> Assume for the moment that we are not going to do the above asymmetric
>>> calculation for every commit.  Then we need some extra, optional syntax  to
>>> carry `enc`, either as an optional field on Commit or as a new Proposal,
>>> which is the agreed mechanism for extending Commits.  (If we do it every
>>> time, we can just make this part of Commit.)  In the below, I’ll assume a
>>> new Proposal, say ExternalInitSecret.
>>> struct { opaque kem_output<0..2^16-1>; } ExternalInitSecret;
>> This is exactly the Proposal we need.
>>> Given the requirement for an Add proposal, the joiner now has to send a
>>> “flight of messages”:
>>> - Proposal(Add) - Proposal(ExternalInitSecret) - Commit
>> I see some more nuance here. The first Add Proposal does not have to be
>> issued by the new joiner, it could very well be an external Proposal. The
>> scenario I had in mind here is the following:
>> A server issues an external Add Proposal for a group. The following things
>> can happen:
>> a) Ideal scenario:
>> - A member of the group comes online, validates the Add Proposal according to
>> the policy and references it in an internal Commit (no External Commit
>> needed) and sends a Welcome message to the new member
>> b) Equally ideal scenario:
>> - The new joiner comes online before anyone else, has access to the public
>> group data but does not need to communicate with the group. Nothing happens.
>> c) Emergency scenario:
>> - The new joiner comes online before anyone else and needs to urgently send a
>> message to the group. The new joiner creates the ExternalInitSecret Proposal
>> and the Commit and sends both to group.
>>> Let’s call this the Separate Option.  It’s a bit heavyweight, since each of
>>> these is signed separately.  It’s duplicative, since the KeyPackage in the
>>> Add is immediately overwritten by the (necessarily different) KP in the
>>> Commit.  And you have potential fate-sharing issues, since all three need
>>> to succeed or fail.
>>> You could also envision a Together Option, where we define another
>>> top-level content type (parallel to Proposal and Commit) for this purpose:
>>> struct { opaque kem_output<0..2^16-1>; UpdatePath path; } ExternalCommit;
>>> That would avoid all of the challenges above, but it optimizes out all of
>>> the flexibility to include other proposals.  So maybe it’s worth
>>> considering an Extensible Together Option, where we can put extra proposals
>>> into an ExternalCommit
>>> struct { Proposal proposals<0..2^32-1>; opaque kem_output<0..2^16-1>; 
>>> UpdatePath path; } ExternalCommit;
>>> Personally, I kind of like the Flexible Together Option, since it provides
>>> simplicity and extensibility.  And to be honest, I’ve been wondering if we
>>> should allow inline proposals in Commit for a while, along just these
>>> lines.  If we do this option, we should probably back-port it to Commit as
>>> well.
>> As mentioned above, I’m all for re-using the existing Commit syntax because
>> of clarity, simplicity and flexibility.
>> I do agree that the amount of signatures is sub-optimal, and this is
>> something that also occurs in other situations. For example, when a member
>> wants to add n new members at once to the group, it needs to compute n+1
>> signatures for that. You mention the idea of inline proposals: Would that be
>> a proposal that doesn’t have a signature, but still all other information? If
>> so, I think it would be worthwhile looking at that separately, because as you
>> say, it could be back-ported to internal Commits as well. I’m all for
>> exploring that idea further.
>>> # Syntax for what the Joiner Needs
>>> The PR notes that the joiner needs to know a bunch of information about the
>>> group in order to make a well-formed ExternalCommit.  In earlier iterations
>>> of this style of join, we had a GroupInitKey that carried the right
>>> information. Following that pattern here, we get something like the
>>> following:
>>> struct { CipherSuite cipher_suite; opaque group_id<0..255>; uint64 epoch; 
>>> opaque tree_hash<0..255>; opaque confirmed_transcript_hash<0..255>; 
>>> Extension extensions<0..2^32-1>; } GroupKeyPackage;
>> This is what the PR currently says, except that the PR used GroupContext and
>> has the full public tree. I agree that the tree hash should be enough and
>> I’ll change the PR accordingly.
>>> Note that this object is essentially the same as a GroupInfo object.  The
>>> only things it is missing are interim_transcript_hash and signature, both
>>> of which might be useful.  So maybe all we need to do here is say that
>>> clients can publish GroupInfo unencrypted if they want to enable self-adds,
>>> in addition to distributing it in encrypted form in Welcome.
>> While the two are awfully similar, the signature on GroupInfo is not required
>> here and might have undesired effects w.r.t deniability if this is publicly
>> accessible. The other reason for not wanting the signature is that it would
>> be an additional signature to compute with every Commit, making them even
>> more expensive. I propose to keep the struct as-is.
>>> In any case, it seems like it would be useful to have some syntax for
>>> this.
>>> Hope this helps, —RLB
>> TL;DR:
>> I propose the following:
>> - Keep the current Commit syntax - Allow all kinds of Proposals for External
>> Commits, same as with internal Commits - Do the HPKE-based init secret for
>> External Commits - Introduce the ExternalInitSecret Proposal and make it
>> mandatory for External Commits - Explore the idea of inline proposals
>> separately - Do not re-use signed GroupInfo struct and keep the adjusted
>> ExternalCommitInfo instead
>> Raphael
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