Re: [MLS] Syntax and mechanics for external commit
Raphael Robert <raphael@wire.com> Wed, 07 October 2020 12:57 UTC
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From: Raphael Robert <raphael@wire.com>
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Date: Wed, 07 Oct 2020 14:57:26 +0200
Cc: mls@ietf.org
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To: Joel Alwen <jalwen@wickr.com>
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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! Raphael > On 7 Oct 2020, at 13:27, Joel Alwen <jalwen@wickr.com> wrote: > > On 7 Oct 2020, at 03:20, Richard Barnes <rlb@ipv.sx> 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 <rlb@ipv.sx> 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 >> >>> _______________________________________________ MLS mailing list >>> MLS@ietf.org https://www.ietf.org/mailman/listinfo/mls >> >> _______________________________________________ MLS mailing list >> MLS@ietf.org https://www.ietf.org/mailman/listinfo/mls >> > > _______________________________________________ > MLS mailing list > MLS@ietf.org > https://www.ietf.org/mailman/listinfo/mls
- [MLS] Syntax and mechanics for external commit Richard Barnes
- Re: [MLS] Syntax and mechanics for external commit Raphael Robert
- Re: [MLS] Syntax and mechanics for external commit Joel Alwen
- Re: [MLS] Syntax and mechanics for external commit Raphael Robert
- Re: [MLS] Syntax and mechanics for external commit Richard Barnes
- Re: [MLS] Syntax and mechanics for external commit Raphael Robert
- Re: [MLS] Syntax and mechanics for external commit Brendan McMillion
- [MLS] Inline proposals Richard Barnes
- Re: [MLS] Inline proposals Raphael Robert
- Re: [MLS] Inline proposals Richard Barnes
- Re: [MLS] Syntax and mechanics for external commit Richard Barnes
- Re: [MLS] Syntax and mechanics for external commit Raphael Robert
- Re: [MLS] Syntax and mechanics for external commit Joel Alwen
- Re: [MLS] Syntax and mechanics for external commit Raphael Robert
- Re: [MLS] Syntax and mechanics for external commit Joel Alwen
- Re: [MLS] Syntax and mechanics for external commit Raphael Robert