Re: [MLS] More clarity around validating proposals

[thomas leavy <[e-mail]>]

Hey Richard,

One thing I think we should be careful about is just ensuring that whatever the spec says that it is prescriptive enough as to not break basic interoperability. What I mean by basic interoperability is that if the same application wanted to use one MLS implementation for a web client, and another implementation for a native client that interoperability would be a success assuming that the application implemented each required customization point the same way on both clients. 

If we leave it too open where one MLS implementation allows filtering proposals on receive, and another only inside processing a commit, then it becomes likely that it is not possible to make multiple MLS implementations work the same way across even across multiple clients of same application. I think the RFC does a good job with this around credential validation, it is very explicit about when the application needs to review, and what should happen if that review fails. 

We would be specifying a flow for first processing the set of available proposals to fixed rules that are already defined, and then saying that the remainder of proposals should be given to the application for additional filtering before the commit is created. On the receiving side we would just be looking for the application to identify rule violations that should result in error given the proposals in the commit. 

Thanks,

- Tom

> On May 13, 2022, at 2:04 PM, Mularczyk Marta <[e-mail]> wrote:
> 
> 
> Hi Richard,
> 
> Thanks for the input! In fact, we’re all for downgrading MUSTs to SHOULDs. I think it still makes sense to collect all the rules / guidance from the RFC into a single section 13.4. It would contain:
> 
> Properties that a committed vector of proposals MUST satisfy. They are required for correctness or security of MLS.E.g.
> There MUST NOT be a proposal removing the committer
> ExternalInit MUST only be used in external commit
> The signature MUST verify
> Properties that a committed vector of proposals SHOULD satisfy, i.e., general guidance). E.g.
> There SHOULD be at most one proposal affecting a leaf
> 
> 
> > Though I have to say I don't love the idea of trying to implement the logic to build such a maximal list.
> 
> Very simple :) Say you have a proposal list L. To build a maximal valid list, start with an empty list LVal. Then for each P in L, do: if LVal with P appended is valid, append P to LVal. Else, continue.
> 
> Best,
> Marta
> 
> From: MLS <[e-mail]> on behalf of Richard Barnes <[e-mail]>
> Sent: Friday, May 13, 2022 5:05:28 PM
> To: Mularczyk Marta
> Cc: [e-mail]
> Subject: Re: [MLS] More clarity around validating proposals
>  
> Hi Marta,
> 
> Thanks for thinking about this stuff, these are good issues to clarify.  I agree with your analysis, and the conclusion that the current text is ambiguous.  Though I think some of the ambiguity may have been intentional, to allow for the sorts of policies you describe.
> 
> It seems like there's a prerequisite question as to whether we need to be prescriptive about these things.  One could imagine leaving the set of proposals committed up to the application.  Different choices by the application would have implications that would be worth discussing in the spec (e.g., the members could refuse to commit a Remove sent by an external proposer), but both the risks and the possibilities to mitigate them seem more specific to application deployment contexts, as opposed to core parts of the protocol.  Personally, I have a pretty strong preference for this option.
> 
> If we were going to do that, I think the major impact would just be to downgrade the MUSTs on this front to SHOULDs.  We should probably provide some notes of the general character you describe, but we wouldn't need to be quite so precise because we would just be providing guidance, not hard requirements.
> 
> If we're going to go down the path you suggest, the text changes you propose sound about right.  Though I have to say I don't love the idea of trying to implement the logic to build such a maximal list.  It might be worth noting that it's possible for there to be multiple lists that are maximal in this sense, e.g. if two proposals are mutually exclusive with each other but compatible with the others.
> 
> --Richard
>> On Thu, May 12, 2022 at 12:44 PM Mularczyk Marta <[e-mail]> wrote:
>> Hi all,
>> 
>> Me, Tom Leavy and Joël Alwen think that the text in the RFC around validating proposals, choosing which proposals go into a commit and how to validate a received commit could do with a bit more work.
>> 
>> The root issue is that Section 13.2 states “The sender of a Commit MUST include all valid proposals that it has received during the current epoch. Invalid proposals include, for example, proposals with an invalid signature or proposals that are semantically invalid, such as an Add when the sender does not have the application-level permission to add new users.” There are two intertwined issues with this text:
>> The qualifier “MUST”
>> The text is too vague about what a “valid Proposal” is. E.g. it could be interpreted to mean a Proposal is “valid” relative to the current group state: Can it be applied to the group as it stands now? Then it’s “valid”.
>> 
>> Put simply, it doesn’t make sense to only validate proposals individually. Instead one should validate if the full list of proposals can be committed. Indeed, other parts of the RFC enforce validation steps that apply to the full list. E.g.
>> A ReInit proposal MUST be the only one in the commit, and other existing proposals SHOULD be preferred (13.2.1).
>> Out of multiple proposals affecting the same leaf, the committer MUST choose an arbitrary one (13.2, “If there are multiple (…)”).
>> Out of multiple PSK proposals with the same ID, the committer MUST choose an arbitrary one (13.2).
>> 
>> In fact, once extensions come into play there are likely many more similar situations, even ones affecting lists of otherwise benign proposals (e.g. Add/Remove). Suppose a chat app is built on MLS with a custom room moderator roles extension. The app wants MLS to enforce two rules: Only mods can Add/Remove. Rooms should always have at least 1 Mod. Here’s how the RFC’s current text can become a problem: A and B are the only mods in a room. A makes proposal P1 that removes B. Meanwhile, B makes prop P2 that removes A. C gets both proposals. What should C do?
>> Option 1: validate & cache both proposals. → Problem: C can no longer issue a commit as it “MUST include all valid proposals”. (Alternatively, it does commit to both as instructed by the RFC which leaves the room without a mod.)
>> Option 2: validate & cache the first prop received (say, P1), but reject P2 as validating both means C can’t commit. → Problem: If someone else commits to P2 by ref. C can’t process the commit as it doesn’t “know” P2.
>> 
>> So, to clean all this up a bit more we are working on a PR with 2 main changes:
>> In 13.2, replace “The sender of a Commit MUST include all valid proposals that it has received“ by ”The sender of a Commit MUST commit to a maximal valid list of proposals. The conditions making a list of proposals 'valid' for an epoch are described in Section 13.4. A valid list is 'maximal' if adding any other received proposal would result in an invalid list.“
>> Add Section 13.4 which collects all rules for validating proposals (currently in 13.2, 13.1, 11.1, 8.3) into 3 procedures: validating an incoming proposal, validating a list of proposals on creating a commit and on receiving a commit.
>> 
>> We’d be happy to hear what people think about this!
>> 
>> Marta
>> 
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