Re: [MLS] More clarity around validating proposals

[Richard Barnes <[e-mail]>]

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:
>
>    1. The qualifier “MUST”
>    2. 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:
>
>    1. 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.“
>    2. 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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