Re: [MLS] Hiding content type

With respect to using an opaque epoch id, I believe this was proposed in
#245 <https://github.com/mlswg/mls-protocol/pull/245> and consensus was
against it because it makes it more difficult to implement the DS. It's
also not clear to me what security properties you want from an opaque epoch
id, because the current PR doesn't provide collision resistance and I'd
expect this to be a source of confusion and bugs.

With respect to encrypting the content type, it also seems to me that this
would cause issues because different content types have different delivery
guarantees. Specifically: messages are allowed to be unordered and lossy,
proposals are allowed to be unordered but not lossy, and commits are both
ordered and not lossy. In the deployment scenarios we've talked about, the
DS essentially always needs to know the content type to provide this.
Conversely, you don't get much additional privacy from encrypting the
content type because an eavesdropper can see the epoch id change and infer
which message was a commit, or see a dropped message getting re-sent and
infer it was a proposal.

On Mon, Jul 13, 2020 at 8:56 AM Richard Barnes <rlb@ipv.sx> wrote:

> Hi all,
>
> Recall that PR#349 does two things (1) use an opaque epoch ID, and (2)
> encrypt the content type of the message (application / proposal / commit)
> [1].
>
> There was some discussion on the call last week that encrypting the
> content type might not be that useful, since an application that relies on
> a server to assure ordering of commits will need to at least be able to
> tell commits from other things.  Of course, even if the content type is
> encrypted by default, the application can add it back in authenticated
> data, or in some unauthenticated wrapper.
>
> Net of those considerations, I'm personally still inclined to merge the
> PR, to have a conservative baseline.  But those on the call thought it
> would be useful to pose this to the group, so please speak up if you have
> concerns.
>
> --Richard
>
> [1] https://github.com/mlswg/mls-protocol/pull/349
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