[MLS] Recommendation for encrypted group operations

[Brendan McMillion <[e-mail]>]

Hi mls@

One of the first topics of discussion at our last interim was on whether or
not to keep the recommendation in the architecture draft to use encrypted
group operations whenever possible. This is issue #210 on the repo [1].

The desire to remove this recommendation comes from the fact that many mls
deployments and the mimi wg do not follow it. It doesn't seem that the
minutes have been uploaded yet, but my memory of the conversation at the
interim is that:
- We generally agree that encrypted group operations would, in an ideal
world, be preferred. I recall several people saying it's the "moral thing
to do."
- Acknowledging that most deployed applications don't encrypt group
operations. We listed the deployments we knew of to prove this.

The after-the-fact answer I came up with for why most applications don't
encrypt group operations is that they can 1.) guarantee strong
transport-layer security, 2.) don't care about leaking group membership to
a central server, and 3.) want to use that central server to provide
certain features. Despite being well-represented in the mls wg,
applications that meet criteria 1, 2 and 3 is actually quite a specific
subset within the space of what MLS was designed to support. Any
decentralized application would not meet these criteria (no transport-layer
encryption). Signal doesn't either (doesn't leak membership).

Given this, my proposal is to keep the recommendation but state that
applications may use unencrypted operations if they have an explicit
reason. I've opened a PR to that effect here:
https://github.com/mlswg/mls-architecture/pull/247

This is in contrast with a PR from Eric that removes the recommendation and
describes the tradeoffs between encrypted and unencrypted group operations
on relatively equal footing:
https://github.com/mlswg/mls-architecture/pull/246

Please say on the list if you have a preference between the PRs. Thank you!

1. https://github.com/mlswg/mls-architecture/issues/210