[MLS] AEAD data in messages

[Peter Slatala <[e-mail]>]

Hello!
I was wondering if you considered allowing additional plaintext but
authenticated data in MLS messages.

While I can't think of immediate, compelling use cases right now, I am
wondering if such extensibility wouldn't be desired. For example, in
encrypted video calls, resolution, framerate, or audio volume can be put in
plaintext so that the selective forwarding unit can decide which streams to
forward to the group members (and the recipient also uses this data).

Here are some use-cases for MLS that I can think of:
* sending a 'sending device identifier' in case if delivery service can't
differentiate different user devices from each other.
* sending 'message type' that server can act upon. For example, delivery
report sent by the recipient to the sender, which also acts as an ACK to
the server that the message was persisted.
* authenticating message id (but make it visible to server to avoid
redelivery),
* other use cases that I can't think of right now.

Have you considered supporting AEAD, or is it already supported and I
missed it?

Thanks,
Peter