[MLS] MLSPlaintext packets aren't authenticated using symmetric key schedule

[Joel Alwen <[e-mail]>]

Hey everyone,

Something thats been bugging Marta Mularczyk and Daniel Jost and me for a bit
now is that handshake messages sent out as MLSPlaintext packets are only
authenticated using signatures, but not using the group's key schedule. For
non-members that makes sense but for group members that's weaker than need be.

Suppose Alice is in a group using signing key pair (spk, ssk). I corrupt her to
learn ssk. Now I loose access to her device again. Later she generates a fresh
key package with her same spk but a new HPKE key for her leaf. She sends out and
update proposal for her new key package and someone commits to the update.

Expected result: she (and the group at large) has achieved PCS again.

Actual result: using her stolen ssk I can still forge a new proposal's (sent as
MLSPlaintext packets) coming from Alice. Some things I could do with this power:
 - I can generate a new key package kp for Alice using her spk and some HPKE key
she doesn't know. Then I forge an update proposal for Alice with kp. If it gets
committed I've effectively kicked her out of the group.
 - I could forge Add's and Remove's coming from Alice, so I could trick the
group into thinking Alice is trying to Add my account to the group or remove
some other group member.

Lemme know if I've missed something here in that scenario...


If I didn't miss anything and the attacks really work as advertised then IMO
this is kinda weak sauce and worth avoiding if possible. So to that end, how
about we modify MLS such that MLSPlaintext packets coming from group members
must also be authenticated using something from the application key schedule.
Now the above attacks fail. As soon as Alice's update is gets committed I no
longer know the group's key schedule and so can't forged packet from Alice. More
generally, this brings the PCS guarantees when using MLSPlaintexts frameing in
line with what we're getting from MLSCiphertext packets.

Any thoughts?

- Joël



PS. For concreteness, we could probably extend the current mechanism for getting
concistancy (the confirmation_tag) to also provide symmetric key authentication.
E.g. include most of the MLSPlaintext content into whats being tagged by
confirmation_tag. That would cover the case of a commit packet and doesn't even
grow the size of MLSPlaintext packets over the current design.

For a proposal packet we could also have a confirmation_tag but this one is
computed using the *current* epoch's confirmation_key and confirmed_transcript_hash.