Finally! After two days of work, I've managed to implement enough of MLS (Messaging Layer Security) to be able to create a new group with a single member, invite another member, then exchange an encrypted message.

I've worked on MLS two years ago, and at the time I got almost all of the test vectors working. This week-end I've fixed the remaining tests, implemented logic required to be an active client (generating groups and various messages), and wired everything up together.