@tinker
Wouldn't it be sufficient if profiles offered to enter xmpp and/or matrix ids? Instances could possibly offer a simple #xmpp server like #prosody. I don't see any other valid #e2e solution for any #federation compatible protocol except using gpg client sided like with email.

Maybe one should discuss to remove any "direct" or "private" message features from protocol in regard to the very common misunderstandings linked 2 those terms?
@facts_the_alt