If you can make it possible for the user to verify identities in a way they can relate to, then you have also solved the key management problem. The identity problem is not just tough for messaging, it pops up any time an attempt is made to apply cryptographic identities to stuff in the real world.

I couldn't wish for a better example than SignalGate. So I wrote an article:

* https://articles.59.ca/doku.php?id=em:sg