This is easy to implement using cryptography. E.g. when firechat assigns a username, they sign your certificate for said username using their key. Then, when s

This is easy to implement using cryptography. E.g. when firechat assigns a username, they sign your certificate for said username using their key. Then, when sending messages, they have to be signed with your key (or outright encrypted), and the receiver verifies the key is correctly signed using firechat's key.