Well, you'll need an independent channel to do this, which also needs to be resistant to MITM. (In-person works, even though it's bothersome - but you only need

Well, you'll need an independent channel to do this, which also needs to be resistant to MITM. (In-person works, even though it's bothersome - but you only need to establish this once) As long as it's a 1:1 chat, there's not much else to it - of course, great complexity comes to multi-user chats.