@Coffee XMPP handles security noninteractively. Signal requires interaction of the user once for each device they add. SSL is noninteractive.

Suppose you have 20 friends. Each has 2 devices, a computer and a phone. You sign out and then sign back in. Now each one of them must on each device verify you, and this requires you and them to interact which means you have to be available at the same time. That's 40 interactions! How is this useable?