This is basically what OTR messaging does. But you're still vulnerable to man-in-the-middle attacks when using DH, unless you're already on a secure channel. An

This is basically what OTR messaging does. But you're still vulnerable to man-in-the-middle attacks when using DH, unless you're already on a secure channel. And the most common solution to authentication is SSL/TLS, which the defeats the purpose of the whole thing. So we need to utilize other channels, like key-signing parties. Or, I suppose you can communicate using something like SRP to avoid this issue, though it sounds like a complicated layering of crypto that is destined to be screwed up at some point. However, this is what ZRTP does, and it seems to be pretty widely adopted.