Comment by h0cked

It's simple unauthenticated Diffie-Hellman key agreement, which is known for MITM attack. Yes, you ask A to accept B's identity upon key exchange, but to what extend A would know B is really B not the server playing along? A plausible method would have A and B exchange certificates separate from the Diffie-Hellman key exchange process, and use those as the identity verification mechanism.