[metacorrector]

my explanation was correct, "common" (but not secret) means that both parties have it (each party also has a secret) and that is the key element to understand what is good about the protocol and what problems it solves. By seeming to correct me, you are actually confusing the people I just explained it to. You can if you wish go ahead and explain what limitations this puts on the protocol, but try to help people understand rather than just throwing confetti in the air.