Today I realised that every key exchange protocol I know, without a priori any shared information or trust relations (i.e. any ability to sign anything), is utterly broken by an active man in the middle attack.
I asked a professor of mine today whether there a proof of this in a formal setting and he said "yes, it's something information theoretic but I can't quite remember what..."
I looked in (what I believe to be) the relevant chapters of a couple of textbooks I have to hand, and done some google-ing and turned up nothing. I was wondering if someone could point me in the direction of either a paper or a text book containing such a proof. Thanks!