Just to point out, though, this is not foolproof. A good impressionist can fool people, especially if they are extremely skilled.

Perhaps. But if it's someone you know well, the imposter may have a hard time passing that particular Turing Test. For example, Jeff Schiller called me the other night, nominally to compare our RSA public keys before signing, but we ended up chewing the fat for nearly an hour. It would be hard for an imposter to duplicate that feat without arousing my suspicion. Another (somewhat more likely) possibility is that the NSA or FBI might be holding a gun to the guy's head when you call him up to verify the key you got with his name on it. Perhaps we need "duress" hash codes. :-) Phil