It occurs to me that perhaps I have been missing a point here when people argue that having a "man in the middle" is not that different from various forms of secure communication, such as where Bob has multiple personalities or is a committee. I have been taking this to mean that we should therefore not worry about MITM attacks, which seems crazy to me. Instead perhaps this was meant as a "reductio ad absurdum" argument for why MITM attacks cannot be prevented in the scenario where people have no out-of-band contact. Anything which could detect and prevent MITM attacks could, by this analogy, detect whether Bob had multiple personalities. Since the latter is obviously impossible, the former must be as well. Hence the problem has no solution and we should not waste much time on it. I don't fully agree with this but at least it is not as bizarre as the first interpretation. Hal