Unconditionally secure two-party bit commitment based solely on the
principles of quantum mechanics (without exploiting special relativistic
signalling constraints, or principles of general relativity or thermodynamics)
has been shown to be impossible, but the claim is repeatedly challenged. The
quantum bit commitment theorem is reviewed here and the central conceptual
point, that an `Einstein-Podolsky-Rosen' attack or cheating strategy can always
be applied, is clarified. The question of whether following such a cheating
strategy can ever be disadvantageous to the cheater is considered and answered
in the negative. There is, indeed, no loophole in the theorem.
