  Bit commitment involves the submission of evidence from one party to another
so that the evidence can be used to confirm a later revealed bit value by the
first party, while the second party cannot determine the bit value from the
evidence alone. It is widely believed that secure quantum bit commitment is
impossible due to quantum entanglement cheating, which is codified in a general
impossibility theorem. In this paper, the scope of this general impossibility
proof is extended and analyzed, and gaps are found. Three specific protocols
are described for which the entanglement cheating as given in the impossibility
proof fails to work. One of these protocols, QBC2, is proved to be
unconditionally secure.
