 It turns out that unconditionally secure two-party bit commitment, based solely on the principles of quantum or classical mechanics (without exploiting special relativistic signaling constraints, or principles of general relativity or thermodynamics) is impossible. See Mayers 1997, Lo and Chau 1997 and Lo’s article “Quantum Cryptology” in Lo, Popescu, and Spiller 1998 for further discussion. (Kent 1999 has shown that one can implement a secure classical bit commitment protocol by exploiting relativistic signaling constraints in a timed sequence of communications between verifiably separated sites for both Alice and Bob.) Roughly, the impossibility arises because at any step in the protocol where either Alice or Bob is required to make a determinate choice (perform a measurement on a particle in the quantum channel, choose randomly and perhaps conditionally between a set of alternative actions to be implemented on the particle in the quantum channel, etc.), the choice can delayed by entangling one or more ancilla particles with the channel particle in an appropriate way. By suitable operations on the ancillas, the channel particle can be ‘steered’ so that this cheating strategy is undetectable. In effect, if Bob can obtain no information about the committed bit, then entanglement will allow Alice to ‘steer’ the bit to either 0 or 1 at will.