  There had been well known claims of unconditionally secure quantum protocols
for bit commitment. However, we, and independently Mayers, showed that all
proposed quantum bit commitment schemes are, in principle, insecure because the
sender, Alice, can almost always cheat successfully by using an
Einstein-Podolsky-Rosen (EPR) type of attack and delaying her measurements. One
might wonder if secure quantum bit commitment protocols exist at all. We answer
this question by showing that the same type of attack by Alice will, in
principle, break any bit commitment scheme. The cheating strategy generally
requires a quantum computer. We emphasize the generality of this ``no-go
theorem'': Unconditionally secure bit commitment schemes based on quantum
mechanics---fully quantum, classical or quantum but with measurements---are all
ruled out by this result. Since bit commitment is a useful primitive for
building up more sophisticated protocols such as zero-knowledge proofs, our
results cast very serious doubt on the security of quantum cryptography in the
so-called ``post-cold-war'' applications. We also show that ideal quantum coin
tossing is impossible because of the EPR attack. This no-go theorem for ideal
quantum coin tossing may help to shed some lights on the possibility of
non-ideal protocols.
