  There had been well known claims of ``provably unbreakable'' quantum
protocols for bit commitment and coin tossing. However, we, and independently
Mayers, showed that all proposed quantum bit commitment (and therefore coin
tossing) schemes are, in principle, insecure because the sender, Alice, can
always cheat successfully by using an EPR-type of attack and delaying her
measurements. One might wonder if secure quantum bit commitment and coin
tossing protocols exist at all. Here we prove that an EPR-type of attack by
Alice will, in principle, break {\em any} realistic quantum bit commitment and
{\em ideal} coin tossing scheme. Therefore, provided that Alice has a quantum
computer and is capable of storing quantum signals for an arbitrary length of
time, all those schemes are insecure. Since bit commitment and coin tossing are
useful primitives 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.
