  We provide a complete proof of the security of quantum cryptography against
any eavesdropping attack including coherent measurements even in the presence
of noise. Polarization-based cryptographic schemes are shown to be equivalent
to EPR-based schemes. We also show that the performance of a noisy channel
approaches that of a noiseless one as the error rate tends to zero. (i.e., the
secrecy capacity $C_s (\epsilon) \to C_s (0)$ as $\epsilon \to 0$.) One
implication of our results is that one can {\it double} the efficiency of a
most well-known quantum cryptographic scheme proposed by Bennett and Brassard
simply by assigning vastly different probabilities to the two conjugate bases.
