  The fundamental security and efficiency considerations for fresh key
generation will be described. It is shown that the attacker's optimal
probability of finding the generated key is an indispensable measure of
security and that this probability limits the possibility of privacy
amplification and the amount of fresh key that can be generated. A new approach
to quantum cryptography to be called KCQ, keyed communication in quantum noise,
is developed on the basis of quantum detection and communication theory for
classical information transmission. KCQ key generation schemes with coherent
states of considerable energy will be described. The possibility of fresh key
generation is demonstrated for binary and N-ary detection systems under
heterodyne attacks. The security issues of these schemes will be discussed and
compared with BB84. The emphasis throughout is on concrete finite bit-length
protocols.
