  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. By the use of a
shared secret key that determines the quantum states generated for different
data bit sequences, the users may employ the corresponding optimum quantum
measurement to decode the data. This gives them a better error performance than
an attacker who does not know the key when she makes her quantum measurement,
and an overall generation of a fresh key may be obtained from the resulting
advantage. This principle is illustrated in the operation of a concrete qubit
system A general information-theoretic description of the overall approach will
be presented, and contrasted with the detection/coding description necessary
for specific protocols.
