  For single-photon quantum key generation between two users, it is shown that
the use of a shared secret key extended via a pseudo-random number generator
may simultaneously enhance the security and efficiency of the cryptosystem.
This effect arises from the intrinsic performance difference between quantum
detectors with versus without knowledge of the key, a purely quantum effect and
a new principle for key generation. No intrusion level estimation is needed and
the method is directly applicable to realistic systems with imperfections.
Quantum direct encryption is also made possible by such use of a secret key.
