  This paper shall investigate Yuen protocol, so called Y-00, which can realize
a randomized stream cipher with high bit rate(Gbps) for long distance(several
hundreds km). The randomized stream cipher with randomization by quantum noise
based on Y-00 is called quantum stream cipher in this paper, and it may have
security against known plaintext attacks which has no analog with any
conventional symmetric key ciphers. We present a simple cryptanalysis based on
an attacker's heterodyne measurement and the quantum unambiguous measurement to
make clear the strength of Y-00 in real communication. In addition, we give a
design for the implementation of an intensity modulation scheme and report the
experimental demonstration of 1 Gbps quantum stream cipher through 20 km long
transmission line.
