  In 2000, an attractive new quantum cryptography was discovered by H.P.Yuen
based on quantum communication theory. It is applicable to direct encryption,
for example quantum stream cipher based on Yuen protocol(Y-00), with high
speeds and for long distance by sophisticated optical devices which can work
under the average photon number per signal light pulse:$<n> = 1000 \sim 10000$.
In addition, it may provide information-theoretic security against known/chosen
plaintext attack, which has no classical analogue. That is, one can provide
secure communication, even the system has $H(K) << H(X)$.
  In this paper, first, we give a brief review on the general logic of Yuen's
theory. Then, we show concrete security analysis of quantum stream cipher to
quantum individual measurement attacks. Especially by showing the analysis of
Lo-Ko known plaintext attack, the feature of Y-00 is clarified. In addition, we
give a simple experimental result on the advantage distillation by scheme
consisting of intensity modulation/direct detection optical communication.
