A quantum symmetric key cipher(Y-00) and key generation (Quantum stream   cipher-Part II)
  What obstructs the realization of useful quantum cryptography is single photon scheme, or entanglement which is not applicable to the current infrastructure of optical communication network. We are concerned with the following question: Can we realize the information theoretically secure symmetric key cipher under "the finite secret key" based on quantum-optical communications? A role of quantum information theory is to give an answer for such a question. As an answer for the question, a new quantum cryptography was proposed by H.P.Yuen, which can realize a secure symmetric key cipher with high speeds(Gbps) and for long distance(1000 Km). Although some researchers claim that Yuen protocol(Y-00) is equivalent to the classical cryptography, they are all mistaken. Indeed it has no classical analogue, and also provides a generalization even in the conventional cryptography.   At present, it is proved that a basic model of Y-00 has at least the security such as $H(X|Y_E)=H(K|Y_E)=H(K)$, $H(K|Y_E,X)\sim 0$ under the average photon number per signal light pulse:$<n> \sim 10000$. Towards our final goal, in this paper, we clarify a role of classical randomness(secret key) and quantum randomness in Y-00, and give a rigorous quantum mechanical interpretation of the security, showing an analysis of quantum collective attack. 
Osamu Hirota, Kentaro Kato, Masaki Sohma, Masaru Fuse