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-OO) is equivalent to the classical cryptography they arc 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-OO has at least the security such as H(X/YE)=H(K/YE)=H(K), H(K/YE,X)~0 under the average photon number per signal light pulse:<n>~10000. Towards our final goal in this paper we clarify a role of classical randomness(secret key) and quantum randomness in Y-OO and give a rigorous quantum mechanical interpretation of the security showing an analysis of quantum collective attack.
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-OO) is equivalent to the classical cryptography they arc 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-OO has at least the security such as H(X/YE)=H(K/YE)=H(K), H(K/YE,X)~0 under the average photon number per signal light pulse:<n>~10000. Towards our final goal in this paper we clarify a role of classical randomness(secret key) and quantum randomness in Y-OO and give a rigorous quantum mechanical interpretation of the security showing an analysis of quantum collective attack.
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 ~ 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). This is a challenge to overcome Shannon limit in the cryptography theory. 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 scheme consisting of intensity modulation/direct detection optical communication.
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>=1000sim10000. 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.
The security in the next generation optical network which realizes "Cloud Computing System Service with data center" is one of the most important problems. In such a network, the encryption in physical layer which provide super security and small delay is a preferable. It has to be applicable, however, to very high speed data because the basic link is operated at 2.5 Gbit/sec ~ 10 Gbit/sec. The quantum stream cipher by Yuen-2000 protocol (Y00) is a completely new type of random cipher, which can exceed the Shannon limit of the symmetric key cipher. This paper extends some theoretical results on the security for quantum stream cipher such as Y00 protocol and generalized Y00. First, the conditions to exceed the Shannon limit are summarized. We formulate a generalized secret capacity in the sense of wire tap channel supported by secret key to clarify a cipher exceeding the Shannon limit. The generalized secret capacities for space communication and fiber communication based on the generalized Y00 are given. When the relaxation of physical constraint or device limit of the eavesdropper is allowed, we point out that a cipher scheme exceeding the Shannon limit can be realized only by the conventional optical system.
We investigate the Yuen 2000 (so-called Y-00)-protocol, which can realize a randomized stream cipher with high bit rate (Gbit/s) for long distances (several hundreds km). The randomized stream cipher with randomization by quantum noise based on the Y-00 protocol is called a 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 a quantum unambiguous measurement to make clear the strength of the Y-00 protocol in real communication. In addition, we give a design for the implementation of an intensity-modulation scheme and report an experimental demonstration of 1Gbit∕s quantum stream cipher through a $20\text{\ensuremath{-}}\mathrm{km}$-long transmission line.
This paper surveys a new way for research of quantum cryptography. The conventional cipher is designed by a mathematical algorithm and its security is evaluated by the complexity of the algorithm for the cryptanalysis and ability of computers. This kind of cipher cannot exceed the Shannon limit of cryptography, and it can be decrypted with probability one in principle by trying all the possible keys against the data length equal to the secret key length. A cipher based on quantum effect so called physical cipher may exceed the Shannon limit of the cryptography. The quantum stream cipher by �/� or Yuen-2000 protocol (Y- 00) which operates at Gbit/sec is a typical example of such a cipher, and may exceed the Shannon limit. In this paper, we show an existence of cipher which exceeds the Shannon limit, and discuss practical realization methods for them. First, the conditions to exceed the Shannon limit are summarized. Then we discuss the generalized secret capacity of the wiretap channel model supported by a secret key to discuss an existence of cipher exceeding the Shannon limit. The generalized secret capacities for space communication and fiber communication are given. In gen- eral, one needs quantum optimum receiver to realize such capacity. But when bandwidth limitation of users is deleted or the eavesdropper has a device limit, a cipher scheme exceeding the Shannon limit may be realized merely by the conventional optical communication devices which are affected by quantum noise at optical signal measurement.
In this talk, we present a design of keyed randomization, and an experiment for quantum stream cipher by Yuen protocol which can realize a randomized stream cipher with Gbps for long distance. A simple explanation of why Y-00 is more secure than any type of the conventional stream cipher is given, showing Y-00 with keyed randomization, for example, overlapping selection keying:OSK. That is, the noiseless Y-00 with OSK is equivalent of the conventional stream cipher, but the noise is inevitable. So one can easily see that the security is enhanced by the noise. In addition, we give a design for the implementation of the intensity modulation scheme and report the experimental demonstration of 1 Gbps quantum stream cipher with OSK randomization through 20~40 km long transmission line. This may provide a way to go toward the broad market applications which require security, high speed, compact size, maintenance free, and low price.