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Physical Review A

Practical security analysis of a quantum stream cipher by the Yuen 2000 protocol

Osamu Hirota

Published Sep 7, 2007• 67 citations
Publisher

Abstract

There exists a great gap between one-time pad with perfect secrecy and conventional mathematical encryption. The Yuen 2000 (Y00) protocol or $\ensuremath{\alpha}\ensuremath{\eta}$ scheme may provide a protocol which covers from the conventional security to the ultimate one, depending on implementations. This paper presents the complexity-theoretic security analysis on some models of the Y00 protocol with nonlinear pseudo-random-number-generator and quantum noise diffusion mapping (QDM). Algebraic attacks and fast correlation attacks are applied with a model of the Y00 protocol with nonlinear filtering like the Toyocrypt stream cipher as the running key generator, and it is shown that these attacks in principle do not work on such models even when the mapping between running key and quantum state signal is fixed. In addition, a security property of the Y00 protocol with QDM is clarified. Consequently, we show that the Y00 protocol has a potential which cannot be realized by conventional cryptography and that it goes beyond mathematical encryption with physical encryption.

DOI
10.1103/physreva.76.032307

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There exists a great gap between one-time pad with perfect secrecy and conventional mathematical encryption. The Yuen 2000 (Y00) protocol or \ensuremathα\ensuremathη\ensuremath{\alpha}\ensuremath{\eta}\ensuremathα\ensuremathη scheme may provide a protocol which covers from the conventional security to the ultimate one, depending on implementations. This paper presents the complexity-theoretic security analysis on some models of the Y00 protocol with nonlinear pseudo-random-number-generator and quantum noise diffusion mapping (QDM). Algebraic attacks and fast correlation attacks are applied with a model of the Y00 protocol with nonlinear filtering like the Toyocrypt stream cipher as the running key generator, and it is shown that these attacks in principle do not work on such models even when the mapping between running key and quantum state signal is fixed. In addition, a security property of the Y00 protocol with QDM is clarified. Consequently, we show that the Y00 protocol has a potential which cannot be realized by conventional cryptography and that it goes beyond mathematical encryption with physical encryption.
This paper presents the security analysis on the quantum stream cipher so called Yuen-2000 protocol (or αηαηαη scheme) against the fast correlation attack, the typical attack on stream ciphers. Although a very simple experimental model of the quantum stream cipher without a random mapper may be decrypted in the information theoretic sense by the fast correlation algorithm, it is not a basic feature of Yuen 2000 protocol. In fact, we clarify that there exists a randomization scheme which attains the perfect correlation immunity against such attacks under an approximation. And in this scheme, the running key correlation from the second randomization that determines the mapping patterns is dismissed also by quantum noise. In such a case, any fast correlation attack does not work on the quantum stream cipher.
A quantum stream cipher by Yuen 2000 protocol (so-called Y00 protocol or $\ensuremath{\alpha}\ensuremath{\eta}$ scheme) consisting of linear feedback shift register of short key is very attractive in implementing secure 40 Gbits/s optical data transmission, which is expected as a next-generation network. However, a basic model of the Y00 protocol with a very short key needs a careful design against fast correlation attacks as pointed out by Donnet et al. This Brief Report clarifies an effectiveness of irregular mapping between running key and physical signals in the driver for selection of MMM-ary basis in the transmitter, and gives a design method. Consequently, quantum stream cipher by the Y00 protocol with our mapping has immunity against the proposed fast correlation attacks on a basic model of the Y00 protocol even if the key is very short.
The previous work showed that the Y00 protocol could stay secure with the eavesdropper's guessing probability on the secret keys being strictly less than one under an unlimitedly long known-plaintext attack with quantum memory. However, an assumption that at least a fast correlation attack is completely disabled by irregular mapping. The present study shows that the Y00 protocol can be information-theoretic secure under any quantum-computational crypto-analyses if the Y00 system is well designed. The Y00 protocol directly encrypts messages with short secret keys expanded into pseudo-random running keys unlike One-Time Pad. However, it may offer information-theoretic security beyond the Shannon limit of cryptography.
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<n> = 1000 \\sim 10000<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)H(K) << H(X)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.
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.
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.