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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Quantum stream cipher part IV: effects of the deliberate signal randomization and the deliberate error randomization

Kentaro Kato, Osamu Hirota

Published Aug 31, 2006• 8 citations
Publisher

Abstract

In a series of articles concerned with quantum stream cipher by Y-00 protocol on this conference, we have claimed that the quantum stream cipher has a pretty good security against several concrete attacks. On the other hand, it has been pointed out that one can improve the security level by using various additional randomization techniques. In this paper, we will show some concrete randomization techniques for quantum stream cipher. First we will sketch the framework of the deliberate signal randomization (DSR) that is realized by randomizing the signals deliberately with true-random numbers or with pseud-random numbers generated by a secret key. Secondly, we will consider about the deliberate error randomization (DER) by using concrete models. It will be shown that these randomization techniques enhance the security level of the quantum stream cipher.

DOI
10.1117/12.680013

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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE•Aug 18, 2005•5 citations
In a series of articles concerned with quantum stream cipher by Y-00 protocol on this conference, we have claimed that the quantum stream cipher has a pretty good security against several concrete attacks. On the other hand, it has been pointed out that one can improve the security level by using various additional randomization techniques. In this paper, we will show some concrete randomization techniques for quantum stream cipher. First we will sketch the framework of the deliberate signal randomization (DSR) that is realized by randomizing the signals deliberately with true-random numbers or with pseud-random numbers generated by a secret key. Secondly, we will consider about the deliberate error randomization (DER) by using concrete models. It will be shown that these randomization techniques enhance the security level of the quantum stream cipher.
This paper considers two issues on the quantum stream cipher by Yuen-2000 (Y-00) protocol. In the fist part of this paper we investigate the optimal modulation scheme for the basic model of the quantum stream cipher by Y-00 protocol, and in the remaining part we study the deliberate signal randomization. For the problem on the optimal modulation scheme, several modulation schemes are investigated for the cipher text-only attacks and the known plaintext attack under the error probability criterion and the information criterion to find the preferable modulation scheme. As a result, it will be shown that the phase shift keying signal yields the best performance among the modulation schemes investigated in our consideration by numerical simulations. After that, the roperty of the randomization technique called the deliberate signal randomization is considered for the cipher text-only attacks and the known plaintext attack in the information criterion. From this, it will be shown by numerical simulations that the amount of leakage of information from the legitimate user to the eavesdropper is reduced by the deliberate signal randomization. At the last section we will mention about the implementation issues of the deliberate signal randomization, taking account of the numerical results.
The notion of the deliberate error randomization (DER) for the Y-00 quantum stream cipher was introduced by Yuen [arXiv:quant-ph/0311061v6], and concrete schemes of DER were proposed by the author for the phase shift keying Y-00 quantum stream cipher (PSK-Y00) [Proc. SPIE 6305, 630508 (2006)]. In this paper, it is shown that one of our DER schemes, which was referred to as Model B in the literature, is applicable to the intensity shift keying Y-00 quantum stream cipher (ISK-Y00) and the brute-force search complexity Q in the case of ISK-Y00 is enhanced by installing DER as well as in the case of PSK-Y00.
As a security parameter of Y-00 quantum stream cipher, a guessing probability by an eavesdropper for a secret key of legitimate users is discussed. Assuming that Eve employs the direct detection and Gaussian intensity distribution of each intensity level of Y-00 cipher signals are same, an analytic solution of probability of correct guessing of the secret key in a case of the ciphertext-only attack is derived. The solution is applied to experimentally measure the probabilities of our Y-00 quantum stream cipher transceiver. A very low probability of the Y-00 cipher transceiver is experimentally confirmed.
A power distribution of optical signals as the ciphertext of Y-00 quantum stream cipher transceivers should be uniform, even when the bit sequence of plaintext is a non-uniform sequence. For examining the uniformity, we experimentally measure powers of optical signals with 4096 intensity levels and calculate the ratio of signal numbers over and under the average power. The ratios for various kinds of bit sequences are 0.5 with measurement error of 1%, which is an evidence of the uniformity of the power distribution. In addition, the effectiveness of a randomization technique of overlapping selection keying for enhancing the security against the known plaintext attack is experimentally observed.
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.