This paper shall investigate Yuen protocol, so called Y-00, which can realize a randomized stream cipher with high bit rate(Gbps) for long distance(several hundreds km). The randomized stream cipher with randomization by quantum noise based on Y-00 is called 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 the quantum unambiguous measurement to make clear the strength of Y-00 in real communication. In addition, we give a design for the implementation of an intensity modulation scheme and report the experimental demonstration of 1 Gbps quantum stream cipher through 20 km long transmission line.
This paper presents the security analysis on the quantum stream cipher so called Yuen-2000 protocol (or $αη$ 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 …
So far, quantum key distribution (QKD) has been the main subject in the field of quantum cryptography, but that is not quantum cryptographic communication, it is only the ability to send keys for cryptographic purposes. To complete cryptographic communication, a technique for encrypting data is necessary, and the conventional cryptographic technique of mathematical symmetric key cipher or One Time Pad (OTP) is adopted in the discussion so far. However, OTP is not the ultimate cipher for data encryption, because it does not satisfy security conditions in the modern cryptology. Around 2000, a ne…
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…
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 hig…
A quantum cipher supported by a secret key so called keyed communication in quantum noise (KCQ) is very attractive in implementing high speed key generation and secure data transmission. However, Yuen-2000 as a basic model of KCQ has a difficulty to ensure the quantitative security evaluation because all physical parameter for the cipher system is finite. Recently, an outline of a generalized scheme so called coherent pulse position modulation(CPPM) to show the rigorous security analysis is given, where the parameters are allowed to be asymptotical. This may open a new way for the quantum key …
The security analysis of physical encryption protocol based on coherent pulse position modulation(CPPM) originated by Yuen is one of the most interesting topics in the study of cryptosystem with a security level beyond the Shannon limit. Although the implementation of CPPM scheme has certain difficulty, several methods have been proposed recently. This paper deals with the CPPM encryption in terms of symplectic transformation, which includes a phase mask encryption as a special example, and formulates a unified security analysis for such encryption schemes. Specifically, we give a lower bound …
This research note suggests a new way to realize a high speed direct encryption based on quantum detection theory. The conventional cipher is designed by a mathematical algorithm and its security is evaluated by the complexity of the algorithm for 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 with quantum effect in physical layer may exceed the Shannon limit of cryptography. The q…
In 2000, an attractive new quantum cryptography was discovered by H.P.Yuen, which can realize secure communication with high speeds and at long distance by conventional optical devices. Recently, a criticism of the Yuen protocol, so called Y-00, was made by Nishioka, and Imai group(Mitsubishi and University of Tokyo), and they claimed Y-00 is essentially a classical stream cipher. This paper shows that the claim is incorrect. In particular, it is shown that the relation $l_i=r_i\oplus \tilde{k}_i$, which is their basis for attack, has no essential role for any security analysis. In addition, w…
Comparison of security mechanisms of Mathematical cipher, Wyner scheme, QKD, and Quantum stream cipher
Optical Quantum Information Technology Laboratory（光学量子信息技术实验室）；Quantum ICT Research Institute, Tamagawa University（多摩大学量子ICT研究所）；Research and Development Initiative, Chuo University（中央大学研发倡议）
Privacy amplification is the key step to guarantee the security of quantum communication. The existing security proofs require accumulating a large number of raw key bits for privacy amplification. This is similar to block ciphers in classical cryptography that would delay the final key generation since an entire block must be accumulated before privacy amplification. Moreover, any leftover errors after information reconciliation would corrupt the entire block. By modifying the security proof based on quantum error correction, we develop a stream privacy amplification scheme, which resembles t…
We review the notion of a classical random cipher and its advantages. We sharpen the usual description of random ciphers to a particular mathematical characterization suggested by the salient feature responsible for their increased security. We describe a concrete system known as AlphaEta and show that it is equivalent to a random cipher in which the required randomization is effected by coherent-state quantum noise. We describe the currently known security features of AlphaEta and similar systems, including lower bounds on the unicity distances against ciphertext-only and known-plaintext atta…
Although quantum key distribution is regarded as promising secure communication, security of Y00 protocol proposed by Yuen in 2000 for the affinity to conventional optical communication is not well-understood yet; its security has been evaluated only by the eavesdropper's error probabilities of detecting individual signals or masking size, the number of hidden signal levels under quantum and classical noise. Our study is the first challenge of evaluating the guessing probabilities on shared secret keys for pseudorandom number generators in a simplified Y00 communication system based on quantum…
QKD-Integrated Quantum Noise Stream Cipher: An Overview
Indian Institute of Technology Roorkee（印度技术学院鲁尔凯拉）；Tamagawa University（多摩川大学）；Northwestern University（西北大学）；Eindhoven University of Technology（埃因霍温理工大学）
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 run…
Nishioka et al claim in [1], elaborating on their earlier paper [2], that the direct encryption scheme called Y-00 [3,4] is equivalent to a classical non-random additive stream cipher, and thus offers no more security than the latter. In this paper, we show that this claim is false and that Y-00 may be considered equivalent to a \emph{random} cipher. We explain why a random cipher provides additional security compared to its nonrandom counterpart. Some criticisms in [1] on the use of Y-00 for key generation are also briefly responded to.
New quantum cryptography, often called Y-00 protocol, has much higher performance than the conventional quantum cryptographies. It seems that the conventional quantum cryptographic attacks are inefficient at Y-00 protocol as its security is based on the different grounds from that of the conventional ones. We have, then, tried to cryptoanalyze Y-00 protocol in the view of cryptographic communication system. As a result, it turns out that the security of Y-00 protocol is equivalent to that of classical stream cipher.
Stream randomness extraction against quantum side information
Future Research Lab, China Mobile Research Institute（中国移动研究院未来研究实验室）；Department of Physics, Renmin University of China（中国人民大学物理系）；QICI Quantum Information and Computation Initiative, Department of Computer Science, The University of Hong Kong（香港大学计算机科学系量子信息与计算倡议）
It is claimed in Phys. Lett. A by T. Nishioka et. al. 327 (2004) 28-32, that the security of Y-00 is equivalent to that of a classical stream cipher. In this paper it is shown that the claim is false in either the use of Y-00 for direct encryption or key generation, in all the parameter ranges it is supposed to operate including those of the experiments reported thus far. The security of Y-00 type protocols is clarified.
