Trojan Horse Attacks, Decoy State Method, and Side Channels of Information Leakage in Quantum Cryptography
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Trojan Horse Attacks, Decoy State Method, and Side Channels of Information Leakage in Quantum Cryptography S. N. Molotkova,b,c,d,* a
Institute of Solid State Physics, Russian Academy of Sciences, Chernogolovka, Moscow oblast, 142432 Russia b Academy of Cryptography of the Russian Federation, Moscow, 121552 Russia c Faculty of Computational Mathematics and Cybernetics, Moscow State University, Moscow, 119899 Russia d Quantum Technology Center, Moscow State University, Moscow, 119899 Russia *e-mail: [email protected] Received October 16, 2019; revised December 23, 2019; accepted December 23, 2019
Abstract—Early proofs of key secrecy in quantum cryptography systems were based on the assumption that the transmitting and receiving stations are completely isolated from the outside world—the eavesdropper. However, this condition cannot be implemented in practice since quantum cryptography systems are open systems in the sense that the eavesdropper may have indirect access, for example, through a fiber communication channel, to the critical elements of the equipment (phase modulators, intensity modulators, etc.) using active probing of the state of these elements. The state of the elements carries information about the transmitted key. In addition, the eavesdropper can use passive detection of side radiation from the receiving and transmitting equipment. Signals in side channels of information leakage may have extremely low intensity and are actually quantum signals. The eavesdropper may use the joint measurement of quantum information states in the communication channel and of states in various side channels of information leakage. The paper considers both passive attacks with measurement of side radiation and active attacks involving the probing of the states of the phase modulator and the intensity modulator, as well as backscattering radiation of single-photon avalanche detectors, which occurs during detecting information states on the receiver side. Combined attacks are also considered. The decoy state method is generalized with regard to active probing attacks, and boundaries for state parameters in side communication channels are obtained that guarantee secret key distribution for a given length of the communication channel. DOI: 10.1134/S1063776120050064
1. INTRODUCTION Information carriers in the classical field are classical signals—the states of classical systems. The laws of classical physics do not impose fundamental constraints on the accuracy of nonperturbing measurements of the states of classical objects—signals. As applied to the problems of information transmission and security, this means that a classical signal can be measured with arbitrary precision and without disturbance. After such a measurement, in principle, the signal can be reproduced, i.e., copied. In fact, this means that, when transmitting information by classical signals, in principle, an undetectable eavesdropping of such signals is possible. That is, within classical physics, there are no prohibitions on an a
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