Trojan-horse attacks threaten the security of practical quantum cryptography

Abstract

A quantum key distribution (QKD) system may be probed by an eavesdropper Eve by sending in bright light from the quantum channel and analyzing the backreflections. We propose and experimentally demonstrate a setup for mounting such a Trojan-horse attack. We show it in operation against the quantum cryptosystem Clavis2 from ID Quantique, as a proof-of-principle. With just a few back-reflected photons, Eve discerns Bobʼs (secret) basis choice, and thus the raw key bit in the Scarani-Acín-Ribordy-Gisin 2004 protocol, with higher than 90% probability. This would clearly breach the security of the cryptosystem. Unfortunately, Eveʼs bright pulses have a side effect of causing a high level of afterpulsing in Bobʼs single-photon detectors, resulting in a large quantum bit error rate that effectively protects this system from our attack. However, in a Clavis2like system equipped with detectors with less-noisy but realistic characteristics, an attack strategy with positive leakage of the key would exist. We confirm this by a numerical simulation. Both the eavesdropping setup and strategy can be generalized to attack most of the current QKD systems, especially if they lack proper safeguards. We also propose countermeasures to prevent such attacks.

Risk Analysis of Trojan-Horse Attacks on Practical Quantum Key Distribution Systems

“…As demonstrated experimentally in Ref. [21], this basis choice can be inferred with more than 90% accuracy even with a few back-reflected photons (Eve  3). Nevertheless, the total attenuation suffered by Eve's pulse is of the order of −57 dB due to which, Eve has to send Trojan-horse pulses containing at least 1.510 6 photons into Bob.…”

“…Moreover, at least one of the onward Trojan-horse pulse or the back-reflected pulse must probe or pass through the basis selector, e.g., a phase modulator [21]. The onward and reverse paths (which may not be the same necessarily [19]) decide the insertion loss, and together with the back-reflection level, determine the total attenuation suffered by Eve's pulse in the double pass.…”

“…Such theory-practice deviations usually arise due to technical deficiencies and operational imperfections in the system hardware or firmware. The field of quantum hacking investigates such deviations [4][5][6] and many proof-of-principle quantum hacking attacks have been devised and performed on practical QKD systems in the last decade [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21].…”

“…Recently, both realistic and real-time Trojan-horse attacks on practical QKD implementations have attracted considerable attention [19][20][21]. The implementations include commercial QKD systems, such as Clavis2 from ID Quantique [27], and Cygnus from SeQureNet [28].…”

“…We then describe some simple spectral measurements that were performed on the receiver system in Clavis2 (Bob), in the quest of improving the attack presented in Ref. [21]. In particular, we also show the results of our estimation of the spectral sensitivity of single-photon avalanche diodes (SPADs) in the 1700 -1800 nm range.…”