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Chinese Physics Letters

Quantum Key Distribution against Trojan Horse Attacks

Qing-yu Cai, Lv Hua

Published May 1, 2007• 7 citations
Publisher

Abstract

Realistic experimental apparatus of quantum cryptography are imperfect, which may be utilized by a potential eavesdropper to eavesdrop on the communication. We show that quantum communication may be improved with quantum teleportation and entanglement swapping, which is robustly secure against the most general Trojan horse attacks. Our scheme is not an improvement of the communication apparatus, but the improvement of quantum communication protocol itself. We show that our modified schemes may be implemented with current technology.

DOI
10.1088/0256-307x/24/5/008

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Realistic experimental apparatus of quantum cryptography are imperfect, which may be utilized by a potential eavesdropper to eavesdrop on the communication. We show that quantum communication may be improved with quantum teleportation and entanglement swapping, which is robustly secure against the most general Trojan horse attacks. Our scheme is not an improvement of the communication apparatus, but the improvement of quantum communication protocol itself. We show that our modified schemes may be implemented with current technology.
Abstract Early security proofs of the keys in quantum cryptography systems were based on the assumption that the transmitting and receiving stations were 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 an eavesdropper can have indirect access, for example, through a fiber communication channel to critical equipment elements (phase modulators, intensity modulators, etc) using active sensing of the state of these elements—Trojan-horse attacks. A new Trojan-horse attack based on joint unambiguous measurements of reflected probing states from an intensity modulator and information states in a quantum communication channel is proposed. Estimates of attack parameters for a number of quantum key distribution protocols are given, when an attack leads to compromise of keys.
Abstract Most security proofs of quantum key distribution (QKD) disregard the effect of information leakage from the users’ devices, and, thus, do not protect against Trojan-horse attacks (THAs). In a THA, the eavesdropper injects strong light into the QKD apparatuses, and then analyzes the back-reflected light to learn information about their internal setting choices. Only a few recent works consider this security threat, but predict a rather poor performance of QKD unless the devices are strongly isolated from the channel. Here, we derive finite-key security bounds for decoy-state-based QKD schemes in the presence of THAs, which significantly outperform previous analyses. Our results constitute an important step forward to closing the existing gap between theory and practice in QKD.
\n It is interesting that counterfactual quantum cryptography protocols allow two remotely separated parties to share a secret key without transmitting any signal particles. Generally, these protocols, expected to provide security advantages, base their security on a translated no-cloning theorem. Therefore, they potentially exhibit unconditional security in theory. In this letter, we propose a new Trojan horse attack, by which an eavesdropper Eve can gain full information about the key without being noticed, to real implementations of a counterfactual quantum cryptography system. Most importantly, the presented attack is available even if the system has negligible imperfections. Therefore, it shows that the present realization of counterfactual quantum key distribution is vulnerable.\n
Quantum key distribution (QKD), which theoretically provides unconditional secure communication, has developed rapidly in the past decades. However, the practical QKD systems still have vulnerabilities due to device imperfections. Polarization-encoding systems are one of the most important branches in the field of QKD. To date, a variety of polarization-encoding schemes have been proposed to meet the requirements of high speed and robust modulation, but the security analysis of these schemes is often ignored. In this paper, we experimentally and theoretically evaluated the security of three typical polarization encoders against the Trojan horse attack (THA), which is recognized as one of the critical quantum attacks. We built a system to implement the THA and collected the reflection from QKD configuration with different encoders. According to the experimental results, we further analyzed the secure key rate and the resistance against THAs of the system while BB84 or measurement-device-independent QKD was applied. Our paper plays an important guiding role in the design of polarization encoders and the construction of secure QKD systems with realistic devices.