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arXiv:1909.10001 · Physical Review Applied

Hacking the Quantum Key Distribution System by Exploiting the Avalanche-Transition Region of Single-Photon Detectors

Yong-Jun Qian, De-Yong He, Shuang Wang, Wei Chen, Zhen-Qiang Yin, Guang-Can Guo, Zheng-Fu Han

Published Dec 28, 2018• 46 citations
arXivHTMLPDF

Abstract

As necessary steps toward practical and secure quantum key distribution (QKD), identifying and patching possible attacks are important and valuable. This study discusses an avalanche-transition-region (ATR) attack on a gated-mode avalanche photodiode (APD) detector, widely used in QKD systems. Unlike other attacks, this one leaves almost no trace; the eavesdropper introduces less than 0.5% quantum bit-error rate, and the photocurrent and afterpulse probability before and after hacking are nearly identical. These findings highlight the importance of detection signals in real-life QKD systems, and are crucial to enhancing the security of practical QKD.

DOI
10.1103/physrevapplied.10.064062

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As necessary steps toward practical and secure quantum key distribution (QKD), identifying and patching possible attacks are important and valuable. This study discusses an avalanche-transition-region (ATR) attack on a gated-mode avalanche photodiode (APD) detector, widely used in QKD systems. Unlike other attacks, this one leaves almost no trace; the eavesdropper introduces less than 0.5% quantum bit-error rate, and the photocurrent and afterpulse probability before and after hacking are nearly identical. These findings highlight the importance of detection signals in real-life QKD systems, and are crucial to enhancing the security of practical QKD.
Quantum key distribution (QKD) allows two users to communicate with information theoretic security by encoding information on single photons. This security is based on the laws of physics and as such can never be broken in theory. However, in practice, components do not always behave according to their theoretical models and these deviations can be exploited by an eavesdropper. In recent years, exposing loopholes in QKD systems, known as quantum hacking, has attracted significant attention. The components most susceptible to being hacked are the single-photon detectors, often avalanche photodiodes (APDs), as they are directly exposed to the optical channel. Whilst measurement-device-independent QKD removes detector vulnerability from the system, secure key rates with this technique can be much lower than point-to-point links. As such, mitigating attacks on QKD systems is a pressing challenge in QKD. In this thesis, the focus is on a special class of detectors, self-differencing APDs (SD-APDs), which have facilitated state-of-the art demonstrations of QKD. The susceptibility of SD-APDs to blinding attacks, the most explored and successful attack to date, was investigated and it was shown that by following best practice for their operation, such an attack would be unsuccessful. We have also proposed and developed a countermeasure such that the onus for appropriate operation could be removed from the user. We have also explored an arguably more dangerous attack, in the form of the after-gate attack. We have shown that delayed detection events, ordinarily considered detrimental in QKD, can provide inherent protection against this attack. Finally, backflashes in GHz-gated APDs were investigated for the first time and it was shown that threat they pose to QKD security is negligible. These results highlight the inherent protection to a number of attacks that self-differencing APDs possess. We stress that the findings presented in this thesis are also applicable to other types of fast-gated InGaAs APDs that don't possess self-differencing circuitry.
Implementation of high-speed quantum key distribution (QKD) has become one of the major focuses in the field, which produces a high key-generation rate for applications. To achieve high-speed QKD, tailored techniques are developed and employed to quickly generate and detect quantum states. However, these techniques may introduce unique loopholes to compromise the security of QKD systems. In this paper, we investigate the loopholes of the self-differencing (SD) avalanche photodiode (APD) detector, typically used for high-speed detection in a QKD system, and demonstrate experimental testing of the SD APD detector under strong-pulse-illumination attack. This attack presents blinding stability and helps an eavesdropper to learn the secret key without introducing extra quantum bit rate error. Based on this testing, we propose a set of criteria for protecting SD APD detectors from the strong-pulse-illumination attack.
Quantum key distribution (QKD) has been proved to be information-theoretically secure in theory. Unfortunately, the imperfect devices in practice compromise its security. Thus, to improve the security property of practical QKD systems, a commonly used method is to patch the loopholes in the existing QKD systems. However, in this work, we show an adversary's capability of exploiting the imperfection of the patch itself to bypass the patch. Specifically, we experimentally demonstrate that, in the detector under test, the patch of photocurrent monitor against the detector blinding attack can be defeated by the pulse illumination attack proposed in this paper. We also analyze the secret key rate under the pulse illumination attack, which theoretically confirmed that Eve can conduct the attack to learn the secret key. This work indicates the importance of inspecting the security loopholes in a detection unit to further understand their impacts on a QKD system. The method of pulse illumination attack can be a general testing item in the security evaluation standard of QKD.
By employing real-time monitoring of single-photon avalanche photodiodes we demonstrate how two types of practical eavesdropping strategies, the after-gate and time-shift attacks, may be detected. Both attacks are identified with the detectors operating without any special modifications, making this proposal well suited for real-world applications. The monitoring system is based on accumulating statistics of the times between consecutive detection events, and extracting the afterpulse and overall efficiency of the detectors in real-time using mathematical models fit to the measured data. We are able to directly observe changes in the afterpulse probabilities generated from the after-gate and faint after-gate attacks, as well as different timing signatures in the time-shift attack. We also discuss the applicability of our scheme to other general blinding attacks.
Although quantum key distribution (QKD) promises information-theoretic security that can never be hacked, several studies have investigated how its security can be compromised by targeting the detectors in the system. Have they found truly fundamental problems with the physics of the scheme, or merely sloppy implementation? This study seeks to define best-practice criteria for these detectors, to distinguish between genuine loopholes and incorrect operation. The authors show that if these directions are followed, many of the attacks previously demonstrated simply do not work---bringing this technology a big step closer to everyday life.