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arXiv:2205.04177 · Physical Review A

Ability of strong-pulse illumination to hack self-differencing avalanche photodiode detectors in a high-speed quantum-key-distribution system

Binwu Gao, Zhihao Wu, Weixu Shi, Yingwen Liu, Dongyang Wang, Chunlin Yu, Anqi Huang, Junjie Wu

Published Sep 29, 2022• 21 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.106.033713

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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) 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.
In recent years, the security of avalanche photodiodes as single photon detectors for quantum key distribution has been subjected to much scrutiny. The most prominent example of this surrounds the vulnerability of such devices to blinding under strong illumination. We focus on self-differencing avalanche photodiodes, single photon detectors that have demonstrated count rates exceeding 1 GCounts/s resulting in secure key rates over 1 MBit/s. These detectors use a passive electronic circuit to cancel any periodic signals thereby enhancing detection sensitivity. However this intrinsic feature can be exploited by adversaries to gain control of the devices using illumination of a moderate intensity. Through careful experimental examinations, we define here a set of criteria for these detectors to avoid such attacks.
Quantum communication, in particular, quantum key distribution (QKD) is moving ever closer to real world implementation. However, for successful QKD system deployment, the QKD system components must be robustly designed and feature highly reliable operation. In this paper we focus on one important aspect of any quantum communication system: the single photon detector. In particular our interest is centered upon the InGaAs avalanche photodiode (APD) single photon detector operating in a self-differencing (SD) mode. Such a detector features high clock frequencies of up to 3GHz, high photon count rates as well as detection efficiencies approaching 20% with low afterpulsing. We show successful operation of a high bit rate QKD system using this SD-APD technology in a real world fiber network.
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.
Quantum cryptography has been developed from being a theoretical proposal to having real world applications, with companies developing and selling quantum key distribution (QKD) instruments commercially. Although laws of quantum physics guarantee perfect security in the key distribution protocol, several vulnerabilities in the practical implementations have been described and demonstrated. In some of these attacks the eavesdropper has been able to acquire the entire secret key without being detected by the two communicating parties. This thesis describes an experimental setup for automatically characterizing and damaging single photon avalanche photodiodes using a high power laser to damage the diode with the intention to find new weaknesses in QKD systems. Hopefully, the discovery of loopholes will stimulate the researchers and commercial manufacturers to fix their implementations, leading to more secure systems. For a silicon avalanche photodiode (APD) in a passive-quenching circuit, a number of interesting results have been found. It has been shown that we are able to change many of the APDs key characteristics by applying strong illumination. We have been able to reduce the dark count rate of several APDs by up to 80% in a predictable way by strong laser illumination, an effect which is believed to be caused by localized annealing. This is an interesting result not only in the field of quantum cryptography, but perhaps also for improving the manufacturing process of different semiconductor components. Other parameters that have been found to change in interesting ways are the dark current and the breakdown voltage. It has been demonstrated that these changes may compromise the security of QKD systems.
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.