Abstract An avalanche single-photon detector blinding attack is one of the methods for quantum hacking of quantum key distribution (QKD) systems. The attack was experimentally demonstrated for both phase- and polarization-encoded QKD systems. During this attack, an eavesdropper knows the entire key, does not produce errors and саnnot be detected. However, the phase encoding has neglected some significant features of the photocount statistics in the receiving party. It is shown in the paper at the level of fundamental principles that this attack changes the photocount statistics and leads to the detection of an eavesdropper. Expressions for the secret key length are obtained for this attack. This does not require any changes in the design and control electronics of the phase-encoded QKD system, and only changes in processing the results of registration of quantum states are sufficient. At the same time, the secret key vulnerability and compromise in polarization-encoded QKD systems is an existing fact rather than a potential menace.
Abstract An avalanche single-photon detector blinding attack is one of the methods for quantum hacking of quantum key distribution (QKD) systems. The attack was experimentally demonstrated for both phase- and polarization-encoded QKD systems. During this attack, an eavesdropper knows the entire key, does not produce errors and саnnot be detected. However, the phase encoding has neglected some significant features of the photocount statistics in the receiving party. It is shown in the paper at the level of fundamental principles that this attack changes the photocount statistics and leads to the detection of an eavesdropper. Expressions for the secret key length are obtained for this attack. This does not require any changes in the design and control electronics of the phase-encoded QKD system, and only changes in processing the results of registration of quantum states are sufficient. At the same time, the secret key vulnerability and compromise in polarization-encoded QKD systems is an existing fact rather than a potential menace.
Abstract An attack exploiting single-photon avalanche diode (SPAD) blinding is one of the effective methods of ‘quantum hacking’ (Lydersen et al 2010 Nat. Photon . 4 686) or cracking quantum key distribution (QKD) systems. This attack was experimentally demonstrated for various QKD systems based on both phase and polarization coding. After such an attack, the eavesdropper knows the whole key, has not produced errors, and is not detected. So far this attack is the only one that was demonstrated in the explicit form on many real QKD systems. It is important that these demonstrations were actually performed in reality, i.e. not in speculations as some other attacks. Therefore, the presence of vulnerability in QKD systems based on polarization coding is an existing fact, rather than just a potential threat. It is often assumed that all systems regardless of the encoding method are vulnerable to such an attack. However, in the case of phase coding, some essential features of photocount statistics on the receiving side make a difference. In this Letter we prove that detector blinding attack, when acts on QKD systems with phase coding, leads to a distortion of the photocounts statistics so the eavesdropper may always be detected. Moreover, one does not need to change the design of the QKD system and/or its control electronics, as it is sufficient to amend only the processing of the quantum states registration results to make the system secure. At the same time, polarization coding-based systems remain vulnerable to such an attack and do not guarantee key security.
Abstract Quantum key distribution (QKD) systems are open systems. Thus, an eavesdropper can actively influence the elements of the system through the communication line, altering their regular operation. One known attack is an attack with detector blinding. With such an attack, the eavesdropper can obtain the entire key and remain undetectable. The paper proposes a fundamentally new method of detecting and protecting quantum cryptography systems with phase coding against the attack with avalanche detector blinding. The proposed method is based on diverting a part of the input signal in the receiving terminal to bypass the interferometer and deliver it to the same avalanche detectors. Measuring the detector response in different time windows, both for the bypassed signal and the normal operation range, one can effectively identify the ongoing detector blinding attack. The method does not require radical changes in existing quantum cryptography systems and is quite universal, since it can be used for any phase-coded quantum key distribution protocol. The method provides a fundamental strategy of detecting and protecting against the attack at the level of the quantum key distribution protocol, and not at the level of technical ‘patches’.
The fundamental quantum mechanics prohibitions on the measurability of quantum states allow secure key distribution between spatially remote users to be performed. Experimental and commercial implementations of quantum cryptography systems, however, use components that exist at the current technology level, in particular, one-photon avalanche photodetectors. These detectors are subject to the blinding effect. It was shown that all the known basic quantum key distribution protocols and systems based on them are vulnerable to attacks with blinding of photodetectors. In such attacks, an eavesdropper knows all the key transferred, does not produce errors at the reception side, and remains undetected. Three protocols of quantum key distribution stable toward such attacks are suggested. The security of keys and detection of eavesdropping attempts are guaranteed by the internal structure of protocols themselves rather than additional technical improvements.
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 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.
Security of an ideal system for quantum key distribution can be formally proved. However, technological imperfections of real systems can be misused by an eavesdropper to get information about the key without causing a detectable change in the error rate. An example of this is the blinding attack where the eavesdropper manipulates detectors of the receiving party. To avoid such an attack, technical modifications of the device are usually required. In this paper we show that an extended measurement, similar to the Bell test measurement, can reveal the blinding attack without any modification of the device.