Abstract Side-channel attacks manipulating single-photon detectors (SPDs) have known to be loopholes in realistic quantum key distribution (QKD) systems. Although measurement-device-independent (MDI) QKD schemes have been proposed and studied to avoid those loopholes, they are not easy to implement in practice because they Require some synchronization between signals sent from two distant parties. In this paper, we propose a new countermeasure against a side-channel attack (control blinding and controlling attacks). It utilizes coincident clicks in differential-quadrature-phase shift (DQPS) QKD systems. Our scheme requires no change in the system configuration of the conventional protocol. Unlike MDI-QKD, side-channel attacks can be found without difficulty in practical implementations.
Abstract Side-channel attacks manipulating single-photon detectors (SPDs) have known to be loopholes in realistic quantum key distribution (QKD) systems. Although measurement-device-independent (MDI) QKD schemes have been proposed and studied to avoid those loopholes, they are not easy to implement in practice because they Require some synchronization between signals sent from two distant parties. In this paper, we propose a new countermeasure against a side-channel attack (control blinding and controlling attacks). It utilizes coincident clicks in differential-quadrature-phase shift (DQPS) QKD systems. Our scheme requires no change in the system configuration of the conventional protocol. Unlike MDI-QKD, side-channel attacks can be found without difficulty in practical implementations.
Abstract Realistic quantum key distribution (QKD) systems suffer from side-channel attacks, which manipulate single-photon detectors. Although measurement-device-independent QKD schemes were proposed to free QKD parties (Alice and Bob) from such measurement devices, these schemes are not easy to be implemented in practice because they require precise synchronization between signals from distant parties. On the other hand, differential phase shift (DPS) QKD is a simple system for practical implementation with current optical equipment. In this study, we propose a simple modification in DPS QKD to prevent side-channel attacks (control blinding and controlling attacks) such that Bob randomly attenuates the incoming signal. This modification allows Bob to utilize photon statistics during attenuated time slots in DPS-QKD systems, using which the side-channel attacks are revealed.
This paper proposes a measurement-device-independent quantum key distribution (MDI-QKD) scheme based on differential phase encoding. The differential phase shift MDI-QKD (DPS-MDI-QKD) couples the advantages of DPS-QKD with that of MDI-QKD. The proposed scheme offers resistance against photon number splitting attack and phase fluctuations as well as immunity against detector side-channel vulnerabilities. The design proposed in this paper uses weak coherent pulses in a superposition of three orthogonal states, corresponding to one of three distinct paths in a delay-line interferometer. The classical bit information is encoded in the phase difference between pulses traversing successive paths. This 3-pulse superposition offers enhanced security compared to using a train of pulses by decreasing the learning rate of an eavesdropper and unmasking her presence with an increased error rate upon application of intercept and resend attack and beamsplitter attack. The proposed scheme employs phase locking of the sources of the two trusted parties so as to maintain the coherence between their optical signal, and uses a beamsplitter (BS) at the untrusted node (Charlie) to extract the key information from the phase encoded signals.
Cryptography is essential for secure communication in the digital era. Today, public-key cryptography is widely employed, and has provided an efficient method for encrypting content and ensuring both confidentiality and authenticity of electronic communications. However, the security of these systems is based on assumptions of computational hardness within the constraints of current computing capability. Thus, as quantum computing becomes a reality, public-key algorithms will be genuinely vulnerable to attack. By contrast, quantum cryptography, which is based on quantum physics instead of mathematical assumptions, is able to achieve information-theoretic security. \n \nAdvances in practical quantum cryptographic systems have not kept pace with theory, where an eavesdropper can relatively easily exploit loopholes in practical implementations to compromise theory-proved security. Bridging the gap between perfect theory and imperfect practice has become a priority for the growing field of quantum key distribution (QKD), which has strived to strengthen the practical security of QKD systems. Among all the countermeasures against quantum hacking, the measurement-device-independent (MDI) QKD protocol is promising because it is immune to all side-channel attacks on measurement devices. However, the MDI QKD protocol has some limitations that critically restrict its practical usefulness. Technically, the MDI scheme is not compatible with existing QKD systems, and produces a low key rate. In addition, the theory underlying MDI QKD security is based on the use of trusted source stations. Thus, this protocol is not a universal solution. This thesis further investigates the practical security of quantum cryptography in and beyond MDI quantum cryptography. \n \nTo overcome the technical limitations of MDI QKD, we first evaluate two other countermeasures against imperfect detections. The first is an industrial patch based on random detection efficiency, recently implemented by ID Quantique in the commercial Clavis2 QKD system. While powerful, experimental testing shows that this countermeasure is not sufficient to defeat the detector blinding attack. The second countermeasure aims to achieve a higher key rate than MDI QKD while maintaining the same security properties. However, our research shows that detector-device-independent (DDI) QKD security is not equivalent to that of MDI QKD and, further, that DDI QKD is insecure against detector side-channel attacks. \n \nWhile this initial work points to the superior performance of MDI QKD systems, core challenges remain. The fundamental security assumption adopted for MDI QKD systems, regarding the exclusive use of trustable source stations, cannot always be satisfied in practice. Our study revealed several side channels of source devices. The first is disclosed from the implementation of a decoy-state protocol, which is widely used in QKD systems with weak coherent sources. The pump-current-modulated intensities result in a timing mismatch between the signal and decoy states, violating the key assumption in the decoy-state QKD protocol. Moreover, an active Eve can break the basic assumption about photon numbers in the QKD system. In this work, we experimentally demonstrate a laser seeding attack on the laser source, which shows that Eve can increase the emission power of the laser diode. Furthermore, by shining a high-power laser into an optical attenuator, Eve can decrease the attenuation values. The increase in laser emission power and the decrease in attenuation leads to an increase in mean photon numbers. \n \nIn summary, MDI QKD is a milestone in quantum cryptography. However, this thesis indicates the importance of continued investigations into the practical security of MDI QKD. The analysis of practical security should be extended to other countermeasures against side-channel attacks and the source stations in MDI QKD systems. Practical quantum hacking and security analysis promote the development of quantum cryptographic systems, which will eventually achieve the unconditional security claimed in theory.
Measurement-device-independent quantum key distribution (MDI-QKD) protocol was proposed to remove all the detector side channel attacks, while its security relies on the trusted encoding systems. Here we propose a one-sided MDI-QKD (1SMDI-QKD) protocol, which enjoys detection loophole-free advantage, and at the same time weakens the state preparation assumption in MDI-QKD. The 1SMDI-QKD can be regarded as a modified MDI-QKD, in which Bob's encoding system is trusted, while Alice's is uncharacterized. For the practical implementation, we also provide a scheme by utilizing coherent light source with an analytical two decoy state estimation method. Simulation with realistic experimental parameters shows that the protocol has a promising performance, and thus can be applied to practical QKD applications.
Measurement-device-independent quantum key distribution (MDI-QKD) is proposed to close the loopholes of the detection part in QKD systems, while it still requires the light source to be trusted. This assumption is not guaranteed in practice, leading to practical secure issues. In this paper, we propose a light source monitoring scheme with single-photon detectors for MDI-QKD, which can guarantee the security of the system under the untrusted source condition with both common decoy state method and the newly proposed five-intensity decoy state method. Moreover, our monitoring scheme performs almost the same as the ideal MDI protocol even considering the imperfect factors in practical systems. Compared to the previous scheme, our work improves the secure key rate and the transmission distance of practical MDI-QKD systems significantly.
Quantum coin tossing (QCT) is an important primitive of quantum cryptography and has received continuous interest. However, in practical QCT, Bob's detectors can be subjected to detector-side channel attacks launched by dishonest Alice, which will possibly make the protocol completely insecure. Here, we report a simple strategy of a detector-blinding attack based on a recent experiment. To remove all the detector side channels, we present a solution of measurement-device-independent QCT (MDI-QCT). This method is similar to the idea of MDI quantum key distribution (QKD). MDI-QCT is loss tolerant with single-photon sources and has the same bias as the original loss-tolerant QCT under a coherent attack. Moreover, it provides the potential advantage of doubling the secure distance for some special cases. Finally, MDI-QCT can also be modified to fit the weak coherent-state sources. Thus, based on the rapid development of practical MDI-QKD, our proposal can be implemented easily.