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Entropy

Counteracting a Saturation Attack in Continuous-Variable Quantum Key Distribution Using an Adjustable Optical Filter Embedded in Homodyne Detector

Shengjie Xu, Yin Li, Yun Jie Mao, Ying Guo

Published Mar 9, 2022• 6 citations
Publisher

Abstract

A saturation attack can be employed for compromising the practical security of continuous-variable quantum key distribution (CVQKD). In this paper, we suggest a countermeasure approach to resisting this attack by embedding an adjustable optical filter (AOF) in the CVQKD system. Numerical simulations illustrate the effects of the AOF-enabled countermeasure on the performance in terms of the secret key rate and transmission distance. The legal participants can trace back the information that has been eavesdropped by an attacker from the imperfect receiver, which indicates that this approach can be used for defeating a saturation attack in practical quantum communications.

DOI
10.3390/e24030383

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A saturation attack can be employed for compromising the practical security of continuous-variable quantum key distribution (CVQKD). In this paper, we suggest a countermeasure approach to resisting this attack by embedding an adjustable optical filter (AOF) in the CVQKD system. Numerical simulations illustrate the effects of the AOF-enabled countermeasure on the performance in terms of the secret key rate and transmission distance. The legal participants can trace back the information that has been eavesdropped by an attacker from the imperfect receiver, which indicates that this approach can be used for defeating a saturation attack in practical quantum communications.
Abstract Saturation attack of imperfect detectors compromises the practical security of continuous-variable quantum key distribution (CVQKD) in implementations. In this paper, we demonstrate the practical security of the CVQKD system with finite-size analysis when considering effects of saturation attack on imperfect detectors at the receiver. We suggest an approach to resisting this attack by embedding an adjustable optical filter in the detector, motivated by characteristics of saturation attack. Numerical simulations illustrate the practical security of the system with finite-size analysis in terms of the achieved secret key rate and maximal transmission distance. It indicates that this approach can be elegantly used for defeating the potential saturation attack in practical quantum networks.
Continuous-variable quantum key distribution (CVQKD) over an atmosphere channel provides the possibility of global coverage, which is threatened by the imperfection of actual devices. Exploiting imperfections of practical devices, an eavesdropper steals the information between legal participants by performing the potential saturation-induced attacks on the homodyne detector. In this paper, we demonstrate the feasibility of the potential saturation attack in discretely modulated (DM) CVQKD over atmospheric turbulence. Subsequently, a countermeasure is suggested by using an adjustable optical filter (AOF) to attenuate the estimated parameter with the measured value in the finite linearity domain, where the attenuated data can be compensated. Numerical simulations illustrate the effects of the saturation attack on the performance of the AOF-based CVQKD system. We find that legal participants evaluate the information that has been eavesdropped because the secret key rate is made negative.
We identify and study a security loophole in continuous-variable quantum key distribution (CVQKD) implementations, related to the imperfect linearity of the homodyne detector. By exploiting this loophole, we propose an active side-channel attack on the Gaussian-modulated coherent-state CVQKD protocol combining an intercept-resend attack with an induced saturation of the homodyne detection on the receiver side (Bob). We show that an attacker can bias the excess noise estimation by displacing the quadratures of the coherent states received by Bob. We propose a saturation model that matches experimental measurements on the homodyne detection and use this model to study the impact of the saturation attack on parameter estimation in CVQKD. We demonstrate that this attack can bias the excess noise estimation beyond the null key threshold for any system parameter, thus leading to a full security break. If we consider an additional criterion imposing that the channel transmission estimation should not be affected by the attack, then the saturation attack can only be launched if the attenuation on the quantum channel is sufficient, corresponding to attenuations larger than approximately 6 dB. We moreover discuss the possible countermeasures against the saturation attack and propose a countermeasure based on Gaussian postselection that can be implemented by classical postprocessing and may allow one to distill the secret key when the raw measurement data are partly saturated.
A homodyne detector, which is also a common element in current telecommunication, is a core component of continuous-variable quantum key distribution (CV-QKD) since it is considered the simplest setup for the distinguishing of coherent states with minimum error. However, the theoretical security of CV-QKD is based on the assumption that the responses of the homodyne detector are always linear with respect to the input, which is impossible in practice. In the real world, a homodyne detector has a finite linear domain, so the linearity assumption is broken when the input is too large. Regarding this security vulnerability, the eavesdropper Eve can perform the so-called homodyne detector-blinding attack by saturating the homodyne detector and then stealing key information without being detected by the legitimate users. In this paper, we propose a countermeasure for the homodyne detector-blinding attack by using an adjustable optical attenuator with a feedback structure. Specifically, we estimate the suitable attenuation value in the data processing of CV-QKD and feed it back to the adjustable optical attenuator before the detector in real time. Numerical simulation shows that the proposed countermeasure can effectively defend against homodyne detector-blinding attacks and ensure the security of the Gaussian-modulated coherent state protocol with finite-size effect.
Abstract Continuous variable quantum key distribution (CVQKD) can achieve the secure communication between two distant legitimate users. The microwave CVQKD exhibits stronger robustness than the optical CVQKD in transmitting quantum signals and accordingly has broad application prospects in quantum networks. Although the microwave CVQKD does not rely on the traditional optical fibers to transmit quantum signals, its practical security is still vulnerable to saturation attacks. In view of this security vulnerability, we generalize saturation attacks to microwave CVQKD and clarify the impact of saturation attacks on the parameter estimation of microwave systems. Two saturation attack schemes are further compared, one based on displacement control alone and the other on joint displacement and gain control. To mitigate the negative impacts caused by this attack, we propose a countermeasure based on the quantum k nearest neighbor algorithm, which can detect saturation attacks by measuring unknown quantum states. In addition, the influence of microwave antennas on the security of the communication system is assessed, confirming that antenna gain plays a key role in determining both the secret key rate and the maximum transmission distance.
In a practical continuous-variable quantum key distribution (CVQKD) system, the optical attenuator can adjust the Gaussian-modulated coherent states and the local oscillator signal to an optimal value for guaranteeing the security of the system and optimizing the performance of the system. However, the performance of the optical attenuator may deteriorate due to the intentional and unintentional damage of the device. In this paper, we investigate the practical security of a CVQKD system with reduced optical attenuation. We find that the secret key rate of the system may be overestimated based on the investigation of parameter estimation under the effects of reduced optical attenuation. This opens a security loophole for Eve to successfully perform an intercept-resend attack in a practical CVQKD system. To close this loophole, we add an optical fuse at Alice's output port and design a scheme to monitor the level of optical attenuation in real time, which can make the secret key rate of the system evaluated precisely. The analysis shows that these countermeasures can effectively resist this potential attack.