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Physica Scripta

Security analysis of continuous-variable source-independent quantum random number generator under light-induced photorefraction attack

Yuanhao Li, Yangyang Fei, Weilong Wang, Xiangdong Meng, Hong Wang, Qianheng Duan, Yu Han, Zhi Ma

Published Jul 16, 2025• 0 citations
Publisher

Abstract

Abstract Continuous-variable source-independent quantum random number generator (CV-SI-QRNG) can produce random numbers with an untrusted source. However, the material properties of trusted measurement devices may lead to potential security risks, such as Lithium niobate (LiNbO3, LN) devices, which must be accurately characterized and considered in security analysis. The performances of LN devices are sensitive to photorefractive effect (PE) in LN, which has been shown to be exploited for malicious attacks on quantum key distribution. However, the research on the security problems associated with loopholes induced by PE has not been carried out in the practical security of CV-SI-QRNG. In this paper, we give a light-induced photorefraction attack against homodyne-based CV-SI-QRNG as well as the corresponding security analysis. Simulation results show that eavesdroppers can use very weak irradiation beam to change the splitting ratio of directional coupler and the phase shift of phase modulator, which leads to an overestimation on the number of extractable random bits. Our work reveals that PE-related security issues should be carefully considered in the LN-based QRNG.

DOI
10.1088/1402-4896/adf0a1

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Abstract Continuous-variable source-independent quantum random number generator (CV-SI-QRNG) can produce random numbers with an untrusted source. However, the material properties of trusted measurement devices may lead to potential security risks, such as Lithium niobate (LiNbO3, LN) devices, which must be accurately characterized and considered in security analysis. The performances of LN devices are sensitive to photorefractive effect (PE) in LN, which has been shown to be exploited for malicious attacks on quantum key distribution. However, the research on the security problems associated with loopholes induced by PE has not been carried out in the practical security of CV-SI-QRNG. In this paper, we give a light-induced photorefraction attack against homodyne-based CV-SI-QRNG as well as the corresponding security analysis. Simulation results show that eavesdroppers can use very weak irradiation beam to change the splitting ratio of directional coupler and the phase shift of phase modulator, which leads to an overestimation on the number of extractable random bits. Our work reveals that PE-related security issues should be carefully considered in the LN-based QRNG.
Modulators based on the Mach-Zehnder interferometer (MZI) structure are widely used in continuous-variable quantum key distribution (CVQKD) systems. MZI-based variable optical attenuator (VOA) and amplitude modulator can reshape the waveform and control the intensity of the coherent state signal to realize secret key information modulation in the CVQKD system. However, these devices are not ideal; internal and external effects like non-linear effects and temperature may degrade their performance. In this paper, we analyzed the security loophole of CVQKD under the photorefractive effect (PE), which originates from the crystal characteristics of lithium niobate (LN). It is found that the refractive index change of modulators because of PE may lead to an overestimate or underestimate of the final secret key rate. This allows Eve to perform further attacks like intercept-resend to get more secret key information. To solve this problem, several countermeasures are proposed, which can effectively eliminate potential risks.
Abstract In a practical continuous‐variable quantum key distribution (CVQKD) system, a new security loophole opened by the photorefractive effect of lithium niobate‐based (LN‐based) modulators is explored. Exploiting this loophole, a quantum hacking strategy called the induced‐photorefraction attack is proposed. This strategy utilizes the induced photorefraction on the LN‐based modulators to conceal a classical intercept‐resend attack. Specifically, the induced photorefraction biases the response curve of the LN‐based modulator, which affects the intensity of the modulated signal. Based on the investigation of channel parameter estimation and the analysis of the secret key rate of the practical CVQKD system, the simulation results show that the communication parties overestimate the secret key rate, which reveals that Eve can actively open a security loophole via the induced‐photorefraction attack to obtain secret key information. To defend against this attack, a random monitoring scheme for modulation variance is proposed to detect the attack, along with an improved optical power limiter to mitigate the irradiation beam. Additionally, a Sagnac‐based IM is suggested to stabilize the practical CVQKD system. Furthermore, the impact of this attack is analyzed on continuous‐variable measurement‐device‐independent QKD systems ((CV‐MDI‐QKD) and on local‐local‐oscillator (LLO) systems and propose countermeasures for each. Finally, the passive‐state‐preparation CVQKD protocol is proposed to fundamentally close the security loopholes originating from the modulators.
Lithium niobate (LiNbO3, LN) devices play critical roles in quantum information processing. However, for special applications like quantum key distribution (QKD), the characteristics of materials and devices and their impact on practical systems must be intensively inquired. For the first time, we reveal that the photorefractive effect in LN can be utilized as a potential loophole to carry out malicious attacks by the eavesdroppers. We take a commercial LN-based variable optical attenuator as an example to demonstrate the method we named Induced-photorefractive attack (IPA) and propose two techniques to enable controllable attacks. Our results show that eavesdroppers can fulfill an efficient source-side attack by injecting an optimized irradiation beam with only several nanowatts, which is realistic when accessing commercial fiber channels. These measure and techniques can be employed for all individual and on-chip LN devices and initially explored a new security branch for system design and standardization of real-life QKD.
Modulators based on the Mach-Zehnder interferometer (MZI) structure are widely used in continuous-variable quantum key distribution (CVQKD) systems. MZI-based variable optical attenuator (VOA) and amplitude modulator can reshape the waveform and control the intensity of coherent state signal to realize secret key information modulation in CVQKD system. However, these devices are not ideal, internal and external effects like non-linear effect and temperature may degrade their performance. In this paper, we analyzed the security loophole of CVQKD under the photorefractive effect (PE), which originates from the crystal characteristic of lithium niobate (LN). It is found that the refractive index change of modulators because of PE may lead to an overestimate or underestimate of the final secret key rate. This allows Eve to perform further attacks like intercept-resend to get more secret key information. To solve this problem, several countermeasures are proposed, which can effectively eliminate potential risks.
Heterodyne-based continuous-variable source-independent quantum random number generator (CV-SI-QRNG) can produce true random numbers without any assumptions on source. However, practical implementations always contain imperfections, which will greatly influence the extractable randomness and even open loopholes for eavesdroppers to steal information about the final output. In this work, based on the theoretical model, we systematically analyzed the effects of imperfect implementations on the practical security of heterodyne-based CV-SI-QRNG. The influences of local oscillator (LO) fluctuation under imbalanced heterodyne detection are first analyzed. The simulation results show that the lower bound of extractable randomness will be overestimated without considering the influence of LO fluctuation, which will threaten the practical security of CV-SI-QRNG system. Moreover, we analyze the effects of the degree of imbalance and the magnitude of LO fluctuation on evaluating the extractable randomness. Finally, we investigate the impact of an imperfect phase modulator on the practical security of CV-SI-QRNG and find it will reduce the extractable randomness. Our analysis reveals that one should carefully consider the imperfections in the actual implementations of CV-SI-QRNGs.
Lithium niobate (LN) devices have been widely adopted in systems for quantum key distribution (QKD), but the photorefractive effect in LN devices can be exploited as a potential loophole for malicious attacks by eavesdroppers. This work points out a vulnerability of LN devices of which manufacturers and users must become aware, so that appropriate countermeasures may be implemented. Moreover, the physical mechanism and techniques introduced in this work open an important avenue to the security analysis of QKD systems.