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.
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.
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.
Lithium niobate materials, which have the potential to fabricate lasers, modulation devices, and photodetectors, are widely used in quantum information processing due to their exceptional optical and electro-optical properties. However, the photorefractive effect in lithium niobate materials may cause the quantum device to deviate from its ideal operation model, which is an important assumption for the security of quantum key distribution. Here, we demonstrate the practical security of the continuous-variable quantum key distribution protocol under the photorefractive effect, where the eavesdropper Eve injects a 488 nm visible light into the source-side variable optical attenuator and excites the photorefractive effect phenomenon in a lithium-niobate-based dual-waveguide. In particular, we derive the photorefractive-effect-induced intensity change of a variable optical attenuator and then the corresponding parameter estimation process, showing the effectiveness of this attack for various irradiation powers and waveguide technologies (including proton-exchanged and annealed-proton-exchanged waveguides). To show its effect on practical continuous-variable quantum key distribution systems, we present the composable finite-size security of one-way continuous-variable quantum key distribution and continuous-variable measurement-device-independent quantum key distribution protocols. Numerical results show that this induced-photorefraction attack can break the security even with a low irradiation power, such as 3 [Formula: see text] (or about 0.21 [Formula: see text]). In addition, we find that Eve's optimal attack strategy against continuous-variable measurement-device-independent quantum key distribution is related to the information reconciliation process by comparing three different attack strategies. Finally, we discuss some possible countermeasures to resist induced-photorefraction attacks to enhance the security of the system.
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.
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.
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.
Continuous-variable measurement-device-independent quantum key distribution (CV-MDI QKD) can defend all detection-side attacks effectively. Therefore, the source side is the final battlefield for performing quantum hacking attacks. This paper investigates the practical security of a CV-MDI QKD system under a light-injection attack. Here, we first describe two different light-injection attacks, i.e., the induced-photorefractive attack and the strong-power injection attack. Then, we consider three attack cases where Eve only attacks one of the parties or both parties of the CV-MDI QKD system. Based on the analysis of the parameter estimation, we find that the legitimate communication parties will overestimate the secret key rate of the system under the effect of a light-injection attack. This opens a security loophole for Eve to successfully obtain secret key information in a practical CV-MDI QKD system. In particular, compared to the laser-damage attack, the above attacks use a lower power of injected light and have a more serious effect on the security of the system. To eliminate the above effects, we can enhance the practical security of the system by doping the lithium niobate material with various impurities or by using protective devices, such as optical isolators, circulators, optical power limiters, and narrow-band filters. Apart from these, we can also use an intensity monitor or a photodetector to detect the light-injection attack.