Quantum Hacking: Induced-Photorefraction Attack on a Practical Continuous-Variable Quantum Key Distribution System

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.