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Journal of Physics B Atomic Molecular and Optical Physics

Performance analysis of practical continuous-variable quantum key distribution systems with weak randomness

Yi Zheng, Peng Huang, Jinye Peng, Yiqun Zhu, Guihua Zeng

Published Feb 20, 2020• 1 citation
Publisher

Abstract

Abstract Ideal randomness is one of most important conditions for the implementation of continuous-variable quantum key distribution (CVQKD). However, imperfect randomness may exist in a practical CVQKD system due to imperfections in Gaussian modulation or in random-number generators. Here we propose a weak randomness attack model in practical CVQKD systems to investigate the practical performance of the systems with imperfect randomness, where the prepared coherent states based on nonrandom state modulation are assumed to be known by Eve. Then, we perform parameter estimation and calculate the secret key rate of the system based on the proposed attack model. The simulation results demonstrate that a small imperfect randomness can dramatically reduce the secret key rate of the system, which may seriously limit the practical performance of the system. Therefore, true random numbers and perfect Gaussian modulation are key for practical CVQKD systems.

DOI
10.1088/1361-6455/ab787d

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Abstract Ideal randomness is one of most important conditions for the implementation of continuous-variable quantum key distribution (CVQKD). However, imperfect randomness may exist in a practical CVQKD system due to imperfections in Gaussian modulation or in random-number generators. Here we propose a weak randomness attack model in practical CVQKD systems to investigate the practical performance of the systems with imperfect randomness, where the prepared coherent states based on nonrandom state modulation are assumed to be known by Eve. Then, we perform parameter estimation and calculate the secret key rate of the system based on the proposed attack model. The simulation results demonstrate that a small imperfect randomness can dramatically reduce the secret key rate of the system, which may seriously limit the practical performance of the system. Therefore, true random numbers and perfect Gaussian modulation are key for practical CVQKD systems.
Quantum key distribution protocol requires true random numbers to prepare and measure quantum states, but the randomness may be weakened by applying the imperfect input random numbers or imperfect quantum devices. In this work, we propose the randomness attack model in the practical quantum key distribution system, and we analyze the final secret key rate under the randomness attack. The security analysis result demonstrates that the imperfect randomness can be amplified by considering the signal loss, thus the quantum key distribution system may be attacked even if the randomness has very small part of imperfection. Combining the practical experimental parameters with the small nonrandom probability parameters 0.10.1%0.1 and 11%1, we prove that the maximal transmission distance will be sharply reduced from 140 km to 60 and 12 km, respectively.
In practical continuous-variable quantum key distribution (CVQKD) systems, due to environmental disturbance or some intrinsic imperfections of devices, inevitably the local oscillator (LO) employed in a coherent detection always fluctuates arbitrarily over time, which compromises the security and performance of practical CVQKD systems. In this paper, we investigate the performance of practical CVQKD systems with LO fluctuating randomly. By revising the measurement result of balanced homodyne detection and embedding fluctuation parameters into security analysis, we find that in addition to the average LO intensity, the fluctuation variance also severely affects the secret key rate. No secret key can be obtained if fluctuation variance is relatively large. This indicates that in a practical CVQKD, LO intensity should be well monitored and stabilized. Our research can be directly applied to improve the robustness of a practical CVQKD system as well as be used to optimize CVQKD protocols.
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.
How to guarantee the practical security of continuous-variable quantum key distribution (CVQKD) system has been an important issue in the quantum cryptography applications. In contrast to the previous practical security strategies, which focus on the intercept-resend attack or the Gaussian attack, we investigate the practical security strategy based on a general attack, i.e., an arbitrated individual attack or collective attack on the system by Eve in this paper. The low bound of intensity disturbance of the local oscillator signal for eavesdropper successfully concealing herself is obtained, considering all noises can be used by Eve in the practical environment. Furthermore, we obtain an optimal monitoring condition for the practical CVQKD system so that legitimate communicators can monitor the general attack in real-time. As examples, practical security of two special systems, i.e., the Gaussian modulated coherent state CVQKD system and the middle-based CVQKD system, are investigated under the intercept-resend attacks.
Abstract For the practical Faraday mirror (FM) in a continuous-variable quantum key distribution (CVQKD) system there exists a rotation angle deviation that can lead to a loss of photon number when a light message passes through the polarization beam splitter. Here we employ the entanglement-based model to research the influence of the FM’s imperfection. Based on parameter estimation, the imperfection can introduce a loophole for Eve to obtain information. The secret key rate of the CVQKD will be impaired. Finally, a novel method is proposed to ensure system performance by compensating optical power, and we further study the relationship between the secret key rate and the modulation variance.
Continuous-variable quantum key distribution (CV–QKD) is a system that provides secret keys for symmetric key systems. In the application of CV–QKD, the practical security of the system is crucial. In this article, we investigate the practical security issues caused by non–ideal Gaussian modulation, which is caused by fitting defects of the amplitude modulator’s (AM) modulation curve. We provide the effect of fitting error on parameter estimation. We also give the relationship between the fitting order and the secret key rate. The simulation results indicate that the system is completely unable to communicate during first–order fitting. During second–order fitting, the system’s performance decreases by more than half. During third–order fitting, the system’s performance will be consistent with the ideal. Therefore, to ensure the performance of the CV–QKD system, the fitting order must be at least three or higher.