# Practical security of continuous-variable quantum key distribution with reduced optical attenuation

## TL;DR

This paper examines the security vulnerabilities in practical continuous-variable quantum key distribution systems caused by reduced optical attenuation, proposing countermeasures to prevent potential intercept-resend attacks.

## Contribution

It identifies a security loophole caused by reduced optical attenuation and introduces a real-time monitoring scheme with an optical fuse to enhance system security.

## Key findings

- Reduced optical attenuation can lead to overestimated secret key rates.
- A practical countermeasure with an optical fuse effectively resists intercept-resend attacks.
- The proposed scheme enables precise evaluation of the secret key rate in CVQKD systems.

## Abstract

In a practical 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.