Security Analysis of Gaussian and Discrete Modulations in FSO/CV-QKD Systems Employing LLO Under Phase and Amplitude Attacks

Continuous variable quantum key distribution (CV-QKD) using a local-local oscillator (LLO) is recently proposed to overcome security issues in conventional CV-QKD systems. However, Eve can manipulate the phase or amplitude of the phase reference pulse (PRP) transmitted over the insecure quantum channel, which has opened the door to new security issues. Maintaining optimal performance and preventing Eve’s activities on the quantum channel depends on such a choice of modulation technique. In this paper, the performance of CV-QKD employing LLO over the free space optical (FSO) channel under weak turbulence conditions is investigated. Channel transmittance is introduced into the system model according to the log-negative Weibull distribution. We have adopted the trusted noise model and included the channel phase distortion in the excess noise calculations. We have also evaluated the secret key rate (SKR) using Gaussian modulation (GM) and discrete modulation (DM) protocols. We have reported the superiority of GM in achieving the highest SKR over long distances with the optimum choice of the modulation variance under Eve’s attack on the PRP. Moreover, we analyzed phase and amplitude attacks on the PRP and showed that phase attacks are more severe and deteriorate the communication link more rapidly than amplitude attacks.