Chaotic Encryption for Secure Image Transmission in Free-Space Optical Systems

Abstract

This work presents a secure Free-Space Optical (FSO) communication system based on a four-dimensional memristor-based hyperchaotic image encryption scheme. The proposed framework combines hyperchaotic cryptography with optical wireless communication to ensure both high-level data security and reliable image transmission under atmospheric disturbances. A 4D hyperchaotic system was designed and analyzed using time-series analysis, phase-space trajectories, and Lyapunov exponents, confirming strong randomness and high sensitivity to initial conditions. Three image encryption methods based on XOR diffusion, arithmetic addition, and QR matrix decomposition were developed and evaluated using entropy, correlation, NPCR, UACI, and key sensitivity analyses. The obtained entropy values reached 7.9987, 7.9983, and 7.9893, while NPCR values exceeded 99.55% and UACI values remained close to 33%, demonstrating excellent security performance and strong resistance against statistical and differential attacks. In addition, the proposed system provided a large key space estimated at approximately ( 10112.). The proposed encryption framework was integrated into an FSO communication system using MATLAB and OptiSystem environments. The obtained results showed that clear weather achieved the best transmission performance, maintaining BER values below ( 10−5.) and Q-factor values above 4 up to approximately 1.6 km, with a BER of (8.68 × 10−22.) and a Q-factor of 9.51 at 1 km. Under moderate rain conditions, the acceptable communication range decreased to approximately 1.1 km with a BER of (4.27 × 10−9.) and a Q-factor of 5.75, while snow conditions reduced the effective range to approximately 0.4 km with a BER of (1.89× 10−4.) and a Q-factor of 3.56 due to severe atmospheric attenuation. Overall, the obtained results demonstrate that the proposed QR-based hyperchaotic encryption scheme provides an effective compromise between cryptographic security, diffusion capability, and transmission robustness, making the proposed framework a promising solution for next-generation secure optical wireless communication systems.