A dynamic physical-layer encryption scheme based on plaintext-adaptive sliding windows for secure transmission over a 60-km hollow-core fiber link

A plaintext-adaptive sliding-window (PASW) physical-layer encryption scheme is proposed for secure orthogonal frequency-division multiplexing (OFDM) transmission over a 60 km hollow-core fiber link. The proposed PASW mechanism enables plaintext-adaptive symbol-level permutation by dynamically varying window partitioning and permutation mapping according to symbol features. Within each window, a dual-layer permutation is performed through key-driven index mapping and quotient polynomial ring automorphism, with keys generated by a cascaded two-dimensional Hénon and iterative chaotic mapping with infinite collapses. Theoretical analysis further characterizes the influence of key PASW parameters on computational complexity, symbol diffusion capability, and window adaptation capability, providing guidance for practical parameter selection. Experimental verification is conducted in a 16-quadrature amplitude modulation (QAM) OFDM system over a 60 km hollow-core fiber link. The results show that the proposed scheme does not significantly affect the legitimate receiver performance, while the bit error rate at the unauthorized receiver remains close to 0.5 over the entire received optical power range. Security analyses show that the proposed key generation scheme exhibits high sensitivity and a large key space, while statistical evaluations verify the high randomness of the generated permutation sequences. Moreover, under 16-QAM modulation, the PASW mechanism achieves a 93.75% constellation-domain symbol change rate and a 47.33% Euclidean-distance-based unified average changing intensity, indicating effective plaintext diffusion and resistance to differential attacks.