Physical security attacks on symbol masking schemes using time-spread cryptographic keys in digital coherent communication

In this study, we propose and evaluate a time-spread cryptographic key scheme for enhancing the physical layer security of optical communication systems. The proposed method diffuses cryptographic key information in the time domain, introducing complexity to the relationship between keys and symbols. This scheme is applied to both symbol-variable and symbol-fixed masking schemes, focusing on Nyquist dual-polarization M-ary quadrature amplitude modulation signals (M = 4, 16). Simulations are conducted to analyze the normalized generalized mutual information (NGMI) characteristics under various conditions, including eavesdropping scenarios with varying wiretap ratios and time-spreading parameters. Results demonstrate that the time-spread cryptographic key scheme effectively degrades the signal quality for eavesdroppers, maintaining NGMI below the forward error correction threshold (NGMI = 0.8) even at high eavesdropping rates of 50 %, 99 %, and 100 %. Furthermore, the scheme preserves the integrity of encryption randomness while requiring minimal modification to existing communication architectures. These findings establish the robustness of the proposed approach, and offer a promising solution for secure optical communication systems.