Secure IM-DD transmission via asymmetric key management and cascaded Josephus-Latin square encryption

This paper proposes a secure encryption scheme for intensity modulation direct detection (IM-DD) data center interconnects employing 4-level pulse amplitude modulation (PAM4). The scheme integrates asymmetric key management with chaos-based symbol-level encryption to enhance physical-layer security. For master key management, a non-interactive Goldwasser-Micali (GM) based distribution is introduced for short-key scenarios. An interactive Elliptic Curve Diffie-Hellman (ECDH) based agreement with forward secrecy is adopted to support longer keys in high-security deployments. The master key is dynamically updated per frame, from which session keys are derived via a hash function to drive chaotic systems for generating chaotic sequences. For symbol-level encryption, we propose a cascaded architecture combining a chaos-driven variable-step Josephus permutation for symbol scrambling with Latin square encryption for symbol value substitution. Experimental results at 100 and 140 Gbit/s over 2 km standard single-mode fiber show that the bit error rate (BER) performance of the encrypted PAM4 signal is basically comparable to that of the unencrypted link, achieving receiver sensitivities of approximately -6.7 dBm and -5.8 dBm, respectively, at the 7% forward error correction threshold. For an eavesdropper without the correct key, the recovered data has a BER of approximately 0.5, rendering the extraction of any meaningful information impossible. Therefore, the proposed encryption scheme enhances physical layer security without requiring dedicated optical hardware or incurring any additional performance penalty.