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OptoElectronics and Communications Conference and International Conference on Photonics in Switching and Computing (OECC/PSC)

Demonstration of Symbol Cipher Block Chain Mode with Polarity-Based Phase Shifting

Reika Suketomo, Keiji Shimada, Hodaka Amano, Takahiro Kodama

Published Jun 29, 2025• 0 citations
Publisher

Abstract

A binary phase shifting symbol block cipher operating in CBC mode on a standard DP16QAM signal revealed a$Q^{2}$penalty of 0.9 dB in a$2240-\text{km} 7$-ch WDM transmission experiment over a nonlinear fiber channel.

DOI
10.23919/oecc/psc62146.2025.11109498

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A binary phase shifting symbol block cipher operating in CBC mode on a standard DP16QAM signal revealed aQ2Q^{2}Q2penalty of 0.9 dB in a2240−km72240-\text{km} 72240−km7-ch WDM transmission experiment over a nonlinear fiber channel.
In recent years, the growing demand for security in optical fiber communications, driven by the development of high-capacity communications based on digital coherent transmission technology, has necessitated advanced physical-layer security measures. This study focuses on symbol block ciphering techniques, evaluating the security performance of applying the cipher block chaining (CBC) mode with a 2-bit key. Specifically, dual-polarized-16 quadrature amplitude modulation signals were encrypted using CBC mode, and the signal quality during transmission over 2240 km of standard single-mode fiber was examined through numerical analysis and experiments. The results demonstrated that CBC mode enables proper signal recovery even in transmission paths affected by nonlinear effects, albeit with slight performance penalties regarding optical signal to noise ratio and transmission distance. Furthermore, by comparing the reception accuracy of an eavesdropper without the encryption key, the study highlighted the potential of CBC mode to provide enhanced security.
We present an experimental demonstration of a physical-layer encryption scheme for high-speed optical communication, focusing on cipher block chaining (CBC)-mode symbol-block phase encryption applied to 100 Gbaud dual-polarization quadrature phase-shift keying (DP-QPSK) signals within a 1.6 Tbps optical waveband. In the proposed approach, each of the two 100 Gbaud subchannels is independently encrypted using distinct 128-bit AES keys, yielding a combined key space of 2^256 and ensuring strong security without key interdependence. Unlike conventional bit-level operations, encryption is performed directly on symbol blocks, which increases confidentiality while remaining compatible with high-baud-rate DSP implementations. Experimental transmission over 560 km of standard single-mode fiber with multiple ROADM nodes confirms the feasibility of this scheme. The results show that, although CBC-mode encryption introduces additional phase variations and slightly degrades Q2performance due to stronger interactions with fiber nonlinearities, the Q2values consistently remained above the FEC threshold, demonstrating reliable transmission. Furthermore, the proposed method benefits from practical integration with forward error correction and exhibits tolerance to cascaded ROADM traversals, with only a modest Q2penalty of ~0.9 dB after 10 nodes. These findings indicate that CBC-mode symbol-block encryption can effectively enhance the confidentiality of high-capacity optical networks while maintaining transmission performance, thereby advancing the physical-layer security of next-generation waveband-based optical transport systems.
We describe in detail our recent demonstration of a 10 Tbit/s secure physical layer transmission that we achieved by using digital coherent QAM quantum noise stream cipher (QNSC) and injection-locked WDM techniques. We used an FPGA-based transmitter and receiver to demonstrate a 165 channel polarization-multiplexed WDM 5 Gbaud 128 QAM/QNSC (70 Gbit/s) on-line transmission over 160 km with a spectral efficiency of 6 bit/s/Hz. In the present system, the original 128 QAM data were encrypted in a 1024 × 1024 QAM format using basis information. The encrypted signal was then masked by a large ASE noise, which reduced the detection “success” probability for an eavesdropper to 0.13% for each symbol. Furthermore, the multiplicity of the original QAM data and the seed keys used to generate the basis information were arbitrarily changed with time, which makes the decryption much more difficult.