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Asia-Pacific Conference on Communications (APCC)

Symbol-Block Encryption Using CBC Mode for 100 Gbaud DP-QPSK Waveband Systems

Takahiro Kodama, Reika Suketomo, Ryosuke Matsumoto, Tadashi Utsunomiya

Published Nov 26, 2025• 0 citations
Publisher

Abstract

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.

DOI
10.23919/apcc64555.2025.11279766

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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.
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 propose a modulation-layer encryption scheme for high-throughput coherent optical systems by extending the Advanced Encryption Standard (AES) with counter (CTR) mode to a symbol-block cipher operating in the modulation domain. AES keystream bits are mapped to per-symbol phase shifts, preserving constellation amplitude and ensuring full compatibility with standard coherent DSP. Independent- and shared-key strategies for multi-subcarrier transmission are analyzed. Simulations for 100-Gsymbol/s DP-4QAM show no OSNR penalty at the forward error correction threshold, while experiments with 96-Gsymbol/s two-subcarrier transmission over 10 ROADM nodes confirm no measurable Q² penalty. The results demonstrate that AES-CTR symbol-block encryption provides scalable modulation-layer encryption with negligible performance impact.
This study investigates a multi-stage digital phase-encryption scheme designed for secure coherent optical communication. While the proposed method is implemented in DSP and can be modeled as a software-level stream cipher, it is explicitly co-designed with and tightly coupled to the physical-layer modulation structure. Unlike conventional XOR-based bitwise schemes, the encryption is applied to the symbol phase domain. It is integrated into the transmitter's modulation process, enabling symbol-level security evaluated in conjunction with transmission physics. By applying K-level phase-shift secret keys toM-ary quadrature amplitude modulation (QAM) signals, this method increases the number of secret-key bits per symbol without altering the signal constellation in its baseline configuration. This approach enhances encryption strength while maintaining signal integrity and reducing quantization overhead in phase-only operation. Experimental evaluations on nonlinear optical transmission channels confirm that baseline digital phase encryption preserves Q-factor stability under self-phase modulation, cross-phase modulation, and four-wave mixing. The proposed scheme ensures error-free transmission at forward error correction thresholds, effectively preventing eavesdroppers from reconstructing symbols under worst-case observation conditions. Additionally, simulations analyzing generalized mutual information and quantization errors demonstrate that the encryption scheme can minimize signal-to-noise ratio penalties, even when employing high-order QAM formats with limited digital-to-analog and analog-to-digital conversion resolutions. As an optional extension, we also analyze pseudo-quantum-noise-based IQ spreading and explicitly quantify the associated trade-off between additional noise masking and increased sensitivity to quantization. These findings position multi-stage digital phase encryption as a practical and power-efficient modulation-layer encryption technique whose behavior is systematically characterized under realistic nonlinear channel and hardware constraints, rather than as an entirely new cryptographic primitive.
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.
Providing physical layer security at the lowest network layer in fiber-optic communication systems is a technical challenge worldwide. Here, we propose and experimentally demonstrate a pure hardware optical encryption scheme based on temporal spreading and self-feedback phase encryption for high-speed and long-distance physical-layer secure optical communication. A record high bit-rate-distance product of 6400 Gb/s km is successfully achieved by the secure transmission of a 32 Gb/s on-off-keying modulated confidential signal over a 200 km optical fiber link. The demonstrated scheme is fully compatible with conventional optical transmission systems and can be operated in a pluggable manner, which may pave a new path to ultra-high-speed physical-layer secure optical communication in the future.