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IEEE Photonics Technology Letters

Symbol-Block Ciphering With CBC Mode for Secure Digital Coherent Optical Communication

Reika Suketomo, Keiji Shimada, Hodaka Amano, Takahiro Kodama

Published Jun 17, 2025• 1 citation
Publisher

Abstract

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.

DOI
10.1109/lpt.2025.3580417

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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.
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.
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 record-long 10,118-km fiber transmission with physical layer encryption is demonstrated utilizing a Y-00 cipher based on signal masking by quantum (shot) noise. The Y-00 cipher enables symmetric-key data encryption to ensure the security of the physical layer of optical communications. Irreducible secrecy without significant negative impact on transmission performance is achieved by the synergistic effect of combining seed-key-based high-order modulation and truly random shot noise inevitable in optical detection. This paper reports a comprehensive study of applying a phase-shift-keying (PSK) Y-00 cipher for ultra-long haul fiber transmission. Theoretical analysis shows that security-enhanced transmission over transoceanic-distance (>10,000 km) fiber is feasible when the quadrature PSK data signal is encrypted by converting to a PSK signal with 218 levels. Subsequently, 10,118-km standard single-mode fiber transmission of 48-Gbit/s line-rate dual-polarization PSK Y-00 cipher with 218 levels is experimentally demonstrated. An adequate signal quality above the Q-factor threshold of soft decision forward error correction is achieved together with sufficient signal masking by shot noise, yielding balanced transmission performance and high security in an ultra-long-haul PSK Y-00 cipher transmission system.
Abstract We provide experimental validation of quantum encryption in phase space using displacement operators in coherent states (DOCS) in a conventional coherent optical communication system. The proposed encryption technique is based on displacing the information symbols in the phase space using random phases and amplitudes to achieve encryption randomly and provide security at the physical layer. We also introduce a dual polarization encryption approach where we use two different and random DOCS to encrypt the X and Y polarizations separately. The experimental results show that only authorized users can decrypt the signal correctly, and any mismatch in the displacement operator coefficients, amplitudes, or phases will lead to a bit error ratio (BER) of approximately 50%. We also compare the performance of the system with and without encryption over 80 km of standard-single mode fiber (SSMF) transmission to assess the added penalty of such encryption. The achieved net bit rates are 224, 448, and 560 Gb/s for QPSK, 16QAM, and 32QAM modulation formats, respectively. The experimental results showcase the efficacy of the DOCS encryption technique in resisting various decryption attempts, demonstrating its effectiveness in ensuring the security and confidentiality of transmitted data in a real-world transmission scenario.
A Quantum Stream Cipher (QSC) using Quadrature Amplitude Modulation (QAM) is presented to greatly increase the secure degree compared with ASK or PSK/QSC. We propose encoding multi-bit data in one symbol with a multi-bit basis state, resulting in QAM/QSC, which employs amplitude and phase encryption of the light beam simultaneously. A 16 QAM/QSC experiment at 10 Gbit/s was successfully carried out over 160 km using a digital coherent optical transmission technique, where 16 QAM data were encrypted in a constellation with 32 × 32~4096 × 4096 symbols. We show experimentally that the Number of Masked Signals (NMS) in the quantum noise Γ(QAM) for QAM/QSC becomes a square multiple larger than Γ(ASK) for ASK/QSC. Γ(QAM) exceeds 10,000. This result indicates that the QSC technique is more robust against eavesdroppers than ASK or PSK/QSC.