We demonstrate shared-key AES-CTR symbol-block encryption for coherent DP-QPSK, achieving penalty-free transmission over 841 km and 10 ROADMs while preserving constellations, maintaining DSP compatibility, and delivering strong physical-layer confidentiality for modern optical waveband secure systems.
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.
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.
A novel digital signal processing (DSP)-based scheme for physical layer security in coherent optical communication systems is proposed and numerically investigated. The optical layer signal encryption is accomplished by two dispersive elements and one phase modulator (PM) driven by a DSP-generated encryption key, whilst signal decryption uses similar components but with inverted dispersion values and security keys. A critical aspect of the DSP-based physical layer security is that the security keys, driving the PMs to hide/recover the data signals, must be highly unpredictable and noise-like, thus orthogonal frequency division multiplexing (OFDM) signals are employed as they possess these characteristics, they can also be easily generated and cover a suitably wide range of unique keys. Numerical simulations are conducted to determine optimum system parameters for achieving a high level of security, the key parameters requiring optimization are the dispersion of the dispersive elements and the bandwidth of the security keys. Using these determined optimum parameters, in-depth investigations are undertaken of encryption/decryption induced transmission performance penalties, sensitives to various parameter offsets and operation over various transmission distances. To observe any data signal dependencies, various performance metrics are investigated for different combinations of modulation formats (DQPSK and 16QAM) and baud rates (40 Gbaud and 100 Gbaud) for the transmitted data signals. The proposed DSP-based physical layer security scheme is shown to have the potential to achieve, in a low-cost and highly effective manner, a high level of physical layer security with acceptable performance penalties for existing coherent optical communication systems.
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.