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International Conference on Optical Communications and Networks (ICOCN)

Ciphertext Mapping Method based on Gray Code in Quantum Noise Stream Cipher

Junjia Li, Yajie Li, Bo Wang, Kai Wang, Yongli Zhao, Jie Zhang

Published Aug 23, 2021• 2 citations
Publisher

Abstract

A ciphertext mapping method based on Gray code is proposed to improve the security of quantum noise stream cipher (QNSC). The experiment is conducted in 1024 × 1024 -quadrature amplitude modulation (QAM) coherent optical communication system over 300 km. Experimental results verify that the proposed scheme does not affect the transmission performance. Meanwhile, the performance analysis reveals that the attacker's bit error rate (BER) increases to about 0.5 per bit position per symbol. The advantage of proposed scheme is that QNSC makes use of basis with fewer bits for encryption/decryption.

DOI
10.1109/icocn53177.2021.9563675

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Journal of Optical Communications and Networking•Jan 7, 2026•0 citations
A ciphertext mapping method based on Gray code is proposed to improve the security of quantum noise stream cipher (QNSC). The experiment is conducted in 1024 × 1024 -quadrature amplitude modulation (QAM) coherent optical communication system over 300 km. Experimental results verify that the proposed scheme does not affect the transmission performance. Meanwhile, the performance analysis reveals that the attacker's bit error rate (BER) increases to about 0.5 per bit position per symbol. The advantage of proposed scheme is that QNSC makes use of basis with fewer bits for encryption/decryption.
Among physical layer encryption schemes, the quantum noise stream cipher (QNSC) has garnered significant attention due to its compatibility with high-speed commercial fiber-optic communication systems. After careful analysis of the encryption scheme, we reveal that QNSC transmission systems exhibit a security vulnerability in their encoding scheme. This vulnerability limits quantum noise to alerting high-order information bits in plaintext-dependent regions, creating structured ciphertext concealment. Numerical simulation and experimental verification both indicate that an eavesdropper can use quantization-attack to crack this vulnerability. Existing security assessment methods will overestimate the system’s security under quantization-attack. In addition, system security demonstrates a strong linear dependence on the plaintext modulation format, rather than the ciphertext modulation format as is widely presumed. To further enhance system security, the probability distribution is further introduced into the encoding process of ciphertext. The experiment results show that we not only achieved random concealment of ciphertext by quantum noise but also enhanced the eavesdropper’s symbol error rate by ∼86 % and maximally expanded the key space of the QAM-QNSC system by 2 27.2 .
The effect of quantum noise on different bit positions of ciphertext is different in Quantum Noise Stream Cipher. This feature makes the QNSC system vulnerable to different attacks, e.g. fast correlation attack. To balance the effect of quantum noise on different bit positions, this paper proposes a ciphertext mapping method based on Bitwise NOT Operation (BNO). The performance of BNO-based 16QAM/QNSC is investigated with different intensity noises and basis states. The simulation results demonstrate that bit error rate in different bit positions of ciphertext is close to 0.5 by using BNO-based 16QAM/QNSC. In addition, the proposed method has little influence on symbol error rate of Bob.
In this letter, we conduct an experiment of quantum noise stream cipher scheme based on 3-channel-QAM mapping and DFTs-OFDM in optical back-to-back and 160km standard single mode fiber. The fusion of 3D mapping and DFTs-OFDM offers sufficient security for QAM/QNSC within the dual polarization I/Q modulator system. Notably, the proposed 3D-mapping approach demonstrates improved minimum Euclidean distance and bit error rate performance compared to conventional 2D mapping. Experimental results confirm that our 3-channel-QAM/QNSC system achieves better transmission and higher security performance at the same base length than conventional QAM/QNSC.
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.
A quantum noise stream cipher scheme (QNSC) based on triangular quadrature amplitude modulation (TQAM) and the secret probabilistic shaping (SPS) is studied. Compared with rectangular quadrature amplitude modulation (QAM), TQAM is a more compact symmetrical structure, which results in better transmission performance and greater noise masking number. Then, SPS is realized, where the probabilistic shaping information as the secret key is masked into noise and transmitted with the encrypted signal simultaneously. The combination of SPS and QNSC makes it impossible for an attacker to obtain the ciphertext, which enhances the security of QNSC scheme without introducing additional key. TQAM and probabilistic shaping improve the transmission performance of QNSC scheme. Under 7% forward error correction (FEC) threshold in back-to-back system, compared with QAM/QNSC systems, TQAM/QNSC provides an OSNR gain of ∼ 0.6 dB, and SPS-TQAM/QNSC provides an OSNR gain of ∼ 1.2 dB. The proposed scheme is demonstrated with the net data rate of 34.28 Gbit/s over 140 km single-span standard single mode fiber.
The quantum noise stream cipher (QNSC) scheme, grounded in Heisenberg’s uncertainty principle, can facilitate secure communication by employing noise to obfuscate multi-base encrypted signals. The current research primarily focuses on point-to-point performance, while studies on point-to-multipoint schemes remain limited due to compatibility issues with multiple access technologies. This study presents a QNSC scheme for passive optical networks based on code-division multiple access (CDMA). In our scheme, different user datasets are combined into one dataset via CDMA and then encrypted using the QNSC protocol. Each user can recover their corresponding dataset from the encrypted signal by employing the same security keys along with a distinct pseudo-noise code (PN code). Through simulation studies, we found that under the condition of being able to decode correctly, as the number of users increases, the optical signal-to-noise ratio (OSNR) at the receiver decreases, which leads to a gradual rise in the eavesdropper’s detection failure probability (DFP). Furthermore, the network capacity is correlated with the number of modulation bits, as the increase in modulation bits directly translates to an enhancement in the data’s carrying capability. We have conducted verification experiments, and the results confirmed that four users can acquire 64 QAM/QNSC plaintext data with an average transmission rate of 600 Mbit/s over a transmission distance of 20 km. These data were encrypted in the constellation with 64×64 symbols, achieving a DFP of up to 99.3%, and each user successfully decoded them via CDMA. We have successfully extended the fundamental point-to-point QNSC scheme to accommodate point-to-multipoint passive optical networks, establishing the foundational framework for the future large-scale deployment of quantum networks.