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Photonics Research

Security analysis of quantum noise stream cipher systems under quantization-attack

Jihui Sun, Lin Jiang, Anlin Yi, Wei Pan, Lianshan Yan

Published Oct 21, 2025• 2 citations
Publisher

Abstract

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 .

DOI
10.1364/prj.574013

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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 .
Quantum noise stream cipher (QNSC) is a physical-layer encryption approach for optical communications, and its security barrier is the "basis" that is used as the secret-key for symbol encryption. Regarding the fact that the basis is actually transmitted through the fiber, we propose a basis precoding scheme based on probabilistic constellation shaping, to better conceal the basis into the noise for security enhancement. Experimental results show our scheme can improve the security of the QAM/QNSC system in terms of Eve’s symbol error ratio.
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.
In this paper, we propose a novel physical layer security optical transmission scheme utilizing randomized quantization noise. The proposed approach encrypts low-order plaintext signals into ultra-high-order ciphertext using principles similar to quantum noise stream cipher (QNSC). While, the ultra-dense quadrature amplitude modulation (QAM) ciphertext waveforms are masked by intrinsic quantization noise. The combination of digital delta-sigma quantization and analog chaotic random scrambling not only produces randomized quantization noise, but also naturally supports one-time processing of masking ciphertext and generating keystream. Whereby, the in-band quantization noise (IBN) conceals nearby ciphertext levels bolstering security, and the out-of-band quantization noise (OOBN) is digitized to generate keystream using the Toeplitz hashing extractor. Experimental results show that 256-QAM plaintext signals were securely transmitted over standard single-mode fiber: 163-Gbps signals over 400 km, and 79.7-Gbps signals over 1800 km. To evaluate system performance, theoretical models for signal-to-noise ratio (SNR), bit error rate (BER), and number of masked signals (NMS) are derived as functions of the oversampling ratio (OSR). Our findings reveal a trade-off between transmission performance and security performance. Our results confirm that this scrambling effectively eliminates the correlation between the masking noise components. Toeplitz hashing extractor can effectively reduce the complexity of keystream generation and obtain a source-independent random keystream.
Quantum noise stream cipher where encrypted signals are masked by quantum noise and ASE noise provides a physical layer of security. It requires the transmitter and the receiver to share a stream cipher that is generated from a PRNG. Yet a correlation attack threatens its security due to the mathematical properties of PRNG. This paper discusses the security of QNSC system under correlation attacks. Our experiment results find that the security of the whole system depends on the cycle to refresh the seed key and the correlation between the incepted running key, original running key, and seed key. Furthermore, it is important to provide security for the QNSC system by maintaining low optical power. Besides, this new analytical method provides quantitative security analysis for a QNSC system under a correlation attack.
Quantum Noise Stream Cipher (QNSC) has emerged as a physical-layer encryption technique that exploits quantum noise and non-orthogonal coherent-state modulation to secure optical communication. However, the security of QNSC relies exceedingly on the secrecy and freshness of its seed key. Quantum Key Distribution (QKD), on the other hand, provides information-theoretically secure key exchange rooted in the laws of quantum mechanics. The convergence of these two paradigms, i.e., integrated QKD-QNSC architectures, offers a compelling solution to each of their limitations. In such integrated systems, QKD continuously supplies and refreshes the secret seed key that governs QNSC modulation. Thus, governing a unified security framework that couples provably secure key establishment with high-speed quantum-enhanced physical-layer encryption. This work presents a comprehensive review of QNSC systems, examining their operating principles, security models under various attacks, and their integration with QKD systems. We analyze the security interplay between the key generation and encryption layers and survey experimental demonstrations and architectural progress toward practical deployment. Furthermore, we identify the open challenges and future research directions that must be addressed to realize fully integrated, quantum-secured optical communication networks at a practical scale.
Abstract Data security is a key feature of future communications networks. Physical layer introduces rich physical mechanisms to increase the complexity of deciphering and provides extensive protection, but faces challenges in compatibility with commercial systems. Quantum noise stream cipher (QNSC) has been proposed as a promising solution to overcome this problem by fusing the stream cryptography regime with the quantum noise masking physical mechanism. However, it has limitations in terms of digital to analog conversion and clock data synchronization of ultra-high-order ciphertext as well as flexible control of masking noise. Here we report a 147.9-Gbps device-compatible quadrature amplitude modulation (QAM) QNSC secure scheme over 75-km fiber. Thanks to delta-sigma modulator, the transmission of 220 × 220-order QAM-QNSC signal are established through the low-order digital signal. We develop a theoretical model for flexibly regulating the transmission rate and security performance. Broadband optical chaos introduces true randomness and acts on the masking noise.