Objective With the rapid development of technologies such as 6G, virtual reality, and artificial intelligence, information transmission and interaction are becoming increasingly frequent, leading to a growing amount of data transmitted through optical fiber networks every year. However, optical fiber transmission systems are vulnerable to illegal eavesdropping, making it crucial to ensure the security of data transmission in optical fiber networks. Quantum noise stream cipher (QNSC) is a new optical network security transmission technology that combines traditional encryption technology with physical layer security technology. Ensuring security requires both the transmitter and the receiver to share a secure key. However, in actual QNSC systems, ensuring the true randomness of the key is challenging, and the exposure of communication behavior may attract the attention of eavesdroppers, greatly increasing the risk of the key being broken by brute force. The imperceptibility of the transmitted signal can be enhanced by hiding the QNSC signal within noise. To further improve the security performance of QNSC communication systems, we propose a quantum noise stream cipher optical covert communication scheme based on differential phase shift keying (DPSK). The QNSC signal is covertly transmitted in a public channel for transmission. By increasing the key bases, the power of the mesoscopic coherent state can be increased, enhancing the transmission performance of the covert channel. The expression of quantum noise masking state bases for the DPSK balanced detection receiver is derived, and the tradeoff between concealment and transmission is discussed. Simulation results confirm the feasibility of covert communication, showing compatibility with wavelength- division multiplexing (WDM) systems. At a transmission distance of 250 km, the covert channel achieves error- free communication. Methods The structure of the DPSK-QNSC optical covert communication system is shown in Fig. 1. Covert communication is realized by injecting the signal into the WDM network. The data to be transmitted is differentially coded at the transmitter and then encrypted with a key. The legitimate transmitter, Alice, and the receiver, Bob, share the same seed key. A Gaussian pulsed laser source is used as the optical carrier signal and is broadened in the time domain due to dispersion. The encrypted ciphertext is loaded onto the optical carrier by a phase modulator and transmitted. A variable optical power attenuator (VOA) attenuates the optical signal to the mesoscopic coherent state. This signal is then injected into the public channel of the WDM system for transmission. The legitimate receiver uses an optical filter to extract the signal from the public channel, and the covert signal is recovered by matching the dispersion and decryption keys. Results and Discussions Simulation results demonstrate that the proposed DPSK-QNSC optical covert communication scheme can be integrated into a public WDM system. When the dispersion in the covert channel is low, it cannot be hidden in the time domain [Fig. 5(a)]. However, increasing the dispersion allows the covert channel to be completely hidden in the time domain [Fig. 5(b)]. The covert channel can be completely hidden in the frequency domain, preventing eavesdroppers from detecting its existence by observing the spectrum (Fig. 6). As the key bases increase, the power of the mesoscopic coherent state of the covert signal increases, reducing the bit error rate for the receiver and improving system performance. The covert channel has negligible impact on the public channel because it is located in the frequency band between two WDM channels. Conclusions To address the issue of communication exposure in QNSC systems, we propose an optical covert communication system based on DPSK-QNSC, offering dual security protection of concealment and confidentiality, thus enhancing the overall system security. The effect of key bases on the performance of the DPSK-QNSC optical covert communication system is explored. Simulation results show that the QNSC signal can be hidden in both frequency and time domains. Increasing the key bases improves the transmission performance of the system but at the cost of decreasing the hiding performance. Although increasing the key bases can raise the upper limit of the power of the mesoscopic coherent state for the covert signal and thus improve the transmission performance, the improvement is not linear. Key bases cannot be increased indefinitely, and hiding the covert channel requires that the transmit power remains below a certain value. Within this range, the optimal balance between hiding performance and transmission performance for the DPSK-QNSC optical covert communication system can be achieved.
