This paper addresses the challenge of achieving information-theoretic security in semantic communication (SeCom) over a wiretap channel, where a legitimate receiver coexists with an eavesdropper experiencing a poorer channel condition. Despite previous efforts to secure SeCom against eavesdroppers, achieving information-theoretic security in such schemes remains an open issue. In this work, we propose a secure digital SeCom approach based on superposition codes, aiming to attain nearly information-theoretic security. Our proposed method involves associating semantic information with satellite constellation points within a double-layered constellation map, where cloud center constellation points are randomly selected. By carefully allocating power between these two layers of constellation, we ensure that the symbol error probability (SEP) of the eavesdropper decoding satellite constellation points is nearly equivalent to random guessing, while maintaining a low SEP for the legitimate receiver to successfully decode the semantic information. Simulation results showcase that the Peak Signal-to-Noise Ratio (PSNR) and Mean Squared Error (MSE) for the eavesdropper's reconstructed data, using our proposed method, can range from decoding Gaussian-distributed random noise to approaching the variance of the data. This validates the ability of our method to achieve nearly information-theoretic security, demonstrating superior data security compared to benchmark methods.

翻译：本文针对窃听信道下语义通信（SeCom）实现信息论安全的挑战，其中合法接收方与信道条件较差的窃听方共存。尽管以往研究致力于抵御窃听者的语义通信安全，但在此类方案中实现信息论安全仍是一个悬而未决的问题。本文提出一种基于叠加码的保密数字语义通信方法，旨在实现近乎信息论安全。所提方法将语义信息与双层星座图中的卫星星座点相关联，同时随机选择云中心星座点。通过谨慎分配这两层星座的功率，我们确保窃听方解码卫星星座点的符号错误概率（SEP）近乎等同于随机猜测，而合法接收方解码语义信息的SEP保持较低水平。仿真结果表明，采用所提方法，窃听方重构数据的峰值信噪比（PSNR）和均方误差（MSE）可从解码高斯分布随机噪声的范围变化至接近数据方差。这验证了所提方法实现近乎信息论安全的能力，相较于基准方法展现出更优的数据安全性。