  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.
