Coverless hiding for multiple color underwater images based on a 4D hyperchaotic system and OAM multiplexing...

To meet the demand for high-security image transmission in resource-constrained underwater environments, this paper proposes a coverless hiding scheme for multiple color underwater images based on what is believed to be a novel four-dimensional (4D) hyperchaotic system and orbital angular momentum (OAM) multiplexing holography. First, a 4D hyperchaotic system with double positive Lyapunov exponents is designed, providing a secure keystream. Second, Bayer sampling is introduced to compress each color image into a single-channel grayscale image, significantly reducing the data volume while preserving the ability to recover color information. Subsequently, by combining OAM multiplexing holography with the complex-domain inverse diffraction pre-compensation (CIDP) algorithm, multiple images are successfully compressed into two ciphertext images. Furthermore, coverless hiding is applied to the encrypted ciphertext, effectively avoiding the security risk of drawing the attention of potential attackers during storage and transmission. Finally, through communication system simulations based on orthogonal frequency-division multiplexing (OFDM) and generalized frequency-division multiplexing (GFDM), the robustness and reliability of the proposed scheme are comprehensively verified, addressing the lack of systematic validation in existing underwater image encryption methods. In the security experiments, the key space reaches up to 2⁵⁰⁸, and the adjacent-pixel correlation coefficients in all three directions are close to 0. In the robustness experiments, both the correlation coefficient (CC) and structural similarity (SSIM) values of the reconstructed images are consistently satisfactory. These results confirm that the proposed scheme not only achieves extremely high security but also exhibits excellent anti-interference capability; in particular, highly reliable image reconstruction is successfully achieved in Rayleigh fading channel transmission tests based on OFDM/GFDM systems. The proposed scheme exploits both the high sensitivity of chaotic systems to initial conditions and the compression advantage of OAM high-dimensional data multiplexing. Through dimension reduction for multi-image fusion, the transmission load on the underwater transmitting end is greatly reduced, providing a novel solution that balances security and lightweight design for high-capacity, high-security underwater information transmission.