Uniquely designed optical cryptosystem for watermarking of double-color images in the fractional Hermite-transform domain

We propose a robust optical cryptosystem for securely watermarking and transmitting two-color images by leveraging the fractional Hermite-transform (FrHMT). The method employs elliptic curves, Arnold’s cat map, and affine transformation for effective pixel scrambling, whereas vortex lens phase masks enhance key security by expanding the key space and simplifying key distribution. The QZ algorithm merges the two images into a single encrypted output, introducing nonlinearity and enabling efficient transmission, whereas dual-layer protection is achieved by embedding the encrypted data within a host image via watermarking, ensuring covert communication. The process begins by converting input color images into indexed formats via colormap extraction. This step reduces computational complexity while preserving essential color information, making subsequent encryption and watermarking more efficient. The scheme is validated and evaluated through extensive experiments, including histogram analysis, mesh plots, and correlation distribution plots, confirming its resilience against statistical attacks. This system is particularly useful for confidential document transmission and multimedia copyright protection. In addition, a comparative analysis with existing encryption schemes highlights the advantages of the proposed cryptosystem in terms of security and efficiency.