Image Encryption Algorithm Based on a Novel 2D Folded Sine-Logistic Map and Concentric-Ring Model.

Digital images, as a widely used multimedia carrier, are vulnerable to various security risks in open network environments. To enhance the security of digital images during storage and transmission, this paper proposes an image encryption algorithm based on a 2D folded sine-logistic map. First, the user master key and the SHA-256 hash of the plaintext image are combined to generate plaintext-related chaotic sequences. Second, a concentric-ring permutation is performed through three-ring partition, cyclic shifting, and route regrouping, with an additional channel crossover for RGB images to enhance inter-channel coupling. Finally, an enhanced hybrid diffusion mechanism with 4-bit nibble-swap perturbation is applied to strengthen pixel-value diffusion and local nonlinearity. Experimental results show that the average ciphertext entropy reaches 7.999, the adjacent-pixel correlation coefficient is reduced to 0.000016, and the NPCR/UACI values are close to their theoretical ideal levels. Combined with comprehensive security analyses, these findings demonstrate that the proposed scheme suppresses statistical characteristics and pixel correlations, resists brute-force, statistical, and differential attacks, and provides an efficient and secure solution for grayscale and color image transmission.