Authenticated RGB image encryption using switched inverse substitution and multiplex Cayley-graph diffusion

Due to the high correlation of RGB images in both spatial and channel domains, the encryption should have the characteristics of nonlinear confusion, spatial and cross-channel diffusion, exact reversibility and integrity protection. This paper proposes SGEM-RGB-24, an authenticated RGB image-encryption scheme in which each pixel is represented as an element of \({\mathbb{F}}_{{2}^{24}}\) . Each round combines position-dependent whitening, a key-dependent switched inverse substitution over \({\mathbb{F}}_{{2}^{24}}\) , MDS mixing of the colour coordinates and two-pass diffusion on a three-layer multiplex Cayley graph based on \(\text{P}\text{S}\text{L}(\text{2,7})\) . Independent material for each component is derived using HKDF-SHA-256 and AES-256-CTR under domain-separated contexts; HMAC-SHA-256 is used to authenticate the encrypted material, the nonce and image parameters before decryption. It is shown that each stage is invertible. For three medical images and three general colour images, all images were recovered exactly, the mean entropy value of the ciphertexts was 7.999955, the mean absolute adjacent-pixel correlation was 0.001336, and, after a one-bit plaintext change, the mean NPCR, UACI and bit avalanche were 99.609521%, 33.463188% and 49.999446%, respectively. All graphs met the specified spectral-gap condition and all tested modifications were rejected. The scheme is described as a research scheme, which needs more formal cryptanalysis before standardization.