ECPD-MIE: An adaptive entropy-controlled chaotic permutation-diffusion scheme for secure multi-image encryption of grayscale and color images.

The rapid growth of multimedia communication has significantly increased the demand for robust encryption techniques capable of securely handling heterogeneous image data. Conventional image encryption schemes typically focus on either grayscale or color images independently, thereby limiting their applicability in real-world scenarios that require simultaneous protection of mixed image types. To address this limitation, this paper proposes ECPD-MIE, an adaptive entropy-controlled chaotic permutation-diffusion scheme for secure multi-image encryption of grayscale and color images within a unified framework. In the proposed method, multiple grayscale and color images of arbitrary dimensions are first adaptively normalized and concatenated into a composite structure, enabling simultaneous encryption while preserving their structural characteristics. High-quality pseudo-random sequences generated from multiple parallel piecewise linear chaotic maps (PWLCMs) are employed to perform circular row and column permutations, ensuring strong confusion properties. To enhance plaintext sensitivity and key security, dynamic initial conditions are derived using SHA-256 hashing of the composite image. Furthermore, an entropy-controlled cross-bit-plane diffusion mechanism is introduced to adaptively improve randomness and diffusion across grayscale intensities and color channels. Comprehensive security analyses demonstrate uniform histogram distributions, near-zero correlations among adjacent pixels, and information entropy values close to the theoretical maximum. The proposed method achieves an average NPCR of 99.62%, UACI of 33.47%, and entropy approaching 8, indicating strong resistance against differential and statistical attacks. Additionally, the encrypted images successfully pass the NIST SP800-22 randomness tests. The scheme exhibits low computational complexity of O(M×N) and supports heterogeneous image datasets, making it a secure and efficient solution for modern multimedia communication systems.