Multilevel Military Image Encryption Based on Tri-Independent Keying Approach

Military image encryption plays a vital role in ensuring the secure transmission of sensitive visual information from unauthorized access. This paper proposes a new Tri-independent keying method for encrypting military images. The proposed encryption method is based on multilevel security stages of pixel-level scrambling, bit-level manipulation, and block-level shuffling operations. For having a vast key space, the input password is hashed by the Secure Hash Algorithm 256-bit (SHA-256) for generating independently deterministic keys used in the multilevel stages. A piecewise pixel-level scrambling function is introduced to perform a dual flipping process controlled with an adaptive key for obscuring the spatial relationships between the adjacent pixels. A dynamic masking scheme is presented for conducting a bit-level manipulation based on distinct keys that change over image regions, providing completely different encryption results on identical regions. To handle the global correlation between large-scale patterns, a chaotic index-map system is employed for shuffling image regions randomly across the image domain based on a logistic map seeded with a private key. Experimental results on a dataset of military images show the effectiveness of the proposed encryption method in producing excellent quantitative and qualitative results. The proposed method obtains uniform histogram distributions, high entropy values around the ideal (≈8 bits), Number of Pixel Change Rate (NPCR) values above 99.5%, and low Peak Signal-to-Noise Ratio (PSNR) over all encrypted images. This validates the robustness of the proposed method against cryptanalytic attacks, verifying its ability to serve as a practical basis for secure image transmission in defense systems.