Security of multiple digital images over an unsecured channel is a current area of research in the field of image cryptography. Several techniques have been proposed for this purpose. However, the main drawbacks of the existing techniques are as follows: The size of the cipher (output) image is bigger than that of plain (input) images; the authors have used a large number of parameters in the encryption/decryption process, besides the secret key; and the cipher/decipher rounds use the same key for encryption/decryption process. The proposed technique, which is based on two-dimensional (2D) Linear Congruential Generator (LCG), silver ratio, and Galois field, is designed not merely to secure multiple digital images, namely, three truecolor, nine grayscale, and seventy-two binary images of different sizes, simultaneously, but also to address the above drawbacks. Here, 2D LCG is employed for generating two arrays, which are further shuffled by a shuffling algorithm based on the exchange principle of a deck of cards. These shuffled arrays are used for permuting rows and columns of a plain image. The silver ratio together with the secret key matrix is utilized for creating diffusion layers in a cipher image, while the Galois field is employed in the diffusion process for computing the multiplication of two block-matrices. The designed technique achieves both confusion and diffusion properties essential for a secure cryptosystem. Also, the technique is empirically assessed via several statistical and security evaluation metrics, such as key sensitivity, histogram, entropy, number of pixel change rate, structural similarity index measure, processing time, etc. The results of these evaluation metrics prove the efficiency and robustness of the designed technique.INDEX TERMS Digital images, multiple image encryption, 2D Linear Congruential Generator (LCG), silver ratio, Galois field.
