Graylevel Images Encrypting Based on the n-Graylevel Encoding Method and the Lowest Common Multiple Technique

This paper proposes an encoding method to encrypt multiple covert images into a host image to form an overt image. The codes encoded in the overt image are classified into seven groups. Identification codes are used to determine whether the overt image contains codes proposed in this paper or not, image number codes are used to indicate the number of covert images encoded in the overt image, base codes are used to indicate the base value which returns pixel gray values of every covert image as a string, digit codes are used to indicate the number of digit that produces pixel gray values of every covert image as a string, gray level codes are used to indicate the gray level numbers of every covert image, dimension codes are used to determine the sizes of every covert image, information codes are used to decode covert images. All the reconstructed covert images do not contain any distortion when the proposed encrypting method is used. Moreover, the overt image looks almost the same as the host image.