Visual cryptography for color images

Visual cryptography, an emerging cryptography technology, uses the characteristics of human vision to decrypt encrypted images. It needs neither cryptography knowledge nor complex computation. For security concerns, it also ensures that hackers cannot perceive any clues about a secret image from individual cover images. Since Naor and Shamir proposed the basic model of visual cryptography, researchers have published many related studies. Most of these studies, however, concentrate on binary images; few of them proposed methods for processing gray-level and color images. This paper proposes three methods for visual cryptography of gray-level and color images based on past studies in black-and-white visual cryptography, the halftone technology, and the color decomposition method. Our methods not only retain the advantages of black-and-white visual cryptography, which exploits the human visual system to decrypt secret images without computation, but also have the backward compatibility with the previous results in black-and-white visual cryptography, such as the t out of n threshold scheme, and can be applied to gray-level and color images easily.