Study On Visual Cryptography Schemes With Covert Image

Visual Cryptography first introduced by Naor and Shamir at EUROCRYPT '94 provides a way to share a secret black-white image. The idea is to encode the secret image into share images and Xerox each of them onto transparency which are distributed to the participants. Stacking all or part of these transparencies will allow visual recovery of the secret image with no special cryptographic computation, unlike almost any other cryptographic techniques.Conventional Visual Cryptography Scheme (VCS) construction is attributed to designing two Boolean matrices, the basic encoding matrices. This method of constructing VCS is complicated and unavoidably results in pixel expansion. Pixel expansion, one of the drawbacks of VCS, enlarges the size of share images and the recovered secret image to many times the size of the original secret image and consequently leads to needing for much more storage space and transmission time and being not convenient for application. Moreover, another drawback of VCS is the share images look as random bunches of black and white pixels but not meaningful images which will probably attract the attention of initiative attacker. So, it is valuable to seek simpler methods to construct VCS which at the same time contributes to meaningful share images and no pixel expansion.Inspired by VCS constructed based on halftoning and the relation of share images and stacking image, a (2, 2) VCS with covert image but without pixel expansion is proposed in this paper. In this scheme, the concealed secret image must be the special black-white image whose background is black, but the secret message on it is white. Halftoning is used to convert two grey covert images to two halftone covert images. Two share images will be attained by modifying the halftone covert images referring to the secret image. Stacking the two share images will reveal the secret image. So, the proposed scheme absolutely accords with the merit of VCS that decoding is very easy. Furthermore, the construction method of the proposed scheme is simpler and the achieved shares are meaningful without any pixel expansion. Subsequently, to improve the quality of share images, an improved (2, 2) scheme is proposed in the paper. The improved scheme is different from the first scheme in that two reversed halftone images are used as covert images. In addition to the property of share images is meaningful and no pixel expansion, the quality of share images of the improved scheme is satisfying.