Cheating prevention in visual cryptography

In 1994, Naor and Shamir proposed a variant of secret sharing called Visual Cryptography (VC) [9], where the shares given to participants are xeroxed onto transparencies. If X is an authorized subset, then the participants in X can visually recover the secret image by stacking their transparencies together without performing any computation. One special property distinguishes VC from conventional secret sharing scheme [16, 17] is that the security of VC is achieved by losing the contrast and the resolution of the secret image. In other words, the quality of the reconstructed secret image is inferior to that of the original secret image. Since the invention of VC, many researchers have devoted themselves to enhancing the contrast and resolution of the reconstructed images [1, 3] and to extending it to general access structures [7]. Moreover, many schemes to visually share nonbinary secret images, such as gray-level secret images [2, 5] and color secret images [12, 13], were also proposed. There are lots of applications based on VC, for example, visual authentication and identification [10], steganography [4, 6, 8], and image encryption [14].