ABSTRACT
Visual cryptography schemes allow the encoding of a secret image, consisting of black or white pixels, into n shares that are distributed to the set P of n
and Secret
participants. The shares are such that only qualified subsets of participants can ”visually” recover the secret image. The secret pixels are shared with techniques based on subdividing each secret pixel into a certain number m, m ≥ 2 of subpixels. Such a parameter m is called the pixel expansion, since the reconstructed shared image becomes m times bigger than the original. This cryptographic paradigm was introduced by Naor and Shamir [16]. They analyzed the case of (k, n)-threshold visual cryptography schemes, in which a black and white secret image is visible if and only if any k transparencies are stacked together.
