A (2, n) and (3, n) Visual Cryptography Scheme for Black and White Images

A visual cryptography scheme (VCS) is a method to encode a secret image into shadow images called shares such that, certain qualified subsets of shares enable the “visual" recovery of the secret image. The “visual" recovery consists of xeroxing the shares onto transparencies, and then stacking them. The shares of a qualified set will reveal the secret image without any cryptographic computation. Here, we analyze the construction of k out of n visual cryptography schemes for black and white images(In such a scheme any k shares out of n will reveal the secret image, but any k -1 shares give no information about the image). The important parameters of a scheme are its contrast, i.e., the clarity of the decoded image, and the number of pixels needed to encode one pixel of the original image. Some methods of construction of (2, n)-schemes having optimal relative contrast using Hadamard matrices and some combinatorial block designs have been discussed. We study the construction of an efficient (3, n)-scheme. We also study the construction of (3,n)-schemes using a design.