On general construction for extended visual cryptography schemes

Numerous visual cryptography schemes (VCSs) have been proposed to protect a secret image with share images. Most VCSs use random-looking shares to code a single binary secret image. Some schemes are designed for color secret images. Droste's [New results on visual cryptography, in: Advances in Cryptology-CRYPTO ’96, Lecture Notes in Computer Science, vol. 1109, Springer, Berlin, 1996, pp. 401–415] (n,n)(n,n)-VCS is introduced for multiple binary secret images. Extended VCS (EVCS), by Ateniese et al. [Extended capabilities for visual cryptography, Theoretical Computer Science 250 (2001) 143–161], for binary secret image uses meaningful (innocent-looking) shares. In this paper, we start with a more concise derivation of matrix extension in the ECVS model. This is implemented by concatenating an extended matrix to each basis matrix. We then present a general construction method for single or multiple binary/grayscale/color secret images using matrix extension utilizing meaningful shares. The result (k,n)(k,n)-visual secret sharing schemes are more general than most existing schemes in terms of the secret/share image types. Using our matrix extension algorithm, any existing VCS with random-looking shares can be easily modified to utilize meaningful shares. The effectiveness of our schemes is demonstrated by real examples.