Step Construction of Visual Cryptography Schemes

Two common drawbacks of the visual cryptography scheme (VCS) are the large pixel expansion of each share image and the small contrast of the recovered secret image. In this paper, we propose a step construction to construct $\hbox{VCS}_{\rm OR}$ and $\hbox{VCS}_{\rm XOR}$ for general access structure by applying (2,2)-VCS recursively, where a participant may receive multiple share images. The proposed step construction generates $\hbox{VCS}_{\rm OR}$ and $\hbox{VCS}_{\rm XOR}$ which have optimal pixel expansion and contrast for each qualified set in the general access structure in most cases. Our scheme applies a technique to simplify the access structure, which can reduce the average pixel expansion (APE) in most cases compared with many of the results in the literature. Finally, we give some experimental results and comparisons to show the effectiveness of the proposed scheme.