Visual Cryptograms of Random Grids for General Access Structures

This paper re-examines the problem of visual secret sharing for general access structures by using visual cryptograms of random grids (VCRG). Given a binary or color secret image shared by a set of n participants with a strong access structure, we devise two effective algorithms to produce a set of VCRG so that the members in each qualified set can reconstruct the secret image by superimposing their shares, while those in any forbidden set cannot. Our algorithms do not require any extra pixel expansion, which is indispensable and grows exponentially as n increases in conventional visual cryptographic schemes. The feasibility, light contrasts, flexibility, and limitations of our algorithms are explored from both theoretical and empirical points of view.