Color Visual Cryptography Schemes Using Linear Algebraic Techniques over Rings

The research on color Visual Cryptographic Scheme (VCS) is much more difficult than that of the black and white VCS. This is essentially because of the fact that in color VCS, the rule for superimposition of two colors is not that simple as in black and white VCS. It was a long standing open issue whether linear algebraic technique in constructing Black and White visual cryptographic schemes could also be extended for color images. It was thought that such an extension was impossible. However, we resolve this issue by providing color VCS in same color model for the threshold access structures by extending linear algebraic techniques from the binary field Z₂ to finite ring Z_c of integers modulo c. We first give a construction method based on linear algebra to share a color image for an (n, n)-threshold access structure. Then we give constructions for (2, n)-threshold access structures and in general (k, n)-threshold access structures. Existing methodology for constructing color VCS in same color model assumes the existence of black and white VCS, whereas our construction is a direct one. Moreover, we give closed form formulas for pixel expansion which is combinatorially a difficult task. Lastly, we give experimental results and propose a method to reduce pixel expansion.