Visual cryptography II: Improving the contrast via the cover base

Abstract

In Eurocrypt 1994 we proposed a a new type of cryptographic scheme, which can decode concealed images without any cryptographic computations, by placing two transparencies on top of each other and using the decoder's (human) visual systems. One of the drawback of that proposal was a toss in contrast: a black pixel is translated in the reconstruction into a black region, but a white pixel is translated into a grey region (half black and half white). In this paper we propose am alternative model for reconstruction with a different set of operation (which we call the "Cover" semi-group) is proposed. In this model we are able to obtain a better contrast than is possible in the previous one.