Paper 2004/287

Almost Ideal Contrast Visual Cryptography with Reversing

Duong Quang Viet and Kaoru Kurosawa

Abstract

A drawback of visual cryptography schemes (VCS) is much loss of contrast in the reconstructed image. This paper shows a new paradigm of VCS in which the original image is almost perfectly reconstructed. A very simple non-cryptographic operation is assumed, reversing black and white, which many copy machines have these days. We first show a $(k,n)$-VCS with {\it reversing} such that white pixels are almost perfectly reconstructed in addition to the perfect reconstruction of black pixels. The proposed scheme is fully compatible with traditional VCS in the following sense: Even if we do not have a copy machine as described above, we can reconstruct the secret image $I$ exactly in the same way as in the underlying VCS. In other words, we use a copy machine as a hedge to obtain better contrast. We next show how to convert a perfect {\it black} $(k,n)$-VCS (with reversing) into a perfect {\it white} $(k,n)$-VCS with reversing. Thirdly, we show a perfect black VCS for any monotone access structure. Finally, we show applications of our idea to colored VCS and grey level VCS, respectively.