A (k; n)-threshold visual cryptography scheme ((k; n)-threshold VCS, for short) is a method to encode a secret image SI into n shadow images called shares such that any k or more shares enable the \visual" recovery of the secret image, but by inspecting less that k share one cannot gain any information on the secret image. The \visual" recovery consists of xeroxing the shares onto transparencies, and then stacking them. Any k shares will reveal the secret image without any cryptographic computation. In this paper we analyze the contrast of the reconstructed image for (k; n)-threshold VCS. We de ne a canonical form for (k; n)-threshold VCS and we also provide a characterizazion of (k; n)-threshold VCS. We completely characterize contrast optimal (n ? 1; n)-threshold VCS in canonical form. Moreover, for n 4, we provide, a contrast optimal (3; n)-threshold VCS in canonical form. We rst describe a family of (3; n)-threshold VCS achieving various values of
Blundo et al. REF provided a contrast optimal (3,n)-VCS and gave a proof on the upper bound on the contrast of any (3,n)-VCS.
Contrast Optimal Threshold Visual Cryptography Schemes
Computer Science Mathematics