Abstract

An extended visual cryptography scheme, (\Gamma Qual ; \Gamma Forb ; m)-EVCS for short, with pixel "expansion" m, for an access structure (\Gamma Qual ; \Gamma Forb ) on a set of n participants, is a technique to encode n innocent looking images in such a way that when we stack together the transparencies associated to participants in any set X 2 \Gamma Qual we get the secret message with no trace of the original images, but any X 2 \Gamma Forb has no information on the shared image. Moreover, after the original innocent looking images are encoded they are still meaningful, that is, any user will recognize the image on his transparency. In this paper we first present a general technique to implement extended visual cryptography schemes, which uses hypergraph colourings. Then we discuss some applications of this technique to various interesting classes of access structures by using relevant results from the theory of hypergraph colourings. Keywords: Visual Cryptography, Secret Sharing ...

Citations