Constructing visual cryptography scheme by hypergraph decomposition

Abstract A threshold visual cryptography scheme, denoted as ( k, n )-VCS for short, can encrypt a secret image into n share images. The physical stacking of any k share images can reveal the secret content while any less than k share images will leak no information of the secret content. Besides the threshold access structure, general access structure VCSs are studied in the following works. In this paper, we first build a hyper star access structure VCS from a hyper star without center access structure VCS. Then we build a normal form hyper star access structure VCS from a full threshold VCS, and prove that its pixel expansion is optimal. At last, we propose a method of constructing general access structure VCS from its several decomposed normal form hyper star access structure VCSs.