摘要
The (t,n )visual cryptography (VC) is a secret sharing scheme where a secret image is encoded into ntransparencies, and the stacking of any tout of ntransparencies reveals the secret image. The stacking of t -1or fewer transparencies is unable to extract any information about the secret. We discuss the additions and deletions of users in a dynamic user group. To reduce the overhead of generating and distributing transparencies in user changes, this paper proposes a (t , n )VC scheme with unlimited nbased on the probabilistic model. The proposed scheme allows nto change dynamically in order to include new transparencies without regenerating and redistributing the original transparencies. Specifically, an extended VC scheme based on basis matrices and a probabilistic model is proposed. An equation is derived from the fundamental definitions of the (t , n )VC scheme, and then the (t,∞)VC scheme achieving maximal contrast can be designed by using the derived equation. The maximal contrasts with t = 2to 6are explicitly solved in this paper. © 2011 IEEE.