The (t,n) visual cryptography (VC) is a secret sharing scheme where a secret image is encoded into n transparencies, and the stacking of any out of transparencies reveals the secret image. The stacking of (t-1) or fewer transparencies is unable to extract any information about the secret. This project discusses the additions and deletions of users in a dynamic user group. To reduce the overhead of generating and distributing transparencies in user changes, this project proposes a (t,n) VC scheme with unlimited n based on the probabilistic model. The proposed scheme allows n to 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=2 to 6 are explicitly solved in this project.
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