Visual Cryptographic scheme (VCS) is a variant form of secret sharing method to encode a secret image into n distinct shares. The dealer distributes shares to each participants in such a way that, while certain qualified subsets of participants can visually recover the secret, but other forbidden sets of participants have no information on the secret. Such a system is the general access structures in VCS. The essential access structure (k, n)* is a type of threshold scheme were one participant is the leader who holds the essential share and the remaining n-1 shares are given to n-1 participants. The secret image cannot be retrieved in the absence of leader and the leader needs the support of at least k-1 remaining participants to recover the secret image. Small pixel expansion and maximum contrast are the basic parameters for measuring the quality of the VCS. In the literature there are constructions for both deterministic and probabilistic (k, n)*-VCS. The following are the important contributions of this paper. (1) A deterministic (k, n)*-VCS with improved contrast. (2) A deterministic (k, n)- VCS for t essential participants with improved contrast and less pixel expansion. (3) An ideal contrast general access structure construction with AND operation for grey scale images. (4) An ideal contrast (k, n)*-VCS construction for both binary and grey scale images. (5) An ideal contrast (k, n)-VCS for t essential participants for both binary and grey scale images. © 2017 Taru Publications.
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K. Praveen, Sethumadhavan, M., and Krishnan, R., “Visual cryptographic schemes using combined Boolean operations”, Journal of Discrete Mathematical Sciences and Cryptography, vol. 20, pp. 413-437, 2017.