Progressive Visual Cryptography with Unexpanded Meaningful Shares

The traditional k-out-of-n Visual Cryptography (VC) scheme is the conception of "all or nothing" for n participants to share a secret image. The original secret image can be visually revealed only when a subset of k or more shares are superimposed together, but if the number of stacked shares are less than k, nothing will be revealed. On the other hand, a Progressive Visual Cryptography (PVC) scheme differs from the traditional VC with respect to decoding. In PVC, clarity and contrast of the decoded secret image will be increased progressively with the number of stacked shares. Much of the existing state-of-the-art research on PVC has problems with pixel expansion and random pattern of the shares. In this article, a novel scheme of progressive visual cryptography with four or more number of unexpanded as well as meaningful shares has been proposed. For this, a novel and efficient Candidate Block Replacement preprocessing approach and a basis matrix creation algorithm have also been introduced. The proposed method also eliminates many unnecessary encryption constraints like a predefined codebook for encoding and decoding the secret image, restriction on the number of participants, and so on. From the experiments, it is observed that the reconstruction probability of black pixels in the decoded image corresponding to the black pixel in the secret image is always 1, whereas that of white pixels is 0.5 irrespective of the meaningful contents visible in the shares, thus ensuring the value of contrast to alwasys be 50%. Therefore, a reconstructed image can be easily identified by a human visual system without any computation.