| Summary: | 碩士 === 國立清華大學 === 資訊工程學系 === 92 === Adi Shamir proposed a well-known technique for secret sharing. This technique can divide data D into n pieces in such a way that any k pieces can easily reconstruct D, but no information about D will be revealed even with complete knowledge of k-1 pieces. Such scheme is called a (k,n) threshold scheme.
Naor and Shamir proposed further the idea of visual cryptography for sharing images. It can be seen as a visual variant of the k out of n secret sharing problem, in which each one of the n users gets a transparency; any k of them can see the image by stacking their transparencies, but any k-1 of them gain no information about it.
In this paper a new construction algorithm of visual cryptography is presented. First, we extend SFCOD (Space Filling Curve Ordered Dither – one of the techniques of halftoning) to transform a gray-level image into an image with fewer grayscale values. In addition, we extend the basic visual cryptography model to handle more than two grayscale values. Then the extended visual cryptography model can be applied to encode this image. This scheme satisfies the security and contrast conditions. It can reveal more details of original images in the decoded images than ordinary visual cryptography scheme. At last, an extension of this scheme for color images is included.
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