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Threshold Visual Cryptography Scheme for Color Images with No Pixel Expansion

Since the introduction of threshold visual cryptography by Naor and Shamir, there have been many other schemes proposed; some of them support color images with a limited number of color levels while a few others achieve the property of no pixel expansion. However, it is unknown if there is a scheme which can satisfy all the following five commonly desired properties: (1) supporting images of arbitrary number of colors; (2) no pixel expansion; (3) no preprocessing of original images (e.g. dithering or block averaging); (4) supporting k-out-of-n threshold setting; and (5) a ‘tunable’ number of color levels in the secret share creation process. In this paper, we answer this question affirmatively by proposing a k-out-of-n threshold visual cryptography scheme which satisfies all these properties. In particular, our scheme uses a probabilistic technique for achieving no pixel expansion and generically converts any k-out-of-n threshold visual cryptography scheme for black-andwhite images into one which supports color images.