New visual secret sharing schemes using probabilistic method

Visual secret sharing (VSS) scheme is a perfect secure method that protects a secret image by breaking it into shadow images (called shadows). Unlike other threshold schemes, VSS scheme can be easily decoded by the human visual system without the knowledge of cryptography and cryptographic computations. However, the size of shadow images (i.e., the number of columns of the black and white matrices in VSS scheme [Naor, Shamir, Visual cryptography, Advances in Cryptology-EUROCRYPT 94, Lecture Notes in Computer Science, vol. 950, Springer-Verlag, 1995, p. 1]) will be expanded. Most recent papers about VSS schemes are dedicated to get a higher contrast or a smaller shadow size. In this paper, we use the frequency of white pixels to show the contrast of the recovered image. Our scheme is nonexpansible and can be easily implemented on a basis of conventional VSS scheme. The term non-expansible means that the sizes of the original image and shadows are the same. 2004 Elsevier B.V. All rights reserved.