An Extended Visual Cryptography Algorithm for General Access Structures
IEEE Transactions on Information Forensics and Security (Impact Factor: 2.41). 02/2012; 7(1):219-229. DOI: 10.1109/TIFS.2011.2167611
Conventional visual secret sharing schemes generate noise-like random pixels on shares to hide secret images. It suffers a management problem, because of which dealers cannot visually identify each share. This problem is solved by the extended visual cryptography scheme (EVCS), which adds a meaningful cover image in each share. However, the previous approaches involving the EVCS for general access structures suffer from a pixel expansion problem. In addition, the visual cryptography (VC)-based approach needs a sophisticated codebook design for various schemes. In this paper, we propose a general approach to solve the above-mentioned problems; the approach can be used for binary secret images in noncomputer-aided decryption environments. The proposed approach consists of two phases. In the first phase, based on a given access structure, we construct meaningless shares using an optimization technique and the construction for conventional VC schemes. In the second phase, cover images are added in each share directly by a stamping algorithm. The experimental results indicate that a solution to the pixel expansion problem of the EVCS for GASs is achieved. Moreover, the display quality of the recovered image is very close to that obtained using conventional VC schemes.
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ABSTRACT: Conventional visual cryptography (VC) suffers from a pixel-expansion problem, or an uncontrollable display quality problem for recovered images, and lacks a general approach to construct visual secret sharing schemes (VSSs) for general access structures (GASs). We propose herein a general and systematic approach to address these issues without sophisticated codebook design. The approach can be used for binary secret images in non-computer-aided decryption environments. To avoid pixel expansion, we design a set of column vectors to encrypt secret pixels rather than use the conventional VC-based approach. We begin by formulating a mathematic model for the VC construction problem to find the column vectors for the optimal VC construction, after which we develop a simulated-annealing-based algorithm to solve the problem. The experimental results indicate that the display quality of the recovered image is superior to that of previous studies.
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ABSTRACT: In this paper we propose the use of colors to improve visual cryptography schemes for black-and-white secret images. The resulting model is called colored-black-and-white visual cryptography (cbw-vc) model. Using this new model we exploit colors to obtain schemes to share b&w images using a smaller pixel expansion. In particular we provide (2,n)-threshold schemes with pixel expansion m=@?log"3n@?, improving on the best pixel expansion attainable in the normal b&w model (bw-vc). For the case of schemes with perfect reconstruction of black pixels we provide a general construction that allows us to transform any bw-vc scheme into a cbw-vc scheme whose pixel expansion is 1/3 of the pixel expansion of the starting bw-vc scheme. We prove that, in the cbw-vc model, it is not possible to construct (2,n)-threshold schemes, for n>=4, and (k,n)-threshold schemes, for k>=3, without pixel expansion. We also prove that there exist schemes with optimal contrast in the subset of schemes that use only full intensity colors; this is a direct consequence of the definition of contrast which distinguishes only black and non-black pixels. We discuss an alternative measure of contrast that takes into account the ''distance'' between colors. We conjecture that also with this definition of contrast there exist schemes that use only full intensity colors and achieve optimal contrast.
Conference Paper: A (m, m)VCS with consistent size reconstruction using full binary tree[Show abstract] [Hide abstract]
ABSTRACT: Visual cryptography method of secret sharing suffers from pixel expansion and low contrast. Such kind of problem persist in approximately all previous existing visual cryptographic algorithm whereas there are few visual cryptographic method which tries to improve but not done with whole similarity. In this proposed method, we have designed a hierarchical visual cryptographic method which doesn't suffer from any pixel expansion and found similar contrast as the original image. It uses the concept of full binary tree for generating share in (m, m) VCS up to n-1th (started from zero) level of its height where only left child in left sub-tree and right child in right sub-tree are further divided. A small size binary image of factor size of one of leftmost share in left sub-tree is used to generate key by doing XOR operation with this share and resultant share works as a key in this process.
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