Journal of Emerging Technologies in Web Intelligence Contents

138 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010The six patterns of shares created based on the abovematrices are shown in figure 1. Note that one pixel of theoriginal image now corresponds to four pixels in eachshare.(c) The second shareFig 1: The six patterns of 4 pixel shares: vertical,horizontal and diagonalA visual cryptography scheme can then beconstructed by picking shares in the following manner:a) If the pixel of the original binary image iswhite, randomly pick the same pattern 0 of f ourpixels for both shares. It is important to pick thepatterns randomly in order to make the patternrandom.b) If the pixel of the original image is black, pick acomplementary pair of patterns, i.e., the patternsfrom the same column in figure 1.It can be easily verified that the resultant scheme hasthe parameters [m = 4; α= 12 ; γ= 6]: any two shares ofC 0 cover two out f our of the pixels, while any pair ofshares from C 1 covers all the f our pixels. An example ofthe above scheme is shown in figure 2. The first image isthe original image, the next two are the shares and the lastimage is the recovered original image obtained byperforming the equivalent of physically stacking twoimage shares on top of each other (assuming that they areprinted on transparencies). It should be noted that the lastthree images in figure 2 are four times as large as the firstone but I have scaled them to the same size as the originalimage.(a) Sample of monochrome image(d) The stacked ImageFig 2. Implementation of existing methodologyIII. RELATED WORKThere has been a steadily growing interest in visualcryptography. Despite its appearance of being a simpletechnique, visual cryptography is a secure and effectivecryptographic scheme. Since the origin of this newparadigm, various extensions to the basic scheme havebeen developed to improve the contrast and the areas ofapplication have also been greatly expanded.In [1], the construction of (n,n)-VCS was extended for(k,n)-VCS. In 1996, the same authors introduced the ideaof cover based semi-group to further improve the contrast[3]. Ateniese et al. [4] provided the first construction of(2, n)-VCS having the best possible contrast for any n·2.Blundo et al. [5] provided a contrast optimal (3,n)-VCSand gave a proof on the upper bound on the contrast ofany (3,n)-VCS. [1] first considered the problem ofconcealing the existence of the secret image. [6] provideda general solution for that problem.The random nature of secret shares makes sharesunsuitable for transmission over an open channel. [6]used a modified scheme to embed some meaningfulimages into the shares. [7] used different moiré patternsto visualize the secret instead of different gray levels. Asfar as extending to color images goes, [8] provided aprimitive scheme for images of 24 colors. Hou [9] thenproposed a novel approach to share color images basedon halftoning. Other interesting topics include visualauthentication [10] and watermarking based on visualcryptography [11]. Recently, there has been an attempt tobuild a physical visual cryptographic system based onoptical interferometry [12]. However, all of these earlierworks result in a decrypted image of reduced quality.(b) The first Share© 2010 ACADEMY PUBLISHER