Journal of Emerging Technologies in Web Intelligence Contents

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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
JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010 139IV. OUR CONTRIBUTIONThe state of the art in visual cryptography leads to thedegradation in the quality of the decoded images, whichmakes it unsuitable for digital media (image, video)sharing and protection. This is quite obvious in figure 2where the white background of the original imagebecomes gray in the decrypted image.Through this paper, I propose a visual cryptographicschemes that not only can support grayscale and colorimages, but also allow high quality images including thatof perfect (original) quality to be reconstructed.The nagging presence of the loss of contrast makestraditional visual cryptography scheme practical onlywhen quality is not an issue which is quite rare. I havetherefore focused our attention on specificallyovercoming this problem by primarily devoting ourefforts towards improving the quality of the reconstructedimages. I first extend the basic scheme from [1] to allowvisual cryptography to be directly applied on grayscaleand color images. Image halftoning is employed in orderto transform the original image from the grayscale/colorspace into the monochrome space which has proved to bequite effective.It is a well known fact that the digital halftoning isalways a lossy process [2], which means that whenever ahalftoning is used for the transformation, it is impossibleto fully reconstruct the original secret image. A newencoding scheme has therefore been developed whichallows for perfectly lossless transformation betweenmonochrome, grayscale and color spaces. This newencoding scheme can be seamlessly incorporated into theproposed scheme for visual cryptography and it allowsthe original secret image to be perfectly restored. Ibelieve this advancement in visual cryptography can beuseful in secret sharing of images, in transmission ofsecret images over multiple untrustworthy channels, in e-commerce of digital media and in digital rightsmanagement of digital media..(a) Sample of monochrome image(b) The first Share(c) The second shareV. OUR APPROACHA. For Monochrome ImagesUsing XOR to Fully Restore Monochrome SecretImages :-I, first made the crucial observation that withjust one additional computational operation; eventraditional visual cryptography can allow full recovery ofthe secret binary image. Normally, when we superimposethe two shares printed on transparencies, this stackingoperation is computationally modeled as the binary ORoperation which causes the contrast level to be lowered.By simply substituting this OR operation with the XORoperation, the original binary image can be recoveredwithout any loss in contrast. Thus, the produced imagecould have a more visually pleasant appearance with lessstorage space requirement. However, the XOR operationneeds computation - the physical stacking process canonly simulate the OR operation. Figure 3 recovers thesame secret image as in figure 2 using the XOR operationand thus it is clearly evident that the contrast of theoriginal image is restored.(d) The stacked Image with XORFig 3. Implementation of proposed methodologyAs we have seen earlier, the application of digitalhalftoning techniques results in some downgrading of theoriginal image quality due to its inherently lossy natureand it is not possible to recover the original image fromits halftone version. We will refer to this proposedscheme as EVCS (Efficient Visual CryptographicScheme).The novelty of my approach is that it not only allowsthe secret image to be just seen but allows the secretimage to be reconstructed with perfect quality. Theadvantage of this approach is that it still retains thecrucial advantages of traditional visual cryptography likesimplicity, visual decoding and perfect security. Theextra feature is that depending on whether additionalcomputing resources are provided, images of different© 2010 ACADEMY PUBLISHER
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