Journal of Emerging Technologies in Web Intelligence Contents

140 JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010quality can be decoded from the same set of shares. Ifonly the stacking operation is allowed (i.e. nocomputations), then our scheme recovers the originalvisual cryptographic quality. If the XOR operation isprovided (instead of the OR operation of stacking), thenwe can fully restore the original quality image.B. For Colored Images (Halftone-based Grayscale andColor Visual Cryptography)Digital halftoning has been extensively used inprinting applications where it has been proved to be veryeffective. For visual cryptography, the use of digital halftoning is for the purpose of converting the grayscaleimage into a monochrome image. Once we have a binaryimage, then the original visual cryptography techniquecan be applied. However, the concomitant loss in qualityis unavoidable in this case.For color images, there are two alternatives forapplying digital halftoning. One is to split the color imageinto channels of cyan, magenta and yellow. Then eachchannel is treated as a grayscale image to whichhalftoning and visual cryptography are appliedindependently. After the monochrome shares aregenerated for each channel, channels are combinedseparately to create the color shares. This is the approachpresented in [9]. The alternative approach would be todirectly apply color halftoning, then perform theseparation into color channels followed by the applicationof visual cryptography to each channel independently.Actually, these two approaches lead to the same resultsfinally. There are many mature halftoning techniquesavailable for selection like dispersed-dot dithering,clustered-dot dithering and error diffusion techniques.Halftoning based visual cryptographic scheme can besummarized as follows:a) Encryption: This stage is for the creation ofshares. This can be further divided into the followingsteps:i. Color halftoning: Standard algorithms such as theones described in [2], [13] and [14] can be usedfor this step. One could do the color channelsplitting first and then do the grayscale halftoningfor each channel:ii.Or one could do color halftoning first followed bythe splitting:Creation of shares: Considering the case of(2,2)-VCS, the steps areb) Decryption: This stage is for the reconstructionof the original secret image. This can be further dividedinto the following steps:i. Stacking of shares: The following stacking (OR)operation needs to be performed:ii. Subsampling for reconstruction: Theseoperations need to be performed where everyblock of f our pixels is sub-sampled into onepixel of the final image. This step is optional andshould be used only with the XOR recoverydescribed in Section III-B.1 to achieve betterquality.Then, for every 2*2 block B(i; j) of I, whereIt is clear that our technique, though independentlydeveloped, is quite similar in spirit to the one described in[9]. So both share the same drawback that digitalhalftoning always leads to permanent loss of informationwhich means that the original image can never beperfectly restored. Inverse halftoning is a possiblesolution that can attempt to recover the image. The bestresults can obtain a restoration quality of 30 dB measuredin PSNR, which is quite good. But this is not sufficientfor applications which require that the original image befaithfully recovered. In fact, in all other cryptographictechniques, it is taken for granted that the decryption of aciphertext perfectly recovers the plaintext. But visualcryptography has been a glaring exception so far.© 2010 ACADEMY PUBLISHER