# I. to 100%. II. BASIC VISUAL CRYPTOGRAPHY SCHEMES 2.1

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INTRODUCTION Visual cryptography is acryptographic technique. It makes visual information to be encrypted in such away that decryption becomes the task of the user to decrypt by means of sightreading. One of the best-known techniqueshas been credited to Naor andShamir, who developed it in 1994. An image was split into n shares so that only someone with all the n shares could decrypt the image, while any n ? 1 shares revealed no information about the original image. Eachshare was printed on a separate transparency, and decryption was performed byoverlaying the shares. When all nshares were overlaid, the original image would appear. Several generalizationsof the basic scheme which included k-out-of-n visual cryptography areavailable.

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Normally, there is an expansion of space requirement in visualcryptography. But if one of the twoshares is structured recursively, the efficiency of visual cryptography can beincreased to 100%.                II.  BASIC VISUAL CRYPTOGRAPHY SCHEMES 2.1(2, 2) VISUAL CRYPTOGRAPHY SCHEME In (2, 2) Visual CryptographyScheme, original image is divided into 2 shares. Each pixel in original imageis represented by non-overlapping block of 2 or 4 sub-pixels in each share.Anyone, having only one share will not be able to reveal any secretinformation.

Both the shares are required to be superimposed to reveal thesecret image.3 In this technique,each pixel in original image is represented by two sub-pixels in each share.While reading the pixels in original image, if a white pixel is encountered,one of the first two rows in Figure 2.1 is selected with probability 0.5, andthe shares are assigned 2 pixel blocks as shown in the third and fourth columnsin Figure 2.1. Similarly, if a black pixel is encountered, one of the last tworows is selected with probability 0.

5, from whicha sub-pixel block is assigned to each share. Two shares when superimposed, iftwo white pixels overlap, the resultant pixel will be white and if a blackpixel in one share overlaps with either a white or black pixel in anothershare, the resultant pixel will be black. This implies that the superimpositionof the shares represents the Boolean OR function. 2 Each pixel of the original imageis divided into two sub-pixels in each share. The last column in Figure 2.1shows the resulting sub-pixel when the sub-pixels of both the shares in thethird and fourth columns are superimposed.                  Figure2.

1: 2 out of 2 using sub pixels per original pixel 2.2 (K, N) VISUALCRYPTOGRAPHY SCHEME In (2, 2) visual cryptography,both the shares are required to reveal secret information. If one share getslost due to some technical problem, secret information cannot be revealed. Sothere is a restriction of keeping all the shares secure to reveal informationand user can not afford to lose a single share. In (k, n) visual cryptographyscheme, n shares can be generated from original image and distributed.

Originalimage is recognizable only if k or more shares stacked together, where value ofk is between 2 to n. If fewer than k shares stacked together, original imagecannot be recognized. It gives flexibility to user. If user loses some of theshares still secret information can be revealed, if minimum k number of sharesis obtained.22.3 MULTIPLE SECRETSHARING SCHEME The previous researches in visualcryptography were focused on securing only one image at a time.

Wu and Chenwere first researchers, who developed a visual cryptography scheme to share twosecret images in two shares. In this scheme, two secret binaryimages can be hidden into two random shares, namely A and B, such that thefirst secret can be seen by stacking the two shares, denoted by A?B, and the second secret can beobtained by rotating A by 90 degree anti-clockwise. J Shyu implemented a schemefor multiple secrets sharing in visual cryptography, where more than two secretimages can be protected at a time in two shares. 1 2.4 HALFTONE VISUALCRYPTOGRAPHY SCHEME Halftone visual cryptography useshalf toning technique to create shares.

Halftone which is a reprographictechnique simulates continuous tone imagery through the use of dots, which mayvary either in size, in shape or in spacing. Zhi Zhou, Gonzalo R. Arce, andGiovanni Di Crescenzo proposed halftone visual cryptography.In halftone visualcryptography a secret binary pixel is encoded into an array of sub pixels,called as halftone cell, in each of the ‘n’ shares. By using halftone cells withan correct size, visually agreeable halftone shares can be obtained. Itmaintains good contrast and security and increases quality of the shares.5 2.5VISUAL CRYPTOGRAPHY SCHEME FOR GREY IMAGES All previous visual cryptographyschemes were only limited to binary images.

These techniques were capable ofdoing operations on only black and white pixels. It is not sufficient for reallife applications. Chang-Chou Lin, Wen- Hsiang Tsai proposed visualcryptography for gray level images. In this scheme a dithering technique isused to convert gray level image into approximate binary image.

Then presentedvisual cryptography schemes for binary images are used to make the shares. 2.6VISUAL CRYPTOGRAPHY SCHEME FOR COLOR IMAGES Visual cryptography schemes wereapplied to only black and white images till year 1997. Verheul and Van Tilborg proposed first colorvisual cryptography scheme. In this visual cryptographyscheme one pixel is distributed into m sub pixels, and each sub pixel isdivided into c color regions.

In each sub pixel, there is exactly one colorregion colored, and all the other color regions are black. 2.7 EXTENDED VISUALCRYPTOGRAPHY SCHEME Shares are created as randompatterns of pixel traditionally. These shares look like a noise.

Noise-likeshares arouse the attention of hackers, as hacker may suspect that some data isencrypted in these noise-like images. So it becomes prone to security relatedissues. It also becomes difficult to manage noise-like shares, as all shareslook alike. Nakajima, M. and Yamaguchi, Y., developed Extended visualcryptography scheme (EVS).

6 An extended visual cryptography(EVC) provide techniques to create meaningful shares instead of random sharesof traditional visual cryptography and help to avoid the possible problems,which may arise by noise-like shares in traditional visual cryptography. Thusthe information can be transmitted securely. 2.8SEGMENT BASED VISUAL CRYPTOGRAPHY SCHEME Traditional visual cryptographyschemes were based on pixels in the input image. The limitation of pixel basedvisual cryptography scheme is loss in contrast of the reconstructed image,which is directly proportional to pixel expansion.

Bernd Borchert proposed anew scheme which is not pixel-based but segment-based. It is useful to encrypt messagesconsisting of symbols represented by a segment display. For example, thedecimal digits 0, 1,….,9 can be represented by seven- segment display.

The advantage of thesegment-based encryption is that, it may be easier to adjust the secret imagesand the symbols are potentially easier to recognize for the human eye and itmay be easier for a non-expert human user of an encryption system to understandthe working.32.9VISUAL CRYPTOGRAPHY SCHEME FOR GENERAL ACCESS In (k, n) visual cryptographyscheme, all n shares have equal importance. Any k out of n shares can revealthe secret information. It may compromise the security of system.

To overcomethis problem, G. Ateniese, C. Blundo, A.

DeSantis, and D. R. Stinson extended(k, n) visual cryptography model to general access structure. 3 In general access structurescheme, given set of n shares is divided into two subsets namely qualified andforbidden subset of shares as per the importance of shares. Any k shares fromqualified subset of shares can reveal secret information, but less than kshares from qualified subset of shares can not reveal any secret information.

Even k or more shares from forbidden set can’t reveal secret information. 2applied to these regions .5 2.12PROGRESSIVEVISUALCRYPTOGRAPHY SCHEME In (k, n) visual secret sharingscheme, it is not possible to recover the secret image though one less than kshares are available. This problem is solved in progressive visual cryptographyscheme developed by D. Jin, W. Q.

Yan, and M. S. Kankanhalli. In progressive visualcryptography scheme, it is not necessary to have at least k shares out of n, asin (k, n)  secretsharing scheme. If more than one share obtained, it starts recovering thesecret image gradually.The quality of recovered image improves, as the number of shares received increases.6