I. INTRODUCTIONVisual cryptography is a cryptographic technique. It makes visual information to be encrypted in such a way that decryption becomes the task of the user to decrypt by means of sight reading.One of the best-known techniques has been credited to Naor and Shamir, 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. Each share was printed on a separate transparency, and decryption was performed by overlaying the shares. When all n shares were overlaid, the original image would appear. Several generalizations of the basic scheme which included k-out-of-n visual cryptography are available. Normally, there is an expansion of space requirement in visual cryptography. But if one of the two shares is structured recursively, the efficiency of visual cryptography can be increased to 100%. II. BASIC VISUAL CRYPTOGRAPHY SCHEMES2.1 (2, 2) VISUAL CRYPTOGRAPHY SCHEMEIn (2, 2) Visual Cryptography Scheme, original image is divided into 2 shares. Each pixel in original image is 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 secret information. Both the shares are required to be superimposed to reveal the secret image. 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, and the shares are assigned 2 pixel blocks as shown in the third and fourth columns in Figure 2.1. Similarly, if a black pixel is encountered, one of the last two rows is selected with probability 0.5, from which a sub-pixel block is assigned to each share. Two shares when superimposed, if two white pixels overlap, the resultant pixel will be white and if a black pixel in one share overlaps with either a white or black pixel in another share, the resultant pixel will be black. This implies that the superimposition of the shares represents the Boolean OR function. Each pixel of the original image is 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 the third and fourth columns are superimposed. Figure 2.1: 2 out of 2 using sub pixels per original pixel 2.2 (K, N) VISUAL CRYPTOGRAPHY SCHEMEIn (2, 2) visual cryptography, both the shares are required to reveal secret information. If one share gets lost due to some technical problem, secret information cannot be revealed. So there is a restriction of keeping all the shares secure to reveal information and user can not afford to lose a single share. In (k, n) visual cryptography scheme, n shares can be generated from original image and distributed. Original image is recognizable only if k or more shares stacked together, where value of k is between 2 to n. If fewer than k shares stacked together, original image cannot be recognized. It gives flexibility to user. If user loses some of the shares still secret information can be revealed, if minimum k number of shares is obtained.2.3 MULTIPLE SECRET SHARING SCHEME The previous researches in visual cryptography were focused on securing only one image at a time. Wu and Chen were first researchers, who developed a visual cryptography scheme to share two secret images in two shares. In this scheme, two secret binary images can be hidden into two random shares, namely A and B, such that the first secret can be seen by stacking the two shares, denoted by A? B, and the second secret can be obtained by rotating A by 90 degree anti-clockwise. J Shyu implemented a scheme for multiple secrets sharing in visual cryptography, where more than two secret images can be protected at a time in two shares. 12.4 HALFTONE VISUAL CRYPTOGRAPHY SCHEMEHalftone visual cryptography uses half toning technique to create shares. Halftone which is a reprographic technique simulates continuous tone imagery through the use of dots, which may vary either in size, in shape or in spacing. Zhi Zhou, Gonzalo R. Arce, and Giovanni Di Crescenzo proposed halftone visual cryptography.In halftone visual cryptography 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 with an correct size, visually agreeable halftone shares can be obtained. It maintains good contrast and security and increases quality of the shares.2.5 VISUAL CRYPTOGRAPHY SCHEME FOR GREY IMAGESAll previous visual cryptography schemes were only limited to binary images. These techniques were capable of doing operations on only black and white pixels. It is not sufficient for real life applications. Chang-Chou Lin, Wen- Hsiang Tsai proposed visual cryptography for gray level images. In this scheme a dithering technique is used to convert gray level image into approximate binary image. Then presented visual cryptography schemes for binary images are used to make the shares.2.6 VISUAL CRYPTOGRAPHY SCHEME FOR COLOR IMAGESVisual cryptography schemes were applied to only black and white images till year 1997. Verheul and Van Tilborg proposed first color visual cryptography scheme. In this visual cryptography scheme one pixel is distributed into m sub pixels, and each sub pixel is divided into c color regions. In each sub pixel, there is exactly one color region colored, and all the other color regions are black. 2.7 EXTENDED VISUAL CRYPTOGRAPHY SCHEMEShares are created as random patterns of pixel traditionally. These shares look like a noise. Noise-like shares arouse the attention of hackers, as hacker may suspect that some data is encrypted in these noise-like images. So it becomes prone to security related issues. It also becomes difficult to manage noise-like shares, as all shares look alike. Nakajima, M. and Yamaguchi, Y., developed Extended visual cryptography scheme (EVS).An extended visual cryptography (EVC) provide techniques to create meaningful shares instead of random shares of traditional visual cryptography and help to avoid the possible problems, which may arise by noise-like shares in traditional visual cryptography. Thus the information can be transmitted securely. 2.8 SEGMENT BASED VISUAL CRYPTOGRAPHY SCHEMETraditional visual cryptography schemes were based on pixels in the input image. The limitation of pixel based visual cryptography scheme is loss in contrast of the reconstructed image, which is directly proportional to pixel expansion. Bernd Borchert proposed a new scheme which is not pixel-based but segment-based. It is useful to encrypt messages consisting of symbols represented by a segment display. For example, the decimal digits 0, 1,….,9 can be represented by seven-segment display . The advantage of the segment-based encryption is that, it may be easier to adjust the secret images and the symbols are potentially easier to recognize for the human eye and it may be easier for a non-expert human user of an encryption system to understand the working.2.9 VISUAL CRYPTOGRAPHY SCHEME FOR GENERAL ACCESS In (k, n) visual cryptography scheme, all n shares have equal importance. Any k out of n shares can reveal the secret information. It may compromise the security of system. To overcome this problem, G. Ateniese, C. Blundo, A. DeSantis, and D. R. Stinson extended (k, n) visual cryptography model to general access structure. In general access structure scheme, given set of n shares is divided into two subsets namely qualified and forbidden subset of shares as per the importance of shares. Any k shares from qualified subset of shares can reveal secret information, but less than k shares from qualified subset of shares can not reveal any secret information. Even k or more shares from forbidden set can’t reveal secret information. 22.10 RECURSIVE THRESHOLD VISUAL CRYPTOGRAPHY SCHEMEIn (k, n) visual secret sharing scheme, a secret of ‘b’ bits is distributed among ‘n’shares of size at least ‘b’ bits each. Since only k out of n shares is needed to reveal secret, every bit of any share conveys at most 1/k bits of secret. It results in inefficiency in terms of number of bits of secret conveyed per bit of shares. To overcome this limitation Abhishek Parakh and Subhash Kak proposed “Recursive threshold visual cryptography”.The basic idea behind Recursive threshold visual cryptography is recursive hiding of smaller secrets in shares of larger secrets with secret sizes doubling at every step, and thereby increasing the information, every bit of share conveys to (n-1)/n bit of secret which is nearly 100 percentage.2.11 REGION INCREMENTING VISUAL CRYPTOGRAPHY SCHEMEIn traditional visual cryptography scheme, one whole image is considered as a single secret and same encoding rule is applied for all pixels of one image. So it reveals either entire image or nothing. It may be the situation that different regions in one image can have different secrecy levels, so we can’t apply same encoding rule to all pixels. Ran-Zan Wang developed a scheme “Region Incrementing Visual cryptography” for sharing visual secrets of multiple secrecy level in a single image. In this scheme, different regions are made of a single image, based on secrecy level and different encoding rules are applied to these regions. 2.12 PROGRESSIVE VISUAL CRYPTOGRAPHY SCHEMEIn (k, n) visual secret sharing scheme, it is not possible to recover the secret image though one less than k shares are available. This problem is solved in progressive visual cryptography scheme developed by D. Jin, W. Q. Yan, and M. S. Kankanhalli. In progressive visual cryptography scheme, it is not necessary to have at least k shares out of n, as in (k, n) secret sharing scheme. If more than one share obtained, it starts recovering the secret image gradually. The quality of recovered image improves, as the number of shares received increases.III.CONCLUSIONProviding security to the digital information shared in day to day life is an important issue in real life. Since the origin of Visual cryptography, various extensions have been developed to improve the things, ranging from (2, 2), (k, n) to progressive visual cryptography models, black and white to color images and random dot like shares to meaningful shares. Starting from the basic model, many visual cryptographic techniques have been evolved day by day. These techniques provide security while sharing information. Thus it helps to prevent the misuse of data and unauthorized access of information by attackers. Visual cryptography seems to be still a potentially useful technique.IV.REFERENCES 1 Suhas B. Bhagate,P.J.Kulkarni “An overview of various Visual cryptography schemes”, International Journal of Advanced Research in Computer and Communication Engineering,Vol. 2, Issue 9, September 2013.2 Young-Chang Hou, “Visual cryptography for color images”, Journal of Pattern Recognition, Vol.36, pp.1619 – 1629, 2003. 3 MoniNaor and Adi Shamir, “Visual cryptography”.In proceedings of advances in cyptology,EUROCRYPT 94, Lecture Notes in computer science,1995. 4 AbhishekParakh and Subhash Kak “A Recursive Threshold Visual Cryptography Scheme”,CoRR abs/0902.2487, (2009). 5 Rehna P Muhammad, “A Secure Approach to Visual Cryptographic Biometric Template”,journal of network security,2011. 6 Kak,S.andChatterjee,A. “On decimal sequences”, IEE transactions on information theory,1981.