Kashmira S. Gulhane 1 , Prof.P.L.Ramteke
2
1
Departement of computer science and Information technology,H.V.P.M. COET,Amravati, gulhanekashmira03 @gmail.com
Abstract -Visual Cryptography is a special encryption technique to hide information in images,which divide secret image into multiple layers. Each layer holds some information. The receiver aligns the layers and the secret information is revealed by human vision without any complex computation. Modern day cryptography entails complex and advance mathematical algorithms that are applied to encryption of text and images as well as other file formats. Visual cryptography is proposed in 1994 byNaor and Shamir who introduced a simple but perfectly secure way that allows secret sharing without any cryptographic computation, which they termed as
Visual Cryptography Scheme (VCS).The simplest Visual Cryptography Scheme is given by the idea of A secret image consists of a collection of black and white pixels where each pixel is treated independently.
In visual cryptography system the pixels of the image to be encrypted can be applied to the image in different manner. There is a set of n participants(image), and the secret image is divided and encoded into n shadow images called shares Each participant encrypted by one share, k out of n participants are needed to combine sharesand see secret image, some time k-1 of shares can not reveals information about secret image. Visual Cryptography is a new Cryptography technique which is used to secure the images. In Visual Cryptography the Image is divided into parts called shares and then they are distributed to the participants. The Decryption side just stacking the share images gets the image. The initial model developed only for the bi-level or binary images or monochrome images. Later it was advanced to suit for the Colour Images means
Gray Images and RGB/CMY Images. For the RGB/CMY Images different methods are developed based on the colour decomposition techniques.
Keywords- Visual Cryptography, Decryption,Encryption

Visual cryptography was pioneered by Moni Naor and Adi Shamir in 1994. They demonstrated a visual secret sharing scheme, where an image was broken up into n shares so that only someone with all 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. Visual cryptography is a new technique which provides information security which uses simple algorithm unlike the complex, computationally intensive algorithms used in other techniques like traditional cryptography. This technique allows Visual information (pictures, text, etc) to be encrypted in such a way that their decryption can be performed by the human visual system, without any complex cryptographic algorithms. In visual cryptography system the pixels of the image to be encrypted can be applied to the image in different manner. There is a set of n participants(image), and the secret image is divided and encoded into n shadow images called shares Each participant encrypted by one share, k out of n participants are needed to combine shares and see secret image, some time k-1 of shares can not reveals information about secret image.
Visual Cryptography is a special encryption technique to hide information in images, which divide secret image into multiple layers. Each layer holds some information. The receiver aligns the layers and the secret information is revealed by human vision without any complex computation. The proposed algorithm is for color image, that presents a system which takes four pictures as an input and generates three images which correspond to three of the four input pictures.
The decoding requires only selecting some subset of these 3 images, making transparencies of them, and stacking them on top of each other, so the forth picture is reconstructed by printing the

three output images onto transparencies and stacking them together. The reconstructed image achieved in same size with original secret image.
As described earlier the VCS model is not applicable for the colour images. Several algorithms were proposed from Ref [ to apply VCS model for Colour Images. They are based on the approach of colour channel decomposition generating the 3-gray versions of the colour images.
Later on applies the Diffusion Filters to get the Halftone images and then applies the VC on them and combines the result in a way producing the colour halftone images.
Proposed Wavelet Model:
The Approach we used in our method is different from the existing ones in a way that we have used the Wavelet Technique[1] to convert the Colour Image to Generate the Gray Colour Image which is the Intensity Image formed from the YCbCr Colour Space. The process of generation of gray image from a colour image using wavelet technique is shown in Fig . The Wavelet Filter used is Antonini 9/7 filter. The Encryption Process can be described in Steps as Follows:
1. Convert the RGB Image to YCbCr Image.
2. Apply the two level discrete wavelet transform on the Y image so that it gets divided in to seven bands. Y-->(s1,sh1,sv1,sd1,sh2,sv2,sd2)
3. Reduce Cb and Cr by ½, construct Cb+,Cb-,Cr+,Cr- and reduce Cb- further to ¼ of its original size. Where Cb+ is Cb if Cb >0 otherwise 0 Cb- is Cb if Cb<0 otherwise 0 Same arrangement for
Cr also.
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5. Take inverse wavelet transform to obtain the grey (Sl,Sh,Sv,Cb-,Cr+,Cb+,Crimage
6. Apply Error-Diffusion Filter on the obtained grey image Y‘.

7. Apply the VCS Model on the generated halftone image from step6.
8. Distribute the shares to the participants visual cryptography system

Flow chart diagram for the encryption process

The Decryption process is very simple take print out of the generated shares on the transparencies. Overlapping the transparencies on top of the other gets the secret image. The pixel displacement and reshuffling of the image in steps between the processes has proven to be really effec-tive. The extra transposition of RGB values in the image file after R G B component reshape has proven the increase of security of the image against all possible attacks available currently.

1.Kester, Q. A., & Koumadi, K. M. (2012, October). Cryptographie technique for image encryption based on the RGB pixel displacement. In Adaptive Science & Technology (ICAST),
2012 IEEE 4th International Conference on (pp. 74-77). IEEE.
2 .Moni Naor and Adi Shamir, ”Visual cryptography”, in Proceedings of Advances in Cryptology
EUROCRYPT 94, LNCS Vol. 950, pages 1-12. Springer-Verlag, 1994.
3.Krishnan, G.S.; Loganathan, D.; , "Color image cryp-tography scheme based on visual cryptography," Sig-nal Processing, Communication, Computing and Networking Technologies
(ICSCCN), 2011 Interna-tional Conference on , vol., no., pp.404-407, 21-22 July 2011
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2010 International Con-ference on , vol., no., pp.1-4, 26-28 July 2010