Journal of Emerging Technologies in Web Intelligence Contents

JOURNAL OF EMERGING TECHNOLOGIES IN WEB INTELLIGENCE, VOL. 2, NO. 2, MAY 2010 137Efficient Visual CryptographyEr. Supriya KingerCSE Department, Chitkara Institute of Engineering and Technology,Rajpura, Punjab, IndiaEmail: ahujasupriya@gmail.comAbstract – Visual cryptography scheme (VCS) is a secretsharingscheme which allows the encryption of a secretimage into n shares that are distributed to n participants.The beauty of such a scheme is that, the decryption of thesecret image requires neither the knowledge ofcryptography nor complex computation. Colour visualcryptography becomes an interesting research topic afterthe formal introduction of visual cryptography by Naor andShamir in 1995. It is a powerful technique which combinesthe notions of perfect ciphers and secret sharing incryptography with that of raster graphics. A binary imagecan be divided into shares which can be stacked together toapproximately recover the original image. Unfortunately, ithas not been used much primarily because the decryptionprocess entails a severe degradation in image quality interms of loss of resolution and contrast. Its usage is alsohampered by the lack of proper techniques for handlinggrayscale and color images. In this paper, I have developeda novel technique which enables visual cryptography ofcolor as well as grayscale images. The physical transparencystacking type of decryption allows for the recovery of thetraditional visual cryptography quality image. An enhancedstacking technique allows for the decryption into a halftonequality image. And finally, a computation based decryptionscheme makes the perfect recovery of the original imagepossible. Based on this basic scheme, I have then establisheda progressive mechanism to share color images at multipleresolutions. I extracted shares from each resolution layer toconstruct a hierarchical structure; the images of differentresolutions can then be restored by stacking the differentshared images together. I have implemented our techniqueand present results.Index Terms – Secret sharing, Color halftoning, imagesharing, multiple resolutions, secret sharing, and visualcryptographyI. INTRODUCTIONVisual cryptography was originally proposed for theproblem of secret sharing. Secret sharing is one of theearly problems to be considered in cryptography. Theidea of the visual cryptography model proposed in [1] isto split an image into two random shares (printed ontransparencies) which separately reveal no informationabout the original secret image other than the size of thesecret image. The image is composed of black and whitepixels. The original image can be recovered bysuperimposing the two shares. The underlying operationof this visual cryptography model is OR.Visual cryptography is a unique technique in thesense that the encrypted messages can be decrypteddirectly by the human visual system. Therefore, a systememploying visual cryptography can be used by anyonewithout any knowledge of cryptography. Anotherinteresting thing about visual cryptography is that it is aperfectly secure cipher. There is a simple analogy of theone time-pad cipher to visual cryptography.II. BACKGROUND ON VISUAL CRYPTOGRAPHYBesides introducing the new paradigm, Naor andShamir also provided their constructions of visualcryptographic solutions for the general k out of n secretsharing problem. One can assume that every secretmessage can be represented as an image, and furthermorethat the image is just a collection of black and whitepixels i.e. it is assumed to be a binary image. Eachoriginal pixel appears in n modified versions (calledshares) of the image, one for each transparency. Eachshare consists of m black and white sub-pixels. Eachshare of sub-pixels is printed on the transparency in closeproximity (to best aid the human perception, they aretypically arranged together to form a square with mselected as a square number). The resulting structure canbe described by a Boolean matrix M = (m i j ) n*m where m i j= 1 if and only if the j-th sub-pixel of the i-th share(transparency) is black. The important parameters of thescheme are:1. m, the number of pixels in a share. Thisparameter represents the loss in resolution fromthe original image to the recovered one.2. α, the relative difference in the weight betweenthe combined shares that come from a whitepixel and a black pixel in the original image.This parameter represents the loss in contrast.3. γ, the size of the collection of C 0 and C 1 . C 0refers to the sub-pixel patterns in the shares fora white pixel and black refers to the sub-pixelpatterns in the shares for the 1 pixel.The constructions can be clearly illustrated by a 2 outof 2 visual cryptographic scheme. Here we define thefollowing collections of 2*4 matrices:C 0 = all the matrices obtained by permuting thecolumns of1 1 0 01 1 0 0C 1 = all the matrices obtained by permuting thecolumns of1 1 0 00 0 1 1© 2010 ACADEMY PUBLISHERdoi:10.4304/jetwi.2.2.137-141