Visual Cryptography Vs Bit Level Secret Sharing For Image Encryption

Secret sharing is a scheme used to distribute secret among a group of users. Rather than making duplicated copies of secrets among users, the secret is divided into a number of pieces, called shares. The secret can be revealed if a certain number of user shares are combined. The method proposed here (i)utilizes bit-level decomposition and stacki ng operations to both e ncrypt and decry pt B -bit image, (ii) preserves all the features of traditional (k, n) sharing schemes, (iii) al lows for perfect reconstruction of the input B-bit image, (iv) encrypts binary, gr ay-scale and color images, and (v) can be effectively implement ed ei ther in software or hardware.


1-Introduction
Secret sharing is one type of key establishment protocols.The Trusted Authority (TA) divides the secret into pieces and distributes the pieces to different users.These pieces are called shares.Shares contain partial information about the secret.
However, shares are constructed in such a way that although the secret can be reconstructed by combining a number of shares, simply examining individual user's share will not reveal the secret information at all [1] Visual cryptography illustrated a new paradigm to solve the (k,n) problem.It was originally proposed by Naor and Shamir [3].The original scheme generates n images (known as shares) based on the secret message (the original image) which can be printed on n transparencies.The original message can then be recovered if any k or more than k of the transparencies are stacked together, but no information about the original image can be gained if fewer than threshold number of k transparencies are stacked.Visual cryptography is a unique technique in the sense that the encrypted messages can be decrypted directly by the human.[3,4] To encrypt a (k1 × k2) binary image using visual cryptography, each binary pixel r(i,j) ( i.e. r(i,j)=1 for white and r(i,j)= 0 for black) is handled separately via an encryption function FEnc(•) to produce a (m1 × m2) block of black and white pixels in each of the n shares.Thus, a (k1×k2) input binary image is encrypted into (n) binary shares S1, S2, . ., Sn each one with resolution of (m1k1×m2k2) pixels.Since the arrangement of the pixels varies from block to block, it is impossible to recover the useful information without accessing a predefined number of shares.[4,5] Let FEnc(•) be the encryption function which maps a reference binary pixel r(i,j ) located at position (i,j) in the original image into (m1×m2) sized blocks in the various shares.Assuming for simplicity a basic (2, 2) scheme with (2 × 2) blocks, the encryption process is given by: For each pixel r(i,j) in the binary image F Enc (r(i,j )) {if r(i,j) = 1 (white) then Select random block from C (Fig. 1) and insert the block at locations: The size of the basis matrices depends on the expansion factor m1m2 and the number of participants, which is given by n.Since m1m2 represents the factor by which each share is larger than the original image, it is desirable to make m1m2 as small as possible [6,7].For a (2, 2) scheme considered here, each pixel in Share1 is equivalent to each pixel in Share2 if r(i, j) = 1, and each pixel in Share1 should complement each pixel in Share2 if r(i,j )=0.The decryption function FDec ( Share1(u,v), Share2(u,v)) is defined as follows:

3-Bit level based secret sharing (Coding algorithm)
The coding algorithm of our proposed system: Step1: Read a digital image (k1 x k2) with B-bit/pixel.(in this paper B=8) Step2: Decompose the input image to eight binary images, ranging from (0) for the least significant bit (LSB) to (7) for the most significant bit (MSB) (b0 … b7) as shown in figure (2).[8] Step3: implement the following procedure: for plane = 0 to 7 for i = 1 to height of image for j = 1 to width of image if b(i, j, plane) = 0 (black) then select random block from C and insert the block at locations:  Step1: Decompose each input share1 and share2 to eight binary images (2k1 x 2k2), ranging from (0) for the least significant bit (LSB) to (7) for the most significant bit (MSB).

4-Visual cryptography vs bit level secret sharing
Visual cryptography of the binary image indicates that: (i) the decrypted image is darker, and (ii) the input image is of quarter size compared to the decrypted output.Visual cryptography (iii) cannot provide perfect reconstruction, either in terms of pixel intensity or spatial resolution, and (iv) is not appropriate for real-time applications as shown in fig (5).Thus, an alternative solution is needed [9,10].The method proposed here (i) utilizes bit-level decomposition and stacking operations to both encrypt and decrypt B-bit image, (ii) preserves all the features of traditional {k, n} sharing schemes, (iii) allows for perfect reconstruction of the input Bbit image, (iv) encrypts binary, grayscale and color images, and (v) can be effectively implemented either in software or hardware as shown in fig.(6,7) .

5-Implementation
We use MATLAB language to implement visual cryptography and B-bit-level algorithm, the following figures (5,6,7) offers a visual comparison between two methods.

Conclusions
A B-bit secret sharing framework that affords perfect reconstruction of the encrypted image input was introduced.The method proposed here (i) utilizes bit-level decomposition and stacking operations to both encrypt and decrypt B-bit image, (ii) preserves all the features of traditional {k, n} sharing schemes, (iii) allows for perfect reconstruction of the input Bbit image, (iv) encrypts binary, grayscale and color images, and (v) can be effectively implemented either in software or hardware.

visual cryptography researches
There has been a steadily growing interest in visual cryptography.Despite its appearance of being a simple technique, visual cryptography is a secure and effective cryptographic scheme.Since the origin of this new paradigm, various extensions to the basic scheme have been developed to improve the contrast and the areas of application have also been greatly expanded.
0)*2 0 …(5) Where u = 2*image height & v= 2 * image width As shown in figure (3) PDF created with pdfFactory Pro trial version www.pdffactory.comDecoding algorithm: To faithfully decrypt the original B-bit image from its shares, the decryption function must satisfy the perfect reconstruction property meaning that the output should be identical to the original input.This can be obtained only if the encryption and decryption operations are reciprocal.