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A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing
A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing
A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing
A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing
A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing
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A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing

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Visual cryptography (VC) is a method of …

Visual cryptography (VC) is a method of
encrypting a secret image into shares such that stacking a
sufficient number of shares reveals the secret image. Shares
are usually presented in transparencies. Each participant
holds a transparency. In this paper, we studied the cheating
problem in VC by malicious adversaries. We considered the
attacks of malicious adversaries who may deviate from the
scheme and create fake shares, whose stacking reveals a
different Secret image. This paper proposes a solution to this
Cheating problem by an Invisible and Blind Watermarking
scheme. This scheme not only provides Authentication for the
VC shares but also makes these secret shares invisible by
embedding them into not so significant Host images. Thus
secret shares are not available for any alterations by the
adversaries who try to create fake shares. In the proposed
invisible and blind watermarking scheme, every pixel of the
binary VC share is invisibly embedded into the individual
blocks of the host image sized 2x2. In this proposed scheme,
the process of watermark extraction necessitates only the
watermarked image and it doesn’t require the original host
image or any of its characteristics, making the proposed
scheme blind. The efficiency of the proposed Cheating
prevention scheme by Invisible Blind watermarking scheme
has been demonstrated via the experimental results. A Perfect
restoration technique is added to the Visual Cryptography
Scheme to improve the quality of the restored secret image as
well as memory space utilization. Hence the proposed
Cheating prevention scheme along with Perfect restoration
techniques provides a Novel Visual Cryptography scheme.

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  • 1. ACEEE Int. J. on Signal & Image Processing, Vol. 01, No. 03, Dec 2010 A Novel Scheme for Mutual Authentication and Cheating Prevention in Visual Cryptography using Image Processing B.Padhmavathi#1, P.Nirmal Kumar*2, M.A.Dorai Rangaswamy#3 # Department of Computer Science & Engineering, Easwari Engineering College, Chennai – 600 089, Tamil Nadu, India 1 padmas9169@yahoo.co.in 3 drdorairs@yahoo.co.in * Department of Electronics & Communications, College of Engineering, Anna University, Chennai – 600 025,Tamil Nadu, India 2 nirmal2100@yahoo.co.inAbstract— Visual cryptography (VC) is a method of In these cases, all participants who hold shares areencrypting a secret image into shares such that stacking a assumed to be honest ie., they will not present false or fakesufficient number of shares reveals the secret image. Shares shares during the phase of recovering the secret image.are usually presented in transparencies. Each participant Thus, the image after stacking them is considered as theholds a transparency. In this paper, we studied the cheatingproblem in VC by malicious adversaries. We considered the real Secret image. Nevertheless, cryptography is supposedattacks of malicious adversaries who may deviate from the to guarantee security even under the attack of maliciousscheme and create fake shares, whose stacking reveals a adversaries who may deviate from this scheme in any way.different Secret image. This paper proposes a solution to this We have seen that it is possible to cheat [1], [2], [3] in VC,Cheating problem by an Invisible and Blind Watermarking though it seems hard to imagine. For cheating, a maliciousscheme. This scheme not only provides Authentication for the participant presents some fake shares such that the stackingVC shares but also makes these secret shares invisible by of fake and genuine shares together reveals a differentembedding them into not so significant Host images. Thus secret image making the genuine participants to believe itsecret shares are not available for any alterations by the to be the original. It is observed that the participant sharesadversaries who try to create fake shares. In the proposedinvisible and blind watermarking scheme, every pixel of the are vulnerable for Cheating attacks. In this paper, webinary VC share is invisibly embedded into the individual consider this problem and provide security to theblocks of the host image sized 2x2. In this proposed scheme, participant shares by hiding them in some insignificantthe process of watermark extraction necessitates only the images. For example, Lena, Peppers etc.,watermarked image and it doesn’t require the original host Watermarking is the technique of embedding a secretimage or any of its characteristics, making the proposed image into a cover image without affecting its perceptualscheme blind. The efficiency of the proposed Cheating quality so that secret image can be revealed by someprevention scheme by Invisible Blind watermarking scheme process. One significant advantage of watermarking is thehas been demonstrated via the experimental results. A Perfect inseparability of the watermark(secret image) from therestoration technique is added to the Visual CryptographyScheme to improve the quality of the restored secret image as cover image. Some of the vital characteristics of thewell as memory space utilization. Hence the proposed watermark are: hard to perceive, resists ordinary distortions,Cheating prevention scheme along with Perfect restoration endures malevolent attacks, carries numerous bits oftechniques provides a Novel Visual Cryptography scheme. information, capable of coexisting with other watermarks, and demands little computation to insert and extractKeywords - Visual Cryptography, Cheating Prevention, Watermarks. Generally, robust watermarking is used toInvisible Watermarking, Blind Scheme, Secret shares, HostImages. resist un-malicious or malicious attacks like scaling, cropping, lossy compression, and so forth. Watermarking I. INTRODUCTION techniques can be categorized into different types based on a number of ways. Watermarking can be divided into Non- Visual Cryptography (VC) is a method of encrypting a blind, Semi-Blind and Blind schemes [4], [5] based on theSecret image into shares such that stacking a sufficient requirements for watermark extraction or detection. Non-number of shares reveals the secret image. Shares are blind watermarking schemes necessitate the original imagebinary images usually presented in transparencies. Each and secret keys for watermark detection. The Semi-Blindparticipant holds a transparency (share). Unlike schemes require the secret key(s) and the watermark bitconventional cryptographic methods, VC needs no sequence for extraction, whereas, the Blind schemes needcomplicated computation for recovering the secret. The act only the secret key(s) for extraction. Another categorizationof decryption is to simply stack shares and view the Secret of watermarks based on the embedded data (watermark) is:image that appears on the stacked shares. visible and invisible. With visible watermarking of images, a secondary image (the watermark) is embedded in a©2010 ACEEE 1DOI: 01.IJSIP.01.03.81_264
  • 2. ACEEE Int. J. on Signal & Image Processing, Vol. 01, No. 03, Dec 2010primary image in such that it is perceptible to a human two identical blocks of four sub-pixels. A black pixel isobserver, whereas the embedded data is not detectable in shared into two complementary blocks of four sub-pixels.case of Invisible watermarking; nevertheless, it can be Fig. 2 illustrates this scheme of encoding one pixel intoextracted by a computer program. four pixels in a (2, 2) VC scheme. All the pixels in the Here our proposed scheme will add the merits of both original image are encrypted similarly using this scheme.visual cryptography as well as Invisible and Blind These shares can be either Vertical or Horizontal orwatermarking techniques , where we will generate the Diagonal Share as shown in the figure.secret shares using basic visual cryptography model andthen we will watermark these shares into some host image                        using invisible and blind watermarking. Thus the secret                        shares are protected from cheating attacks. The decryptionwill be same as in the visual cryptographic model i.e. by Vertical Shares Horizontal Shares Diagonal Sharesstacking of the shares after the secret shares have been Fig 2. Pixel encoding in (2,2) Visual Cryptography schemeextracted by a simple watermark extraction technique. Theproposed watermarking scheme doesn’t necessitate the B. Cheating Behaviour in Visual Cryptographyoriginal image or any of its characteristics for the A cheating process against a VCS [9] consists of theextraction of watermark, and hence the proposed scheme is following two phases:blind. The experimental results have been demonstrated for 1) Fake Share Construction Phase, where the cheaterefficiency of the proposed Invisible and Blind participant generates the fake shares.Watermarking scheme for Binary images in [6],[7]. 2) Image Reconstruction Phase, where the Fake image The rest of this paper is organized as follows. The appears after stacking the genuine shares with fakemodels for Visual Cryptography scheme(VCS), Cheating shares.behaviors in VC and Digital watermarking for the In the case of cheating, honest participants who presentprotection of digital images are discussed along with brief their shares for recovering the secret image are not able toreview of their previous works in Section 2. We then distinguish fake shares from genuine shares. Adescribe the proposed Innovative Invisible and Blind reconstructed image is a perfect image indistinguishableWatermarking scheme for Cheating prevention in Section 3. from the original. The key point of cheating is how toThe experimental results are provided in Section 4 and predict and rearrange the positions of black and white subconclusions are summed up in Section 5. pixels in the victim’s and cheater’s share. Fig. 3 shows the whole cheating process and Table 1 shows how the II. REVIEW OF RELATED TOPICS cheaters create fake shares to change the decoded image.A. (2,2) Visual Cryptography Scheme Visual Cryptography (VC) was first introduced byMoni Noar and Shamir at Eurocrypt’94 [8]. It involvedbreaking up the image into n shares so that only someonewith all n shares could decrypt the image by overlayingeach of the shares over each other. To encode a secretemploying a (2, 2) VC Scheme, the original image isdivided into two shares such that each pixel in the originalimage is replaced with a non-overlapping block of two orfour sub-pixels as shown in Fig.1. Fig 3. Cheating Process in VCS TABLE I FAKE SHARE PIXEL CREATION Pixel in Pixel in Pixel in Pixel in Pixel in Pixel in Pixel in Secret Share Cheat Share SA Share SC Share SA’ Share SB’ Image SB Image Fig 1. A (2,2) Visual Cryptography scheme white [1 0 0] [1 0 0] [1 0 0] white [1 0 0] [1 0 0] Anyone who holds only one share will not be able toreveal any information about the secret. To decode the white [1 0 0] [1 0 0] [1 0 0] black [0 1 0] [0 0 1]image, each of these shares is Xeroxed onto a transparency.Stacking both these transparencies will permit visualrecovery of the secret. black [1 0 0] [0 1 0] [0 0 1] white [0 0 1] [0 0 1] There are several schemes of encoding the pixels of thesecret image. In our scheme, each pixel in the secret image black [1 0 0] [0 1 0] [0 0 1] black [1 0 0] [0 1 0]is broken into four sub pixels. A white pixel is shared into©2010 ACEEE 2DOI: 01.IJSIP.01.03.81_264
  • 3. ACEEE Int. J. on Signal & Image Processing, Vol. 01, No. 03, Dec 2010C. Digital Watermarking into a single block of the host image. The watermark Our work has been motivated by a copious number of embedding process involves: mean calculation, embeddingearlier works available in the literature that utilize digital strength (β) and signum function. Each non-overlappingimage watermarking for protecting copyrights of digital block is converted into a vector, and the mean value of theimages. Debasish Jena1Sanjay Kumar has proposed Data vector is computed and divided with the embeddinghiding in halftone images using conjugate ordered dithering strength (β) Since, the watermark is a binary image; the(DHCOD) algorithm is given, which is a modified version process of watermark embedding involves two cases:of existing Data hiding in halftone image by conjugate embedding pixel value ‘1’ and embedding pixel value ‘0’.error diffusion (DHCED) algorithm. Lu et al. [10] have Two different inserting mathematical operations are carriedpresented a digital watermarking technique that is intended out for embedding pixel value ‘0’ and ‘1’.to solve some vital issues of the digital world, such as B. Watermark Extractioncopyright protection, copy protection, and image This sub-section details the steps involved in theauthentication.Wei-Che Chen [11] have presented a digital extraction of the binary watermark image from theimage copyright protection scheme on the basis of visual watermarked image. As the proposed scheme is blind,cryptography (VC) and singular value decomposition (SVD) watermark extraction necessitates the watermarked image,techniques. Initially, the scheme applies SVD to a host size of watermark image and the embedding strength,image to construct a master share. This scheme embeds the whereas it doesn’t require the host image or any of itssecret image without effecting any modification of the host characteristics. Initially, non overlapping blocks sized 2x2image. All the above mentioned schemes provide copyright are extracted from the watermarked image and the numberprotection to the VC Secret image but where as VC secret of blocks extracted varies based on the size of theshares are still vulnerable to the Cheating attacks. In this watermark image. The blocks that are extracted are storedresearch, we propose an innovative Invisible and Blind in a vector. Subsequently, all the extracted blocks arewatermarking scheme [6], applied to VC shares to secure converted into a vector and the mean value of the vector isthem against Cheating attacks by the adversaries. computed. Afterwards, the mean values of all the blocks are divided by the embedding strength (β). The resultant III. PROPOSED MUTUAL AUTHENTICATION AND value is made use of in the extraction of watermark. Finally, CHEATING PREVENTION SCHEME USING a matrix of the size of the watermark image is created and INVISIBLE BLIND WATERMARKING the extracted pixel values are placed in it so as to obtain the This section presents the proposed Invisible and blind watermark image.watermarking Algorithm for the protection of Secret shares C. Phases of the Proposed Schemein VC. The proposed scheme utilizes the binary VC secretshare image as watermark data . With the aid of the In the proposed scheme we shall generate the VC-embedding strength and signum function, the scheme shares using basic visual cryptography model and thendiscussed, embeds this binary watermark image pixel into embed them into a cover image using a Invisible Blindevery 2x2 non-overlapping block of some insignificant host Watermarking technique, so that the secret shares will beimages. This process protects the VC secret shares from more secure and meaningful. And the shares are protectedany adversary manipulations, which may result in fake from the malicious adversaries who may alter the bitshares. Also, during extraction, the embedded binary sequences to create the Fake shares. During the Decryptionwatermark is extracted from the watermarked host image phase, the secret shares are extracted from their coverusing watermark image size and the embedding strength. images without needing any of the cover imageAs this digital watermarking scheme doesn’t require the characteristics to provide mutual authentication.original image or any of its characteristics for watermark Proposed scheme consists of three phases which areextraction, it is said to be blind. The VC Secret shares are described in the following subsections.completely hidden from malicious users. Hence it is proved Phase I – Visual Cryptographic Encryption : In this verythat above stated Cheating behaviour in VC is prevented by first phase we will do visual cryptography encryption. Itthis novel watermarking algorithm. In our paper, we have consists of generation of shares using any basic visualchosen the user’s signature images for VC encryption cryptography model. Visual cryptographic solutionsphase and this provides mutual authentication during any operate on binary inputs. In our proposed scheme, atransaction at the VC Decryption phase. The following (2,2)VC share creation is performed. During encoding duesubsections describe the steps involved in the watermark to Pixel expansion the size of the shares will be large. Sinceembedding and extraction processes and also various each pixel in the original image is replaced by four sub-phases of the entire scheme. pixels in each share, the width of the decoded image is more than twice that of the original image. But afterA. Watermark Embedding applying the Perfect Restoration techniques [12], the This sub-section presents the process of binary VC original size of the secret image can be restored. So thesecret share image embedment into the host image. The result of this phase will be 2 unintelligible shares of blacksize of the host image chosen is dyadic (2nx2n) and the and white pixels. In real time applications share 1 willwatermark used is a binary image. Firstly, non-overlapping available with Client and share 2 will be saved in theblocks sized 2x2 are extracted from the host image and Server database for future authentication purposes. Fig. 4every pixel of the watermark VC share image is embedded©2010 ACEEE 3DOI: 01.IJSIP.01.03.81_264
  • 4. ACEEE Int. J. on Signal & Image Processing, Vol. 01, No. 03, Dec 2010shows the result of Share 1 creation with perfect restorationtechnique applied. Secret Image Phase I a)Original b) Share 1 c) Share 1 after Perfect Share 1 Share 2 Restoration technique Fig 4. Share 1 creation using (2,2) VC EncryptionPhase II - Hiding the Shares using invisible blind Embed in Host Image Embed in Host image 1 2 Phase IIwatermarking : This is the second phase of our approachwhich will embed shares of the client and server, generatedfrom the first phase into some host images. Forwatermarking we will use Invisible and BlindWatermarking scheme discussed under section 3. Use ofwatermarking will give an added advantage of double Extracted Extractedsecurity over other visual cryptographic schemes. The Share 1 Share 2watermarked host images are robust against variousattacks like Blurring, Cropping,, Sharpening, JPEG Lossycompression. This forbids any malicious participant toaccess the secret shares and modify them into Fake shares Phase IIIthus preventing Cheating problem in VC. The result of thisphase would be innocent looking watermarked host imagesthat does not seem to contain any secret in them because ofthe invisible watermarking methodology applied. The Decoded Secret Imagewatermarked host image 1 and 2 will be available with theclient and server databases.Phase III - Visual Cryptographic Decryption andAuthentication: This is the last phase of proposed scheme. Fig. 5. Structure of the Proposed SchemeIn this phase we extract the binary watermarked sharesfrom the host images of the client and server. The proposed IV. SIMULATION RESULTSwatermarking scheme doesn’t necessitate the originalimage or any of its characteristics for the extraction of For simulation we have used MATLAB 7.0 tool andwatermark, and hence the proposed scheme is blind. Then tested with images of different sizes. The proposed schemewe apply the visual cryptographic decryption. As we achieves effective embedment of the binary share imagesknow that visual Cryptographic decryption does not need into the host images. Also, the proposed scheme depictsany type of decryption algorithm or computation. It uses efficient extraction of the embedded watermarks from thehuman visual system for decryption which is the core watermarked images. The watermarked images possessadvantage for which visual cryptography was developed. good Peak Signal to Noise Ratio (PSNR) and good visualNow we can decrypt the original secret image by quality. Fig. 6 depicts the results obtained onoverlapping or stacking the shares. When the two shares experimentation of the entire proposed Visualare stacked together, the black pixels in the original image Cryptography scheme.remain black and the white pixels become grey. Although The results include original secret image, encodedsome contrast loss occurs, the decoded image can be secret shares, host image, watermarked images and theclearly identified. Now again the perfect restoration decoded secret image. Thus the encoded secret shares havetechnique is used to remove the greying effect to give more been authenticated to prevent Cheating attacks.contrast. Hence the result of this phase will be an imageconsisting original secret image. Fig 5 is the structure ofproposed scheme.©2010 ACEEE 4DOI: 01.IJSIP.01.03.81_264
  • 5. ACEEE Int. J. on Signal & Image Processing, Vol. 01, No. 03, Dec 2010 image characteristics because the watermark extraction technique is blind.. The overlapping of these shares reveal a) Secret image the original secret. The decoded secret signature image quality is improved by using perfect restoration technique. Yet many possible enhancements and extensions can be made to improve further. REFERENCES [1] H.Yan, Z. Gan, and K. Chen,“A cheater detectable visual cryptography scheme,” (in Chinese) J. Shanghai Jiaotong Univ., vol. 38, no. 1, 2004. c) Share 1 [2] G.-B. Horng, T.-G. Chen, and D.-S. Tsai, “Cheating inb) Host Image 1 d) Watermarked host image 1 visual cryptography , ” Designs, Codes, Cryptog., vol. 38, no. 2, pp. 219–236, 2006. [3] M. Naor and B. Pinkas, “Visual authentication and identification,” in Proc. Advances in Cryptology, 1997, vol. 1294, LNCS, pp. 322–36. [4] P. Tao and A. M. Eskicioglu, “A Robust Multiple Watermarking Scheme in the DWT Domain,” Optics East 2004 Symposium,Internet Multimedia Management Systems f) Share2 V Conference, Philadelphia, PA, pp. 33-144, October 25-28, 2004.e) Host Image 2 g) Watermarked [5] Ersin Elbasi and Ahmet M. Eskicioglu, “A Semi-Blind host image 1 Watermarking Scheme for Color Images Using a Tree Structure," in proc. of IEEE Sarnoff Symposium, March, 2006 [6] M.A.Dorairangaswamy, “A Novel Invisible and Blind Watermarking Scheme for Copyright Protection of Digital Images,” International Journal of Computer Science and Network Security (IJCSNS), Vol. 9 , No. 4, pp. 71-78, 2009. h) Extracted & decoded Secret [7] M.A.Dorairangaswamy, “A Robust Blind Image Watermarking Scheme in Spatial Domain for Copyright Protection,” International Journal of Engineering and Fig. 6. Experimentation Results Technology (IJET), Vol. 1, No.3, pp. 249 - 255, August 2009. [8] M. Naor and A. Shamir, .Visual Cryptography,. Advances V. CONCLUSION in Cryptography -EUROCRYPT94, Lecture Notes in Visual cryptography is the current area of research Computer Science 950, 1995, pp. 1-12.where lot of scope exists. Currently this particular [9] C. M. Hu and W. G. Tzeng, .Cheating Prevention in Visualcryptographic technique is being used by several countries Cryptography, .IEEE Transaction on Image Processing, vol. 16, no. 1, Jan- 2007, pp. 36-45.for secretly transfer of hand written documents, financial [10] Zhe-Ming Lu, Wei-Min Zheng, Jeng-Shyang Pan and Zhendocuments, text images, internet voting etc. There are Sun, “Multipurpose Image Watermarking Method Basedvarious innovative ideas and extensions exist for the basic on Mean-removed Vector Quantization", Journal ofvisual cryptographic model introduced till now. One such Information Assurance and Security, Vol. 1, pp. 33-42,enhancement we are trying to do in terms of preventing 2006.cheating attacks. In the existing VC schemes no security is [11] Ming-Shi Wang and Wei-Che Chen, "Digital imageprovided to the secret shares and adversaries can alter its copyright protection scheme based on visual cryptographybit sequences to create fake shares. And in our proposed and singular value decomposition", Optical Engineering,scheme, the vulnerability of these binary secret shares is Vol. 46, No. 6, 2007. [12] Duo Jin, Wei-Qi Yan, Mohan S. Kankanhalli, “ Progressiveovercome by hiding them invisibly into some host images. Color Visual Cryptography”, SPIE Journal of ElectronicDuring the decryption phase, the secret shares are extracted Imaging (JEI/SPIE) Jan,2006from their cover images without needing any of the cover©2010 ACEEE 5DOI: 01.IJSIP.01.03.81_264

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