International Journal of Knowledge Management & e-LearningVolume 3 • Number 1 • January-June 2011 • pp. 29-34

NOVEL SECURE TECHNIQUE USING VISUAL CRYPTOGRAPHY ANDADVANCE AES FOR IMAGESM. Arun Kumar1 & K. Jhon Singh2

1School of Information Technology, VIT University, Vellore, India, (arunkumarm88@gmail.com)2Prof., School of Information Technology, VIT University, Vellore, India, (Jhonsingh.k@vit.ac.in)

Abstract: The novel technique is proposed, that uses both Visual Cryptography and Advance AES Encryptionmethods simultaneously for color images. The Secrete image is transmitted by secrete sharing using visualcryptography technique and the shares are encrypted using secret key and thus provides double layer protectionfor a digital Image. Experimental results show that the proposed system is resilient to attacks and constantlyprotects the ownership rights. The Rudin-Osher-Fatemi de-noising algorithm is used to reduce the noise andproduce less distortion image. The main goal is to protect the image from unauthorized use and to provideauthentication for digital information continuous basis with less distortion.

Keywords: Advanced AES, ROF algorithm, Visual cryptography.

1. VISUAL CRYPTOGRAPHYVISUAL CRYPTOGRAPHY is a cryptographictechnique which allows visual information (pictures)to be encrypted in such a way that the decryptioncan be performed by the human visual system,without the aid of computers. The visual cryptographyis based on selection of particular pixels from theoriginal cover image instead of random selection ofpixels.

1.1. Visual Cryptography Scheme for BinaryImage

[6] In 1994, Naor and Shamir proposed the conceptof visual cryptography. The visual cryptographydoes not use any complex computation. Each pixelis sub divided into 4 sub pixels in to two shares.Share 1 is a key and share 2 is a cipher. The sub pixelsof the share are aligned using XOR to get half blackpixel and full black pixels. The visual cryptographystarts with selecting random cells in 6 choices shownbelow.

The selected random cell is a key. Share 1 doesnot provide any information. The cipher share2 isgenerated by choosing complementary cell for blacksub pixel and same cell for white sub pixel. Thentwo shares are stacked to view the originalinformation. The key share is generated by randomselection of 6 pixels. The pixel is selected from the

secrete message, if the pixel is black then thecomplementary cell is selected in cipher share forcorresponding cell in the key and vice versa, thusthe entire secrete message is divided in to two shares.The two shares are stacked XOR’ed to view the entiresecrete image. The sizes of shares generated by thevisual cryptography are double the original image.So get the image of original size the combined sharesare sub sampled [2].

1.2. Visual Cryptography Scheme for Color Image[8] In1997 ’Verheul and VanTilborg’ proposed visualcryptography for color image. Color image is seenas array of pixels. Image can look up with graylevels. Each pixel is expanded to m sub pixels. Eachpixel can take one of the color after look up withgray level. After all shares are stacked, and color i isrevealed, if corresponding sub pixel of all shares areof color i. Otherwise, the level of black is revealed

Figure 1: Random Secrete Key for Visual Cryptography

2. HALFTONINGHalf toning is process image reproduction forvarious computer devices with limited colors.According to physical properties of different mediauses different methods to represent a color level of

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image. The general dot matrix and laser printers canonly control a single pixel to be printed. The halftoning is applied to image to render the illusion ofcontinuous tone image on the devices that is capableof producing only binary image. This illusion isachieved because our eyes perform spatialintegration.

2.1. Error Diffusion[7] Error diffusion is used in our scheme to convertthe gray component to binary image. The Jarvis errorfilter is used to distribute the error to error toneighboring pixels to obtain the continuous halftoneimage.. The Floyd Steinberg error filter uses 4 tagsfor error distribution, but Jarvis error filter uses 12tag filter. Thus Jarvis error diffusion providessharper halftone image.

The above diagram shows the Jarvis error filter.Jarvis algorithm uses 12 tag for error filter uses 12tags for error distribution.

3. ROF DENOISING[4]The error distributed by Jarvis error diffusionmethod is removed using ROF de noising. Thegeneral idea behind most of denoising method is toget noisy image f as being obtained by corrupting anoiseless image u. The desire image u is then asolution of inverse process. Most of de noisingprocedures employ some sort of regularization. Avery successful algorithm is that of Rudin, Osher,and Fatemi which uses Total variation regularization.The ROF algorithm is based on Total variance [5].The main characteristics of TV procedure is asfollows.

(1) It has simple fixed filter structure and anexact formula for the filter coefficients thatexactly encode the edge information.

(2) Unlike most filters, The digital filter is basedon functional analysis and geometry.

(3) The digital TV filter is quite flexible in thesense that it applies to general data on agraph, signals like color images and non flatimage such as chromaticity.

Let be the U0(X) noise contaminated signal of aclean signal

U(X).

U0(X)=U(X)+n(X) [2]

Here, denotes random noise with mean 0 andvariance[2]

En(X)=0,En2(X)=σ2 (1)

The TV anisotropic diffusion model was inventedby Rudin, Osher and Fatemi and is now one of themost successful tools for image restoration. That is,one minimizes the total variation.

TV [U] =∫Ω |∆u| dx (2)

Ω Here denotes the continuous signal domain, the∆u gradient, and dx the area element of Ω.

The assumptions on the noise (1) now lead totwo constraints for the minimization of the TV [2]

∫Ω u dx ∫Ωuodx 1/|Ω|∫Ω Ù(u-u0)2 dx = σ2 (3)

Where |Ω| is the area of the image domainÙ.Therefore, (2) and (3) together define a constrainedoptimization problem.

Figure 2: Error Diffusion

The pixel j[n] of a continuous tone digital imageis processed from right to left and from top tobottom. At each step of algorithm the gray valuecurrent pixel represented by 0 to 255 is compared tothreshold value. If the gray value is greater thanthreshold then the pixel is black and output i[n]value is set to 1, else if the value is lesser thanthreshold then th pixel is white and output i[n] isset to 0. The difference between the original graypixel and threshold is error. To achieve continuoustone effect the error is distributed to 12 neighboringpixels that have to process yet using Jarvis errorfilter.[3] The Jarvis halftone is substantially sharperthen Floyd Steinberg halftone.

Figure 3: Jarvis Error filter

Novel Secure Technique Using Visual Cryptography and Advance AES for Images 31

Due to the translation invariance of the TV norm:TV[u+c]=TV[u] for any constant c, the first constraintis in fact automatically encoded. We only needconsider the second fitting constraint. By introducingthe Lagrange multiplier ë, one can define a newenergy functional

j[u]=∫Ω|∆u |dx + λ/2∫Ω (u-uo)2 dx (4)

The Euler-Lagrange equation of j is

-∆α (∆u/|∆u|)+ λ(u-uo)2 = 0 (5)

To avoid singularities in flat region equation (5)is regularized to

|∆u|α- = √ (|∆u|2+ α2)

for some small positive parameter. Then themodified Euler-Lagrange equation (5) in factminimizes the regularized energy functional.

j[u]α=∫Ω|∆uα|dx + λ/2∫Ω (u-uo)2 dx.

4. ENCRYPTION[9] The AES Algorithm is a symmetric key cipher inwhich both the sender and receiver uses same keyfor encryption and decryption. The data block isfixed size of 128 bits, the key length can vary from128,192,256 bits.AES algorithm is an iterativealgorithm. Each iteration is called round. Thenumber of rounds vary from 10, 12, 14 depends onthe key size 128, 192, 256 respectively. The 128 bitimage block is divided in to 16 bytes. These bytesare mapped in to 4x4 arrays called states. All theinternal operations of AES algorithm are performedon the states. In AES encryption and decryption eachround consist four transformations. The transformationperformed on the state are similar among allversions but the number of transformation roundsdepends on the cipher key length. The final Aes

differ slightly from the first Nr-1 rounds as it hasone less transformation performed on the state. Eachround of Aes cipher consists of following rounds.

• Sub bytes• Shift rows• Mix columns• Add round key.Sub bytes: Operates in each byte of state

independently. Each byte is substituted bycorresponding byte in S-box

Shift rows: Cyclically shift the rows of the stateover different offsets.

Mix columns: This transformation operates oncolumn of the state. Each column of states areconsidered as polynomial over GF(28) and aremultiplied with fixed polynomial. The mix columndoes not operate in last round of the algorithm.

Add round key: This involves bit wise XORoperation.

The key used in AES algorithm can be easilyhacked using statistical analysis, so we have to gofor advanced AES encryption [1]. This scheme usesrandom key generator to generate 128 bit key, usingthis 128 bit key generated the shares are encrypted.This scheme provides better security.

Figure 4: AES Encryption

Figure 5: Advance AES Encryption [1]

The above figure shows the Advanced AESscheme. The 128 bit key generator is used to generatekey, using this key the plain image is converted into cipher image using traditional AES encryptionalgorithm as discussed above.

5. PROPOSED SYSTEMIn visual cryptography scheme for color image, thesecrete image is separated in to individual R,G,Bcomponent and Jarvis error diffusion algorithm isapplied to each component to get binary halftoneimage, then 2x2 visual cryptography is applied toeach halftone component to get secret shares of eachcomponent, then the secrete shares are encryptedusing Advanced AES encryption and the encryptedshares are transmitted to receiver side.

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Thus our proposed scheme uses both visualcryptography technique and Advance AESalgorithm to provide double layer security forimages with less distortion.

6. RESULT & DISCUSSIONThe R, G, B component are separated for the secreteimage then, the each channel is treated as gray-scaleimage to which the halftone and Visual cryptographyare applied independently. After the binary sharesare generated for each channel, the channels afterde-noising are combined to create the RGB image.

In receiver sides the encrypted shares aredecrypted and decrypted shares are super imposedusing XOR operation and then the superimposedshares are sub sampled to get the original halftoneR, G, B component.ROF de noising algorithm isapplied to each halftone component, then R,G,Bcomponents are combined to get the original secreteimage with less distortion.

Figure 6: Flow Diagram of Proposed System-sender Side

Figure 7: Flow Diagram of Proposed System-receiver Side

Figure 8: RGB Components of Original Image

Figure 9: Halftone RGB Components

Novel Secure Technique Using Visual Cryptography and Advance AES for Images 33

The shares are decrypted in receiver side andthe shares are superimposed using XOR operationto get the original secrete image .The halftoneimages contain noise so Rudin-Osher-Fatemi (ROF)de-noising algorithm is applied to reduce noise. Thedenoised RGB components are combined to get theoriginal secrete image.

Figure 10: Secrete Sub Sampled Shares of RED Component

Figure 11: Secrete Shares of GREEN Component

Figure 12: Secrete Shares of BLUE Component

The above generated shares are encrypted usingadvanced AES encryption and then send to receiver.The encrypted shares of RED, GREEN, BLUE sharesare shown below respectively.

Figure 13: Encrypted Sub Sampled RED Shares

Figure 14 Encrypted Sub Sampled GREEN Shares

Figure 15 Encrypted Sub Sampled BLUE Shares

Figure 16: Final Image Obtained After Denoising

Table 1PSNR Values

Components PSNR value for PSNR valueimage using color for image by

error diffusion proposed systemRED 25.49 29.9104GREEN 25.45 29.5785BLUE 25.43 29.5289

The PSNR values are calculated shown abovefor image obtained by color error diffusion methodand for the image obtained by the proposed system.The values calculated shows that the proposedsystem gives high resolution output

7. CONCLUSION AND FUTURE WORKThe proposed visual cryptography scheme for thecolor images uses the Jarvis error diffusion ditheringon color channels. The use of ROF algorithm

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improves the quality of image. The XOR operation isused in stacking which produces the better qualityof image and there is no expansion in the size ofimage. The quality of decrypted image will be betterthan the other schemes which use different ditheringtechniques. This has been proved by experimentalresults obtained by using PSNR. The use of advancedAES encryption provides better security, the sharesgenerated by visual cryptography is encrypted usingadvanced AES so the secrete image is transferred withdouble layer security. Further work can be done toreduce the time taken to encrypt the shares generatedand improve the quality of halftone shares.

REFERENCES[1] A Modified AES Based Algorithm for Image

Encryption ‘M. Zeghid, M. Machhout, L. Khriji, A.Baganne, and R. Tourki’, 2007.

[2] D.L. Phillips, A Technique for The numerical Solutionof Certain Integral Equations of the First Kind, J. ACM,9, 1962, 84.

[3] ”Modelling and Quality Assessments of HalftoningError Diffusion“, Thomas D. Kite,Brain L. Evans,Senior member, IEEE and Alan c. Bovik, FellowIEEE.

[4] L. Rudin S. Osher And E. Fatemi, Nonlinear TotalVariation Based Noise Removal Algorithm, PhysicaD, 60, 1992, 259-268.

[5] The Digital TV Filter and Nonlinear De Noising by’Tony F. Chan, Member, Stanley Osher, and JianhongShen’, Vol. 10, No. 2, February 2001.

[6] Visual Cryptography for Color Image UsingColor Error Diffusion by ‘Nagaraj V. Dharwadkar,B. B. Amberker, Sushil Raj Joshi’, Vol. 10, February2010.

[7] Zhigang Fan, A Simple Modification of Error-diffusionWeights. Proceedings of SPIE’, 92.

[8] Verheul, Van Tilborg, Construction and Propertiesof k out of n Visual Secret Sharing SchemeDesigns, Codes and Cryptography, Vol. 11, 1997, pp.179-196.

[9] William Stallings, “Cryptography and NetworkSecurity”, 4th Edition.