A Comparative Study between Proposed and Traditional Visual Cryptography Scheme

International Journal of Engineering, Management & Sciences (IJEMS) ISSN: 2348 –3733, Volume-2, Issue-1, January 2015

A Comparative Study between Proposed and Traditional Visual Cryptography Scheme Priyanka Agrawal, Vijay Kumar Sharma, Shikha Chaudhary, Shweta Parashar

Abstract— Visual cryptography is an encryption technique which is used to hide information of an image in multiple secret shares. These shares make the secrecy levels. For n level region incrementing visual cryptography scheme, image is divided in n regions. Each region consists of one level information. For implementing visual cryptography in n levels we need to encode (n+1) shares in such a way so that any single share is not able to show the information and by combining any two shares, first level information would be visible. Similarly by superimposing any three shares, information upto second level could be seen. In similar way, for revealing whole information all the (n+1) shares are superimposed. In this paper, we are comparing many visual cryptography algorithms. Traditional visual cryptography Algorithms were based on permutation whereas modern approach are based on random column selection. In Proposed algorithm, problem of pixel expansion and poor contrast has beenremoved and further it is modified to generate the levels automatically which is named as automatic region incrementing visual cryptography.

Index Terms— Encryption, Security, Traditional RIVC, Visual Cryptography

I. INTRODUCTION Digital communication of information with secure way has become a crucial issue in business and administration.

Cryptography provides essential techniques for securing information and protecting data.Visual cryptography uses the same concept except that it is applied to images. Visual cryptography is an encryption technique which is used to hide information which is present in an image. Visual cryptography scheme was developed by Naor and Shamir [1] in 1994. In this scheme shares are developed. Shares are transparent images developed by the randomization. One of the share is made of random pixels in which black and white pixels are of equal number. Second share is made according to first share. When these two shares are superimposed, information is revealed, Some smaller blocks are used in place of single pixel of original image which is needed to be encrypted. If two blocks are used for representing one pixel of original image, one of the blocks will be white and second one will be black. In the similar way, if four blocks are used in place of one pixel, two of those pixels will be white and remaining will be black. [2]

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Fig 1.Black pixel and White pixel in visual cryptography with two sub pixel layout.

Now two layers are created. One layer will have all the pixels in random state, one of the six possible states. Share 2 is same as share 1, except for the information pixels. These pixels are in opposite state in share 1. When both shares are superimposed, the pixels identical will be seen as gray and remaining will be completely black.[3]

. Fig. 2. Black pixel and White pixel in visual cryptography with four subpixel layout

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A Comparative Study between Proposed and Traditional Visual Cryptography Scheme

II. TRADITIONAL VISUAL CRYPTOGRAPHY SCHEME

times greater than original image. Also contrast of third level is very low. [7]

Visual cryptography (VC), invented by Noar and Shamir [1], is a method for protecting image-based secrets that has a computation-free decoding process.For each pixel of secret image a matrix is obtained by the permutation of columns of LKj0 and LKj1. Ck0 is used to encode a white pixel and Ck1 is used to encode a black pixel. Now first row of this matrix, according to secret image's pixel is given to first share, second row is given to second share and so on. As there are four columns in any row, four pixel will occupy the place of share corresponding to one pixel of secret image [4]. This will result in pixel expansion four.

Wang [5] provided the basis matrices for the construction of 2-level RIVC scheme.

LK10= LK20= Fig. 4. A 2-out-of-2 VCS with no PE: (a) Secret Image S, ( b) First Share S1, (c) Second Share S2, and (d) Reconstructed Image by Superimposing S1 and S2.

LK11 =

LK21 =

III. PROPOSED REGION INCREMENTING VISUAL CRYPTOGRAPHY SCHEME

We propose the (2, n)-RIVCS in which user does not need to address the area of different levels manually. Levels are created automatically. User needs only to use a particular level information with a particular size of text. A generalized form of traditional region incrementing visual cryptography with extra features added as no pixel expansion and improved contrast is proposed in this scheme.

Basis matrices for proposed 2-level scheme are shown belowLK10 =

LK20 =

LK11 = Fig. 3. A 2-out-of-2 VCS with 4-subpixel layout: (a) Secret Image S, ( b) First Share S1, (c) Second Share S2, and (d) Reconstructed Image by Superimposing S1 and S2.

In this scheme, there are some drawbacks. First drawback is that the address of different levels are given by the user manually everytime. Another drawback is that revealed image bears from pixel expansion m i.e the revealed image is m

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LK21 = 3.1 Performance evaluation of Manual Level Detection Scheme Results of 2 level traditional region incrementing visual cryptography is shown in Fig5.

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International Journal of Engineering, Management & Sciences (IJEMS) ISSN: 2348 –3733, Volume-2, Issue-1, January 2015

Fig. 5. Results of traditional two-level RIVC scheme: (a) Secret image, (b) to (d) three encoded shares, (e) to (g) superimposing any two of the three shares, and (h) superimposing all three shares.

Table1: Tabular Representation of Information on No. of Pixels Image Secret image Share1 Share2 Share3 Share1+Share2 Share1+Share3 Share1+Share2 Share1+Share2+Share3

No. of Columns 37 150 150 150 150 150 150 150

No. of Rows 37 150 150 150 150 150 150 150

No. of Black Pixels 593 11162 11295 11364 16562 16651 16636 16923

No. of White Pixels 776 11338 11205 11136 5938 5849 5864 5577

Total Pixels 1369 22500 22500 22500 22500 22500 22500 22500

Fig. 6. Results of an experiment of proposed two level RIVC scheme: (a) Secret image, (b) to (d) three encoded shares, (e) to (g) superimposing any two of the three shares, and (h) superimposing all three shares.

Table2.Tabular Representation of Information on No. of Pixels Image

No. of

No. of

No. of Black

No. of White

Total

Secret image Share1 Share2 Share3 Share1+Share2 Share1+Share3 Share1+Share2 Share1+Share2+Share3

Columns 150 150 150 150 150 150 150 150

Rows 150 150 150 150 150 150 150 150

Pixels 2373 12224 12164 12141 16840 16866 16833 17012

Pixels 20127 10276 10336 10359 5660 5634 5667 5488

Pixels 22500 22500 22500 22500 22500 22500 22500 22500

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A Comparative Study between Proposed and Traditional Visual Cryptography Scheme

It is evident from Fig. 6 and Table 2 that the proposed scheme is better than traditional region incrementing visual cryptography. This improvement is due to the following facts: 1. The reconstructed image's size is same as original image while in traditional region incrementing visual cryptography bears from pixel expansion four i.e. reconstructed image is four times larger than original image.

2. Contrast of information pixel in our reconstructed image is always 1 for each level information, whereas contrast of first level information pixels in traditional region incrementing visual cryptography is half and that of second level black pixel is one. [8]

used as secret image and noise is the error introduced in reconstructed image. Higher the PSNR value, higher will be the similarity between secret image and reconstructed image.

PSNR value is calculated from the following equationPSNR= 10. log10 (R2/MSE) where R represents the maximum pixel value and MSE is mean squared error.

Traditional RIVC

Proposed RIVC

IV. ANALYSIS OF EXPERIMENT RESULTS

+1.55

+2.62

+0.81

+1.96

By analyzing Table 1 and Table 2, the difference between the number of black pixel and white pixel is increased. Therefore, the contrast of proposed scheme is better than traditional RIVC. Fig. 7 represents Table 1 in graphical form. In graph of Fig.5 the black bar shows the black pixel and white bar represents white pixels. It is clear from the graph of Fig. 4.3 that there exist pixel expansion. Fig. 6 represents Table 2 in graphical form.

Table 3.Comparison between PSNR values of Traditional RIVC and Proposed RIVC PSNR in different levels PSNR in dB of 2 level RIVC PSNR in dB of 3 level RIVC

These results show that proposed scheme is better than traditional RIVC scheme.

V. CONCLUSION The quality assessment of the traditional RIVC and the proposed RIVC scheme have also been carried out. The performance of these schemes have been computed in terms of Peak Signal to Noise Ratio (PSNR) values and the comparison between PSNR values clearly indicates that the proposed RIVC scheme outperformed the traditional RIVC scheme Proposed automatic RIVC considers the levels as horizontally aligned. Future work can be done on automatic RIVC if levels are vertically as well as horizontally aligned.

In this work, the efforts are made to overcome the above shortcomings of the traditional region incrementing visual cryptography. In the proposed modified Region Incrementing Visual Cryptography (RIVC), the numbers of pixels in the reconstructed image are same as those in original image. The algorithms have also been modified in such a way that the resultant reconstructed images do have the better contrast information.

REFERENCES

Fig.7. Graphical representation of Table 1.

Fig..8. Graphical representation of Table 2.

To measure the quality of reconstructed image Peak Signal to Noise Ratio (PSNR) value is calculated. The term peak signal-to-noise ratio (PSNR) is an expression for the ratio between the maximum possible value (power) of a signal and the power of distorting noise that affects the quality of its representation. It is the ratio of signal to noise. Here signal is

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[1] Naor M. and Shamir. A. Visual Cryptography, 1995, p1.

[2] Monoth T. and Babu AP. Contrast Enhanced Visual Cryptography Schemes based on Additional Pixel Patterns.Presented at International Conf. on Crberworld, 2010, p. 171-178 [3] Blesswin J. and Joselin, Recovering secret image in visual cryptography.Presented at International Conf. on Communication and Signal Processing, 2011,p.538-542.

[4] Nakajima M. and Yamaguchi Y. Extended visual cryptography for natural images. Journal of WSCG, vol. 10(2), 2002. p. 303-310.

[5] OdedGoldreich, Foundations of Cryptography, Basic Tools, Cambridge University Press, vol. 1, 2001.

[6] Wang R. Z. Region incrementing visual cryptography, Signal Processing Letters, IEEE, vol. 16(8), 2009, p. 659-662.

[7] Hao L. and Faxin Y., Data Hiding in Image Size Invariant Visual Cryptography, Presented at International Conf. on Innovative Computing Information and Control, ICICIC '08. 3rd international Conference on, 2008, p. 25-25.

[8] Blundo C.; De Santis A. and Stinson D. R., Contrast in Visual Cryptography Schemes. Journal of Cryptology, vol. 12(4), 1999, p.261-289.

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