Review of Visual Cryptography

ISSN: 2395-0560 International Research Journal of Innovative Engineering

www.irjie.com Volume1, Issue 3 of March 2015

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Review of Visual Cryptography Ankita G. Shirodkar

1Electronics, Shah and Anchor Kuttchi Engineering College, Mumbai, 400088, India

Abstract Cryptography is a technique used for secure transformation of information that is secret data in presence of unauthenticated interceptor. Visual Cryptography Scheme is a method that converts secret data like images or written text into binary images and then encodes them into n images, also called as shares of images. For decoding only some of the n shares or all n shares are required which are stacked on each other. The purpose of this review paper is to make clear the idea of visual cryptography schemes and continued work in the area. Keywords Visual Cryptography, Extended Visual Cryptography, Halftoning

1. Introduction Peoples have always a passion of keeping secret information away from the others. As the world becomes more con-

nected, the demand for sharing confidential information on internet is increasing rapidly. Secret information like Credit card information, Bank transaction statements, personal data, passwords of any important account are transmitted over the Internet conveniently. While transmitting secret information an important topic to consider is Security of data. Hackers may hack the data from the weak links over the internet and may change that data. So, Confidentiality, Integrity and Availability are basic information security goals.

As technology progresses, the need for sophisticated methods of protecting data has increased. One important technique to protect secret data is Visual Cryptography Scheme (VCS). VCS was invented by Moni Naor and Adi Shamir in 1994 at the Eurocrypt Conference [1]. In k-out-of-n VCS, secret image is encoded into n shares of binary patterns. These shares are distributed among n participants. Participant doesnt have idea about the share of other participants. Secret is revealed by k or more participants by superimposing k shares. The secret cannot be revealed by superimposing less than n shares.

This paper introduces the visual cryptography schemes in section 2. Section 3 will describe the applications in the field of Visual Cryptography. Section IV concludes the paper.

2. Visual Cryptography Schemes VCS describes the different ways in which an image is encrypted and decrypted. Basically VCS are divided into Black and

White VCS, Extended VCS, Halftone VCS and Color VCS. Each one of these is discussed below with example. 2.1. Black and White VCS

M. Naor and A. Shamir initially assume that secret image I(x, y) is a collection of binary data 0 and 1 which is displayed as black and white pixels. Secret image I(x, y) is divided into n shares. Where, each pixel of secret image is represented by m black and white sub pixels in each of the n shared image. Original secret image is recovered by stacking together all or less than n shares which are distributed among participants. But, size of image is increased by m times. A various visual cryptography schemes [2] are discussed below:

2.1.1. two-out-of-two VCS

In this scheme secret image is encoded into two shares and both are required for decryption. Here, secret image is en-coded into 2 shares share 1 and share 2. If pixel in secret image is white or black, then one of the two rows corresponding to white or black pixel from Table 1 is chosen to generate sub pixel in share 1 and share 2. Any share pixel alone from any one of the two shares, dont give idea about original pixel in secret image whether it is white or black. Secret Image is re-vealed only when both the shares are superimposed.




But, 2 sub pixels per pixel can distort the aspect ratio of the original image. Thus, it is recommended to use 4 sub pixels to represent each pixel of secret image. Example of 2-out-of-2 VCS is shown in figure 1.

Table 1.Pixel pattern for 2-out-of-2 VCS with 2-subpixel

Original Pixel

Pixel Value

Share 1

Share 2

Share1 + Share2

0

0

1

1

2.1.2. n-out-of-n VCS

In this scheme secret image is encoded into n shares. By superimposing i shares where i < n will not reveal any infor-mation of the secret image [2].

2.1.3. k-out-of-n VCS

In this scheme secret image is encoded into n shares out of which any k shares are required for decryption (k < n) [2]. The secret image is invisible if less than k shares are stacked together. Figure (a) Secret Image

Figure (b) Share 1 Figure (c) Share 2 Figure (d) Overlapping of Share1 and Share 2 Figure 1.Example of 2-out-of-2 VCS

2.2. Extended VCS

Ateniese et al. [3] proposed an Extended VCS (EVCS). In EVCS the shares are meaningful. They dont have random noise on the shares. In an EVCS, for an access structure (Qual, Forb) n base images are encoded in such a way that secret image is reveled when the shares associated with the participants in any set X Qual are stacked together. No secret image is revealed when shares associated in any set Y Forb are stacked together.




2.3. Halftone VCS

The halftone VCS can be applied to grayscale and color images. Halftone VC is based on the principle of void and clus-ter dithering [4]. A halftone image is obtained by applying any halftoning method such as classical screening, dithering with blue noise, error diffusion, direct binary search etc [5-7].

Zhi Zhou and G. Arce [8] proposed halftone VCS based on error diffusion algorithm. Halftone image is obtained by ap-plying error diffusion algorithm on gray level image. It is assigned to participant 1 and its complementary image is ob-tained by reversing all black/white pixels to white/black pixels and assigned to participant 2. A secret binary pixel p is encoded into an array of Q1 x Q2 sub-pixels called as halftone cells, in each of the n shares. The two secret information pixels should be at same position in the two shares. If p is white, matrix M is randomly selected from collection of ma-trices C0 and if it is black, matrix M is randomly selected from matrices C1 where C0 and C1 are given below.

C0 = 0 10 1 1 01 0 (1)

C1 = 0 11 0 1 00 1 (2)

To select location in share, for secret information pixel is random, which may introduces white noise and leads to poor

visual quality. Hence, void and cluster algorithm is applied to choose these pixel locations.

2.4. Colour VCS

Color VC allows the use of natural color images to secure some type of information. It helps to reduce the risk of alert-ing someone to the fact that information is hidden within it. In 1996, Naor and Shamir published an article [9], which con-tains several important changes from previous work. They use two opaque colours and one white colour for sub pixel in share. The first difference from previous work is that, the order in which shares are stacked. The second difference is that each participant have more than single share.

Figure (a).Additive Colour Model Figure (b) Subtractive Colour Model

Figure 2.Fundamental Colour Model

Y. C. Hou [10] proposed colour VC scheme, in which original secret image is decomposed into three color images, un-der subtractive model, namely, C(Cyan), M(Magenta) and Y(Yellow). The size of three images is equal to original image. After that each colour image is transformed into halftone image. Randomly generated half black and white mask is select-ed, for example-. There are six possible patterns of mask. According to selected mask, three halftone colour shares are generated such that after stacking the three shares secret image is revealed. Shares generated by proposed method of Hou are larger than original image, as secret pixel is represented by several colour subpixels. Number of these subpixels is referred as pixel expansion. Therefore, Yang and Chen proposed another method which uses Additive colour model.




3. Applications of Visual Cryptography There are various applications of Visual Cryptography. Visual Cryptography applications include authentication and

identification, copyright protection and watermarking [12-13]. VCS is being used for authenticating customers signature [11], where customers signature is divided into different

shares. One share is kept in the banks database and other shares are given to customer. Customer needs to provide his shares during every transaction which are overlapped with the share present in the banks database to authenticate that cus-tomer.

4. Conclusion

Internet is used in all the aspects for sharing data to reduce time consumption. Visual Cryptographic Schemes provide security during such data transfer. Hence, to know about the different Visual Cryptographic Schemes and their applications different papers based on it are studied in this review paper.

REFERENCES [1] Moni Naor and Adi Shamir, Visual Cryptography, advances in cryptography-Eurocrypt, 1994, Lecture Notes in Computer Science,

Vol. 950, pp 1-12. [2] Debashish Jena and Sanjay Kumar Jena, A Novel Visual Cryptographic Scheme, International Conference on Advanced Computer

Control, pp. 207-211, 2009. [3] G. Ateniese, C. Blundo, A. De Santis and D. R. Stinson, Extended Capabilities for Visual Cryptography, Theoretical Computer

Science, Vol.250 (2001) 143-161. [4] Meera Kamath, Arpita Parab, Extended Visual Cryptography for color images using coding tables ,2012 International Confer-

ence on Communication, Information & Computing Technology (ICCICT), Oct. 19-20, Mumbai, India. [5] Emi Myodo, Shigeyuki Sakazawa and Yasuhiro Takishima, Visual Cryptography based on void-and-cluster halftoning technique.

In ICIP, pages 97-100, 2006 [6] Emi Myodo, Shigeyuki Sakazawa and Yasuhiro Takishima, Halftone Visual Cryptography embedding a natural grayscale image

based on error diffusion technique. In ICME, pages 2114-2117, 2007. [7] Zhongmin Wand and Gonzalo R. Arce, Halftone Visual Cryptography through error diffusion. In ICIP, pages 109-112, 2006. [8] M. Naor and B. Pinkas, Visual authentication and identification Crypto, Lecture Notes Comput. Sci., vol. 1294, pp. 322340, 1997. [9] Moni Naor and Adi Shamir, Visual Cryptography II: Improving the contrast via the cover base. In Proceedings of the International

Workshop on Security Protocols, pages 197-202, London, UK, 1997. Springer-Verlag. [10] Neha Gupta, Manish Gupta, Abhishek Mishra. Journey of VCS from Black and White Images to Colored Images with their Per-

formance Analysis, International Journal of Computer Applications (0975-8887), Volume 79-No 9, October 2013. [11] B.Srikanth, G.Padmaja, Dr.Syed Khasim, Dr.P.V.S.Lakshmi, A.Haritha. Secured Bank Authentication using Image Processing and

Visual Cryptography, International Journal of Computer Science and Information Technologies, Vol. 5 (2) , 2014, 2432-2437. [12] C. Chang and H.Wu, A copyright protection scheme of images based on visual cryptography, Imag. Sci. J., vol. 49, no. 3, pp.

141150, 2001. [13] C. Wang, S. Tai, and C. Yu, Repeating image watermarking technique by the visual cryptography, IEICE Trans. Fundamentals

Electronic Communication.

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