Visual Cryptography Schemes for Secret Colour Images Sharing

DOI : 10.17577/IJERTV2IS110526

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Visual Cryptography Schemes for Secret Colour Images Sharing

Vol. 2 Issue 11, November – 2013

Mr.Thorat N.N , Mr.Patil P.P, Prof.Thakur Ritesh ,Ms.Kiranmai B

Dept. of Computer Engineering, Nishitha college of Engineering and Technology, Kandukur, AP, India Dept. of Computer Engineering, Institute Of Knowledge College Of Engineering, Pune, India

Dept. of Computer Engineering, Institute Of Knowledge College Of Engineering, Pune, India

Dept. of Computer Engineering, Nishitha college of Engineering and Technology, Kandukur, AP, India


Visual cryptography is one of the techniques used to encrypt the colour images by dividing the original image into transparencies. The transparencies can be sent to the intended person, and at the other end the transparencies received person can decrypt the transparencies using our tool, thus gets the original image. Our proposed Visual cryptography provides the demonstration to the users to show how encryption and decryption can be done to the images. In this technology, the end user identifies an image, which is not the correct image. That is, while transmitting the image the sender will encrypt the image using our application here sender gets the two or more transparencies of the same image. Our application provides an option to the end user of encryption. The end user can divide the original image into number of different images. Using our application we can send encrypted images that are in the format of GIF and PNG. The encrypted transparencies can be saved in the machine and can be sent to the intended person by other means [source].


Visual Cryptography, encryption, decryption, gif and png, transparencies

  1. Introduction

    In this technology, the end user identifies an image, which is going to act as the carrier of data. The data file is also selected and then to achieve greater speed of transmission the data file and image file are compressed and sent. Prior to this the data is embedded into the image and then sent. The image if hacked or interpreted by a third party user will open up in any image previewed but not displaying the data. This protects the data from being invisible and hence is secure during transmission. The user in the receiving end uses another piece of code to retrieve the data from the image.

    System provides a friendly environment to deal with images. Generally tools supports only one kind of image formats. Our application supports .gif and .png (portable Network

    graphics) formatted images and our application has been developed using swing and applet technologies, hence provides a friendly environment to users.

    The basic principle of the visual cryptography scheme (VCS) was first introduced by Naor and Shamir. VCS is a kind of secret sharing scheme that focuses on sharing secret images. The idea of the visual cryptography model proposed in [3] is to split a secret image into two random shares (printed on transparencies) which separately reveals no information about the secret image other than the size of the secret image. The secret image can be reconstructed by stacking the two shares. The underlying operation of this scheme is logical operation OR. In this paper, we call a VCS with random shares the traditional VCS or simply the VCS. In general, a traditional VCS takes a secret image as input, and outputs shares that satisfy two conditions: 1) any

    qualified subset of shares can recover the secret image; 2) any forbidden subset of shares cannot obtain any information of the secret image other than the size of the secret image. An example of traditional (2,2)-VCS can be found in Fig. 1, where, generally speaking, a VCS means any out of shares could recover the secret image. In the scheme of Fig. 1, shares (a) and (b) are distributed to two participants secretly, and each participant cannot get any information about the secret image, but after stacking shares (a) and (b), secret image can be observed visually by the participants. VCS has many special applications, for example, transmitting military orders to soldiers who may have no cryptographic knowledge or computation devices in the battle field. Many other applications of VCS, other than its original objective (i.e., sharing secret image), have been found, for example, authentication and identification [4], watermarking [5] and transmitting passwords [6] etc.

  2. Literature Review

    Literature survey is the most important step in software development process. Before developing the tool it is necessary to determine the time factor, economy n company strength. Once these things r satisfied, ten next step is to determine which operating system and language can be used for developing the tool. Once the programmers start building the tool the programmers need lot of external support. This support can be obtained from senior programmers, from book or from websites. Before building the system the above consideration r taken into account for developing the proposed system.

    The associated secret sharing problem and its physical properties such as contrast, pixel expansion, and color were extensively studied by researchers worldwide. For example, Naor et al. [3] and Blundo et al. [7] showed constructions of threshold VCS with perfect reconstruction of the black pixels. Ateniese et al.

    [8] gave constructions of VCS for the general access structure. Krishna et al., Luo et al., Hou et al., and Liu et al. considered color VCSs [9][12]. Shyu et al. proposed a scheme which can share multiple secret images [13]. Furthermore, Eisen et al. proposed a construction of threshold VCS for specified whiteness levels of the recovered pixels [14].

    The term of extended visual cryptography scheme (VCS) was first introduced by Naor et al. in [3], where a simple example of (2,2)-VCS was presented. In this paper, when we refer to a corresponding VCS of an EVCS, we mean a traditional VCS that have the same access structure with the VCS. Generally, an EVCS takes a secret image and original share images as inputs, and outputs shares that satisfy the following three conditions: 1) any qualified subset of shares can recover the secret image; 2) any forbidden subset of shares cannot obtain any information of the secret image other than the size of the secret image; 3) all the shares are meaningful images. EVCS can also be treated as a technique of steganography. One scenario of the applications of EVCS is to avoid the custom inspections, because the shares of EVCS are meaningful images, hence there are fewer chances for the shares to be suspected and detected.

    There have been many EVCSs proposed in the literature. Droste [15], Ateniese et al. [16], and Wang et al. [17] proposed three EVCSs, respectively, by manipulating the share matrices. Nakajima et al. [18] proposed a (2,2)-EVCS for natural images.

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    Tsai et al. [19] proposed a simple EVCS, where its shares were simply generated by replacing the white and black sub pixels in a traditional VCS share with transparent pixels and pixels from the cover images, respectively. Furthermore, Zhou et al. [20] presented an EVCS by using half toning techniques, and hence can treat gray-scale input share images. Their methods made use of the complementary images to cover the visual information of the share images. Recently, Wang et al. proposed three EVCSs by using an error diffusion half toning technique [21] to obtain nice looking shares. Their first EVCS also made use of complementary shares to cover the visual information of the shares as the way proposed in [20]. Their second EVCS imported auxiliay black pixels to cover the visual information of the shares. In such a way, each qualified participants did not necessarily require a pair of complementary share images. Their third EVCS modified the halftoned share images and imported extra black pixels to cover the visual information of the shares.

  3. Classification

    The VCS techniques we are going to discuss are broadly classified into

    1. Visual Cryptography for General Access Structure by Multi-pixel Encoding with Variable Block Size:

      Authors: Haibo Zhang, Xiaofei Wang, Wanhua Cao,Youpeng Huang

      Multi-pixel encoding is an emerging method in visual cryptography for that it can encode more than one pixel for each run. However, in fact its encoding efficiency is still low. This paper presents a novel multi-pixel encoding which can encode variable number of pixels for each run. The length of encoding at one run is equal to the number of the consecutive same pixels met during scanning the secret image. The proposed scheme can work well for general access structure and chromatic images without pixel expansion. The experimental results also show that it can achieve high efficiency for encoding and good quality for overlapped images.

    2. Halftone Visual Cryptography:

      Authors: Zhi Zhou, Member, IEEE, Gonzalo R. Arce, Fellow, IEEE, and Giovanni Di Crescenzo.

      Visual cryptography encodes a secret binary image (SI) into shares of random binary patterns. If the shares are xeroxed onto transparencies, the secret image can be visually decoded by superimposing a qualified subset of transparencies, but no secret information can be obtained from the superposition of a forbidden subset. The binary patterns of the shares, however, have no visual meaning and hinder the objectives of visual cryptography. Extended visual cryptography [1] was proposed recently to construct meaningful binary images as shares using hypergraph colorings, but the visual quality is poor. In this paper, a novel

      technique named halftone visual cryptography is proposed to achieve visual cryptography via halftoning. Based on the blue- noise dithering principles, the proposed method utilizes the void

  4. Proposed Method

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    and cluster algorithm [2] to encode a secret binary image into halftone shares (images) carrying significant visual information. The simulation shows that the visual quality of the obtained halftone shares are observably better than that attained by any available visual cryptography method known to date.


      Authors: Wei-Qi Yan, Duo Jin, Mohan S Kankanhalli

      Visual cryptography is not much in use in spite of possessing several advantages. One of the reasons for this is the difficulty of use in practice. The shares of visual cryptography are printed on transparencies which need to be superimposed. However, it is not very easy to do precise superposition due to the fine resolution as well as printing noise. Furthermore, many visual cryptography applications need to print shares on paper in which case scanning of the share is necessary. The print and scan process can introduce noise as well which can make the alignment difficult. In this paper, we consider the problem of precise alignment of printed and scanned visual cryptography shares. Due to the vulnerabilities in the spatial domain, we have developed a frequency domain alignment scheme. We employ the Walsh transform to embed marks in both of the shares so as to find the alignment position of these shares. Our experimental results show that our technique can be useful in print and scan applications.


      Authors: Ming Sun Fu. Oscar C. Au

      In this paper, we discuss how to use watermarking technique for visual cryptography. Both halftone watermarking and visual cryptography involve a hidden secret image. However, their concepts are different. For visual cryptography, a set of share binary images is used to protect the content of the hidden image. The hidden image can only be revealed when enough share images are obtained. For watermarking, the hidden image is usually embedded in a single halftone image while preserving the quality of the watermarked halftone image. In this paper, we proposed a Joint Visual-cryptography and watermarking (JVW) algorithm that has the merits of both visual cryptography and watermarking.


    Authors: R.Youmaran, A. Adler, A. Miri

    Visual Cryptography is based on cryptography where n images are encoded in a way that only the human visual system can decrypt the hidden message without any cryptographic computations when all shares are stacked together. This paper presents an improved algorithm based on Changs and Yu visual cryptography scheme for hiding a colored image into multiple colored cover images. This scheme achieves lossless recovery and reduces the noise in the cover images without adding any computational found so that each location in one image can be mapped to a new location in the second.

    Proposed system Visual cryptography provides a friendly environment to deal with images. Generally cryptography tools supports only one kind of image formats. Our application supports .gif and .png (portable network graphics) formatted images and our application has been developed using swing and applet technologies, hence provides a friendly environment to users.


        1. Interface design using Applet frame work

    In this module, we design user interface design using applet frame work. The user interface should be very easy and understandable to every user. So that anyone can access using our system. It must be supportable using various GUIs. The user interface also consists of help file. The help file assists on every concepts of the embedded visual cryptography. Help file should clearly depict the details of the project developed in simple language using various screen shoots.

        1. Visual cryptography Implementation:-

          This module is the core for the project, where we implement the Visual Cryptography. We used LZW Data Compression algorithm. The LZW data compression algorithm is applied for the gray scale image here. As a pre- processing step, a dictionary is prepared for the gray scale image. In this dictionary, the string replaces characters with single quotes. Calculations are done using dynamic Huffman coding. In compression of greyscale image select the information pixels. Then generate halftone shares using error diffusion method. At last filter process is applied for the output gray scale images. Filters are used to improve the quality of reconstructed image to minimize the noises for sharpening the input secret image.

        2. Encoding

          A high level view of the encoding algorithm is shown here:

          1. Initialize the dictionary to contain all strings of length one.

          2. Find the longest string W in the dictionary that matches the current input.

          3. Emit the dictionary index for W to output and remove W from the input.

          4. Add W followed by the next symbol in the input to the dictionary.

          5. Go to Step 2.

            A dictionary is initialized to contain the single-character strings corresponding to all the possible input characters (and nothing else except the clear and stop codes if they're being used). The algorithm works by scanning through the input string for successively longer substrings until it finds one that is not in the dictionary. When such a string is found, the

            index for the string less the last character (i.e., the longest substring that is in the dictionary) is retrieved from the dictionary and sent to output, and the new string (including the last character) is added to the dictionary with the next available code. Thelast input character is then used as the next starting point to scan for substrings.

        3. Decoding

          The decoding algorithm works by reading a value from the encoded input and outputting the corresponding string from the initialized dictionary. At the same time it obtains the next value from the input, and adds to the dictionary the concatenation of the string just output and the first character of the string obtained by decoding the next input value. The decoder then proceeds to the next input value (which was already read in as the "next value" in the previous pass) and repeats the process until there is no more input, at which point the final input value is decoded without any more additions to the dictionary.

          In this way the decoder builds up a dictionary which is identical to that used by the encoder, and uses it to decode subsequent input values. Thus the full dictionary does not need be sent with the encoded data; just the initial dictionary containing the single-character strings is sufficient (and is typically defined beforehand within the encoder and decoder rather than being explicitly sent with the encoded data.)

        4. Creating Transparencies

    This scheme provides theoretically perfect secrecy. An attacker who obtains either the transparency image or the screen image obtains no information at all about the encoded image since a black-white square on either image is equally likely to encode a clear or dark square in the original image. Another valuable property of visual cryptography is that we can create the second layer after distributing the first layer to produce any image we want. Given a known transparency image, we can select a screen image by choosing the appropriate squares to produce the desired image. One of the most obvious limitations of using visual cryptography in the past was the problem of the decoded image containing an overall gray effect due to the leftover black sub pixel from encoding. This occurred because the decoded image is not an exact preproduction, but an expansion of the original, with extra black pixel. Black pixel in the original document remains black pixel in the decoded version, but White pixel becomes gray. This resulted in a lot of contrast to the entire image. The extra black sub pixel in the image causes the image to become distorted.

    D – Secret information. K – Number of shares generated from D. share – piece of information.

    Divide data D into n pieces in such a way that D is easily reconstruct able from any k pieces, but even complete knowledge of any k-1 pieces reveals no information about D. Stacking two pixels (each consists of four sub-pixels) can occur for example the following two cases: Secret sharing scheme is a method of sharing secret information among a group of participants. In a secret sharing scheme, each participant gets a piece of secret information, called a share. When the allowed coalitions of the participants pool their shares, they can recover the shared secret; on the other hand, any other subsets, namely non-allowed coalitions, cannot recover the secret image by pooling their shares. In the last decade, various

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    secret sharing schemes were proposed, but most of them need a lot of computations to decode the shared information.

    The basic 2 out of 2 visual cryptography model consist of secret message encoded into two transparencies, one transparency representing the cipher text and the other acting as a secret key. Both transparencies appear to be random dot when inspected individually and provide no information about the original clear text. However, by carefully aligning the transparencies, the original secret message is reproduced. The actual decoding is accomplished by the human visual system. The original is encrypted into 2 transparencies you need both transparencies to decode the message.

        1. Un-hiding Image from Transparency

          The simplest form of visual cryptography separates an image into two layers so that either layer by itself conveys no information, but when the layers are combined the image is revealed. One layer can be printed on a transparency, and the other layer displayed on a monitor. When the transparency is placed on top of the monitor and aligned correctly, the image is revealed. For each image pixel, one of the two encoding options is randomly selected with equal probability. Then, the appropriate colorings of the transparency and screen squares are determined based on the color of the pixel in the image.

        2. Testing and integration

    This is the final module, which consists of integration of Visual cryptography implementation module into interface design using applet viewer. Then we need to test with various images and formation of transparencies. The transparencies should be able to save and load into the user interface.

  5. Future Enhancement

    We want to survey visual cryptography schemes of gray scale and color images. Extend the capability of our program such that it will be suitable for any kinds of input image.

  6. Conclusion

    In this paper, we proposed system provides a friendly environment to deal with images. Security is provided in the form obscurity. It supports .gif and .png formats. Our application has been developed using swing and applet technologies, hence provides a friendly environment to users.



  7. References

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    3. M. Naor and A. Shamir, Visual cryptography, in Proc. EUROCRYPT 94, Berlin, Germany, 1995, vol. 950, pp. 112, Springer-Verlag, LNCS.

    4. M. Naor and B. Pinkas, Visual authentication and identification, in Proc. CRYPTO97, 1997, vol. 1294, pp. 322336, Springer-Verlag LNCS.

    5. T. H. Chen and D. S. Tsai, Owner-customer right protection mechanism using a watermarking scheme and a watermarking protocol, Pattern Recognit., vol. 39, pp. 1530 1541, 2006.

    6. P. Tuyls, T. Kevenaar, G. J. Schrijen, T. Staring, and M. Van Dijk,

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    7. C. Blundo, A. De Bonis, and A. De Santis, Improved schemes for

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    8. G. Ateniese, C. Blundo, A. De Santis, and D. R. Stinson, Visual cryptography for general access structures, Inf. Computat., vol. 129, pp. 86106, 1996.

    9. N. K. Prakash and S. Govindaraju, Visual secret sharing schemes for color images using halftoning, in Proc. Int. Conf. Computational Intelligence and Multimedia Applications (ICCIMA 2007), 2007, vol. 3, pp. 174178.

    10. H. Luo, F.X.Yu, J. S. Pan, and Z. M. Lu, Robust and progressive color image visual secret sharing cooperated with data hiding, in Proc. 2008 Eighth Int. Conf. Intelligent Systems Design and Applications, 2008, vol. 3, pp. 431436.

    11. Y. C. Hou, Visual cryptography for color images,

      Pattern Recognit., vol. 1773, pp. 111, 2003.

    12. F. Liu, C. K. Wu, and X. J. Lin, Color visual cryptography schemes, IET Inf. Security, vol. 2, no. 4, pp. 151165, 2008.

    13. S. J. Shyu, S. Y. Huang, Y. K. Lee,R. Z.Wang, andK.Chen, Sharing multiple secrets in visual cryptography, Pattern Recognit., vol. 40, no. 12, pp. 36333651, 2007.

    14. P. A. Eisen and D. R. Stinson, Threshold visual cryptography

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      schemes with specified whiteness levels of reconstructed pixels, Designs, Codes and Cryptography, vol. 25, pp. 1561, 2002.

    15. S. Droste, ew results on visual cryptography, in Proc. CRYPTO96, 1996, vol. 1109, pp. 401415, Springer-Verlag Berlin LNCS.

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    21. Z.M.Wang, G. R. Arce, and G. Di Crescenzo, Halftone visual cryptography via error diffusion, IEEE Trans. Inf. Forensics Security, vol. 4, no. 3, pp. 383396, Sep. 2009.



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