Different Visual Cryptographic Schemes and Their Past, Present and Future Aspects Ms. Urvashi Yadav 1, Mr. Nisheeth Saxena 2 1 MTech Student, 2 Assistant Professor, 1,2 Mody University of Science and Technology Laxmangarh, Sikar, India, Pin- 332311 1 urvashi.yadav16@gmail.com, 2 nisheethsaxena.fet@modyuniversity.ac.in Abstract: Visual Cryptography is a technique for encrypting an image with the help of encryption algorithms, but for decryption no algorithm is required. Decryption is done only by the human visions. The image is divided into the shares and by placing an appropriate number of shares in right order, decryption can be performed. This technique is used for the visual information such as pictures, texts etc. to be encrypted in such a manner that it can be decrypted only by the human visions, without any help of any decryption algorithms. This paper is the survey for many visual cryptographic algorithms based on the past studies. Keywords: Visual cryptography, Visual cryptography schemes, Meaningful shares, Pixels. 1. INTRODUCTION With the emerging transmitting multimedia data over the internet, the security of the data has given much more importance. As a result, there is a rapid advancement in the different types of Internet technologies, the information can be transmitted all over the world from any place through the Internet. Some data that is to be transmitted online can be more important as the secret image, so before sending the data and trusting the speed of the Internet or its attractive offers, the sender have to be more concern about the security issues. With the growing era of electronic commerce, there is the requirement to solve the issue of information security or safety in today s expanding open network scenario. The encrypting technologies of the standard cryptography are used to make the information much secure or to protect the information security. With these technologies, after encryption, the data turns into the unstable form and then can be retrieved by using a correct key. If the key is wrong, the data cannot be retrieved and the unauthorized access of data cannot be done. So the encrypted data cannot be retrieved without the correct key. Visual cryptography is an emerging cryptographic technology, which was proposed by Naor and Shamir [1] in 1994. This technique was based on the human vision for the decryption of the encrypted image. A person who has no knowledge of cryptography can also decrypt the image. It does not need any cryptographic algorithms or other complex calculations. It was also secure, so that no hacker could perceive any idea about the secret image or the cover image. They proposed the scheme named as ( k, n) -threshold visual secret sharing scheme, and also can referred to as visual cryptography (VC) [1]. The main characteristic of this scheme was that the decryption of the secret image can be done with the human vision only without making effort like to solve any complex computation. Their scheme could hide the secret image into n distinct shares that can also be called as n distinct images. The secret image can be reformed by placing or stacking as many as k shares. Each share is the collection of random pixels, in which each share is meaningless if counted Page 127
individually. Single share revels nothing if it is not stacked with the other shares. So, one share individually is meaningless. Doing so, the security of the secret image increased, that is, transmitted through Internet. VC technique is a simple technique in which the data D is divided into n shares, after encryption D can be reconstructed from any of the k shares out of n. If k -1shares are stacked together, then it will reveal no information about the data D.it is compulsory to stack total k shares out of n. It can understand with the help of a simple example. Suppose there are 6 thieves having the same account in the bank. The password is divided in such a way that less than 4 thieves cannot be able to access the account, it is compulsory to present 4 thieves at a time to access the account. There are many VC schemes or methods have been proposed after the scheme of Naor and Shamir [1]. The basic scheme takes binary images only as secret images which can be black and white designs. The basic 2 out of 2 scheme of visual cryptography is shown in Fig Later on, researchers work on the colored images. security model. To solve the subject of the basic model leads to the general access structures given by Atenise et al [3], in which an access structure is the identification of all authorized and unauthorized subsets of n shares. The secret image can be decrypt with the subsets of k or more authorized shares but, if the shares are stacking together which is less than the number of authorized shares or if unauthorized shares are stacked then no information or data about the secret image is revealed. This scheme of general access structure for the pixel expansion is much better than the previous ones [1]. 2.2 Visual cryptography for gray level images Earlier the achievements in visual cryptography were restricted to the binary images only which was inadequate in the real time applications. The new technique in visual cryptography for the gray level images with the use of different dithering techniques was proposed by Chang et al [4]. For the conversion of the gray scaled images to the binary images dithering technique is used in place of using gray sub pixels to the shares. Then the existing visual cryptography schemes are applied to create shares. The results of this scheme is average with respect to the relative size and decrypted image quality, if the gray levels counts of the original image reaches to 256. This can be understood with the help of Fig. 2. Figure 1: A Visual Cryptography 2 out of 2 Scheme [2] 2. VISUAL CRYPTOGRAPHY SCHEMES Some of the Visual Cryptography Schemes are as follows [2]: 2.1 Visual cryptography for general access structures When any k shares are used to decode the secret image from ( k, n) Basic model results decrease in Figure 2: Visual cryptography for gray level [4] Page 128
2.3 Recursive threshold visual cryptography Abhishek et al [5] proposed a new approach for the recursive threshold visual cryptography removes the problem of inefficiency in terms of the number of bits of image per bit of shares. This scheme hides short secrets in shares of the images of larger sizes which can be double at each step. Network load found very less while make use of recursive threshold visual cryptography into network applications. Fig 3 shows the regeneration of smaller images from the shares. while [6] forms the extended visual cryptography scheme appropriate for natural images. Figure 4: Basic idea for proposed system [6] 2.5 Halftone visual cryptography The shares were of poor quality that was generated in [6] and the suspicion of data encryption increases, again. To increase the quality of the meaningful shares Zhou et al [7] proposed a new technique based on halftone visual cryptography. In halftone visual ' P ', which is cryptography as shown in Fig 5, a pixel secret as well as binary, is encoded into an array of 'n share, mentioned to Q sub pixels in each ' 1 Q 2 as halftone cells. Visually satisfying halftone shares can get by the use of halftone cells of proper size. This method helps in maintaining the security and the contrast. Figure 3: Regeneration of smaller images from the shares [5] 2.4 Extended visual cryptography for natural images There were limitations in all of the visual cryptographic algorithms which hold the objectives of the VC. There were the random patterns which carries no visual information, lifting the doubt of data encryption. Then extended visual cryptography for natural images has been proposed by Nakajima et al [6], which define meaningful shares of binary images as shown in Fig 4. It helps in reducing the suspect of the cryptanalyst from an individual shares. Only binary images was handled by the earlier researches, Figure 5: Halftone visual cryptography scheme (2 out of 2) [8] 2.6 Visual cryptography for colored images The quality of the decoded binary images was degraded by the new researches in visual cryptography which makes it inappropriate for the Page 129
security of colored images. Then a new approach is proposed for the colored images in visual cryptography by Liu et al [9]. They proposed three new approaches as given below: 1. In the first approach, the colors from the secret image of the shares prints with the same method that is used in basic model. Larger pixel expansion method was used which degrades the quality of the decrypted colored image. 2. In the second approach, color image is converted into the black and white images on the basis of three color channels; they are red, green, blue or equivalent to cyan, magenta and yellow. Then black and white VCS are applied to each color channels. This decreases the pixel expansion but the quality of the decoded image is reduced due to halftone process. 3. In the third approach, the pixel of the color image is represented in the binary format and at the bit-level, the secret image encrypts. This approach gives better quality of the image but requires a decoding device to decode the image. Based on the above mentioned approaches when applied on an image give results as shown properly in Fig. 6. contrast, when the quality of the image is not only an issue but quite rare. The quality of the original image degrades by the use of digital half-toning techniques. The quality degrades due to its intrinsic lossy nature and to recover the original image from half-toning stage is nearly impossible. A new encoding method is proposed by Jin et al [10] that enable transforming the gray scale and color images into monochrome images without any information loss. By integrating the new technique of encoding into visual cryptography scheme results in exact recovery of the gray scale or the color image. 2.8 Regional incrementing visual cryptography In traditional VC schemes, all the pixels are shared in the secret image by the use of a single encoding rule. This sharing strategy disclose either the whole image or no image, therefore it limits to have the same secret properties in an image. So, a new technique of sharing the secrets in multiple secrecy level in an image has been proposed by Wang et al [11] known as Region Incrementing Visual Cryptography. This scheme has ' n' level. An image 'S' has been appointed to numerous areas connected with secret levels, and encodes shares with the following features: (a) No secrets from 'S' can be obtained to the shares. (b) Any t( 2 t n 1) shares can used to show ( t 1) levels of secrets. (c) Users are unknown of the number and locations of not so far revealed secrets. (d) If all of the ( n 1) shares are available together then all the secrets in 'S' can be revealed. The results of the RIVC scheme that applies on an image are shown in Fig 7. Figure 6: Experimental results for colored images [9] 2.7 Progressive visual cryptography In the traditional schemes for the color visual cryptography, VCS comes in light due to loss of Page 130
Jagdeep et al [14] proposed an approach to secure the image shares by the use of watermarking techniques. The shares can be made invisible by hiding them into the host images and authentication is provided for the shares. This can be done with the use of digital watermarking. Discrete Cosine Transform (DCT) is used to embed the shares into the host image. This produces a less distorted image as compared to the original. This scheme makes the share more secure, meaningful and robust against attacks like blurring, motion blurring and sharpening etc. These different attacks on the image are shown in Fig 8. Figure 7: Results of RIVC Scheme [11] 2.9 Segment based visual cryptography All the visual cryptography schemes discussed above is based on the pixels of the input image. The main drawback of the pixel oriented visual cryptography is loss in the decoded image which is directly related to the pixel expansion. Borchert et al [12] proposed a new approach based on the small segments which encrypts the smallest unit i.e. pixel. The benefit of this scheme based on segment over pixel is to make easy for the human eye to identify the symbols. The encoded messages contains numbers fundamentally manages only binary images. 2.10 Visual Cryptography in images using steganography Khalil et al [13] propose a new approach to make the security better with the use of steganography. To hide the messages in image the existing methods is merge with the steganalysis technique. By using this, the level of secrecy and security becomes high with the limitations of course. Two methods has been propose, the first was all about combining two techniques together i.e. steganography and cryptography, if cryptanalysis is not used to retrieve the text from the secret image then it will be harder to retrieve the message again. The second method relies only on steganography, no use of cryptographic techniques. 2.11 Visual cryptographic technique to secure image shares Shares can be generated from the image with the help of visual cryptographic techniques. But it is more important to secure the generated image shares. Figure 8: Images after different attacks (a) blur, (b) motion blur, (c) sharpening [14] 3. APPLICATIONS Visual Cryptography Schemes are able to decode the hidden images based on human visual systems, without any help of any cryptographic algorithm. This property can be used in many cryptographic encryption applications. This section includes some applications of Visual Cryptography Schemes such as E-voting system, Financial documents and its copyright protections [15]. a. Electronic Balloting System In the present age, mostly computer systems are used to manage the voting. The voters are expected to trust the voting machines for their invaluable vote that their votes are recorded correctly without any invoice from the machine. This problem can be solved by generating an invoice or a receipt to ensure the voters that their votes are counted. Nevertheless, this could wrongly impact on the voters, which creates coercion or vote selling problems. To solve this muddle, Chaum [16] proposed a Secret-Ballot Receipts which Page 131
is based on (2, 2) threshold binary VCS. It generates an encrypted receipt to each and every voter which let them to confirm the election outcome-even if all the election computers and records were compromised. A double layer receipt that prints voter s voting decision will be received by you, at the polling station. Then you will be asked to present one layer to the poll operator who will destroy it instantly with paper shredder and the other one layer will become unreadable. The serial number will be provided on the receipt which you can later check on the election Website to ensure that your vote is not altered or deleted. It will generate a posted receipt that will look similar as the previous one. No software is required to verify this; just simply print the receipt and inlay it with the original receipt. There are two plus points of this system. First, if an election system is failed then the receipt that is not posted can be used as physical evidence. Second, guarantee of the votes is given to the voters that are recorded correctly at the polling booth. Nobody can decode it without knowing the secret keys with the help of any decryption algorithm, but this can be done only after surrendering of a layer of the receipt. Third, if the election systems are compromised, there is limited number of ways that the system could alter the voting. b. Encrypting Financial documents Transmission of confidential documents over the internet can be done with the VCS principle. This kind of system is proposed by Hawkes et al [17] which is named as VCRPT. VCRYPT can encode the image with the specified ( k, n) VCS. Each of the encoded n shares sent to the receiver with the help of Emails or Ftp. The decoding of the VCRYPT needs bitwise OR operation on all shares in the stored directory that requires no effort of cryptographic calculation. No attacker which can intercepts into the m of n shares where m k will not gain any data or information about the financial documents. Besides, it is not possible to change the contents of the documents until all shares are stopped, changed and re-introduce over the network. As we all know, financial documents consists of a lot of digits, hence, after implementing VCS, we expect that the greying effect will block us from acknowledge the fuzzy digits in decoded documents. This problem can be solved with the post altering method, proposed by the VCRYPT, to return the decoded image exactly to its original form. It calculates each set of m sub-pixels in contrast to the encoding threshold and in result; it shows the last pixel as black, if the number of the black sub-pixels is greater than the threshold, otherwise the last pixel will be white. 4. CONCLUSION This paper is the survey of the visual cryptographic schemes, which gives us a basic idea about the visual cryptography. This paper provides an on the spot glimpses of past and current research trends in the field of visual cryptography. This paper will also give a future direction to the current researchers in the same field. REFERENCES [1] M. Naor, A. Shamir, in: A. De Santis (Ed.), Visual Cryptography, Advances in Cryptology: Eurpocrypt 94, Lecture Notes in Computer Science, Vol. 950, Springer, Berlin, 1995, pp. 1 12. [2] Chandramathi S., Ramesh Kumar R., Suresh R. and Harish S., An overview of visual cryptography, International Journal of Computational Intelligence Techniques, ISSN: 0976 0466 & E-ISSN: 0976 0474 Volume 1, Issue 1, PP-32-37, 2010. [3] G.Ateniese, C.Blundo, A.DeSantis, D.R.S tinson, Visual cryptography for general access structures, Proc.ICALP96, Springer, Berlin, 1996, pp.416-428. [4] Chang-Chou Lin, Wen-Hsiang Tsai, Visual cryptography for gray-level images by dithering techniques, Pattern Recognition Letters, v.24 n.1-3. [5] Abhishek Parakh, Subhash Kak: A Recursive Threshold Visual Cryptography Scheme CoRR abs/0902.2487: (2009). [6] Nakajima, M. and Yamaguchi, Y., Extended visual cryptography for natural images, Journal of WSCG. v10 i2. 303-310. Page 132
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