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Visual cryptography provides secured digital transmission which is used only for one time. In order to make visual cryptography reusable Diffie and Hellman (D-H) key is used. The original images can be reuse by using this scheme. It is effortless and uncomplicated technique to execute the secret image for shadow images. The shadow images are the shrunken version of the original image, in which the secret image share is embedded. These are used to guard the data and secret images in the internet so that it is not accessed by any unauthorized persons. Visual cryptography divides the image into secret shadow images. After this these shadow images are distributed in the original image. Recovering of secret image is done by human visual system by piling all the shadow images. As this Visual cryptography is used only once D-H key agreement was used to make it reusable by using D-H key scheme in it.
Provision of security for hidden images
Reusing of the original image is possible by this technology
Images should be visible only to human visual system
In this project a visual cryptography scheme is used for encrypting the information. Visual cryptography is an encryption method which is used to hide the information in an image, decrypted can be done by human visual system. By using only this scheme the reusing is not possible. The image which is recovered after decryption will not be same as original image so it cannot be reused. For the reuse of the visual cryptography Diffie and Hellman (D-H) key agreement method and Toral Automorphism (TA) can be used. In this both secret and symmetry-key representation can be used (Chao-Wen Chan and Yi-Da Wu, 2008). Visual cryptography is the scheme which facilities the secret sharing of the pictures or data. The pixels of the pictures or data that need to be transmitted secretly will be treated as an individual secret which can be shared using a secret sharing scheme. The picture is first split into two or more shared images the secret data are embedded. At the decryption side images are joined on the transparent paper to get the secret picture or data. This is the way in which the first visual cryptography was started (Borivoje Furht, Edin Muharemagic and Daniel Socek, 2005). Using current methods the Visual cryptography scheme will allow the encoding of the secret image into shares; this is done in the form of transparencies. These shares are distributed to the participants, such that only the qualified subsets of participants can recover the secret image visually. This is done by superimposing the share images one over the other which are obtained from the original image (Carlo Blundo and Stelvio Cimato, 2005). In this scheme the devices are used for encoding of secret information and the decoding is done with human vision directly. For encoding, the visual cryptography will cryptographically encode the binary secret data into shares of the pattern which will be random. Then the shares are copied on to the transparencies with the same number of transparencies as shares. The transparencies are distributed among the participants, one for each participant. The decoding of the data or picture is done only when all of the participants superimpose their transparencies (Alan Conrad Bovik, 2005). The importance of this scheme is that there is no need for data expansion. The resolution of the data will not be lost by using this scheme. By using this scheme the image will be secure against the most important cryptanalytic attacks. The computation complexity will be less since it involves only XOR operations. By using XOR operations the share images are combined to form the encrypted images. The authentication of the image will be based on the global visual effect, so local defects due to noise will not affect the final result. The most important aspect of this scheme is that the decoding of the secret data or image can be done with human vision without any decoding equipment (Kiyoharu Aizawa, Yuichi Nakamura and Shinichi Satoh, 2004). The previous technologies that came into existing before visual cryptography are two-out-of two visual threshold schemes, two-out-of-n visual-threshold scheme. In a two-out-of two visual threshold scheme the secret can be any type of data (Abhishek Parakh and Subhash Kak, 2006, p.1). For an image comprising of white and black pixels, this image can be encoded as a binary string. In this scheme the 0 will represent a white pixel and 1 represents a black pixel and the result will be sent in two shares. The security problem by this is having only 50 percent of resolution will be obtained because the image is divided into only two shares and if the new shares of data are found the image can be decrypted very easily. A two-out-of-n visual-threshold scheme is more secure when compared with the first. In this scheme also the pixels are divided into two, but the shares will be 'n'. By this the security will increase. When compared with the D-H key agreement the resolution of these schemes will be less and security will also be less (Doug Stinson, 1999).
The D-H key agreement is used to make possible the reuse of visual cryptography. The Diffie-Hellman key agreement used an interface known as D-H key interface. This interface is used for password based Encryption. These interfaces typically can be used by the programmer who is implementing a Cryptographic provider or who wants to implement a cryptography algorithm (David Flanagan, 2005). Diffie- Hellman Key agreement is also known as exponential key exchange. This key provides the solution to the key distribution problems, allowing two parties, never having met in advance to share key material for establishing a shared secret by exchanging messages over an open channel. This is a fundamental technique providing unauthenticated key agreement. The main goal of an authenticated Key establishment protocol is to distribute key of data. The established key should have precisely the same attributes as a key established face to face, it should be distributed uniformly at random from the key space, and an unauthorized entity will not be able to learn anything about the key (Alfred J.Menezes, Paul C. Van Oorschot, and Scott A. Vanstone, 1997). D-H key agreement protocols involve the derivation of the shared secret information based on compatible D-H keys between the sender and recipient. The information is then converted into the cryptographic keying material for other algorithms. A variation of the Diffie-Hellman is used for converting the shared secret data into an arbitrary amount of keying materials (Manuel Mogollon, 2008). According to Michael Baake and John A G Roberts (2001, p.1) "Toral Automorphisms will be represented by the uni-modular integer matrices, are investigated with the help of symmetries and also reversing symmetries group of matrices with a simple spectrum through their connection with unit groups in orders of algebraic number fields. The reversibility will derive the necessary conditions in terms of the distinctive polynomial and the polynomial invariants". This shows that the Voiculescu- Brown entropy of the non-commutative toral automorphism arising from a matrix S in is at least half the value of the topological entropy of the corresponding classical toral automorphism. This is a new method used to prove the position limit laws in the theory of dynamical systems, which is based on the Chen-Stein method combined with the analysis of the homo clinic Laplace operator and some other homo clinic considerations (Massimo Franceschetti and Ronald Meester, 2002, p.2). The main use of this is to generate a disorder in the arrangement of digital images. The equation (1) define the two dimensional matrix.
The coordinates (x', y') denotes the new position of the two dimensional matrix after randomly selecting its permutation, matrix is denoted as the size of the image, is denotes the secret key of the Toral Automorphism matrix. Let assume that secret image size is pixels, and is 4 (Chao-Wen and Yi-Da Wu, 2008). Then the Toral Automorphisms matrix and the pixels value of the image are given as:
- represent the TA matrix.
The new technology that can be used for the visual cryptography is adaptive order dithering technique. By using this technique the decrypted image size can be reduced and also the quality of decryption image can be improved. In this technique the technique will get adaptive to the data that is used (Nagaraj V. Dharwadkar, B.B. Amberker and Sushil Raj Joshi, 2009). From the above context it can be understood that visual cryptography is a scheme in which the secret data can be transmitted without getting decoded by others.
Chapter 1: Introduction
Visual Information system significantly enlarges the applications scope of information systems and typically deals with the general purpose multimedia operating system that is supported by multimedia hardware and high bandwidth network. Visual information system involves variety of facilities for efficient visual information presentation and visualization. This system actually provides inter related visual and multimedia information to support the operations, management and decision making functions in an organization. These are text oriented which provides reports, decision-making information and documents for all levels of hierarchy in the organization. With the increase in multimedia platforms, visual information has become more available (Clement H. C. Leung, 1997). In recent years, there has been a rapid growth of information technology for human to communication on the Internet. Since Internet is public, with the advancement of information technology, communication through internet has increased rapidly. As internet can be accessed by everyone and anyone can easily access the information and transmit it without any protection there is possibility of grabbing the information as it became secondary data. So in order to avoid hacking of sensitive information, it should be encrypted before transmission. In order to protect the information Diffie and Helman (D-H) proposed a key agreement scheme that is implemented when two parties agree on a common session key. This key agreement also helps in improving the reusable process. This method is used to compute a common image for encryption and decryption with two parties (Chao-Wen Chan and Yi-Da Wu, 2008). The D-H algorithm does not encrypt data or make a digital signature. This is exclusively used for the generation of shred secrets. The DH key agreement can be split into three parts; they are parameter generation that generates non secret public value and is expensive process, next is phase1generatres two key exchange values for the parties and phase2 is the originator and recipient that computes the functions in the given image (Blake Dournaee, 2002). DH key agreement has a nice property that it is not sensitive to off-line attacks but it is sensitive to man-in-the-middle attacks (Christian Gehmann, Joakim Persson and Ben Smeets, 2004). In Diffie and Hellman (D-H) key agreement method both the secret and symmetry key are represented in binary image. It is simple and easy to be implemented especially for shadow images. Thus, it can be applied in many electronic business applications. There after Naor and Shamir proposed a methodology named visual secret sharing that provides secrecy to the information and avoids illegal activities; this process is called as Visual Cryptography. This process provides secrecy by partitioning the secret digital image into several shadow images that are recovered by the human visual system by piling all those images together (Jeng-Shyang Pan, Hsiang-Cheh Huang and L. C. Jain, 2004). Hence it can be stated that in order to safeguard the images visual cryptography and DH key agreement are used where the images are split into shadow images and sent which then will be combines and viewed by the human visual system. It is secured by encryption and decryption of images with a key to be presented. Visual cryptography is seen as a one - time pad system that cannot be reused, in further sections of the study the implementation of Diffie and Hellman (D-H) key agreement method is researched in detail so that visual cryptography can be reused.
1.2. Aims and Objectives
Aim: To research on visual information encryption by D-H Key agreement and visual Cryptography
Provision of security for hidden images
Reusing of the technology again and again
Images should be visible only to human visual system
Chapter 2: Literature Review
Visual Cryptography is a particular encryption method used to hide information in images in such a way that it can be decrypted by the human visual system if the correct key image is applied. This method was suggested by Naor and Shamir in 1994. Visual Cryptography applies two transparent images. One image consist random pixels and the other image consist of the secret information. In visual cryptography it is not possible to recollect the secret information from one of the images. Both transparent images and layers are involved to reveal the information (Kiyoharu Aizawa, yuichi Nakamura and shichi satoh, 2004). The simplest method to implement Visual Cryptography is to bring out the two layers against a transparent sheet. When the random image contains genuinely random pixels then it can be seen as a onetime pad system and will offer infrangibly encryption. In visual cryptography pixel is divided into white and black blocks. In this chapter, importance of visual cryptography is discussed and a New Visual information Encryption Scheme is proposed followed by the explanation of Security analysis on images with the previous technologies.
2.2. Visual Cryptography
Visual Cryptography refers to a secret sharing method that will encrypt the secret message into a number of shares and does not require any computer or calculations for decrypting the secret image rather the secret message will be reconstructed visually by overlaying the encrypted shares the secret image will become clearly visible. Visual cryptography scheme on a set P of n participants can be defined as a method of encoding the secret image into n shares so that the original image will be obtained by stacking specific combination of shares onto each other. Visual cryptography technique was initiated by the Naor and Shamiri at Eurocrypt '94. It is a process used for separating one image into various shares. Visual cryptography solutions functions on binary inputs. Visual cryptography is a special technique used to hide information in images in such a way that it can be decrypted by the human vision when the correct image is used. It uses two images one image contains secret information, in visual cryptography it is not possible to retrieve the secret information from one of the images, both transparent images are required to reveal the information. To print two layers into the transparent sheet is the simplest way to implement Visual cryptography (jeng shying pan, Hsiang cheh Huang and L. C. Jain, 2004). If the random image contains random pixel then it can be seen as a onetime pad system and will offer unbreakable encryption. These two layers slide over each other until they are correctly aligned where the hidden information appears. This type of visual cryptography which restores the image by stacking some significant images together is known as extended visual cryptography. Generally, visual cryptography suffers from the deterioration of image quality. The basic two cryptography models consists of a secret message encoded into two transparencies', one transparency representing the cipher text and the other acting as a secret key. Both transparencies appear to random dots when inspected individually and does not provide any information about the original clear text. By carefully aligning the transparencies the original message is reproduced (Borivoje Furht, Edin Muharemagic and Daniel socek, 2005). The original decoding is accomplished by the human visual system. Visual cryptography scheme is a visual secret sharing problem in which the secret message is viewed only in black and white pixels. Each pixel in the original image is represented by at least one sub pixel in each of the n transparencies or shares generated. Each share is comprised of collection of m black and white sub pixels where each collection represents a particular original pixel. The contrast of Visual Cryptography Scheme is the difference between the minimum number of black sub pixels in a reconstructed (secret) black pixel and the maximum number of black sub pixels in a reconstructed (secret) white pixel (Kiyoharu Aizawa, Yuchi Nakamura and Shinichi Satoh, 2004). The main instantiation of visual cryptographic realizes a cryptographic protocol called secret sharing. The important thing in secret sharing is it relies on a human visual system to perform the decryption. In a conventional secret sharing a secret shared among n participants can pull their shares and recover the secret but the subsets of the forbidden can obtain no information about that. Visual secret sharing schemes inherits all applications of conventional secret sharing schemes most notably access control.
Figure 1: Shows construction of 2- out-of-2 visual secret sharing scheme (Alan Conrad Bovik, 2005, p.1113)
An example of the encoding of white and black pixels in a 2 out of 2 scheme can be seen in Figure 1. Here two shares out of the two generated would be needed to recover the original image. Since only two shares are generated, n = 2. White box represents a single white or black pixel in the original image. Sub pixel assignments that would be given to shares #1 and #2 respectively. The number of sub pixels per share used to represent the original pixel is four Finally, the overall visual effect when shares #1 and #2 are correctly aligned on top of one another. Notice that when the shares in this example are combined the original black pixel is viewed as black; however, the original white pixel takes on a grey scale. The structure obtained from either white or black pixel representation can be described by an n. Similar to the area of the secret sharing, more generally structures have been studied in the visual cryptography. The basic model of the visual cryptography consists of a several umber of transparency sheets. On each transparency a cipher text is printed which is identical from random noise. The hidden message is reconstructed by stacking a certain number of the transparencies and viewing them. The system can be used by anyone without any knowledge of cryptography and without performing any cryptographic computations. Naor and Shamir have developed the Visual Secret Sharing Scheme (VSSS) to implement this model [Naor95] (Mizuho Nakajima and Yasushi Yamaguchi, 2002). Thus, from the above discursion it can be stated that Visual cryptography is a special technique used to hide information in images in such a way that it can be decrypted by the human vision when the correct image is used. It is a simple and clear scheme that can partition the secret image into a number of shadow images and the dealers can distribute these shadow images to participants. The Visual cryptography scheme makes use of human visual system for recovering secret images by staking various shadow images.
2.3. Importance of Visual Cryptography
Visual Information system importantly elaborates the diligences scope of information systems and deals with the multimedia operating system which is supported by multimedia hardware and high bandwidth network. These are text oriented which provides reports, decision-making information and documents for all levels of hierarchy in the organization (Carlo Blundo and Stelvio, 2005). With the increase in multimedia platforms, visual information has become more available with the advancement of information technology; contact through internet has increased rapidly. Visual information system requires many kinds of facilities for effective visual information presentation and visualization. This system actually supplies inter related visual and multimedia information to support the operations, management and decision making functions in an organization. As internet can be accessed by everyone and anyone can easily access the information and transmit it without any protection there is possibility of grabbing the information as it became secondary data. So in order to avoid hacking of sensitive information, it should be encrypted before transmission (Yongfei Han, 1997). In order to protect the information Diffie and Helman (D-H) proposed a key agreement scheme that is implemented when two parties agree on a common session key. This key agreement also helps in improving the reusable process. This technique is utilized to calculate a common image for encryption and decryption with two parties. The D-H algorithm does not encrypt data or make a digital signature. This is exclusively used for the generation of shred secrets. The DH key agreement can be split into three parts; they are parameter generation that generates non secret public value and is expensive process, next is phase1generatres two key exchange values for the parties and phase2 is the originator and recipient that computes the functions in the given image. DH key agreement has a nice property that it is not sensitive to off-line attacks but it is sensitive to man-in-the-middle attacks. There after Naor and Shamir proposed a methodology named visual secret sharing that provides secrecy to the information and avoids illegal activities (Jeng-Shyang Pan, Hsiang-Cheh Huang and L. C. Jain, 2004). This process is called as Visual Cryptography. This process provides secrecy by partitioning the secret digital image into several shadow images that are recovered by the human visual system by piling all those images together. It is secured technique where the data cannot be traced by others. In classic secret sharing, algorithms for splitting and sharing secret information are a stem of cryptography. In the most general case, their objective is to generate such parts for the data and that could be shared by multiple authorized persons. This problem of splitting information in a manner allowing its reconstruction by a certain n-person group interested in the reconstruction of the split information. Algorithm solutions developed to achieve this objective and at the same time none of the groups of participants in such a protocol, whose number is lesser than the required m persons. It leads to opposing to read the split message. The algorithms for dividing information make it possible to split it into chunks known as shadows that are later distributed among the participants of the protocol. So that the shares of certain subsets of users collective together and these are capable of reconstructing the original information. There are two groups of algorithms for dividing information, namely, secret splitting and secret sharing. In the first technique, information is distributed among the participants of the protocol, and all the participants are required to put together their parts to have it reconstructed. A more universal method of splitting information is the latter method, i.e. secret sharing (Marek R. Ogiela and Urszula Ogiela, 2009). Thus, from the above discursion it can be stated that visual cryptography plays a vital role in securing the data and also in sharing the secret message as the data transferred through internet has been increased. rapidly.
Advantages of visual cryptography
Visual cryptography is simple to be implemented.
Encryption incase of visual cryptography does not require any hard problem dependency.
Specifically decryption algorithms are not required because a person aware of cryptography can easily decrypt the message.
Cipher text can be Fax or e-mail in case of visual cryptography
2.4. Security analysis on images with the previous technologies
The security analysis is the major aspect of the encryption. Analysis to the field of security will encounter the serious obstacle that deal is by nature not an exact science. The previous technologies which where there before the D-H key agreement encryption are manual encryption, transparent encryption, symmetric encryption and asymmetric encryption (Fedora Documentation Project, 2009). Visual cryptography can be seen as a one-time pad system. Then, it cannot be reused. Diffie and Hellman (D-H) key agreement method and Toral Automorphism (TA) such that visual cryptography can be reused. Both secret and symmetry-key are represented in binary image. The proposed scheme is simple and easy to be implemented for shadow images. Therefore, it can be used in many electronic business applications (Chao-Wen Chan and Yi-Da Wu, 2008). The Manual Encryption is a technique that will involve the use of encryption software. In this computer programs are used for encrypt various bits of information digitally. In this technique the encryption key is provided later in the process. The main security issues are the potential leakage through security insensible program and also the bad implementation may lead to re-encrypt with same key for same password (Bruice Schneier, 2007). Transparent encryption is one of the computer software encryption technique.In this encryption technique the data will be downloaded automatically to encrypt the downloaded data auomatically. Each every encrypted application and files in the computer has an encrypted copy that hold the power surges and protect the data of encrypted data in case of computer is stolen.
. The security issues is the data which is automatically encrypted can be taken if the key provided is known (Cetin K. Koc, David Naccache and Christof Paar, 2001). The Symmetric Encryption is in which a letter or number coincides with another letter or number in the encryption code. This technique is commonly referred to as secret-key encryption. In this only single key is used for encrypting and decryption. In a situation where large numbers of people want to communicate securely, like modern internet commerce it is impossible for everyone to share a secret key. So the security of the data will not be possible in this type of technique (Ivan Ristic, 2005). The Asymmetric Encryption is a technique which is generally done electronically. This is also called as public key encryption. This will involves pairs of keys: a public key which can be made openly available, and a private key. Once information has been encrypted with the public key, nobody but the holder of the private key can decrypt it. In reverse, if the private key is used for encryption, anyone with the public key can decrypt it. Asymmetric encryption has a property that figuring out the key from the other should be as hard as decrypting the message without the key. Asymmetric encryption is slower than symmetric encryption, even on fast computers, so most modern encryption uses a combination of both methods (Eric Cole, Ronald Krutz, 2005). When compared with the asymmetric encryption D-H key agreement is the technique which is more advantages is security issues. Hence from the above context it can be understood that the manual encryption, transparent encryption, symmetric encryption and asymmetric encryption are the technique which are many security issues, so the advantage technique in this is D-H key agreement encryption. This technique has the les security problems when compared with the other techniques. Hence from the above discussion it can be understood that security analysis is the important aspect in encryption process. D-H key agreement is the fundamental technology in image encryption process. The main security problems are the potential leakage during security insensible program and the bad implementation may lead to re-encrypt with same key for same password. So many techniques are used to encrypt the image. Transparent Encryption is most important type of computer software encryption. In this technique the data can be downloaded onto a computer to encrypt automatically. D-H key agreement is the best technique; it provides more advantages are security issues.
The overlapping of image in the space is the basic operation of visual secret sharing scheme. The effect of persistence of vision in human vision system will bring the stacking while fast showing two images in order. The human vision property is utilized by proposed in video secret sharing scheme. A sequence of frames are expected after the video secret sharing process, that reveal nothing individually but display a secret while playing successive frames speedily. By knowing the frames an eyedropper can't get information that is discontinuous.fi is a processed from ith frame, the frame which is reconstructed from the ith is frei (fi is analogous to shares in VSS.)
For implementing the concept, a VSS is applied for for each frame in original video sequence, such that fi + fi+1 = frei.
The secret will be reconstructed by the required scheme which is shown in the experimental results and will also the pixels are randomly distributed so that they reveal nothing about the original secret. This scheme also has some weakness. First, video file size is quite large after this process. The worse, thing is compression don't work since the random distribution nature of pixels in these frames (Horst Reichel and Sophie Tison, 2000). Second, a small amount of information leakage occurs due to the dependency between fi and fi+2.