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Visual Authentication and Identification
 Lecture Notes in Computer Science
"... The problems of authentication and identification have received wide interest in cryptographic research. However, there has been no satisfactory solution for the problem of authentication by a human recipient who does not use any trusted computational device. The problem of authentication arises for ..."
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Cited by 62 (1 self)
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The problems of authentication and identification have received wide interest in cryptographic research. However, there has been no satisfactory solution for the problem of authentication by a human recipient who does not use any trusted computational device. The problem of authentication arises for example in the context of smartcardhuman interaction, in particular in the context of electronic wallets. The problem of identification is ubiquitous in communication over insecure networks. This paper introduces visual authentication and visual identification methods, which are authentication and identification methods for human users based on visual cryptography. These methods are very natural and easy to use, and can be implemented using very common "low tech" technology. The methods we suggest are efficient in the sense that a single transparency can be used for several authentications or for several identifications. The visual authentication methods we suggest are not limited to auth...
On the Contrast in Visual Cryptography Schemes
 Journal of Cryptology
, 1999
"... Visual cryptography schemes allow the encoding of a secret image, consisting of black or white pixels, into n shares which are distributed to the participants. The shares are such that only qualified subsets of participants can ‘visually ’ recover the secret image. The secret pixels are shared with ..."
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Cited by 49 (3 self)
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Visual cryptography schemes allow the encoding of a secret image, consisting of black or white pixels, into n shares which are distributed to the participants. The shares are such that only qualified subsets of participants can ‘visually ’ recover the secret image. The secret pixels are shared with techniques that subdivide each secret pixel into a certain number m, m ≥ 2 of subpixels. Such a parameter m is called pixel expansion. Recently Yang introduced a probabilistic model. In such a model the pixel expansion m is 1, that is, there is no pixel expansion. The reconstruction of the image however is probabilistic, meaning that a secret pixel will be correctly reconstructed only with a certain probability. In this paper we propose a generalization of the model proposed by Yang. In our model we fix the pixel expansion m ≥ 1 that can be tolerated and we consider probabilistic schemes attaining such a pixel expansion. For m = 1 our model reduces to the one of Yang. For big enough values of m, for which a deterministic scheme exists, our model reduces to the classical deterministic model. We show that between these two extremes one can trade the probability factor of the scheme with the pixel expansion. Moreover, we prove that there is a onetoone mapping between deterministic schemes and probabilistic schemes with no pixel expansion, where contrast is traded for the probability factor. 1.
Visual Cryptography for Graylevel Images by Dithering Techniques
 Pattern Recognition Letters 24 (2003) 349358. ELSEVIER. Available at Computer Science Web, powered by SCIENCE DIRECT. http://www.cis.nctu.edu.tw/~whtsai/Journal%20Paper%2 0PDFs/Lin_&_Tsai_PRL_2003.pdf
, 2003
"... A ðk; nÞthreshold visual cryptography scheme is proposed to encode a secret image into n shadow images, where any k or more of them can visually recover the secret image, but any k 1 or fewer of them gain no information about it. The decoding process of a visual cryptography scheme, which differs ..."
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Cited by 45 (0 self)
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A ðk; nÞthreshold visual cryptography scheme is proposed to encode a secret image into n shadow images, where any k or more of them can visually recover the secret image, but any k 1 or fewer of them gain no information about it. The decoding process of a visual cryptography scheme, which differs from traditional secret sharing, does not need complicated cryptographic mechanisms and computations. Instead, it can be decoded directly by the human visual system. Previous efforts in this topic are almost restricted in processing binary images, which are insufficient for many applications. In this paper, a new visual cryptography scheme suitable for graylevel images is proposed. Instead of using gray subpixels directly to construct shares, a dithering technique is used first to convert a graylevel image into an approximate binary image. Then existing visual cryptography schemes for binary images are applied to accomplish the work of creating shares. The overall effect of the proposed method is the achievement of visual encryption and decryption functions for graylevel images. Some comparisons with a previously proposed method are also made. Some experimental results are shown to prove the feasibility of the proposed method. Finally, an application is mentioned to show its practicability.
Extended Capabilities for Visual Cryptography
, 1999
"... An extended visual cryptography scheme, EVCS for short, for an access structure (\Gamma Qual ; \Gamma Forb ) on a set of n participants, is a technique to encode n images in such a way that when we stack together the transparencies associated to participants in any set X 2 \Gamma Qual we get the sec ..."
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Cited by 45 (0 self)
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An extended visual cryptography scheme, EVCS for short, for an access structure (\Gamma Qual ; \Gamma Forb ) on a set of n participants, is a technique to encode n images in such a way that when we stack together the transparencies associated to participants in any set X 2 \Gamma Qual we get the secret message with no trace of the original images, but any X 2 \Gamma Forb has no information on the shared image. Moreover, after the original images are encoded they are still meaningful, that is, any user will recognize the image on his transparency. The main contributions of this paper are the following: ffl A tradeoff between the contrast of the reconstructed image and the contrast of the image on each transparency for (k; k)threshold EVCS (in a (k; k)threshold EVCS the image is visible if and only if k transparencies are stacked together). This yields a necessary and sufficient condition for the existence of (k; k)threshold EVCS for the values of such contrasts. In case a scheme e...
Extended Visual Cryptography for Natural Images
 Department of Computer Science, University of Toronto
"... Extended Visual Cryptography[Ateni01] is a type of cryptography which encodes a number of images in the way that when the images on transparencies are stacked together, the hidden message appears without a trace of original images. The decryption is done directly by the human visual system with no s ..."
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Cited by 37 (0 self)
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Extended Visual Cryptography[Ateni01] is a type of cryptography which encodes a number of images in the way that when the images on transparencies are stacked together, the hidden message appears without a trace of original images. The decryption is done directly by the human visual system with no special cryptographic calculations. This paper presents a system which takes three pictures as an input and generates two images which correspond to two of the three input pictures. The third picture is reconstructed by printing the two output images onto transparencies and stacking them together. While the previous researches basically handle only binary images, this paper establishes the extended visual cryptography scheme suitable for natural images. Generally, visual cryptography suffers from the deterioration of the image quality. This paper also describes the method to improve the quality of the output images. The tradeoff between the image quality and the security are discussed and assessed by observing the actual results of this method. Furthermore, the optimization of the image quality is discussed.
Extended Schemes for Visual Cryptography
, 1995
"... An extended visual cryptography scheme, (\Gamma Qual ; \Gamma Forb ; m)EVCS for short, with pixel "expansion" m, for an access structure (\Gamma Qual ; \Gamma Forb ) on a set of n participants, is a technique to encode n innocent looking images in such a way that when we stack together th ..."
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Cited by 32 (6 self)
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An extended visual cryptography scheme, (\Gamma Qual ; \Gamma Forb ; m)EVCS for short, with pixel "expansion" m, for an access structure (\Gamma Qual ; \Gamma Forb ) on a set of n participants, is a technique to encode n innocent looking images in such a way that when we stack together the transparencies associated to participants in any set X 2 \Gamma Qual we get the secret message with no trace of the original images, but any X 2 \Gamma Forb has no information on the shared image. Moreover, after the original innocent looking images are encoded they are still meaningful, that is, any user will recognize the image on his transparency. In this paper we first present a general technique to implement extended visual cryptography schemes, which uses hypergraph colourings. Then we discuss some applications of this technique to various interesting classes of access structures by using relevant results from the theory of hypergraph colourings. Keywords: Visual Cryptography, Secret Sharing ...
Contrast Optimal Threshold Visual Cryptography Schemes
, 1998
"... A (k; n)threshold visual cryptography scheme ((k; n)threshold VCS, for short) is a method to encode a secret image SI into n shadow images called shares such that any k or more shares enable the "visual" recovery of the secret image, but by inspecting less that k share one cannot gain an ..."
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Cited by 28 (4 self)
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A (k; n)threshold visual cryptography scheme ((k; n)threshold VCS, for short) is a method to encode a secret image SI into n shadow images called shares such that any k or more shares enable the "visual" recovery of the secret image, but by inspecting less that k share one cannot gain any information on the secret image. The "visual" recovery consists of xeroxing the shares onto transparencies, and then stacking them. Any k shares will reveal the secret image without any cryptographic computation. In this paper we analyze the contrast of the reconstructed image for (k; n) threshold VCS. We define a canonical form for (k; n)threshold VCS and we also provide a characterizazion of (k; n)threshold VCS. We completely characterize contrast optimal (n \Gamma 1; n)threshold VCS in canonical form. Moreover, for n 4, we provide, a contrast optimal (3; n)threshold VCS in canonical form. We first describe a family of (3; n)threshold VCS achieving various values of Research partially ...
Constructions of Generalized Superimposed Codes with Applications to Group Testing and Conflict Resolution in Multiple Access Channels.
 Theor. Comp. Sc.
, 2003
"... Abstract In this paper we introduce a parameterized generalization of the well known superimposed codes. We give algorithms for their construction and provide nonexistential results. We apply our new combinatorial structures to the e cient solution of new group testing problems and access coordina ..."
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Cited by 27 (1 self)
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Abstract In this paper we introduce a parameterized generalization of the well known superimposed codes. We give algorithms for their construction and provide nonexistential results. We apply our new combinatorial structures to the e cient solution of new group testing problems and access coordination issues in multiple access channels.
Constructions and Bounds for Visual Cryptography
 of &quot;Lecture Notes in Computer Science
, 1996
"... . A visual cryptography scheme for a set P of n participants is a method to encode a secret image SI into n images in such a way that any participant in P receives one image and only qualified subsets of participants can "visually" recover the secret image, but nonqualified sets of partic ..."
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Cited by 19 (7 self)
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. A visual cryptography scheme for a set P of n participants is a method to encode a secret image SI into n images in such a way that any participant in P receives one image and only qualified subsets of participants can "visually" recover the secret image, but nonqualified sets of participants have no information, in an information theoretical sense, on SI. A "visual" recover for a set X ` P consists of stacking together the images associated to participants in X. The participants in a qualified set X will be able to see the secret image without any knowledge of cryptography and without performing any cryptographic computation. In this paper we propose two techniques to construct visual cryptography schemes for any access structure. We analyze the structure of visual cryptography schemes and we prove bounds on the size of the image distributed to the participants in the scheme. We provide a novel technique to realize k out of n visual cryptography schemes. Finally, we consider graph...
Cheating Prevention in Visual Cryptography
 IEEE Trans. on Image Processing
"... Abstract—Visual cryptography (VC) is a method of encrypting a secret image into shares such that stacking a sufficient number of shares reveals the secret image. Shares are usually presented in transparencies. Each participant holds a transparency. Most of the previous research work on VC focuses on ..."
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Abstract—Visual cryptography (VC) is a method of encrypting a secret image into shares such that stacking a sufficient number of shares reveals the secret image. Shares are usually presented in transparencies. Each participant holds a transparency. Most of the previous research work on VC focuses on improving two parameters: pixel expansion and contrast. In this paper, we studied the cheating problem in VC and extended VC. We considered the attacks of malicious adversaries who may deviate from the scheme in any way. We presented three cheating methods and applied them on attacking existent VC or extended VC schemes. We improved one cheatpreventing scheme. We proposed a generic method that converts a VCS to another VCS that has the property of cheating prevention. The overhead of the conversion is near optimal in both contrast degression and pixel expansion. Index Terms—Cheatpreventing, cheating, secret sharing, visual cryptography. I.