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Showing papers on "Visual cryptography published in 2003"


Journal ArticleDOI
TL;DR: Three methods for visual cryptography of gray-level and color images based on past studies in black-and-white visual cryptography, the halftone technology, and the color decomposition method are proposed.

463 citations


Journal ArticleDOI
TL;DR: A (k, n)-threshold 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.

268 citations


Journal ArticleDOI
TL;DR: An upper bound on the contrast of any (3,n)-threshold VCS is proved and a scheme in the described family has optimal contrast and is completely characterize a contrast optimal (n-1,n) VCS in canonical form.
Abstract: A (k,n)-threshold visual cryptography scheme (VCS) 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. However, by inspecting less than k shares one cannot gain any information on the secret image. The "visual" recovery consists of copying 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 a (k,n)-threshold VCS. We define a canonical form for a (k,n)-threshold VCS and provide a characterization of a (k,,n)-threshold VCS. We completely characterize a contrast optimal (n-1,n)-threshold VCS in canonical form. Moreover, for $n\geq 4$, 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 contrast and pixel expansion. Then we prove an upper bound on the contrast of any (3,n)-threshold VCS and show that a scheme in the described family has optimal contrast. Finally, for k=4,5 we present two schemes with contrast asymptotically equal to 1/64 and 1/256, respectively.

186 citations


Journal ArticleDOI
TL;DR: A set of encryption codes is developed to maintain the designed viewing zone and a display is demonstrated that provides a limited viewing zone that ensures the security of visual information.
Abstract: We propose a secure display technique based on visual cryptography. The proposed technique ensures the security of visual information. The display employs a decoding mask based on visual cryptography. Without the decoding mask, the displayed information cannot be viewed. The viewing zone is limited by the decoding mask so that only one person can view the information. We have developed a set of encryption codes to maintain the designed viewing zone and have demonstrated a display that provides a limited viewing zone.

51 citations


Journal ArticleDOI
TL;DR: A novel approach to secret image sharing based on a (k,n)-threshold scheme with the additional capability of share data re- duction is proposed, which is suitable for certain application environments, such as the uses of mobile or handheld devices, where only a small amount of network traffic and space for data storage are allowed.
Abstract: A novel approach to secret image sharing based on a (k,n)-threshold scheme with the additional capability of share data re- duction is proposed. A secret image is first transformed into the fre- quency domain using the discrete cosine transform (DCT), which is ap- plied in most compression schemes. Then all the DCT coefficients except the first 10 lower frequency ones are discarded. And the values of the 2nd through the 10th coefficients are disarranged in such a way that they cannot be recovered without the first coefficient and that the inverse DCT of them cannot reveal the details of the original image. Finally, the first coefficient is encoded into a number of shares for a group of secret- sharing participants and the remaining nine manipulated coefficients are allowed to be accessible to the public. The overall effect of this scheme is achievement of effective secret sharing with good reduction of share data. The scheme is thus suitable for certain application environments, such as the uses of mobile or handheld devices, where only a small amount of network traffic for shared transmission and a small amount of space for data storage are allowed. Good experimental results proving the feasibility of the proposed approach are also included. © 2003 Society

46 citations


Book ChapterDOI
08 Dec 2003
TL;DR: This paper proposes a new scheme for a (2,n)-visual threshold scheme (VTS) for color images that achieves a better color ratio than the schemes proposed by Koga et al in 1998 and 2001.
Abstract: In this paper we propose a new scheme for a (2,n)-visual threshold scheme (VTS) for color images. Our scheme achieves a better color ratio than the schemes proposed by Koga et al in 1998 and 2001. The pixel expansion of our scheme is reasonably good and we also give a lower bound on the color ratio of our scheme.

41 citations


Book ChapterDOI
13 Oct 2003
TL;DR: Chaum as discussed by the authors proposed an electronic voting scheme that combines visual cryptography and digital processing, which was designed to meet not only mathematical security standards, but also to be accepted by voters that do not trust electronic devices.
Abstract: Recently, David Chaum proposed an electronic voting scheme that combines visual cryptography and digital processing. It was designed to meet not only mathematical security standards, but also to be accepted by voters that do not trust electronic devices.

33 citations


Posted Content
Pim Tuyls1, Tom A. M. Kevenaar1, Geert J. Schrijen1, Toine Staring1, Marten van Dijk1 
TL;DR: In this article, a low-tech and user friendly solution for secure two-way communication between two parties over a network of untrusted devices is presented, in which displays play a central role.
Abstract: In this paper we describe a low-tech and user friendly solution for secure two-way communication between two parties over a network of untrusted devices. We present a solution in which displays play a central role. Our approach guarantees privacy and allows to check the authenticity of information presented on displays. Furthermore, we provide the user with a secure return channel. To this end we propose to provide every user with a small decryption display which is, for example, integrated in a credit card and requires very limited computing power. The authentication and security are based on visual cryptography which was first introduced by Naor and Shamir in 1994. We solve some practical shortcomings of traditional visual cryptography and develop protocols for two-way authentication and privacy in untrusted environments.

30 citations



Proceedings ArticleDOI
24 Nov 2003
TL;DR: Halftone visual cryptography is proposed, where all shares are halftones of grey level images carrying significant visual information, and the proposed methods utilize blue-noise dithering principles to construct halftone shares having visually pleasing attributes.
Abstract: Visual cryptography encodes a secret image SI into n shares of random patterns. If the shares are xeroxed onto transparencies, we can visually decode the secret image by superimposing a qualified subset of transparencies, but no secret information can be obtained from the superposition of a forbidden subset. Such a scheme is mathematically secure, however, it produces random patterns which have no visual meaning, raising the suspicion of data encryption. In this paper, to achieve a higher level of security, we propose halftone visual cryptography, where all shares are halftones of grey level images carrying significant visual information. The proposed methods utilize blue-noise dithering principles to construct halftone shares having visually pleasing attributes.

18 citations


Patent
Pim Tuyls1, Marten van Dijk1
17 Nov 2003
TL;DR: In this article, a system for visual cryptography comprises a server (1) for encrypting a series of images using a set of keys, a terminal (2) for displaying the encrypted images, a transmission medium (4) for transmitting the encrypting images from the server to the terminal, and a decryption device (3) for decrypting the encrypted image displayed on the terminal.
Abstract: A system for visual cryptography comprises a server (1) for encrypting a series of images using a set of keys, a terminal (2) for displaying the encrypted images, a transmission medium (4) for transmitting the encrypting images from the server to the terminal (2), and a decryption device (3) for decrypting the encrypted image displayed on the terminal. A key identification is produced by the server (1) and displayed by the terminal (2). The decryption device (3) senses the key identification and selects the corresponding key from its key set.

Patent
Geert J. Schrijen1, Pim Tuyls1
31 Oct 2003
TL;DR: In this paper, a system for visual cryptography comprises a server (1) for encrypting a series of images using a set of keys, a terminal (2) for displaying the encrypted images, a transmission medium (4) for transmitting the encrypting images from the server to the terminal, and a decryption device (5) for decrypting the encrypted image displayed on the terminal.
Abstract: A system for visual cryptography comprises a server (1) for encrypting a series of images using a set of keys, a terminal (2) for displaying the encrypted images, a transmission medium (4) for transmitting the encrypting images from the server to the terminal (2), and a decryption device (3) for decrypting the encrypted image displayed on the terminal. Subsequent images are encrypted using different keys chosen from the set of keys. These encrypted images and a feedback mechanism are provided to test whether the server and the terminal utilize the same keys at a particular instant. Preferably at least two encrypted images are provided simultaneously as parts of a larger image, thus allowing a user of the decryption device to indicate which key decrypts the image correctly.

Journal ArticleDOI
TL;DR: The uniqueness and validity of this model are proved, the minimum rows and columns for realizing the scheme are obtained, and it can share two secret images with the same size of pixels expansion without losing the contrast.
Abstract: A new visual cryptography scheme of two secret images, which is named the shift visual cryptography of two secret images, is given. The compound function family of a general construction model is also introduced. The uniqueness and validity of this model are proved, and the minimum rows and columns for realizing the scheme are obtained. Compared with visual cryptography of one secret image, the shift visual cryptography scheme can share two secret images with the same size of pixels expansion without losing the contrast. It has randomness and various forms, and furthermore, we can also visually recover two secret images even if the two shares are different in form and color.

Proceedings ArticleDOI
04 Aug 2003
TL;DR: It is proved that there exists a contrast optimal scheme that is a member of a special set of schemes, which are called canonical schemes, and that satisfy strong symmetry properties.
Abstract: Visual cryptography schemes allow the encoding of a secret image into n shares which are distributed to the participants, such that only qualified subsets of participants can "visually" recover the secret image. In colored threshold visual cryptography schemes, the secret image is composed of pixels taken from a given set of c colors. We study c-color (k, n)-threshold visual cryptography schemes and provide a characterization of contrast optimal schemes. More specifically, we prove that there exists a contrast optimal scheme that is a member of a special set of schemes, which we call canonical schemes, and that satisfy strong symmetry properties. Then we use canonical schemes to provide a constructive proof of optimality, with respect to the pixel expansion, of c-color (n, n)-threshold visual cryptography schemes.


Proceedings Article
01 Jan 2003
TL;DR: A neural-network approach for visual authorization, which is an application of visual cryptography (VC), that contains a key-share and a set of user-shares and can visually recognize the authority assigned to a particular user by viewing the information appearing in the superposed image of key- share and user-share.
Abstract: In this paper, we propose a neural-network approach for visual authorization It is an application on visual cryptography. The scheme contains a key-share and a set of user-shares. The administrator owns the key-share, and each user owns a user-share issued by the administrator from the usershare set. The shares in the user share set are visually indistinguishable, i.e., they halve the same pictorial meaning. However, the stacking of the key-share with different user-shares will reveal significantly different images. Therefore, the administrator (in fact, only the administrator) can visually recognize the authority assigned to a particular user by viewing the information appearing in the superposed image of key-share and user-share.

Proceedings ArticleDOI
18 Dec 2003
TL;DR: Based on blue-noise dithering principles, a global optimization method is proposed to improve the overall visual quality of all n halftone shares so that the adversaries are less likely to suspect the presence of hidden cryptographic information.
Abstract: Visual cryptography encodes a secret binary image SI into n shares of random binary patterns. The secret image can be visually decoded by superimposing a qualified subset of shares, but no secret information can be obtained from the superposition of a forbidden subset. Such a scheme is mathematically secure, however, the binary patterns of the n shares have no visual meaning, raising the suspicion of data encryption. In order to achieve a higher level of security, halftone visual cryptography was proposed to encode a secret binary image into n halftone shares (images) carrying significant visual information. The method is further extended in this paper. Based on blue-noise dithering principles, a global optimization method is proposed to improve the overall visual quality of all n halftone shares. Thus, the adversaries are less likely to suspect the presence of hidden cryptographic information.

Proceedings ArticleDOI
17 Jun 2003
TL;DR: This new approach tolerates registration error for the third image and eases the difficulty, by adopting concentric-circular subpixel arrangement and continuous grayscale subpixel values, and becomes considerably robust to the registration error.
Abstract: Extended Visual Cryptography is a method which encodes a number of images so that when the images are superimposed, the hidden image appears without a trace of original images. The decryption is done directly by human eyes without cryptographic calculations. The proposing system takes three pictures as input and generates two images which correspond to two of the input pictures. The third picture is perceived by superimposing the two output images. Previous methods are based on halftoning and Boolean operations. Transparency values must be quantized before encryption, and a pixel is halftoned by a fixed numbers of completely transparent and opaque subpixels. Then a transparency of the superimposed pixel is controlled by changing the subpixel arrangements of the two output pixels. Since the subpixel arrangement is basically random, a tradeoff exists that to express the more graylevels, each subpixel must become the smaller, making it the more difficult to superimpose by hand. Our new approach tolerates registration error for the third image and eases the difficulty, by adopting concentric-circular subpixel arrangement and continuous grayscale subpixel values. The system becomes considerably robust to the registration error. Also, it achieves quality improvement for all three images, by explicitly dealing with continuous graylevels.

Posted Content
TL;DR: In this paper, extended visual cryptography schemes are studied, i.e. shared secret systems where any subset of $P$ shares its own secret.
Abstract: Visual cryptography schemes have been introduced in 1994 by Naor and Shamir Their idea was to encode a secret image into $n$ shadow images and to give exactly one such shadow image to each member of a group $P$ of $n$ persons Whereas most work in recent years has been done concerning the problem of qualified and forbidden subsets of $P$ or the question of contrast optimizing, in this paper we study extended visual cryptography schemes, ie shared secret systems where any subset of $P$ shares its own secret