Showing papers on "Visual cryptography published in 2007"

Cheating Prevention in Visual Cryptography

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 cheat-preventing 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

Segment-based Visual Cryptography

Abstract: A version of Visual Cryptography is presented which is not pixel-based but segment-based. It is used to encrypt messages consisting of symbols which can be represented by a segment display. For example, the decimal digits 0, . . . , 9 can be represented by the well-known sevensegment display. The advantage of the segment-based encryption is that it may be easier to adjust the secret images and that the symbols are potentially easier to recognize for the human eye, especially in a transparency-on-screen szenario.

Digital image copyright protection scheme based on visual cryptography and singular value decomposition

Abstract: A digital image copyright protection scheme based on visual cryptography VC and singular value decomposition SVD techniques is proposed. In the proposed scheme, a master share is first constructed by applying SVD to a host image. Then, the master share is used to- gether with a secret image to construct an ownership share, according to a two-out-of-two VC scheme. The secret image for ownership identifica- tion can be revealed by stacking the master share, and the ownership share. The proposed scheme embeds the secret image without modify- ing the host image. In addition, the hidden secret image can be extracted without resorting to the original host image and the aid of computers. Experimental results show that the proposed scheme, compared with existing schemes, achieves stronger robustness against several com- mon attacks. © 2007 Society of Photo-Optical Instrumentation Engineers.

Recursive Visual Cryptography Using Random Basis Column Pixel Expansion

Abstract: In this paper we present a new method for a visual cryptography scheme that uses random basis column pixel expansion technique. This method encoded the secret image into 'n' share images in a recursive manner, which have dimensions identical to the secret image. In this scheme, the encoded n shares further encoded into number of subshares recursively. It will increase the level of security of the encrypted message.

Visual Cryptography in reversible style

Abstract: We proposed in this paper a brand new type of visual cryptography (VC), namely, the VC in reversible style. For any two given secret images, two corresponding transparencies S1 and S2, also known as shares, can be produced. Both transparencies look noisy. However, if we stack the front views of both transparencies, then the first secret image is unveiled. On the other hand, if we stack the front view of S1 with the back view (the turn-over) of S2, then the second secret image is unveiled.

Design of visual cryptographic methods with smoothlooking decoded images of invariant size for grey-level images

Abstract: Most visual secret sharing (VSS) schemes need to encrypt a pixel of the secret image into m subpixels on the share; obviously, the shares are enlarged and so are the stacked images. A handful of studies try to solve the problem of pixel expansion, but little information is available on improving the visual effect of the stacked image. In addition, most of them do not mention how to deal with grey-level images. Since the secret is decoded by the human eye, the visual effect of the stacked image is an important issue in the study of the VSS scheme. This paper proposes two visual cryptographic methods to solve the problem of pixel expansion and to improve the visual effect of the stacked image at the same time. Unlike in previous studies, multiple pixels are simultaneously encoded each time. With the help of halftoning, the methods can be applied to encoding grey-level images. The experimental results show that these methods have a better visual effect on the stacked image compared with other researc...

Self Watermarking based on Visual Cryptography

Abstract: We are proposing a simple watermarking method based on visual cryptography. The method is based on selection of specific pixels from the original image instead of random selection of pixels as per Hwang [1] paper. Verification information is generated which will be used to verify the ownership of the image without the need to embed the watermark pattern into the original digital data. Experimental results show the proposed method can recover the watermark pattern from the marked data even if some changes are made to the original digital data. Keywords—Watermarking, visual cryptography, visual threshold.

Halftone Visual Cryptography Embedding a Natural Grayscale Image Based on Error Diffusion Technique

Abstract: This paper proposes a novel visual cryptography method that embeds a natural secret image into two natural carrier images with high quality at low computational complexity. The secret image is decoded visually by superposing the two carrier images. For some applications such as entertainment use, all three images should be high quality natural images and low computational complexity is also required. However, in the conventional methods, picture quality and processing speed have been tradeoff relations. To solve this problem, we propose a method which is based on conventional visual cryptography using error diffusion halftoning and introduce an additional feedback mechanism to embed the high quality natural image. Experimental results show that the proposed method can achieve higher quality natural images of a superposed image and carrier images while computational complexity is maintained low when compared with the conventional methods.

Visual Secret Sharing Schemes for Color Images Using Halftoning

Abstract: This paper proposes a method of encoding a color image into n meaningful halftone shares using the scheme of halftone visual cryptography. Visual cryptography is a secret sharing scheme where a secret image is encrypted into the so-called shares which are noise-like secure images that can be transmitted or distributed over an untrusted communication channel. The image can be reconstructed "visually" by superimposing the shares. The proposed method encrypts the color image into high-quality n halftone images or halftone shares (color). This scheme achieves lossless recovery and reduces the noise in the shares without any computational complexity.

A Visual Cryptography and Statistics Based Method for Ownership Identification of Digital Images

Abstract: In this paper, a novel copyright protection scheme for digital images based on Visual Cryptography and Statistics is proposed. In our scheme, the theories and properties of sampling distribution of means and visual cryptography are employed to achieve the requirements of robustness and security. Our method does not need to alter the original image and can identify the ownership without resorting to the original image. Besides, our method allows multiple watermarks to be registered for a single host image without causing any damage to other hidden watermarks. Moreover, it is also possible for our scheme to cast a larger watermark into a smaller host image. Finally, experimental results will show the robustness of our scheme against several common attacks. Keywords—Copyright protection, digital watermarking, sampling distribution, visual cryptography.

Method for tap-proof transmission of character string from client to server through computer network, involves producing shadow image on screen through server according to visual cryptography process

Abstract: The method involves producing a shadow image on a screen through a server according to visual cryptography process. An image with pushbutton is produced and marked corresponding to the random principle with characters, by the server. An input of string of characters on the marked pushbutton is realized by the client through mouse clicks to reconstruct the character string entered from the client over a server. Independent claims are also included for the following: (1) a computer program for executing the method (2) a plastic or paper screen for executing the method.

An Innocuous Visual Cryptography Scheme

Abstract: An innocuous visual secret sharing scheme over natural images is presented in this paper. Secret sharing scheme allows a group of participants to share a secret (i.e., an image) among them. In case of(k, n) visual cryptography any group of k qualified participants among n (where k les n) can reconstruct the secret. This paper presents (n, n) visual cryptography scheme. This scheme does not apply dithering techniques to hide a secret image. Thus, it does not degrade quality of the secret image and in particular, this scheme is far from negative photo effect. Instead of dithering, this scheme spreads data by applying simple arithmetic operations. The rationale of arithmetic operations is presented.

Extended visual cryptography schemes

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, i.e., shared secret systems where any subset of P shares its own secret.

Bounds for Visual Cryptography Schemes

Abstract: In this paper, we investigate the best pixel expansion of the various models of visual cryptography schemes. In this regard, we consider visual cryptography schemes introduced by Tzeng and Hu [13]. In such a model, only minimal qualified sets can recover the secret image and that the recovered secret image can be darker or lighter than the background. Blundo et al. [4] introduced a lower bound for the best pixel expansion of this scheme in terms of minimal qualified sets. We present another lower bound for the best pixel expansion of the scheme. As a corollary, we introduce a lower bound, based on an induced matching of hypergraph of qualified sets, for the best pixel expansion of the aforementioned model and the traditional model of visual cryptography realized by basis matrices. Finally, we study access structures based on graphs and we present an upper bound for the smallest pixel expansion in terms of strong chromatic index.

Special type of circular visual cryptography for multiple secret hiding

Abstract: A special type of circular visual cryptography scheme for hiding multiple secrets is proposed. The circular shares are divided into two concentric rings: the outer ring and the inner ring. On the outer ring, two different secrets are hidden, where the second secret could only be revealed by rotating one share through a θ° angle off the other. In the inner ring, two more secrets are hidden. To reveal the first secret, one of the shares must be flipped over before stacking. The second secret can only be revealed after one of the shares is rotated through a β° angle in the flipped position. Through flipping and rotating, four secrets are encrypted in the shares. The secrets in the shares are secure, as the angles to rotate or which share to flip must be known beforehand.

A Novel Subliminal Channel Found in Visual Cryptography and Its Application to Image Hiding

Abstract: Visual cryptography techniques have been extensively investigated since the invention and extension to numerous applications such as image encryption, visual authentication, digital watermarking and image hiding. In this paper, a new subliminal channel found in visual cryptography (SCVC) is presented. This subliminal channel is a secure way to convey an extra secret message in the plain form of an ordinary visual cryptography mechanism. To show the superioritysuperiority of the new perspective, a SCVC-based application for image hiding is demonstrated as an example. In such a way, this discovery implies the possibility of enhancing hiding capacity to image hiding. It's worthwhile to note that the extra computational cost is extremely low upon increasing the extra hiding capacity.

Watermarking-Based Transparency Authentication in Visual Cryptography

Abstract: This paper proposes two transparency authentication schemes used in visual cryptography, which are based on watermarking techniques. In the first scheme, a secret image can be perceptible when stacking two transparencies. In addition, another watermark image is visible when shifting one transparency to an appropriate position and stacking it with the other transparency. In the second scheme, both the secret image and the watermark are of the same size, while in the first scheme, the watermark is half of the size of the secret image. No computer aid is needed in decryption in the first scheme while simple computation is required in the second one. The secret image and the watermark are encrypted at the same time in the two schemes. Experimental results demonstrate the two schemes are effective and practical.

Multi-secret Sharing Threshold Visual Cryptography Scheme

Abstract: Original VCS can only share a secret image at a time When we want to share more secret images, we need to preserve a number of shares which increase our burden Based on (k, n)-VCS and (k-1, k-1)-VCS, we propose a multi-secret sharing threshold VCS that a participant only preserves a share to share more than one secret and expands the scope of application

Hiding Multiple Watermarks in Transparencies of Visual Cryptography

Abstract: This paper proposes a scheme to hide multiple watermarks in transparencies of visual cryptography. On one hand, the main properties of the traditional visual cryptography are still maintained. On the other hand, not only the encrypted image can be visible when stacking the transparencies, but also two extra watermarks can be extracted with simple computations. The scheme can be applied to hide some affiliated information of the secret image. Experimental results demonstrate the scheme is effective and practical.

Watermarks and Text Transformations in Visual Document Authentication

Abstract: Integrity of digital documents is a very important issue if they are to be legally binding. Common solutions, like digital signatures or message authentication codes, are based on cryptography and require computers or similar hardware to be produced. They can be trusted only as long as the employed hardware can be trusted. We present a complementary scheme for document authentication in untrusted environment. The scheme combines visual authentication, human- recognizable watermarks, and image transformations, and is suitable also for larger text document. Because it relies on visual cryptography, it requires practically no computational power on the receiver side. To prevent potential attackers from obtaining signatures without author’s knowledge, we propose using a simple challenge-response protocol.

Visual cryptography schemes with dihedral group access structure for many images

Abstract: A new construction of visual cryptography scheme (VCS) with dihedral group access structure for two shares and many secret images is proposed Let D2n be a dihedral group of order 2n, and let {Image(τ) | τ ∈ D2n} be 2n secret images corresponding to D2n In a VCS with dihedral group access structure, two shares (two transparencies) A and B are constructed so that for any element τ of D2n, A and τ (B) reconstruct the secret image Image(τ) This new VCS is perfect and has contrast 1/(6n2)

A visual cryptography based system for sharing multiple secret images

Abstract: A Visual Cryptography (VC)-based system for sharing multiple secret images is proposed. Several weighted transparencies are generated so that people can reveal multiple secret images by stacking qualified subsets of some transparencies together. The transparency with relatively larger weight decides which secret image will be revealed. The proposed method has the following characteristics: decoding without computation, multiple secret images recovery, max-weight dominance, and quality-control design.

Secure display that limits the viewing space by use of optically decodable encryption

Abstract: We propose a secure display that limits the viewing space three-dimensionally by use of optically decodable encryption. A secret image is encrypted into a displayed image and a decoding mask. The decryption process is based on optical logic and performed without calculation. The secret is visible within the limited viewing space. However, the viewed results outside the viewing space look like random-dot pattern. When viewed at a distance different from the designed viewing distance, only the central area of the secret is faintly visible for a limited direction. When a user watches the secure display, in practice, the user's head prevents peeping at the secret. The relationships between the size and the position of the displayed image and the viewing space have been analyzed and confirmed experimentally. The proposed technique has been demonstrated by use of two-layered liquid-crystal displays. Furthermore, 3D displacement of the viewing space have been realized only by renewing the displayed image, i.e., without any mechanical movements. By using the decoding mask as the key for decryption, no one can view the secret image without the knowledge of the decoding mask pattern, and the viewed image is secure against prying eyes behind the viewer.