Showing papers on "Visual cryptography published in 2005"

Progressive color visual cryptography

Abstract: Visual cryptography is a powerful technique that combines the notions of perfect ciphers and secret sharing in cryptography with that of raster graphics. A binary image can be divided into shares that can be stacked together to approximately recover the original image. Unfortunately, it has not been used much primarily because the decryption process entails a severe degradation in image quality in terms of loss of resolution and contrast. Its usage is also hampered by the lack of proper techniques for handling gray-scale and color images. We develop a novel technique that enables visual cryptography of color as well as gray-scale images. With the use of halftoning and a novel microblock encoding scheme, the technique has a unique flexibility that enables a single encryption of a color image but enables three types of decryptions on the same ciphertext. The three different types of decryptions enable the recovery of the image of varying qualities. The physical transparency stacking type of decryption enables the recovery of the traditional visual cryptography quality image. An enhanced stacking technique enables the decryption into a halftone quality image. Finally, a computation-based decryption scheme makes the perfect recovery of the original image possible. Based on this basic scheme, we establish a progressive mechanism to share color images at multiple resolutions. We extract shares from each resolution layer to construct a hierarchical structure; the images of different resolutions can then be restored by stacking the different shared images together. Thus, our technique enables flexible decryption. We implement our technique and present results.

Copyright protection scheme for digital images using visual cryptography and sampling methods

Abstract: A novel copyright protection scheme for digital images based on visual cryptography and statistics is proposed. The proposed method employs sampling distribution of means and visual cryptography to achieve the requirements of robustness and security. Our method can register multiple secret images without altering the host image and can identify the rightful ownership without resorting to the original image. Moreover, the proposed method enables the secret images to be of any size regardless of the size of the host image. Finally, experimental re- sults show that the proposed scheme can resist several common attacks. © 2005 Society of Photo-Optical Instrumentation Engineers.

Optimal colored threshold visual cryptography schemes

Abstract: Visual cryptography schemes allow the encoding of a secret image 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. Usually the secret image consist of black and white pixels. In colored threshold visual cryptography schemes the secret image is composed of pixels taken from a given set of c colors. The pixels expansion and the contrast of a scheme are two measures of the goodness of the scheme.In this paper, 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.Finally, we provide constructions of c-color (2,n)-threshold schemes whose pixels expansion improves on previously proposed schemes.

A Visual Cryptographic Technicque for Chromatic Images Using Multi-Pixel Encoding Method.

Abstract: Visual cryptography is a secret sharing method that uses human eyes to decrypt the secret. Most visual cryptographic methods utilize the technique of pixel expansion, which causes the size of the shares to be much larger than that of the secret image. This situation is more serious for grey-level and chromatic images. In this paper, we propose a multi-pixel encoding method for grey-level and chromatic images without pixel expansion. We simultaneously encrypt r successive white or black pixels each time. The probability of these r pixels being coloured black depends on the ratio of blacks in the basis matrices. Afterward, we incorporate the techniques of colour decomposition and halftoning into the proposed scheme to handle grey-level and chromatic images. The experimental results show that the shares are not only the same size as the secret image, but also attain the requirement of security. The stacked images have good visual effect as well. Besides, our method can be easily extended to general access structure.

Sharing a Secret Two-Tone Image in Two Gray-Level Images

Abstract: In this paper, we shall propose two spatial-domain image hiding schemes with the concept of secret sharing. The two new schemes use the two-out-of-two visual secret sharing technique to generate two shares for hiding a secret two-tone image. These two secret shares are embedded into two gray-level cover images by the proposed embedding scheme. To decode the hidden messages, we can superimpose the extracted shares from the secret-share-carrier images (namely the embedding images). The advantages of our scheme are simple computation and good security, and thus it is very suitable for applications involving low power verification systems. Besides, our scheme can support two participants to share one secret two-tone image. According to our experimental results, the two proposed schemes are capable of offering satisfactory embedding image quality

Size-Adjustable Visual Secret Sharing Schemes

Abstract: Visual secret sharing (VSS) scheme is a perfect secure method that protects a secret image by breaking it into shadows. Unlike other secret sharing schemes, the VSS scheme can be easily decoded by the human visual sight when staking the shadows. We replace a pixel in the secret image by m sub pixels in the shadow image and the value m is called as pixel expansion. In general, most papers are dedicated to find the minimum m for a VSS scheme, i.e. a smaller shadow size. However, it seems that no one studies how to trade the shadow size for the contrast. In this paper, we take the lead in studying size-adjustable VSS schemes such that one can choose appropriate shadow size and the recovered image contrast for practical use.

New visual cryptography system based on circular shadow image and fixed angle segmentation

Abstract: We introduce a new secret sharing scheme, where the novelty lies in the use of a fixed angle segmentation technique to create circular shadow images called shares. In this way, the shares can be stacked in different angles to reveal different secret messages. The participants must be in possession of both the shares and the stacking angles, which creates the additional degree of protection.

Visual Cryptography for Digital Watermarking in Still Images

Abstract: Visual cryptograph can represent the secret image by several different shares of binary images. It is hard to perceive any clues about a secret image from individual shares. The secret message is revealed when parts or all of these shares are aligned and stacked together. In this paper, a novel public robust digital watermarking scheme based on visual cryptography is proposed. A binary logo is used to represent the ownership of the host image. The logo is used to generate a private sharing image and a public sharing image by visual cryptography algorithms. We use the public sharing image as the watermark embedded in the host image. An error correction-coding scheme is also used to protect the watermark. Simulation shows that our proposed watermarking algorithm is robust against various attacks indeed. Our algorithm and public share image can be open to the public, but only the owner, who uses the private sharing image of the ownership logo, can retrieve the logo for ownership. If there is an argument between the owner and the attacker the private sharing image can be provided to the arbitrator to resolve the ownership issue more convincingly.

Hiding a secret colour image in two colour images

Abstract: Visual cryptography is different from traditional cryptography. That is, neither time-consuming computation nor complex cryptographic knowledge is needed. Stacking is the only operation required to recover a secret image, and the individual image does not give the hackers any information about the secret image. None of researches tried to deal with meaningful colour share transparencies. Hence, two methods are proposed for hiding a colour image in two meaningful colour share transparencies in this paper. To achieve this goal, the colour decomposition approach and halftone technology are first applied to cope with secret colour images. Then the concept of the human visual system is utilized to generate two colour meaningful sharing transparencies. To support various applications, two variants are presented. The first proposed method, method-1, is suitable for simple colour images, and the second, method-2, provides better visibility of complex colour images.

New size-reduced visual secret sharing schemes with half reduction of shadow size

Abstract: Visual Secret Sharing (VSS) scheme proposed by Naor and Shamir is a perfect secure scheme to share the secret image. By using m subpixels to represent one pixel, the secret image is divided into several shadow images whose size is m times than the secret image. The value of m is known as the pixel expansion. In this paper, we propose the new size-reduced VSS schemes and dramatically decrease the pixel expansion to a half and meantime the contrast is not compromised.

Secure Information Display with Two Limited Viewing Zones Using Two Decoding Masks Based on Visual Secret Sharing Scheme

Abstract: We propose a secure display technique using visual cryptography. The proposed technique ensures the security of visual information. Encrypted information is perceived when viewed through a mask pattern that is composed of dot patterns of a decoding key. In order to improve security of the decoding key, the proposed display technique employs two decoding masks. Without both decoding masks, the encrypted information presented on a display cannot be decoded. We have developed a set of encryption codes so that information of a secret image can be shared among two decoding masks and a displayed image. The pixel arrangements of the decoding masks and the displayed image are designed to provide two limited viewing zones. Each viewing zone is limited by the stacked decoding masks in front of the display so that only one person can view the information at each viewing zone.

General access structures in audio cryptography

Abstract: We propose and analyze a new type of cryptographic scheme, which extends principles of secret sharing to Morse code-like audio signals. The proposed "audio cryptography scheme" (ACS) is perfectly secure and easy to implement. It relies on the human auditory system for decoding. "Audio sharing schemes" (ASS) proposed earlier were based on disguising secret binary message with a cover sound. Moreover, only 2-out-of-n audio sharing schemes have ever been proposed. Our scheme correlates strongly, and is analogous to schemes in well-studied visual cryptography. Consequently, we were able to use the existing visual cryptography constructions and obtain not only k-out-of-n audio sharing schemes, but also the most general audio cryptography schemes for qualified subsets. In audio cryptography scheme for qualified subsets, a subset of participants can recover the secret audio signal only if some qualified subset of participants is its subset

Visual cryptography and voting technology using a pair of enhanced contrast glyphs in overlay

Abstract: In some embodiments, techniques for voting and visual cryptography may include various enhancements.

Compatible ideal contrast visual cryptography schemes with reversing

Abstract: Recently, Viet and Kurosawa proposed a VCS with reversing, which is a Visual Cryptography Scheme(VCS) where every participant is allowed to change black pixels on the transparency to white pixels and vice-versa. The contrast of their scheme is almost ideal, depending on the number of the reconstruction runs performed. Before long, S. Cimato et al. proposed two VCSs with reversing where the contrast of the reconstructed secret image is ideal. However, both Cimato et al.'s schemes cannot be decrypted solely with the human eye. In this paper we propose a new ideal VCS with reversing which is compatible and requires less stacking and reversing operations compared to all previous schemes. Each participant is required to store only two transparencies to reconstruct the ideal contrast secret image.

A new visual cryptography using natural images

Abstract: In this paper an (n, n) visual cryptography scheme without dithering is proposed. This scheme takes n grayscale input images to cover a target image across n grayscale images and produces n grayscale output images which are very close to the input images, respectively. Since the output images are visibly innocuous and natural, it may be easy to pass visual inspection, which is a very desirable property in terms of the steganography aspect. This method is different from the existing schemes by the fact that it keeps the input images almost intact.

Colored digital watermarking technology based on visual cryptography

Abstract: Summary form only given. For a period of time, it was popular to use gray-level and black-and-white authentication logos for digital watermarking. With the changes of times, there are more and more digital watermarking techniques using colored logos for the index of its authentication. However, the algorithm must be more complicated for using colored logos to develop digital watermarking. It depends on a lot of complicated mathematical operations to embed the colored digital watermark. In this paper, an application of using visual cryptography technology on the making of colored digital watermarks is proposed. The proposed scheme is promoted by the great advantage of visual cryptography as it is without complicated mathematical operations. Using the human visual system, the secret can be distinguished. Our approaches not only keep the advantage of visual cryptography, but also will not destroy the protected image while making the colored digital watermark.

Halftone image watermarking based on visual cryptography

Abstract: The advancement of digital image emphasizes the need for copyright protection. Digital watermarking is one of the most popular techniques and a very active research area for copyright protection of documents and media. In this paper, we propose a watermark technique for halftone image based on visual cryptography technique. The proposed method is simple and efficient. We introduce error diffusion techniques for halftoning; three most known filters of error were used: Floyd and Steinberg, Jarvis and Judice and Ninke and Stucki methods. The system security is guaranteed by using a visual cryptography approach to generate two random shares of a watermark; public watermark and secret watermark.

Hiding Data in Halftone Images

Abstract: Most recent papers about visual cryptography for halftone images are dedicated to get a higher contrast decoded image. However, the hidden visual pattern often blends into the background image and leads to a confused image. In this paper, we propose an improved method for halftone image hiding. By using the proposed method, the background image can be eliminated and the hidden visual pattern can be revealed precisely. Experimental results show that the decoded visual patterns could reveal good visual quality under various kinds of input patterns. Furthermore, better visual quality can be obtained when more halftone images are overlaid.

Perfect reconstruction of black pixels revisited

Abstract: It has been shown recently in [5] that the visual secret sharing scheme proposed in [1] leads to the largest possible visual contrast among all schemes that perfectly reconstruct black pixels. The main purpose of this paper is to demonstrate that the largest optimal contrast (for this kind of schemes) equals the smallest possible error when we try to approximate a polynomial of degree k on k + 1 interpolation points by a polynomial of degree k - 1. Thus, the problem of finding a contrast-optimal scheme with perfect reconstruction of black pixels boils down to a well-known problem (with a well-known solution) in Approximation Theory. A second purpose of this paper is to present a tight asymptotic analysis for the contrast parameter. Furthermore, the connection between visual cryptography and approximation theory discussed in this paper (partially known before) may also find some interest in its own right.

Perfect reconstruction of black pixels revisited

Abstract: It has been shown recently in [5] that the visual secret sharing scheme proposed in [1] leads to the largest possible visual contrast among all schemes that perfectly reconstruct black pixels. The main purpose of this paper is to demonstrate that the largest optimal contrast (for this kind of schemes) equals the smallest possible error when we try to approximate a polynomial of degree k on k + 1 interpolation points by a polynomial of degree k – 1. Thus, the problem of finding a contrast-optimal scheme with perfect reconstruction of black pixels boils down to a well-known problem (with a well-known solution) in Approximation Theory. A second purpose of this paper is to present a tight asymptotic analysis for the contrast parameter. Furthermore, the connection between visual cryptography and approximation theory discussed in this paper (partially known before) may also find some interest in its own right.

A Cheater Detectable Visual Cryptography Scheme

Abstract: The visual cryptography scheme(VCS) presented by Naor and Shamir[1] provides a method to encrypt a black-white image to different shares and decrypt the secret without any computations. But if there is a cheater who gives a faked share in the decrypting process, the secret image will fail to be decrypted.this paper gives a construction , which can find out who is the cheater in the decrypting process.

Multi-watermarking scheme for group users based on Visual Cryptography

Abstract: All watermarks are shared into shadow images using VCS(Visual Cryptography Scheme). Only one specific shadow (as a new watermark) is embedded into the host signal and the other ones are distributed to every user in each group as a key. In the watermarking extraction procedure, users in different group can obtain different watermark by combining their shadows with the extracted one from the watermarked signal. Analysis and experimental results show that the new watermarking method is novel, secure and robust.

An error resilient image camouflaging scheme for secret image transmission

Abstract: In the proposed error resilient image camouflaging scheme, a secret image is camouflaged at the sender into multiple virtual JPEG-2000 images, resulting in multiple cipher-images. Then, instead of the secret image, only the cipher-images are transmitted to the receiver. Because both the secret and cipher-images are significant, the cipher-images do not pique the interest of illegal users who may try to get the secret image. At the receiver, if all the cipher-images are completely received, the secret image can be recovered from the cipher-images, whereas if the cipher-images are not completely received, the secret image cannot be recovered. Additionally, because the transmitted JPEG-2000 cipher-images may be corrupted by transmission errors, to enhance the degree of transmission reliability, three proposed techniques, namely, side-match vector quantization, round-robin like data embedding, and duplicate data embedding, are employed to provide error resilience for robust image transmission.

Alternatives to visual cryptography for colored images

Abstract: This paper presents three simple methods alternatives to visual cryptography for colored images. In the first method, an RGB colored image is converted to HSI image generating 8 sub-images. Then each sub-image is divided to an arbitrary number of slides (8 slides) sub-images and encrypted using XOR operation with HSI random keys. A substitution process is applied for decryption followed by HIS/RGB conversion. With the second method and after RGB/HIS conversion, a bit by bit is taken from the original image and inserted them in each key image (8 images). The third method converts the original image and the key image into 3 sub-images (R,G,B) and takes a bit by bit for each pixel from the red, green, blue bands and insert them in the LSB in each of the key image (8 images). Finally the results of the three methods are compared.