Visual Secret Sharing Scheme Research Articles

Friendly Color Visual Secret Sharing by Random Grids

The visual secret sharing (VSS) approach by random grids (RG-based VSS) has drawn people's attention again recently. Nevertheless, the current RG-based VSS approaches suffer from the problem of meaningless share images (cipher-grids) so that it is not user-friendly to manage noise-like share images. This paper presents a novel color RG-based VSS scheme aiming at providing the friendly color cipher-grids with all the advantages of traditional VSS schemes. To our best knowledge, this paper is the first effort to develop the friendly technique. The experimental results show that the scheme does work.

Visual secret sharing by random grids revisited

Recently, the visual secret sharing (VSS) technique by random grids (RG), proposed by Kafri and Keren in 1987, has drawn attention in academia again to remove the abovementioned drawbacks. However, Kafri and Keren's scheme is a 2-out-of-2 VSS scheme but neither n -out-of- n nor 2-out-of- n ( n > 2 ) . In this paper, novel n -out-of- n (Method 1) and 2-out-of- n (Method 2) secret image sharing schemes based on RG are proposed to encrypt the secret into n cipher-grids without pixel expansion and additional codebook required. In the decryption phase, while participants superimpose all (Method 1) or at least two (Method 2) cipher-grids without any extra computation, the secret is recognizable by the human visual system. To our best knowledge, this paper is the first attempt in the literature to develop new RG-based VSS schemes by means of extending the basic 2-out-of-2 scheme to the n -out-of- n as well as 2-out-of- n ones. To demonstrate the correctness of the proposed schemes, the formal analysis is given while the experimental results show the proposed schemes do work well.

On generating meaningful shares in visual secret sharing scheme

In the k-out-of-n visual secret sharing scheme (VSSS) proposed by Naor and Shamir, a binary secret image is encoded into n shares called transparencies. Each share, consisting of black and white pixels, is in the form of noise and has size larger than that of the secret image. The binary secret image can be decoded by using the visual system through superimposing any k of n transparencies without performing any cryptographic computation. In practice, meaningless shares, however, might invite the adversary's attention and to manage numerous increasing transparencies belonging to different secrets is also a problem. In the present paper, a simple method to transform meaningless shares in VSSS into natural colourful images is proposed. Experiment results show that this method does provide a general solution to the k-out-of-n VSSS with few additional pixel expansions and low computational power as compared with others.

Visual secret sharing scheme: prioritizing the secret pixels with different pixel expansions to enhance the image contrast

A visual secret sharing (VSS) scheme is a secret image sharing scheme that divides a secret pixel into several subpixels (the number of subpixels is called the pixel expansion). The secret image encrypted by the VSS technique can be visually revealed without additional computations. However, the contrast of recovered image is poor and thus the VSS framework is always a research issue and does not have practical applications. We introduce a new framework that prioritizes the pixels with different pixel expansions to enhance the image contrast for practical use. Also, by integrating the optical character recognition with the proposed VSS scheme we show a new visual authentication scheme.

EXTENDED VISUAL SECRET SHARING SCHEMES: IMPROVING THE SHADOW IMAGE QUALITY

Visual secret sharing (VSS) scheme is a perfectly secure method to divide a secret image into several noise-like shadow images by splitting a secret pixel into black and white subpixels. Unlike other secret sharing schemes, the VSS scheme can be easily decoded by the human visual sight when staking shadow images. However, noise-like shadows are unusual and suspected by censors when delivered by e-mail or fax. Also, noise-like shadows are difficult to identify and manage when distributed. The problem was solved by adding the extended capability, a meaningful shadow image, in the so-called extended visual secret sharing (EVSS) scheme. In this paper, we present a new EVSS scheme by using gray and white subpixels to represent the secret pixel. Our proposed scheme still has the capability of visually revealing the secret image by stacking shadow images without the help of hardware and complex computation. When compared to the previous EVSS scheme, our new scheme has less number of subpixels to represent a secret pixel and the clearer shadow images.

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

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 researchers' methods. The methods are based on two basis matrices and hence can satisfy the security and contrast conditions required by the VSS scheme.

Basic Properties of the (t, n)-Threshold Visual Secret Sharing Scheme with Perfect Reconstruction of Black Pixels

In this paper we consider the (t, n)-threshold visual secret sharing scheme (VSSS) in which black pixels in a secret black-white images is reproduced perfectly as black pixels when we stack arbitrary t shares. This paper provides a new characterization of the (t, n)-threshold visual secret sharing scheme with such a property (hereafter, we call such a VSSS the (t, n)-PBVSSS for short). We use an algebraic method to characterize basis matrices of the (t, n)-PBVSSS in a certain class of matrices. We show that the set of all homogeneous polynomials each element of which yields basis matrices of the (t, n)-PBVSSS becomes a set of lattice points in an (n?t+1)-dimensional linear space. In addition, we prove that the optimal basis matrices in the sense of maximizing the relative difference among all the basis matrices in the class coincides with the basis matrices given by Blundo, Bonis and De Santis [3] for all n? t ? 2.

Visual Secret Sharing Schemes for Multiple Secret Images Allowing the Rotation of Shares

In this paper, a method is proposed to construct a visual secret sharing (VSS) scheme for multiple secret images in which each share can be rotated with 180 degrees in decryption. The proposed VSS scheme can encrypt more number of secret images compared with the normal VSS schemes. Furthermore, the proposed technique can be applied to the VSS scheme that allows to turn over some shares in decryption. From the theoretical point of view, it is interesting to note that such VSS schemes cannot be obtained from so-called basis matrices straightforwardly.

New Size-Reduced Visual Secret Sharing Schemes with Half Reduction of Shadow Size

The Visual Secret Sharing (VSS) scheme proposed by Naor and Shamir is a perfectly secure scheme to share a secret image. By using m sub pixels to represent one pixel, we encrypt the secret image into several noise-like shadow images. The value of m is known as the pixel expansion. More pixel expansion increases the shadow size and makes VSS schemes impractical for real application. In this paper, we propose new size-reduced VSS schemes and dramatically decrease the pixel expansion by a half.

XOR-based Visual Cryptography Schemes

A recent publication introduced a Visual Crypto (VC) system, based on the polarisation of light. This VC system has goodresolution, contrast and colour properties.Mathematically, the VC system is described by the XOR operation (modulo two addition). In this paper we investigate Threshold Visual Secret Sharing schemes associated to XOR-based VC systems. Firstly, we show that n out of n schemes with optimal resolution and contrast exist, and that (2,n) schemes are equivalent to binary codes. It turns out that these schemes have much better resolution than their OR-based counterparts. Secondly, we provide two explicit constructions for general k out of n schemes. Finally, we derive bounds on the contrast and resolution of XOR-based schemes. It follows from these bounds that for k<n, the contrast is strictly smaller than one. Moreover, the bounds imply that XOR-based k out of n schemes for even k are fundamentally different from those for odd k.

Efficient visual secret sharing scheme for color images

A k -out-of- n visual secret sharing scheme (VSSS) resolves the visual variant of the k-out-of- n secret sharing problem where only k or more out of n participants can reveal the secret by human visual system without any cryptographic computation. The best pixel expansion of the general k-out-of- n VSSS for c-colored images was c × m by Yang and Laih [New colored visual secret sharing schemes, Des Codes Cryptogr. 24 (2000) 325–335] where m is the pixel expansion of an existing binary k-out-of- n VSSS. Regarding the c-colored n-out-of- n scheme, the best pixel expansion is ( c - 1 ) 2 n - 1 - c + 2 and c ( c - 1 ) 2 n - 2 - c when n is odd and even, respectively, by Blundo et al. [Improved schemes for visual cryptography, Des Codes Cryptogr. 24 (2001) 255–278]. In this paper, we propose a new c-colored k-out-of- n VSSS by using a pixel expansion of ⌈ log 2 c ⌉ × m that is more efficient than ever.

Size-Adjustable Visual Secret Sharing Schemes

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.

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

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.

Aspect ratio invariant visual secret sharing schemes with minimum pixel expansion

Visual secret sharing (VSS) scheme is a perfect secure method to hide a secret image by breaking it into shadow images and one can decode it easily by the human visual system. We use m, the pixel expansion, subpixels to represent a pixel. Suppose the secret image is a circle and m is not a square value, i.e., the aspect ratio is changed. After performing the VSS technique, the circle will be changed to an ellipse and consequently lead to the loss of information. To avoid distorting the image, the dummy subpixels are added to keep the aspect ratio unchanged. In this paper, we propose a novel method to dramatically reduce the number of extra subpixels to construct the aspect ratio invariant VSS schemes.

New visual secret sharing schemes using probabilistic method

Visual secret sharing (VSS) scheme is a perfect secure method that protects a secret image by breaking it into shadow images (called shadows). Unlike other threshold schemes, VSS scheme can be easily decoded by the human visual system without the knowledge of cryptography and cryptographic computations. However, the size of shadow images (i.e., the number of columns of the black and white matrices in VSS scheme [Naor, Shamir, Visual cryptography, Advances in Cryptology-EUROCRYPT’94, Lecture Notes in Computer Science, vol. 950, Springer-Verlag, 1995, p. 1]) will be expanded. Most recent papers about VSS schemes are dedicated to get a higher contrast or a smaller shadow size. In this paper, we use the frequency of white pixels to show the contrast of the recovered image. Our scheme is non-expansible and can be easily implemented on a basis of conventional VSS scheme. The term non-expansible means that the sizes of the original image and shadows are the same.

New Colored Visual Secret Sharing Schemes

Visual secret sharing (VSS) schemes are used to protect the visual secret by sending n transparencies to different participants so that k-1 or fewer of them have no information about the original image, but the image can be seen by stacking k or more transparencies. However, the revealed secret image of a conventional VSS scheme is just black and white. The colored k out of n VSS scheme sharing a colored image is first introduced by Verheul and Van Tilborg [1]. In this paper, a new construction for the colored VSS scheme is proposed. This scheme can be easily implemented on basis of a black & white VSS scheme and get much better block length than the Verheul-Van Tilborg scheme.