Visual secret sharing for multiple secrets

Conventional visual secret sharing schemes are designed for a single secret image so it is inefficient to generate numerous share images for multiple secret images simultaneously. Therefore, a novel visual secret sharing scheme for multiple secret images is proposed in this paper. In the proposed encryption process, a stacking relationship graph of secret pixels and share blocks is generated to indicate the encryption functions, and a set of visual patterns is defined to produce two share images according to this graph. Based on the stacking properties of these patterns, the secret images can be obtained from the two share images at aliquot stacking angles. The proposed scheme makes the number of secret images not restricted and further extends it to be general. As a result, the proposed scheme enhances visual secret sharing schemes’ ability for multiple secrets.     

Reduce shadow size in aspect ratio invariant visual secret sharing schemes using a square block-wise operation

An aspect ratio invariant visual secret sharing (ARIVSS) scheme is a perfectly secure method for sharing secret images. Due to the nature of the VSS encryption, each secret pixel is expanded to m sub-pixels in each of the generated shares. The advantage of ARIVSS is that the aspect ratio of the recovered secret image is fixed and thus there is no loss of information when the shape of the secret image is our information. For example, a secret image of a circle is compromised to an ellipse if m does not have a square value. Two ARIVSS schemes based on processing one and four pixel blocks, respectively, were previously proposed. In this paper, we have generalized the square block-wise approach to further reduce pixel expansion.     

Visual multiple secret sharing based upon turning and flipping

The secret sharing schemes in conventional visual cryptography are capable of sharing one secret image into a set of random transparencies (called shares) in the form of rectangles, which reveal the secret image to the human visual system when they are superimposed. Recently, visual secret sharing schemes involving multiple secrets have attracted much attention. By adopting rotations on one of the two encoded circle shares, more than two secrets could be shared. Yet, the encoding and decoding processes of circle shares need more sophisticated mechanisms than those of rectangular or square ones. In this paper, we explore the possibilities of visual multiple secret sharing using simply two rectangular or square shares. Specifically, we define some operations onto a transparency based upon turning over or flipping around. Then we propose visual cryptographic schemes that are able to encode two or four secrets into two rectangular shares and up to eight secrets into two square shares such that the secrets cannot be obtained from any single share, whereas they are revealed by stacking the two shares under various combinations of turning or flipping operations. The proposed schemes, which solidly elaborate the relationship between the encoded shares and the shared secrets, broaden the research scope and enrich the flexibility and applicability of visual cryptography or image encryption theoretically and practically.     

Probabilistic visual secret sharing schemes for grey-scale images and color images

Visual secret sharing (VSS) scheme is an encryption technique that utilizes the human visual system in recovering the secret image and does not require any cryptographic computation. Pixel expansion has been a major issue of VSS schemes. A number of probabilistic VSS schemes with minimum pixel expansion have been proposed for binary secret images. This paper presents a general probabilistic (k, n)-VSS scheme for grey-scale images and another scheme for color images. With our schemes, the pixel expansion can be set to a user-defined value. When this value is 1, there is no pixel expansion at all. The quality of reconstructed secret images, measured by average contrast (or average relative difference), is equivalent to the contrast of existing deterministic VSS schemes. Previous probabilistic VSS schemes for black-and-white images can be viewed as special cases in the schemes proposed here.     

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 that is more efficient than ever.     

Image secret sharing method with two-decoding-options: Lossless recovery and previewing capability

Visual cryptography scheme (VCS) is a research area in image secret sharing, where one can easily stack shadow images and decode a black-and-white secret image through the human visual system without computation. Although VCS does not provide a competitive reconstruction quality, its stacking-to-see property clearly has the intended applications. To develop the ease of decoding of VCS and simultaneously overcome its weakness, Lin et al. recently proposed a novel two-in-one image secret sharing scheme (TiOISSS) with two decoding options based on VCS and a polynomial-based image secret sharing scheme (PISSS). In this TiOISSS, the first decoding stage has the stacking-to-see property to preview a vague image, and the second decoding stage is to obtain the original gray-level secret image. In this paper, we design a new TiOISSS by combining VCS and PISSS in a different way; in specific, our new scheme reduce shadow image size, and is more suitable for faster transmission within a distributed multimedia system.     

Threshold visual secret sharing by random grids

A new visual secret sharing (VSS) approach by random grids (RG-based VSS), proposed by Kafri and Keren (1987), has drawn close attention recently. With almost all advantages of visual cryptography-based VSS, RG-based VSS benefits more from keeping the same size of secret images without the problem of pixel expansion from which VC-based VSS suffer. In this paper, a threshold RG-based VSS scheme aiming at providing the wide-use version is presented. This is the first effort to develop the technique. The experimental results and theoretically analysis in visual quality and security show that the proposed scheme performs well.     

A new visual cryptography with multi-level encoding

Visual secret sharing (VSS) is a visual cryptography scheme which decodes secret messages into several enlarged shares, and distributes them to different participants. The participants can recover the secret messages by stacking their shares, and then secret message can be revealed by human visual sensitivity. Afterward some researchers start to research size invariant scheme, and apply to encode grayscale images such as scenic photos or pictures, not only binary messages. Owing to the gray values distribution of pictures are different, extreme distribution may cause blurred revealed image. In this paper, we proposed a size invariant VSS scheme which is suitable for different distribution of image's gray values. Experiment results show that the reconstructed images of our method, for brighter, darker, and normal images, have clearer and higher contrast, and without apparent artifact and unexpected contour.     

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-. 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.     

Two secret sharing schemes based on Boolean operations

Traditional secret sharing schemes involve complex computation. A visual secret sharing (VSS) scheme decodes the secret without computation, but each shadow is m times as big as the original. Probabilistic VSS solved the computation complexity and space complexity problems at once. In this paper we propose a probabilistic (2,n) scheme for binary images and a deterministic (n,n) scheme for grayscale images. Both use simple Boolean operations and both have no pixel expansion. The (2,n) scheme provides a better contrast and significantly smaller recognized areas than other methods. The (n,n) scheme gives an exact reconstruction.     

一种基于排列的(2,n)可视门限方案

像素扩展度和对比度是视觉加密方案中两个重要的指标，同时使二者达到最优在理论上是非常困难的。运用排列的方法设计出一种(2，n)方案，从理论上证明了其存在性，并给出了该方案的构造方法。软件仿真结果表明，该方案能够折衷考虑二者之间的矛盾，在像素扩展度较小的情况下，使对比度接近最优。     

Image encryption by multiple random grids

In this paper, we first give a formal definition to the visual cryptograms of n (?2) random grids (VCRG-n) for encrypting an image. Specifically, a set of VCRG-n with respect to image P consists of n random grids printed on transparencies such that only when the n transparencies are superimposed altogether can P be recognized by human vision (without any computing device), while any group of less than n transparencies obtains no information about P. Then, we design novel VCRG-n encryption schemes for binary, gray-level and color images, validate the correctness by formal proofs and demonstrate the feasibility by computer simulations. Our schemes do not require any extra pixel expansion neither any encoding basis matrix, which are necessary and inevitable in the approaches of conventional visual cryptography. These attractive advantages make our schemes more informative in theoretical interests and much applicable in practical applications.     

A multiple-level visual secret-sharing scheme without image size expansion

In traditional VSS schemes, the size of the share image is substantially expanded since each pixel of the secret image is mapped onto a block consisting of several pixels. In addition, the quality of the reconstructed secret image is normally degraded in contrast, especially for halftone images. This study proposes a VSS scheme that maps a block in a secret image onto one corresponding equal-sized block in each share image without image size expansion. Two types of techniques, including histogram width-equalization and histogram depth-equalization, are proposed to generate the corresponding share blocks containing multiple levels rather than two levels based on the density of black pixels on the blocks for a secret block. In the former technique, the gray-scale image histogram is obtained by uniformly splitting the range of the pixel gray levels in the secret image, while in the latter the buckets are created so that the area of each bucket is roughly constant by containing approximately the same number of pixels. The proposed schemes significantly improve the quality of the reconstructed secret image compared to several previous investigations.     

具有优化对照度的灰度图像的可视分存方案

在可视分存方案（VCS）中,其解密过程通过人的视觉系统完成。对照度是VCS中重要的研究主题。现有复制设备提供了反色复制的基本功能,黑白图像的全黑反色方案（PBVCS）通过叠加一定数目的分存图像可以精确重构密图,从而为解决VCS重构图像质量问题提供了一种新的途径。给出了灰度图像的反色方案不能直接使用已有的全黑反色PBVCS来构造的原因和存在的问题,进而给出一般灰度图像的反色(k,n)-VCS,该方案的有效性被证明,使用我们的灰度反色VCS,对分存图执行有限次反色和叠加操作可正确重构密图。     

A Study on Information Reservation for Visual Secret Sharing Method with Reversible Style

This paper discussed three conventional visual secret sharing methods with reversible property systematically and proposed a brand new information-reserved, non-expansion and reversible style visual secret sharing method. In all these approaches, fake contour effect has been tested and discussed, while on the other hand, an interleave approach has also been adopted. The proposed method could overcome such problems as information leak and slight fake contour effect that might occur while using the conventional visual secret sharing scheme in reversible style. Furthermore, the new method could enable the size of secret images to be equal to that of transparencies.     

Sharing multiple secrets in visual cryptography

The secret sharing schemes in conventional visual cryptography are characterized by encoding one shared secret into a set of random transparencies which reveal the secret to the human visual system when they are superimposed. In this paper, we propose a visual secret sharing scheme that encodes a set of x 2 secrets into two circle shares such that none of any single share leaks the secrets and the x secrets can be obtained one by one by stacking the first share and the rotated second shares with x different rotation angles. This is the first true result that discusses the sharing ability in visual cryptography up to any general number of multiple secrets in two circle shares.