Sharing more information in gray visual cryptography scheme

Visual cryptography scheme (VCS) shares a binary secret image into several binary shadows, and the secret image can be visually revealed by stacking qualified shadows without computation. From the point of view of sharing secret information, VCS is not efficiency because of the large size expansion and low visual quality. In this paper, we introduce a general gray visual cryptography scheme, which can share more information, called Sharing More Information Gray Visual Cryptography Scheme (SMIGVCS). All the shadow pixels of VCS embed additional information to generate gray shadows of SMIGVCS, and the embedded information comes from the shadows of a polynomial-based secret sharing scheme (PSSS). In the revealing process, a vague secret image is visually decoded by stacking qualified shadows, and more information is revealed by computation. Compared with the two-in-one image secret sharing scheme (TiOISSS), our SMIGVCS can achieve smaller shadow size with acceptable visual quality.     

(t,k, n) XOR-based visual cryptography scheme with essential shadows

Visual cryptography scheme (VCS) shares a binary secret image into multiple shadows, only qualified set of shadows can reveal the secret image by stacking operation. However, VCS suffers the problems of low visual quality of the revealed image and large shadow size. A (t, k, n) XOR-based visual cryptography scheme (XVCS) shares the secret image into n shadows including t essentials and n-t non-essentials. A qualified set of shadows contains any k shadows including t essentials. The revealing process is implemented by XOR operation on the involved shadows. In this paper, we propose a construction method for (t, k, n)-XVCS with essential shadows. The secret image can be revealed perfectly, and the shadow size is small compared with VCS. Theoretical analysis and experimental results show the security and effectiveness of the proposed scheme.     

HP-VCS: A high-quality and printer-friendly visual cryptography scheme

Visual Cryptography Scheme (VCS) is a secret-sharing scheme which aims to encrypt a secret message into multiple shares and transmit them to participants over an untrusted communication channel. Although human vision can easily reveal the secret message by stacking a sufficient number of shares, this scheme reduces the visual quality of recovered images. This paper presents a novel high-quality and printer-friendly VCS. When providing high-quality recovery, this scheme keeps the size of the shares the same as the secret image. Experimental results show that, compared with previous work, the proposed scheme significantly improves the performance of recovered images.     

具有掩盖图像的（2，2）可视密码方案

为解决传统可视密码像素膨胀及分存图像无意义等问题,文中提出了一个具有掩盖图像的（2,2）可视密码方案。方案中密图为黑白反色图像,利用半色调技术将两个灰度图像处理后的半色调图像作为掩盖图像,根据密图修改掩盖图像生成分存图像,叠加分存图像恢复密图。方案符合可视密码解密简单的特性且分存图像有意义,没有引入任何像素膨胀。     

基于误差扩散算法的（2，2）可视密码方案

为解决传统可视密码存在像素膨胀及分存图像无意义等问题,研究并实现了将黑白反色的密图嵌入到两个灰度图像的方法。通过采用对灰度图像进行预处理增加分存图像叠加后,黑色像素恢复的概率以及在误差扩散的过程中同时嵌入密图信息、适当降低白色像素恢复概率以及恢复黑色像素时随机选择修改其中一个分存图像的方法,有效地提高了分存图像的质量。方案符合可视密码解密简单的特性且分存图像有意义,没有引入任何像素膨胀。     

Construction of General (k, n) Probabilistic Visual Cryptography Scheme

Visual cryptography scheme (VCS) is a secure method that encrypts a secret image by subdividing it into shadow images. Due to the nature of encryption VCS is categorized into two types: the deterministic VCS (DVCS) and the probabilistic VCS (PVCS). For the DVCS, we use m (known as the pixel expansion) subpixels to represent a secret pixel. The PVCS uses only one subpixel to represent a secret pixel, while the quality of reconstructed image is degraded. A well-known construction of (k, n)-PVCS is obtained from the (k, n)-DVCS. In this paper, we show another construction of (k, n)-PVCS by extending the (k, k)-PVCS.     

多重门限的图像秘密共享方案

 针对传统的基于视觉密码的图像秘密共享方案存在像素扩张导致其只能共享小尺寸的秘密图像、信息隐藏效率较低的问题,提出一种能够提高信息隐藏容量的(t,k,n)多重门限图像秘密共享方案.该方案利用秘密图像信息控制视觉密码方案中共享矩阵的选取,从而实现秘密图像在视觉密码方案中的隐藏.在秘密图像恢复的第一阶段,任意t个参与者直接叠加其影子图像后可以视觉解密出低质量的秘密图像信息;在第二阶段,任意k个参与者可以从影子图像中提取出隐藏的信息,并通过计算恢复出精确的灰度秘密图像.相对于传统的视觉密码方案,本文方案在不影响视觉密码恢复图像的视觉质量前提下,可以隐藏更多的秘密图像信息,而像素扩张尺寸较小.     

XOR-based visual cryptography scheme with essential shadows

Visual cryptography scheme with essential shadows (EVCS) is of great significance since it provides different levels of the importance to shadows. In this paper, we propose a general construction method for (t, s, k, n)-VCS with essential shadows based on XOR operation ((t, s, k, n)-EXVCS), which originates from the partition of access structure. The secret image is encrypted into s essential shadows and n-s non-essential shadows. Any k shadows including at least t essentials can cooperate to decode the secret image and the decoding process is implemented by XOR operation on the involved shadows. Our scheme achieves perfectly reconstruction of secret image in the revealed image and the less size of shadows and revealed images. The experiments are conducted to testify the feasibility and practicability of the proposed scheme.     

Improving the visual quality of size invariant visual cryptography scheme

In order to reduce the pixel expansion of visual cryptography scheme (VCS), many size invariant visual cryptography schemes (SIVCS’s) were proposed. However, most of the known SIVCS’s have bad visual quality and thin line problems, hence the known SIVCS’s are only suitable to encrypt coarse secret images. In this paper, we notice that the variance of the darkness levels of the pixels also reflects the visual quality of the recovered secret image, as well as the average contrast. We verify, analytically and experimentally, the effectiveness of the variance to be a criterion for evaluating the visual quality of the recovered secret image. Furthermore, we propose two multi-pixel encryption size invariant visual cryptography schemes (ME-SIVCS’s) which improve the visual quality of the recovered secret image by reducing the variance of the darkness levels. In addition, the proposed ME-SIVCS’s can be used to encrypt fine secret images since they avoid some known thin line problems. Experimental results and comparisons are also given to show the effectiveness of the proposed ME-SIVCS’s. Finally, we give suggestions on obtaining good visual quality for the recovered secret image.     

A blind watermarking algorithm based on fractional Fourier transform and visual cryptography

This paper presents a robust copyright protection scheme based on fractional Fourier transform (FrFT) and visual cryptography (VC). Unlike the traditional schemes, in our scheme, the original image is not modified by embedding the watermark into the original image. We use the visual secret sharing scheme to construct two shares, namely, master share and ownership share. Features of the original image are extracted using SVD, and are used to generate the master share. Ownership share is generated with the help of secret image (watermark) and the master share, using VC technique. The two shares separately give no information about the secret image, but for ownership identification, the secret image can be revealed by stacking the master share and the ownership share. In order to achieve the robustness and security, the properties of VC, FrFT and SVD are used in our scheme. The experimental results show that the proposed scheme is strong enough to resist various signal processing operations.     

Multi-watermarking scheme for group users based on Visual Cryptography

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.     

一种无损多秘密分享视觉密码方案

针对分享多幅秘密图像存在信息损失的问题,该文给出(n, n)无损多秘密分享视觉密码的定义,在此基础上基于环状共享份设计了一种(n, n)多秘密视觉密码方案,使秘密图像的信息损失为零。实验结果表明,该方案不仅实现了在多个参与者之间分享多幅秘密图像,而且秘密图像能够完全恢复。     

多分存彩色图像秘密共享（2，n）方案

文献[2]中提出了一种基于异或（XOR）操作的彩色图像秘密共享（2,n）方案,简单易于实现,但恢复密图的效果较差。通过分析此方案,文章提出一个多分存的彩色图像秘密共享（2,n）方案,通过给用户增加分存图像的方法改善了恢复密图的质量。实验分析表明所提方案不仅取得了很好的恢复效果,而且保持了安全性和算法的简单性。     

Improvements of a two-in-one image secret sharing scheme based on gray mixing model

Yang and Ciou recently proposed a two-in-one image secret sharing scheme (TiOISSS), which can easily preview a vague image by human eyes, but also provide a perfect reconstruction of the original image by computation. However, their scheme cannot recover the lossless image by computation as they claimed. In this paper, we resolve the problem of lossless reconstruction. In addition, we improve the visual quality of the previewed image. Also, we introduce a new definition of contrast to evaluate the visual quality of the previewed image. Compared with Yang and Ciou’s TiOISSS, our scheme can gain the lossless secret image and meantime enhance the contrast of previewed image.     

Secret Sharing Using Visual Cryptography

Compared with the traditional cryptography,visual cryptography(VC) decrypts secret images referring to the characteristics of human vision,rather than the cryptography knowledge or complex computations.Furthermore,seeing to the freeness of the secret key,the whole process of encryption as well as decryption for the visual cryptography meets a fast dealing course.As to the security concern,it is able to guarantee that no one can have access to any clues about the content of a secret image from individual cover images.Thus,owing to the studies on this area,the target of light-weighted cryptography is reached.Now the visual cryptography has been developed from the meaningless shadows to the meaningful ones.Seeing to the highly developed technique,some advanced VC techniques are introduced in this survey,respectively.     

两种新的彩色图像(n, n)分存方案

提出了两种彩色图像的（n,n）．分存方案，解密过程仅需要执行XOR运算。这两种方案重构密图的复杂度与可视分存方案等价，更为重要的是该方案没有像素膨胀，并且重构密图的质量优于彩色可视分存方案。     

基于累积矩阵的可防欺骗视觉密码方案

该文基于累积矩阵提出一种可防止欺骗的视觉密码方案,在不泄露秘密信息的前提下,不需要其他额外信息,可发现多个独立欺骗者的存在,及时阻止欺骗行为。与以往方案相比,该方案构造更为简单,能发现更多的欺骗者。     

基于迭代算法的可验证视觉密码

通过改变验证图像的分享和恢复方式,该文提出了一种基于迭代算法的可验证视觉密码方案。该方案设计专用算法分享验证图像,利用算法的迭代优化验证过程,不仅大幅减小了像素扩展度,而且显著提高了验证效率。同时,通过引入异或操作实现了验证图像的完全恢复。     

Constructions of general (k,n) reversible AMBTC-based visual cryptography with two decryption options

In this paper, a general method of $(k\text{,}n)$ threshold Reversible Absolute moment block truncation coding Visual Cryptography Scheme (RAVCS) is introduced for sharing a binary secret image into multiple absolute moment block truncation coding (AMBTC) shadows. A $(k\text{,}k)$ RAVCS is firstly proposed to encode a secret by referencing one ABMTC image. Then, the proposed $(k\text{,}k)$ RAVCS is adopted to share the same secret into multiple groups of shadows by referencing multiple images. Those multiple groups of shadows are distributed to participants according to a matrix generated by the proposed shadow distribution algorithms. When any k or more participants share their shadows, the secret image is revealed by OR or XOR decryption. Further, those AMBTC shadows can be losslessly reverted to their original forms. Sufficient theoretical analysis and extensive experimental results are provided in this paper, showing the effectiveness and advantages of the proposed method.     

Halftone visual cryptography.

Visual cryptography encodes a secret binary image (SI) into n shares of random binary patterns. If the shares are xeroxed onto transparencies, the secret image can be visually decoded by superimposing a qualified subset of transparencies, but no secret information can be obtained from the superposition of a forbidden subset. The binary patterns of the n shares, however, have no visual meaning and hinder the objectives of visual cryptography. Extended visual cryptography [1] was proposed recently to construct meaningful binary images as shares using hypergraph colourings, but the visual quality is poor. In this paper, a novel technique named halftone visual cryptography is proposed to achieve visual cryptography via halftoning. Based on the blue-noise dithering principles, the proposed method utilizes the void and cluster algorithm [2] to encode a secret binary image into n halftone shares (images) carrying significant visual information. The simulation shows that the visual quality of the obtained halftone shares are observably better than that attained by any available visual cryptography method known to date.