基于半色调技术的（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,提出一个基于半色调技术的（2,2）可视密码方案。方案中密图为黑白反色图像,利用半色调技术将一幅灰度图像处理后的半色调图像作为掩盖图像一,然后对其进行反色运算得到掩盖图像二,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。该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。     

基于恢复函数和误差扩散的灰度图像分存方案

基于((n,n)-阈值的灰度图像分存方案利用恢复函数和误差扩散技术将一张秘密灰度图像分存到n张有意义的灰度分存图像中。所提方案选择n张有意义的灰度图像作为分存图像,以增强秘密图像的隐蔽性,达到保护图像信息安全的目的;采用误差扩散技术,可以产生具有良好视觉质量的分存图像。所生成的分存图像没有像素膨胀,其大小与秘密图像相等。此外,方案的重构过程简单快速,而且是无损的。实验结果和理论分析表明,所提方案能为秘密图像分存提供一个高安全和有效的机制。     

非等量备份和双认证自修复有限域图像分存

传统有意义图像分存方案存在认证能力偏低、攻击后不具备修复能力或修复能力整体较弱以及嵌入掩体视觉质量不高等问题.针对以上问题,提出一种结合非等量备份和双认证自修复有限域图像分存方案.所提方案包含分存和恢复阶段.在分存阶段,首先对密图做1级离散小波变换,取LL子带按密钥置乱,并对置乱后LL子带每个系数比特按比特位重要程度分组进行非等量备份来构造与密图等大备份图;然后对密图和备份图每个像素及其对应7K-13位认证信息在GF（2⁷）有限域进行（K,N）分存,将产生的7位分存信息和使用密钥产生的1位认证信息使用优化LSB法嵌入到N个掩体2×2分块中;最后对密钥进行（K,N）分存,将子密钥对应MD5值公开到第3方公信方并将子密钥和嵌入掩体分发给参与者.在恢复阶段,首先对参与者提供的子密钥真实性进行检验,利用检验通过子密钥对密钥进行恢复;其次对分发掩体2×2分块嵌入的分存信息和1位认证信息使用密钥进行第1重认证,利用第1重认证通过分存信息重建GF（2⁷）有限域分存多项式,提取出密图和备份图每个像素及其对应的7K-13位认证信息并对其进行第2重检验和构造初步密图、备份图以及认证图;再次由备份图和认证图重构密图LL子带,然后对其做逆置乱和逆离散小波变换得到密图修复参考图;最后对认证图每一个认证不通过秘密像素,根据其周围像素认证情况选择多项式插值拟合或进行修复参考图像素替代修复.理论和实验表明,同现有方法相比,所提方法具备更好认证能力,并能充分使用双认证和自然图像邻近像素相关性来提升其攻击后修复能力,且分发掩体具备较高视觉质量.     

灰度可视秘密共享方案分析与设计

针对现有灰度可视秘密共享（GVSS,gray visual secret sharing）将原始图像转化为黑白二值图像进行处理时产生像素信息损失的问题,提出了一种基于灰度叠加的GVSS,对于每一个灰度级像素,分别设计相应的加密矩阵,生成灰度共享份图像,在此基础上设计了(2,2)、(2,n)、(n,n)方案。与现有方案相比,所提方案有效降低了像素扩展度并实现了灰度密图的无损恢复。     

用VC++对BMP格式图像文件编程的技巧与实践

本文以实际中用VC＋＋对BPM格式图像文件处理时,使用几何算法（旋转,缩放等）造成图像数据区数据排列错误,导致处理后的图像发生畸变的问题进行了总结,并给出了解决的方法,因为现在的图像算法大多是基于256色灰度图的,所以我们这里仅讨论256色灰度图,对于别的颜色的位图（如24位真彩色位图）,也很容易类推得到。     

结合调整差值变换的(K,N)有意义图像分存方案

目的 针对传统有意义分存方法存在的像素扩张和分发掩体图像视觉质量不高等问题,提出一种结合调整差值变换的(K,N)有意义图像分存方案,该方案可用于与掩体等大且同为自然图像的密图分存.方法 在分存阶段,首先用调整差值变换将密图转换为差值图和位置图;其次将差值图和位置图进行(K,N)分存,分别嵌入到掩体图像中,并使用密钥确定位置图分存信息的嵌入位置和根据位置图中不同的差值类型选择不同的差值嵌入方法;再次对密钥进行(K,N)分存,将子密钥和分发掩体对应的MD5值公布到第3方公信方作为认证码;最后将子密钥和分发掩体分发给参与者进行保管.在恢复阶段,首先核对参与者的子密钥和分发掩体对应的MD5值,若认证通过的人数小于K,则恢复失败;否则,使用认证通过的子密钥还原出密钥,然后根据密钥提取并恢复出位置图;其次根据位置图中的差值类型来提取和恢复出差值图;最后使用逆调整差值变换还原出最终密图.结果 同现有方法相比,所提策略不存在像素扩张且分发掩体图像视觉质量较高,具有较强的恶意参与者检测能力.结论 本文方法的掩体图像与密图等大且同为自然图像,同经典有意义图像分存方案相比,克服了像素扩张问题,嵌入信息后的掩体图像具有较高的视觉质量,使用第3方公信方存储的MD5值作为认证码,具有较强的恶意参与者识别能力.     

基于误差扩散的图像分存方案

提出一种基于误差扩散的(n,n)阈值图像分存方案。使用该分存方案可以将一幅秘密的二值图像分存成n幅有意义的半色调图像。使用这n张半色调的分存图可以无损恢复出原来的秘密二值图像,而任何少于n幅的分存图都无法得出原秘图的任何信息。实验表明,使用这个分存方案,能够产生出视觉质量较好且有意义的半色调图像,而且当任何一张分存图缺失时,恢复出来的图像都只是噪声图,分存方案的安全性得到保证。     

图视秘密的任意分存

在图视秘密分散存储（简称分存）中，任意分存方案可比一般的分存方案更有效地隐蔽秘密信息，因而有着重要的实用意义．本文在Ｓｈａｍｉｒ提出的图机密码的基础上，明确给出了任意分存的概念，并提出了根据（ｋ，ｎ）分存方案构造任意（ｋ，ｎ）分存方案的方法，此外，还给出了由（ｋ，ｋ）分存方案构造任意（ｋ，ｋ）分存方案的简便方法，并证明了其有效性．     

彩色图像的可视分存方案

利用矩阵并的思想给出了一般存取结构下分存图有意义的可视分存方案（称作扩展可视分存方案）,得到像素膨胀值,并且构造了一般存取结构的彩色图像扩展可视分存方案,得到彩色（k,n）扩展方案的最优像素膨胀.     

一个新的图像单幅可视隐藏方案

作为一种数字水印的技术方法,提出了一种新的单幅图像可视隐藏方案.在该方案中,仅使用一幅加密图便能可视恢复密图,加密图本身既是编码又是解码.与已有的基于移位的方案不同,该方案通过加密图逆时针旋转90°后和自身的叠加来恢复原图.给出了实现单幅图像可视隐藏的新方案的设计方法,对方案的性能进行了分析,并且指出新方案具有较好的抗压缩性、抗破损性、抗放缩性,且能充分利用密图的容量.     

Ideal contrast visual cryptography schemes with reversing

A visual cryptography scheme (VCS) for a set of n participants is a method to encode a secret image, consisting of black and white pixels, into n transparencies, one for each participant. Certain qualified subsets of participants can “visually” recover the secret image by stacking their transparencies, whereas, other, forbidden, subsets of participants, cannot gain any information about the secret image.Recently, Viet and Kurosawa proposed a VCS with reversing, which is a VCS where the participants are also allowed to reverse their transparencies, i.e., to change black pixels to white pixels and vice-versa. They showed how to construct VCSs with reversing where the reconstruction of black (white, respectively) pixels is perfect, whereas, the reconstruction of white (black, respectively) pixels is almost perfect. In both their schemes there is a loss of resolution, since the number of pixels in the reconstructed image is greater than that in the original secret image.In this paper we show how to construct VCSs with reversing where reconstruction of both black and white pixels is perfect. In our schemes each participant is required to store a certain number of transparencies, each having the same number of pixels as the original secret image. Moreover, our schemes guarantee no loss of resolution, since the reconstructed image is exactly the same as the original secret image. Finally, compared to the schemes of Viet and Kurosawa, our schemes require each participant to store a smaller amount of information.     

Visual cryptographic techniques for secret image sharing: a review

ABSTRACT

A visual cryptography scheme (VCS) allows one to decrypt images without any knowledge of cryptography and computational efforts. VCS allows secret image sharing such that we can divide the original image into meaningful or nonmeaningful shares. The shares are distributed among participants; during decryption, the original secret image is recovered through stacking all or some of the shares by the human visual system. Various techniques of visual cryptography were constructed mainly for binary images but later, they were enhanced to handle gray-scale or color images. This article focuses on the study of various visual cryptographic schemes and analyses the performance on the basis of various parameters such as, pixel expansion, type of shares generated, image format, number of secret images, encryption method, etc.. In the proposed work, we give a precise and complete review of various visual cryptographic schemes based on different research works related to this area and cite the relevant literature.     

A (2, 2) Visual Cryptography Technique with Improved Contrast

ABSTRACT

Visual cryptography is a technique used to share a secret among many participants. The problem with visual cryptography technique is that the secret is not perfectly recovered, i.e. a black secret pixel is perfectly recovered but a white secret pixel is recovered with less contrast. Because of this, the overall quality of recovered secret is poor. So to improve the quality of recovered secret image, we need to improve the contrast. In this paper, we have proposed a (2, 2) visual cryptography technique with improved contrast as compared to the traditional (2, 2) visual cryptography scheme (VCS). In the proposed technique, we have used additional basis matrices for encrypting the secret pixels. The quality of the recovered secret image is compared with traditional (2, 2) VCS using various image quality metrics.     

用提升格式构造二次平均插值小波

提升格式是小波研究的新热点，提升格式的运用有两个不同的层次，即用提升格式来实现已有的小波变换和构造新的小波，首先用提升格式构造了二次平均插值小波，并用构造出的小波对灰度图象做变换，然后对变换后的系数采用最优比特率分配的方法来做编码，取得了与JPEG2000推荐的D9/7双正交小波图象编码基本一致的结果，但是，二次平均插值小波的提升格式要比D9/7双正交小波的提升格式简单得多，这意味着消耗的计算资源也少得多。     

Aspect ratio invariant visual cryptography by image filtering and resizing

In visual cryptographic scheme (VCS), each secret pixel is expanded to m subpixels in shadow images to encrypt a secret image. Apparently, we should put these m subpixels as a rectangle such that the blocks can be arranged compactly with each other. However, if the aspect ratio is viewed as important information of the secret image, the distortion occurs at the case that m is not a square. An aspect ratio invariant VCS (ARIVCS) was accordingly proposed to address the arrangement of subpixels. However, the existing ARIVCSs relied on adding dummy subpixels to the shadow images. In addition, their method needs a mapping pattern that reduces the number of dummy subpixels, but how to design a mapping pattern is a huge challenge. In this paper, we propose an easy solution to construct ARIVCS by image filtering and resizing.     

On general construction for extended visual cryptography schemes

Numerous visual cryptography schemes (VCSs) have been proposed to protect a secret image with share images. Most VCSs use random-looking shares to code a single binary secret image. Some schemes are designed for color secret images. Droste's [New results on visual cryptography, in: Advances in Cryptology-CRYPTO ’96, Lecture Notes in Computer Science, vol. 1109, Springer, Berlin, 1996, pp. 401-415] (n,n)-VCS is introduced for multiple binary secret images. Extended VCS (EVCS), by Ateniese et al. [Extended capabilities for visual cryptography, Theoretical Computer Science 250 (2001) 143-161], for binary secret image uses meaningful (innocent-looking) shares. In this paper, we start with a more concise derivation of matrix extension in the ECVS model. This is implemented by concatenating an extended matrix to each basis matrix. We then present a general construction method for single or multiple binary/grayscale/color secret images using matrix extension utilizing meaningful shares. The result (k,n)-visual secret sharing schemes are more general than most existing schemes in terms of the secret/share image types. Using our matrix extension algorithm, any existing VCS with random-looking shares can be easily modified to utilize meaningful shares. The effectiveness of our schemes is demonstrated by real examples.     

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.     

Secret sharing schemes from binary linear codes

In principle, every linear code can be used to construct a secret sharing scheme. However, in general, determining the access structure of the scheme is very hard. On the other hand, finding error correcting codes that produce secret sharing schemes with efficient access structures is also difficult. In this paper, we study a set of minimal codewords for certain classes of binary linear codes, and then determine the access structure of secret sharing schemes based on these codes. Furthermore, we prove that the secret sharing schemes obtained are democratic in the sense that every participant is involved in the same number of minimal access sets.     

一种在医学图像中挖掘非对称区域的方法

提出一种利用图像刚性配准算法和数字形态学调整近似对称区域、有效挖掘出图像非对称区域,并自动判断头部炎症类疾病的算法.首先扩展适用于二值图像的自反射刚性配准算法到灰度医学图像上,然后对配准后的图像使用条件腐蚀算子调整那些近似对称区域的边界,在不同的精度下进行迭代以获得最终的非对称区域.在人类头部CT的胆脂瘤检测实验中,该方法显示出良好的挖掘效果,检测成功率达到95%.