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.     

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.     

Voronoi-based image representation applied to binary visual cryptography

A common processing scheme of Visual Cryptography (VC) consists of generating two ciphered images from a secret image. The sender transmits these key images via two different channels. The receiver collects the key images and recovers the secret image by superimposing them. In this article, Voronoi tessellation is exploited in Visual Cryptography for the first time to the best of our knowledge. The proposed Voronoi-based Visual Cryptography (VVC) technique permits to drastically reduce the amount of encoded and transmitted information by comparison with a traditional VC scheme. Hence, the technique can be used for transmissions of secret images with enforcement of the transmission reliability by enhancing the redundancy of information or with communication networks and technologies having low bandwidth and memory. Moreover, the random arrangement of the Voronoi polygons makes the transmission of the secret image more secure since the sender and the recipient use a private generator of random points. In addition, shape reconstruction properties of the image-based Voronoi representation are stated.     

Distributed three-level QR codes based on visual cryptography scheme

Owing to the large storage and fast machine recognition, QR codes have been widely utilized in many fields such as mobile payment, website navigation and user identity authentication. However, any QR code reader can access to the message contained in the QR code, the security becomes a major challenge to QR codes for privacy usage scenarios. Moreover, the management of QR codes for users are also inconvenient, since the human vision is hard to distinguish a QR code from the others. To solve the security and management problems, we propose the three-level QR codes for a group of participants. The first-level management information and the second-level public information are recognizable for the human vision and QR code reader device, respectively. The third-level privacy information is protected using visual cryptography scheme, and can be decoded using simple and non-cryptography computations. Furthermore, the shares can be stored or transferred in not only e-format but also print-format and photo-format, leading to the broad applicability. Experimental results and analysis demonstrate that the proposed scheme can encode three-level information into several distributed QR codes, and has more advantages compared with the previous schemes.     

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.     

视觉密码方案概述

视觉密码(VC)是一种用于分享秘密图像的加密方案,与传统的密码技术相比具有简单性、安全性、隐蔽性等优点.其加密是将图像编码为分享,将分享打印在透明胶片上或以数字形式存储;其解密过程是对分享进行叠加,然后通过人类的视觉能力来实现.本文介绍了视觉密码的基本概念和各种不同的视觉密码方案,以及每种方案中使用的技术.此外,对视觉...     

一种基于可视密码的轮廓小波图像零水印方案

为了实现数字图像的版权保护,提出了一种基于可视密码的零水印方案.该方案首先实现了一种轮廓小波变换(CWT)的多尺度几何分析方法,然后通过在变换域能量最高的中频子带适当选取系数,结合预定义的可视密码码书,将二值水印图像藏匿于两个分享中,从而构造出零水印信号.实验结果表明,该方案不仅能在不改变原始图像的情况下实现版权保护,而且鲁棒性强,能抵抗JPEG压缩、剪切、加噪、锐化及旋转等图像处理攻击.     

(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.     

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.     

New privilege-based visual cryptography with arbitrary privilege levels

Recently, Hou et al. introduced a novel (2, n) privilege-based visual cryptography scheme (PVCS) with various privilege levels of shadow images. In this scheme, a shadow with a higher privilege contributes more recovered information, while a lower privileged shadow has the less recovery capability. Moreover, the visual quality of stacked result depends on the total sum of privilege levels for all involved shadows in reconstruction. Unfortunately, the PVC scheme has the inconsistency of the contrast of recovered image and the sum of privilege levels. Accordingly, an enhanced Hou et al.’s (2, n)-PVC scheme (EPVCS) is proposed to solve this inconsistency problem. However, the EPVCS is not a general solution to implement all PVCSs with arbitrary privilege levels, and it also has the unequal whiteness of shadows. In this paper, we first extend Hou et al.’s (2, n)-EPVCS with a correct privilege levels achieving the consistency of the contrast and the sum of privilege levels. Then we construct a (2, n)-PVCS to allow arbitrary privilege levels and provide the equal whiteness for each shadow.     

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.     

Multitone reconstruction visual cryptography based on phase periodicity

The current no-computation grayscale image visual cryptography (VC) can only achieve halftone reconstruction but cannot truly achieve multitone. To solve this problem, we propose the concept of phase periodicity of the $\lambda /2$ retarder film and calculate the optical axis angle set with phase periodicity. According to the phase periodicity, we propose a $\lambda /2$ retarder film phase periodicity visual cryptography (RPP-VC). In RPP-VC, the secret pixels are encoded as the optical axis angles of $n$ $\lambda /2$ retarder films and distributed to $n$ shares. The decoding process does not require computation. The reconstructed image has no pixel expansion and can reach up to 23 tones. The quality of the reconstructed images has been greatly improved and the evaluation indicators of perceived quality are nearly doubled compared with other grayscale image VC schemes. The experimental results verify the feasibility of RPP-VC.     

A zero-watermark algorithm for multiple images based on visual cryptography and image fusion

At present, it is difficult for the multiple images zero-watermark algorithm to protect all the images in the image set, and repeated operations will reduce the efficiency of the algorithm. To solve these issues, the proposed algorithm can design a reasonable copyright protection scheme according to the number of images in the image set to realize the protection of all images, and reduce the cost of time and storage. The gray-weighted average image fusion method is used to fuse multiple normalized standard images into one image. The LWT(Lifting the Wavelet Transform)-QR decomposition is applied to the effective area of the fusion image to obtain the robust feature image. Non-extended visual cryptography is used to enhance the security of the algorithm. A zero-watermark image is obtained by using the XOR manipulation for the feature image and the public shared image. Experimental results demonstrate that the proposed algorithm has good performance.     

A feature-based digital watermarking scheme for halftone image

It is a challenging work to design a robust halftone image watermarking scheme against desynchronization attacks. In this paper, we propose a feature-based digital watermarking method for halftone images with low computational complexity, good visual quality and reasonable resistance toward desynchronization attacks. Firstly, the feature points are extracted from host halftone image by using multi-scale Harris–Laplace detector, and the local feature regions (LFRs) are constructed according to the feature scale theory. Secondly, discrete Fourier transform (DFT) is performed on the LFRs, and the embedding positions (DFT coefficients) are selected adaptively according to the magnitude spectrum information. Finally, the digital watermark is embedded into the LFRs by quantizing the magnitudes of the selected DFT coefficients. By binding the watermark with the geometrically invariant halftone image features, the watermark detection can be done without synchronization error. Simulation results show that the proposed scheme is invisible and robust against common signals processing such as median filtering, sharpening, noise adding, and JPEG compression, etc., and desynchronization attacks such as rotation, scaling, translation (RST), cropping, local random bend, and print-scan, etc.     

General digital rights management solution based on white-box cryptography

Digital rights management(DRM) applications are usually confronted with threats like key extraction, code lifting, and illegal distribution. White-box cryptography aims at protecting software implementations of cryptographic algorithms and can be employed into DRM applications to provide security. A general DRM solution based on white-box cryptography was proposed to address the three threats mentioned above. The method is to construct a general perturbation-enabled white-box compiler for lookup-table based white-box block ciphers, such that the white-box program generated by this compiler provides traceability along with resistance against key extraction and code lifting. To get a traceable white-box program, the idea of hiding a slight perturbation in the lookup-table was employed, aiming at perturbing its decryption functionality, so that each user can be identified. Security analysis and experimental results show that the proposed DRM solution is secure and practical.