On Real-valued Visual Cryptographic Basis Matrices

Visual cryptography (VC) encodes an image into noise-like shares, which can be stacked to reveal a reduced quality version of the original. The problem with encrypting colour images is that they must undergo heavy pre-processing to reduce them to binary, entailing significant quality loss. This paper proposes VC that works directly on intermediate grayscale values per colour channel and demonstrates real-valued basis matrices for this purpose. The resulting stacked shares produce a clearer reconstruction than in binary VC, and to the best of the authors’ knowledge, is the first method posing no restrictions on colour values while maintaining the ability to decrypt with human vision. Grayscale and colour images of differing entropies are encrypted using fuzzy OR and XOR, and their PSNR and structural similarities are compared with binary VC to demonstrate improved quality. It is compared with previous research and its advantages highlighted, notably in high quality reconstructions with minimal processing

Fourier-based automatic alignment for improved visual cryptography schemes

International audienceIn Visual Cryptography, several images, called "shadow images", that separately contain no information, are overlapped to reveal a shared secret message. We develop a method to digitally register one printed shadow image acquired by a camera with a purely digital shadow image, stored in memory. Using Fourier techniques derived from Fourier Optics concepts, the idea is to enhance and exploit the quasi periodicity of the shadow images, composed by a random distribution of black and white patterns on a periodic sampling grid. The advantage is to speed up the security control or the access time to the message, in particular in the cases of a small pixel size or of large numbers of pixels. Furthermore, the interest of visual cryptography can be increased by embedding the initial message in two shadow images that do not have identical mathematical supports, making manual registration impractical. Experimental results demonstrate the successful operation of the method, including the possibility to directly project the result onto the printed shadow image

Web Application Authentication Using Visual Cryptography and Cued Clicked Point Recall-based Graphical Password

Alphanumerical usernames and passwords are the most used computer authentication technique. This approach has been found to have a number of disadvantages. Users, for example, frequently choose passwords that are simple to guess. On the other side, if a password is difficult to guess, it is also difficult to remember. Graphical passwords have been proposed in the literature as a potential alternative to alphanumerical passwords, based on the fact that people remember pictures better than text. Existing graphical passwords, on the other hand, are vulnerable to a shoulder surfing assault. To address this shoulder surfing vulnerability, this study proposes an authentication system for web-applications based on visual cryptography and cued click point recall-based graphical password. The efficiency of the proposed system was validated using unit, system and usability testing measures. The results of the system and unit testing showed that the proposed system accomplished its objectives and requirements. The results of the usability test showed that the proposed system is easy to use, friendly and highly secured