Improved visual cryptography schemes without images size expansion

Askari, Nazanin (2012) Improved visual cryptography schemes without images size expansion. Masters thesis, Memorial University of Newfoundland.

[English] PDF - Accepted Version Available under License - The author retains copyright ownership and moral rights in this thesis. Neither the thesis nor substantial extracts from it may be printed or otherwise reproduced without the author's permission. Download (84MB)

Abstract

Over the past few years, increasing concern about the privacy of information shared in computer systems has increased interest in data security. Visual cryptography is a secure secret sharing scheme that divides secret images into shares which on their own reveal no information of the original secret. Recovery of the secret image can be performed by superimposing the shares using transparencies. Hence, the process does not require any special software or hardware for cryptographic computations and the decoding process is done by human visual system. -- In the application of visual cryptography, pixel expansion and contrast are two primary issues. Pixel expansion can result in many problems such as the problem of image distortion and the requirement of more storage space. In addition to the problem of pixel expansion, loss of contrast is also a problem for visual cryptography. Since the secret is recovered visually, making a good visual effect is as important as keeping the image size invariant. A few studies pay attention to solving the problem of pixel expansion; however, such schemes may not be suitable because the visual effect of the recovered image is poor. -- In this research, we propose a novel visual secret sharing scheme for sharing one secret image. The purpose of the proposed method is to solve a problem of pixel expansion and generate a recovered image such that its visual quality is very similar to the original secret image. Our proposed non-expansion scheme is perfectly secure and experimental results show that it provides better contrast and visual cryptography schemes. -- Furthermore, we extend our research to find a solution to deal with the problem of pixel expansion and the visual quality of the recovered images for advanced schemes such as multiple image visual cryptography and extended visual cryptography. This is done by using algorithms that process the original secret image prior to applying the visual cryptography schemes. The resulting recovered images in a multiple image visual cryptography scheme and the share images in an extended visual cryptography scheme are significantly visually improved over other approaches. The improvement in visual quality is particularly critical for half-toned images which are derived originally form grayscale images. Three novel methods are proposed to improve the visual quality of the recovered share images when using grayscale secret images.

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