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Title: A study on efficient chaotic image encryption schemes
Other Titles: Gao xiao neng hun dun tu xiang jia mi fang an de yan jiu
Authors: Kwok, Sin Hung (郭善雄)
Department: Dept. of Electronic Engineering
Degree: Master of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Data encryption (Computer science)
Image processing -- Security measures
Notes: CityU Call Number: TA1637.K88 2007
Includes bibliographical references (leaves 104-108)
Thesis (M.Phil.)--City University of Hong Kong, 2007
xii, 109 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: With the advancements of mobile communication technologies, the utilization of audio-visual information in addition to textual information becomes more prevalent than the past. Cryptographic approaches are therefore necessary for secure multimedia content storage and distribution over open networks such as the Internet. A traditional way to resist statistical and differential cryptanalyses is to employ permutation and diffusion alternatively. Recently, research on image encryption using chaos theory has been emerged. Some chaotic image encryption schemes use a multi-dimensional chaotic map for pixel permutation in the spatial domain while taking another one-dimensional (1D) chaotic map for keystream generation in the diffusion function. Various image encryption schemes under this architecture have been proposed in the literature. There are still two realization constraints of the above architecture which hinder the system performance. First, the confusion and diffusion effect is solely contributed by the permutation and the diffusion stage, respectively. Consequently, more overall rounds than necessary are required to achieve a certain level of security. Second, in particular to diffusion stage, real-valued chaotic sequence is commonly treated as a pseudo-random keystream. However, a considerable amount of computation load is sacrificed for real-valued computation and consequent integer quantization. In this thesis, the typical structure of chaos-based image encryption schemes has been studied. The concept of introducing certain diffusion effect in the confusion stage by simple sequential Add-and-then-shift operations is proposed. The purpose is to mix the pixel values over the entire image to achieve similar effect of diffusion. The explicit diffusion function then contributes the second level diffusion effect which leads to fewer overall rounds and hence a faster encryption. Moreover, a more efficient diffusion function using simple table lookup techniques as a light-weight replacement to real-valued chaotic maps is also suggested. Instead of floating point computation, the diffusion process is accomplished by mutual lookup of a static two-dimensional (2D) permutation table and a dynamic 2D diffusion table. Both the position and the value of each permuted image pixel are used to locate a secret mask. Eventually, each permuted pixel value is added to the random mask drawn from the table. Simulation results show that at a similar performance level, the proposed cryptosystem requires around one-third the encryption time of an existing cryptosystem. The effective acceleration of the encryption speed is therefore achieved which is then more applicable to real-time image encryption.
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