Research And Design On Image Encryption Algorithm Based On Chaotic Theory

With the development of the modern communications technology and network technology, image information has become an important carrier of the communication for information. In order to ensure the security of information, a higher demand about the encryption performance of information has been proposed. Since image data is informative and high redundancy, the existing encryption algorithm of text message is not suitable for image encryption. Therefore, how to guarantee the security of digital image information has become into a hot research issue.Considering the characteristics of image data and as well as for improving the efficiency and security of image encryption, chaos theory has been widely used to the current image encryption algorithms. Chaotic system is a highly complex nonlinear dynamic system, with pseudo-randomness and uncertainties, as well as the extreme sensitivity of the initial conditions and system parameters, and generated chaotic sequence with non-cyclical and stochastic characteristics. At the same time chaotic sequence has good cryptographic properties, and is very suitable for image information encryption. Using chaotic sequences to encrypt image information, encrypted image has lot of characteristics such as the even distribution of gray value level, the small association of adjacent pixels and the good effect and high security of encryption.This thesis first introduces the definition of chaos, the basic concepts, characteristics and identification methods and several typical chaotic systems. In this paper, the current of several common image encryption methods have been studied. On this basis, an image encryption algorithm based on permutation-diffusion structure was studied and analysied. This thesis have designed and implemented an image encryption algorithm based on the permutation-diffusion structure, that is the image encryption algorithm based on3D chaotic cat map. This algorithm uses the3D cat map toshuffle the positions of image pixels and use another chaotic map to confuse the relationship between the cipher-image and the plain-image, this has improved the security of the algorithm. This thesis has also pointed out the shortcomings of the algorithm and proposed a novel chaos-based image encryption algorithm with variable control parameters. In each round of the encryption process, the control parameters used in the permutation stage are related not only to the key, but also to the certain pixels in the plain-image. And the keystream employed in the diffusion stage are generated from a chaotic maps which is related to the plain-image and influenced by the ciphertext.This can make sure the keys for each pixel in the diffusion stage different from each other. As a result, the algorithm can effectively resist all known attacks against permutation-diffusion architectures. Theoretical analyses and computer simulations both confirm that the new algorithm possesses high security for practical image encryption.To test and verify the encryption performance of two encryption algorithms, using Matlab to carry out encryption and decryption experiments to these two encryption algorithms, and carried out a safety analysis, the results proved the feasibility and effectiveness of the encryption algorithms.