| With the rapid development of Internet and digital multimedia technology, the transmission of multimedia information especially image information is becoming increasingly important. Digital technology can bring us a lot of convenience, but also gives the illegal users an opportunity to attack, so the method of image encryption has become a major concern. Different from text files, image has its unique properties intuitive, data volume and strong correlation between pixels, and other characteristics, so, some of the traditional methods of data encryption, such as DES, AES, IDEA and RSA are no longer suitable for digital image encryption. As the chaos has high security, low computational complexity, acutely sensitive to initial conditions and system parameters of the nature of good, thus provides a new method for digital image encryption.Therefore, this paper based on the two-dimensional discrete nonlinear equations, by analyzing and calculating Lyapunov exponents, proposed a two-dimensional hyperchaotic system, and research and analyse the characteristics of two-dimensional hyperchaotic system theoretically, including draw the maps attractor, analyse balance point and stability, solve and draw Lyapunov exponents and bifurcation diagram, and sensitivity analysis. The theoretical analyse and experimental results show that the two-dimensional hyperchaotic system has the characteristics of the hyperchaotic system, phase space complex, highly sensitive to initial conditions, good randomness, large key space, good safety and other advantages, it also does not have complex computation like the high-dimensional chaotic systems, slow encryption speed, a large amount of the generated cipher image data and other defects.Secondly, this paper through the analysis of the characteristics of the existing chaotic sequence preprocessing methods, combined with multiple segmentation method, designed a multiple segmentation method with two-dimensional hyperchaotic output sequence’s features, to achieve the discretization on the kinetics of degradation of chaotic system output sequence, and have random tests. Experimental results show that the sequence after the method preprocessing have passed FIPS140-2test, correlation test and power spectrum test, and the output sequence have more randomness and complexity, can effectively used for image encryption.Finally, aim at the main defect that the present digital image encryption algorithm based on chaotic is difficult to resist the known/choose plaintext attack, In this paper, an improved algorithm based on two-dimensional hyperchaotics system was proposed. Different from traditional Fridrich architecture of confusion and diffusion, an improved encryption algorithm architecture with confusion-diffusion process was proposed, and the original image’s MD5value is XORed to control two-dimensional hyperchaotics system’s parameters and initial values respectively, increase the encryption algorithm’s sensitivity of original image, as a result the encryption algorithm can effectively resist known/choice plaintext attack. Experimental results show that the improved algorithm has good encryption efficacy and security, and better results in original image’s sensitivity analysis, can effectively resist known/choose plaintext attack. |
