Study And Inplement Of Chaotic Stream Ciphers Based On Perturbation

With the rapidly development of computer application, network information security has become to an important issue in our society. It relates to every aspect of people's daily life. As a very important branch of network information security, cryptography is being more and more researched and pay attention to by the researchers and scholars. All of these have prompted the rapid development of cryptography. Chaos has a complex nonlinear nature. Many of its properties have shown consistent with the cryptography required. So, it deserves many cryptography researchers'attractions. In this case, chaotic cryptography as a new discipline was born. Chaotic system has showed the excellent characteristics when it used for encryption, no matter for the traditional text messages or new types of images, multimedia information and so on. However, though chaotic systems shown their fit with the encryption system, it exposed some of the inherent shortcomings at the same moment: chaotic system is running in a continuous domain of nonlinear dynamic systems, when it is digitized with cryptography require, some of its important dynamics had degenerated. For example, a lot of chaotic systems have an infinite cycle; but if they run in discrete domain, the cycle become to a finite number. So, when it happens, cryptanalysts can attack the chaotic cryptography system by mathematic methods. It means that the system becomes insecurity. We called this phenomenon finite precision effect (FPE).So, how to improve the dynamics of digital chaotic system under the finite precision effects becomes an important issue of chaos-based cryptography. This is also the major work of this paper doing. At the beginning of the paper, some preliminary knowledge of chaotic cryptography is introduced, and then concluded the development of chaos theory, to study and discuss some primary characters which describe the chaotic systems. On the basis of that, study some modal of digital chaos systems, for example one dimensional Logistic map and two dimensional Arnold map and so on. After that deeply analysis their chaotic behaviors combined with simulations and experiments. Next take a review of the development of stream cipher technique, introduce the concept of random sequence. It is an in-depth study on the subject of how to design a non linear random sequence generator based on Linear Feedback Shift Register (LFSR). From the point of view of theoretical randomness, mainly analyses the mathematics nature of the sequence, such as how to calculating the linear complexity of generated sequence.During this process, focus on a encrypt method of using pseudo-random sequence to perturbation on the digital chaotic system. The first step of this algorithm is using nonlinear sequence generators generate pseudo-random perturbation sequences. Secondly, take one dimensional Logistic map for digital chaotic system; use the pseudo-random sequences to perturb on the system from both inputs and outputs. When come to the process of translating the real values sequences generated by chaos system to 0-1 bit sequences used in encrypt, we introduce the concept of one way function. With this step, a good quailed key sequence has been generated, and it can be used in encryption. After the first encryption step, put the cipher text feedback; bring the information of plaint text to pseudo-random sequence generator and influent the following encryption process. By this step, we can break the linear nature of output sequence, so it will be a big trouble for cryptanalyst. After the completion of design algorithm, take some simulations and experiments. With the requirements of Golomb randomness assumption and Shannon cipher designing, we test the encryption output. The results show that the algorithm meets the theatrical requirements.At the end of this paper, the encryption towards digital image is introduced, discuss two dimensional General Arnold map using in image encryption. Then, use perturbation based chaotic stream cipher system for encryption to improve the security performance. The encryption system is running slower when it is implemented in software, but if it is implemented in hardware this problem will not exist. So, the system hardware will have a practical value.