Several Chaotic Digital Symmetric Cryptography Systems

Nonlinear science is a foundational discipline which concerns the common properties of nonlinear phenomena. Particularly, chaos theory is one of important subdisciplines of nonlinear science. The research has studied the relative problems of chaotic cryptography using the method of numerical simulation. The main contents are as follows:A symmetric key encryption algorithm based on one-dimensional chaotic map is proposed. This algorithm uses the random-like property and ergodicity of chaotic systems. In the process of encryption or decryption, this algorithm generates a chaotic pseudo-random sequence, changes the initial iterative times and the increment to encrypt the plaintext.A new block encryption algorithm is proposed. The chaotic trajectories are computed by weighting. Then the result is used to mask the plaintext. Multiple blocks of plaintext are encrypted at the same time that decreases the chaotic iterations. So the encryption speed is improved to some extend.Because of the sensitivity of chaotic systems on initial conditions/control parameters, when chaotic systems are realized in a discrete space with finite states, the dynamical properties will be far different from the ones described by the continuous chaos theory, and some degradation will arise. This problem will cause the chaotic trajectory eventually periodic. In order to solve the problem, a new binary stream-cipher algorithm based on one-dimensional piecewise linear chaotic map is proposed in this paper. In the process of encryption and decryption, we employ a secret variant to perturb the chaotic trajectory and the control parameter to lengthen the cycle-length of chaotic trajectory. In addition, we design a nonlinear principle to generate a pseudo-random chaotic bit sequence as key stream.A new encryption algorithm is proposed, which encrypts the plaintext based on alternant of the stream cipher and block cipher. A pseudo-random number is used to control which encryption mode is chosen. Using this algorithm, multiple kinds of files, such as TXT, DOC, WMA and JPEG, are encrypted and decrypted, and the performance of the proposed cryptosystem is analyzed.Compared with the low dimensional chaotic systems, the high dimensional chaotic systems have many advantages as more complex mechanism, increasing the key space, more ruleless for the time series of the system variable, etc. Based on these reasons, a new encryption cryptosystem based on Lorenz system is proposed. Moreover, the other low dimensional chaotic system is used to perturb the trajectory of Lorenz system to enhance the encryption speed.