Analysis And New Designs Of Digital Chaotic Cryptosystem

We analyze the security of two chaotic ciphers, and design a chaos-based image encryption algorithm. The results could be summarized as follows:For an image encryption algorithm based on chaotic sequence proposed by Gao Fei, we point out its flaw in design. A known-image attack algorithm to solve the equivalent key of the cipher is presented. The computational complexity of the attack is 28+2×M×N for the image with M×N pels. So the image encryption algorithm is insecurity under the known-image attack.The security of a stream cipher based on Logistic map is investigated, which was proposed by Deng Shao-jiang et al.. We prove that the keystream of the cipher is unbalanced and that there are many equivalent keys of the cipher. Furthermore, we find that the most significant bits of the first several states in chaotic sequence are not sensitive to the least significant bits of chaos initial state. Based on the above information leakage, we propose a divide-and-conquer attack. At last, we give some improved methods on the cipher to resist our divide-and-conquer attack.By analyzing and researching several digital chaotic image encryption algorithms, we design a symmetric image encryption scheme based on composite dispersed chaotic dynamics system. The chaotic sequence of Logistic map and Chebyshev map is used as the initialization sequence, and the chaotic encryption of image is realized by processing image incise stretch and fold. The analysis method is not in effect which is usually used in analyzing these two chaotic maps, and the algorithm can encrypt in a higher speed.