| Chaos is an important branch of scientific research in the field of non-linear, it becomes to be the link between deterministic system and random phenomena. Since chaos has the characteristics of extremely sensitive to parameters, ergodicity, pseudo-randomness etc., it is used in the field of image encryption more and more. Recently, a lot of image encryption algorithms are proposed, but some of them are defective more or less. This paper focuses on cryptanalysis of two of them, find defects, and gives the improvements, and finally propose a novel image encryption algorithm. The specific research work are as follows:(1) A chaotic image encryption method based on coupled map lattices proposed by Sun et al is analyzed, and find two drawbacks within it:①the key stream in the encryption process is only determined by the key.②The type of encryption operation is only determined by the key. Base on these two drawbacks, a chosen plaintext attack algorithm is proposed, and later give the improvement measures to enhance the security of the original algorithm.(2) A parallel sub-image encryption method based on high-dimensional chaos proposed by Mirzaei et al is analyzed, and find three drawbacks within it:①the rearrangement operation of sub-images is ony determined by the key.②The total shuffling process is insecure.③The changing order of the pixes’values in the diffusion process is fixed, and everyone knows that. Based on these three drawbacks, a chosen plaintext attack algorithm is proposed, and then give some improvements to enhance the security of the original algorithm.(3) A new image encryption algorithm based on improved cat map and parameter perturbation method is proposed. The improved cat map in the scrambling process overcomes two existing inherent flaws of the original cat map, but maintains its advantages; parameter perturbation method in the diffusion process makes the control parameter associated with the ciphertext image, which enhances the security of image. |
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