Abstract: Recently,an image encryption algorithm based on bit scrambling and hyperchaos was proposed,whose main idea is as follows.First,a chaotic sequence is used to scramble pixel values of plaintext image.Then,according to the size of two adjacent elements in a random sequence,internal bit of each pixel is scrambled.In the end,the final ciphertext image is obtained by the diffusion and confusion of the scrambling sequence,and chaotic sequence,and plain image information can be well hidden.In this paper,the security of the above encryption algorithm were analyzed.It is found that the security of the algorithm depends entirely on three chaotic sequences by choosing plaintext attack.The three chaotic random sequences in the original algorithm were obtained,thus the plaintext image was restored.Theoretical analysis and experimental results verify the feasibility of the chosen plaintext attack strategy.At the same time,the improved method was given.In the improved algorithm,the initial value of the chaotic system is related to the SHA-256 hash value of the plaintext image,so that the key stream is related to the plaintext image.Then the algorithm is resis-tant to the attack of chosen plaintext.

Key words: Hyperchaotic image encryption,Cryptanalysis,Bit scrambling,Chosen plaintext attack,SHA-256 hash value