Cryptanalysis And Improvement Of Chaotic Image Encryption Based On Nonrandom Diffusion

In the context of the rapid development of the Internet,more and more digital images have been applied to all walks of life.How to improve the security of transmission and storage of digital images has received the attention of researchers.Chaotic systems are characterized by ergodicity,determinism and sensitivity to initial conditions and control parameters,which are in line with the requirements of image encryption mechanism.Therefore,more and more chaotic image encryption mechanism have appeared in recent years.Due to the high redundancy of image data and the strong correlation between adjacent pixels,chaotic image encryption algorithm has stronger advantages than traditional encryption algorithms in efficiency and security.However,some chaotic image encryption algorithms based on non-random diffusion have been proved to be insecure by cryptanalysts.Through cryptanalysis on chaotic image encryption algorithm,researchers check whether there are security holes in it,so as to crack the encryption algorithm,and propose a more secure image encryption algorithm,thus promoting the continuous development and improvement of cryptography.This paper conducts cryptanalysis on two latest chaotic image encryption algorithms based on nonrandom diffusion,and finds the security vulnerabilities in them,confirms that the nonrandom diffusion they use is not secure enough,and proposes corresponding attack schemes based on these vulnerabilities.In order to make up for these vulnerabilities and improve the security of these algorithms,this paper also proposes a multi-round chaotic image encryption algorithm based on random diffusion.The main research contents of this paper are summarized as follows:(1)This paper cryptanalyzed the bit-level color chaotic image encryption algorithm based on oneround of permutation-diffusion structure proposed by Pak et al.and found that the scheme uses nonrandom diffusion and secret key independent of the plaintext.Then this paper proposed an attack scheme with computational complexity of (?),and conducted the simulation experiment on the proposed attack scheme,successfully cracked the encryption algorithm,verified the effectiveness of the attack scheme and further proved that nonrandom diffusion is not safe enough.Finally,this paper also put forward three reasonable suggestions for improvement.(2)In this paper,the cryptanalysis of the multi-round chaotic image encryption algorithm based on the permutation-bit diffusion structure proposed by Mondal et al.is carried out.It is found that the scheme uses non-random diffusion and secret key that has nothing to do with the plaintext,and proved that the scheme is equivalent to one-round of chaotic image encryption algorithm with permutation-bit diffusion structure.Then this paper proposed two attack schemes with computational complexity of (?)respectively,and simulated experiments on them respectively,which further proved that non-random diffusion is not safe enough and the feasibility of the two attack schemes.Finally,this paper also gives three reasonable suggestions for improvement.(3)Based on the cryptanalysis of the above schemes,an improved multi-round chaotic image encryption mechanism based on random diffusion is proposed.The mechanism uses the secret key related to the plaintext image,adds a substitution process and uses a random diffusion function in each round of encryption,thereby enhancing the ability to resist chosen-plaintext attack.Then this paper also uses our two attack schemes to conduct attack testing and cryptanalysis on the encryption mechanism.Finally,this paper has tested a series of evaluation indicators to prove that the improved encryption mechanism has higher security and stronger robustness,and can resist chosen-plaintext attacks.