Abstract: Various image encryption algorithms based on the permutation-diffusion structure have been proposed in the past few years．However,permutation and diffusion are considered as two separate stages,making image encryption vulnerable to attacks．Moreover,the algorithms,in general,have low diffusion efficiency and high computational complexity in key scheming．To solve these problems,this paper proposed a SP(Substitution-Permutation) network image encryption algorithm based on chaotic map.The proposed algorithm adopts the following three strategies:(1) To further enhance the security of the cryptosystem,XOR and S-box operations are introduced in the beginning of each encryption round；(2) An bidirectional permutation-diffusion strategy is proposed to accelerate the spreading process；(3) Simple circular bit shift and XOR operations are used to improve the efficiency of key scheming．We conducted a rich set of cryptanalyses on the proposed algorithm,e.g.,key space analysis,key sensitivity analysis,various statistical analyses and differential analysis．Analytical results demonstrate that the proposed algorithm reaches the same security level of previously proposed counterparts in merely three iterations with high efficiency,and is thus applicable for secure image encryption．

Key words: SP network,Image encryption,Chaos,3D arnold map,Logistic map