Security analysis of a color image encryption scheme based on skew tent map and hyper chaotic system of 6th-order CNN against chosen-plaintext attack

Recently, a color image encryption scheme based on skew tent map and hyper chaotic system of 6th-order CNN was proposed. It consists of two phases including one-time confusion and many times diffusion, which are controlled by skew tent map and hyper chaotic system of 6th-order CNN, respectively. It was claimed that the scheme can resist common attacks. However, this scheme is found to be insecure against chosen-plaintext attack. In this paper, we analyze the security weakness of the scheme. Although many keys were utilized, they keep fixed so that the corresponding chaotic keystreams are also consistent when encrypting different color images. All the keystreams used for each encryption can be wholly revealed through the derivation. In spite of many times of iterations in the diffusion phase, cryptanalytic results show that it is the same as only once. Experiment also verifies the proposed assertion.