Cryptanalysis of an image encryption algorithm based on cellular automata and chaotic skew tent map

In this paper, a security analysis of Image Encryption Algorithm based on Cellular Automata and Chaotic Skew Tent Map (IEA-CACSTM) is presented. The security of IEA-CACSTM has been tested through numerical analysis. The algorithm claims to be secure and resistant to various attack methods, including the chosen-plaintext attack. However, our cryptanalysis results reveal that the two index matrices and three random numbers generated by the IEA-CACSTM do not depend on the plaintext, which means equivalent permutation keys may exist. In particular, when subjected to a chosen-plaintext attack, it is possible to decipher the result of the modulus operation linked to one of the random numbers. Some initial conditions can also be deciphered. On this basis, a row-by-row deciphering method is adopted to decipher the ciphertext image, and then the whole permuted image can be obtained. Finally, the plaintext images of different sizes can be decrypted using the equivalent permutation keys. Theoretical analysis and numerical simulations confirm the effectiveness of our decryption method.