Improving security of an image encryption algorithm based on chaotic circular shift

An image encryption algorithm based on chaotic circular bit shift is proposed recently. This paper analyses the security of this algorithm and point out that the key space is not as large as they alleged and the algorithm can not resist chosen-plaintext attack or difference attack. The amount of chosen-plaintexts to carry out an attack is very few. This paper also introduces two methods to improve its security by changing chaotic sequences generators, altering orders of permutation and substitution, and applying feedback link mode. The improved algorithm has variable key space and has very good avalanche effect to resist chosen-plaintext attacks, chosen-ciphertext attacks, or difference attacks. The computation cost of improved algorithm is very low.