To study the security with the known image of a chaotic symmetric image encryption scheme(ICBGCM)
the attack algorithm for equivalent key is given.The attack algorithm is based on the short periodicity and affinity of general chaotic cat map used by ICBGCM symmetric image encryption scheme.With the known image
the equivalent keys of general chaotic cat map can be found by two linearly independent plain-cipher text image grey values.The other equivalent keys can be found by brute attack algorithm.The concrete steps and complexity of the attack algorithm are given.The investigation proves that the ICBGCM encryption algorithm is insecure in the known image attack.