Subhayan Sen et al. have proposed a block cipher system CAC (cellular automata based cryptosystem) based on cellular automata theory, but they have not given a detailed description of some of its modules. From the point of view of application, one module of CAC is fixed, and the variant is called SMCAC (same major-CA CAC). The analysis of SMCAC is given, and the results show that the variant of this cipher is very insecure under chosen-plaintext attacks. Using cryptanalysis of the variant for reference, attacks on the cipher itself may be found when some of the design details of the cipher are known.