A fast key-changing hardware apparatus, which generates one sub-key each clock cycle, which is used by advanced encryption system (AES) algorithm block cipher, is independent from the AES algorithm block cipher. The invented apparatus automatically generate expanded keys from the input cipher key, store them in the key expanded RAM and ready to be used by the AES algorithm block cipher. If the key changing and the key expanding speed in the fast key-changing device is as fast as the data block (i.e. 128, 192, or 256 bits) processing speed in the AES algorithm block cipher, the cipher system has the characteristic of one-time pad perfect cryptography. When using this device with a fixed key cipher system, the original input cipher key can be detached or destroyed from the system and guarantees the safety of the cipher key.