An encryption processor for encrypting and decrypting data that ensures data security by preventing illegal deciphering or leakage of the data. The encryption processor includes a CPU for running a program, a ROM for storing the program run by the CPU, a RAM used as a work area while the CPU is running the program, an I/O section for sending/receiving data to/from an external device, and an encryption section for decrypting encrypted data and encrypting plain text data.