Semiconductor Device and Encryption Key Writing Method

A microcomputer employed for an IC card, etc. is provided, in which the security level to an attack which tries to read an encrypted program illegally is improved. A semiconductor device is provided with a CPU, an EEPROM, and a ROM. The ROM is provided with an encryption area and a non-encryption area and the encrypted firmware is stored in the encryption area. The semiconductor device is provided with a decrypter which holds the encryption key, decrypts the encrypted firmware, and supplies the decrypted firmware to the CPU. The EEPROM is provided with a system area to which an access from the CPU is forbidden in a user mode. The encryption key is divided into split keys of plural bit strings, and stored in the distributed address areas in the system area. An encryption key reading program which is not encrypted is stored in the non-encryption area of the ROM. Executing the encryption key reading program, the CPU reads and reconfigures plural split keys stored in the EEPROM in a distributed manner to restore the encryption key. The CPU supplies the restored encryption key to the decrypter.