Enciphering, deciphering and information processing apparatus and methods

An enciphering/deciphering apparatus and method are provided by which illegal copying can be prevented with a greater degree of certainty. Data enciphered by a 1394 interface (26) of a DVD player (1) is transmitted to a personal computer (2) and a magneto-optical disk apparatus (3) through a 1394 bus (11). In the magneto-optical disk apparatus (3) with which a change to a function is open to a user, the received data is deciphered by a 1394 interface (36). In contrast, in the personal computer (2) with which a change to a function is open to a user, the enciphered data is deciphered using a time variable key (50) by a 1394 interface (49), and a result of the decipherment is further deciphered using a session key by an application section.