To improve reliability on secrecy protection of a control key, without increasing actual loads.
An STB 100 is equipped with an enciphering means 101 to allow a work key Kw to encipher AV data, a transmission side recognition means 102, which performs recognition with a VTR 200 and enciphers the Kc, a Kc generation function selection means 103 which incorporates plural functions and their function identifiers in advance and selects any one of the functions, a random numbers generation means 104 to generate random numbers Kc' to be used in generating the Kc, a Kc storage means 105 for storing the already generated Kc, a Kc generation means 106 for generating a new Kc by using part of the Kc generated in the past, the outputted random numbers Kc' and a selected function fi with these as variable, and a Kw enciphering means 107 for enciphering the Kw by using the generated Kc.
IIZUKA HIROYUKI
YAMADA MASAZUMI
GOTO SHOICHI
TAKECHI HIDEAKI
