| 摘要 |
PROBLEM TO BE SOLVED: To utilize contents even when the term of the validity of a delivery key distributed from an information transmitter or other information required for utilizing contents is expired in an information distribution system for distributing contents. SOLUTION: The information transmitter enciphers a contents key with an individual key peculiar to the information transmitter and transmits the contents key enciphered by the individual key and an individual cryptographic key, which is formed by enciphering the individual key with the delivery key to be updated in a prescribed cycle, supplied from the outside to an information receiver at least. The information receiver decodes the individual key with the previously given delivery key before updating the delivery key, decodes the contents key with the decoded individual key and preserves the decoded contents key, so that contents can be decoded after the delivery key is updated. Therefore, the reserve purchase contents can formally be purchased regardlessly of the validity term of the delivery key. |
