Key generating method, contents providing method, ciphered-contents deciphering method, pirate identifying method, contents providing system, user system, trace system, ciphering apparatus, deciphering apparatus, and computer program

摘要

A contents providing system for providing ciphered-contents to users, comprising a generating unit (12) which generates header information based on a generating method, wherein the generating method is a method of generating a decipher key in a system which adds header information enabling to obtain a session key be using a decipher key assigned to a user receiving the contents, to contents ciphered with the session key and provides the ciphered-contents and the header information to the user, obtains the session key by using the header information and the decipher key assigned to the user, and deciphers the ciphered-contents by using the session key, characterized by comprising: dividing a user identification information group of users into subgroups; assigning the respective subgroups with different key generation polynomials; and generating a decipher key by substituting the user identification information in the key generation polynomial assigned to the subgroup to which the user identification information of the user belongs.