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 tracing system for identifying one or plural users of one or plural legal user systems which are sources of production of an unauthorized user system, the legal user systems and the unauthorized user system receiving header information enabling to obtain a session key by using contents ciphered by a session key and a decipher key assigned to a user for receiving the contents, obtaining the session key by using the header information and the decipher key assigned to the user, and deciphering the ciphered-contents by using the session key, comprising a generating unit (41) which generates header information based on a generating method, the header information disabling to obtain the session key by using a decipher key assigned to a specific user and enabling to obtain the session key by using a decipher key assigned to the other users, 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 by using a decipher key assigned to a user receiving the contents and comprises: 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.