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

A user system for deciphering ciphered-contents provided from a contents providing system, the user system comprising:

• a receiving unit which receives header information enabling to obtain a session key by using contents ciphered by a predetermined session key and a decipher key assigned to a user for receiving the contents, the ciphered-contents and the decipher key being transmitted from the contents providing system;
• a session key reproducing unit which obtains the session key by using the received header information and a decipher key assigned to the user;
• and
• a contents deciphering unit which deciphers the received ciphered-contents by using the obtained session key,
• wherein the session key obtains unit obtains the decipher key by dividing a user identification information group of users into subgroups, assigns subgroups with different key generation polynomials, and substitutes user identification information of a specified user in a key generation polynomial assigned to a subgroup to which the user identification information of the specific user belongs.

A method of deciphering ciphered-contents comprising:

• receiving header information enabling to obtain a session key by using contents ciphered by a predetermined session key and a decipher key assigned to a user for receiving the contents, the ciphered-contents and the decipher key being transmitted from the contents providing system;
• obtaining the session key by using the received header information and a decipher key assigned to the user;
• and
• deciphering the received ciphered-contents by using the obtained session key,
• wherein the session key obtains unit obtains the decipher key by dividing a user identification information group of users into subgroups, assigns subgroups with different key generation polynomials, and substitutes user identification information of a specified user in a key generation polynomial assigned to a subgroup to which the user identification information of the specific user belongs.

A computer program for deciphering ciphered-contents comprising:

• a program code for receiving header information enabling to obtain a session key by using contents ciphered by a predetermined session key and a decipher key assigned to a user for receiving the contents, the ciphered-contents and the decipher key being transmitted from the contents providing system;
• a program code for obtaining the session key by using the received header information and a decipher key assigned to the user;
• and
• a program code for deciphering the received ciphered-contents by using the obtained session key,
• wherein the session key obtains unit obtains the decipher key by dividing a user identification information group of users into subgroups, assigns subgroups with different key generation polynomials, and substitutes user identification information of a specified user in a key generation polynomial assigned to a subgroup to which the user identification information of the specific user belongs.

A deciphering device for a contents providing system transmitting to a user contents ciphered by a predetermined session key and header information enabling to obtain the session key by using a decipher key assigned to the user, the device comprising:

• a session key obtaining unit which obtains the session key by using the header information and the decipher key assigned to the user;
• and
• a contents deciphering unit which deciphers the ciphered-contents by using the deciphered session key,
• wherein the session key obtaining unit divides a user identification information group of users into subgroups, assigns the respective subgroups with different key generation polynomials, and obtains the decipher key assigned to a specific user by substituting user identification information of the specific user in a key generation polynomial assigned to a subgroup to which the user identification information of the specific user belongs.