Digital watermark system, digital watermark method and program

A digital watermark method executed by a digital watermark system that realizes digital watermark for a cryptographic function, the method comprising the steps of:

• a key generating process in which a security parameter is taken as input and a watermark embedding key for embedding information representative of a watermark in a circuit that realizes the cryptographic function, and a watermark extraction key for extracting information representative of the watermark from a circuit in which a watermark is embedded are generated;
• an embedding process in which a master private key used for the cryptographic function, the watermark embedding key, and information representative of a watermark are taken as input, and generating a circuit, wherein the circuit comprises the watermark and a program code to perform the cryptographic function, and the cryptographic function comprises encryption of data, decryption of data, and generation of a signature of the data;
• and
• an extraction process in which the watermark extraction key and the circuit, are taken as input, and information representative of a watermark is extracted from the circuit for authenticating the program code.