To provide a method which generates a plurality of different codes from an original code respectively by a plurality of obfuscation techniques, a program upgrading method by which another certification process is unnecessary, and to provide a system for these methods.

In this system, after a plurality of obfuscated codes are generated from an original code using a plurality of obfuscation techniques and a program unique number is generated, the tamper-resisting technique of the program provides with users the codes using the one of the obfuscated codes according to the unique number. The upgrading method generates a plurality of the obfuscated codes by the obfuscation techniques; compares them with the existing obfuscated codes, and generates a plurality of patching cords by the difference. The patching cords are selected based on the unique number input by the users.