Methods and apparatus permit: reading a rewritable storage medium, the rewritable storage medium including digital content and a flag indicative of whether the digital content is encrypted; determining whether the digital content of the rewritable storage medium is encrypted based on the flag; encrypting the digital content of the rewritable storage medium using an identification (ID) number as an encryption key when the flag indicates that the digital content is not encrypted, the ID number being unique to a processing apparatus operable to execute the digital content; and overwriting the digital content of the rewritable storage medium with the encrypted digital content.