A security system for protecting information stored in portable storage media by checking identifiers assigned to each medium, system, and terminal. Medium IDs are identifiers written by manufacturers of the storage media. System IDs, or corporate IDs, are assigned to enterprise-wide computer systems each consisting of a host computer and terminals. Terminal IDs are affixed to the respective terminals. Data is stored in the media in encrypted form. The security system checks the validity of the medium ID, system ID, and terminal ID, manages before allowing the user to perform any data access, thus protecting the contents from unauthorized access. The security system also writes permission data into each storage medium to manage the decryption of encrypted data stored therein. The permission data contains an encrypted key which is necessary for decrypting the data, and the key can be decoded only with a valid medium ID and unit ID, thus protecting the data in the medium against any attempt to decrypt it with unqualified equipment.