The pervasive use of optical discs-the preferred media for storing and distributing digital information-is motivated by their low cost, versatility,and durability. Unfortunately, circuit technology advances have made it easyto create high-fidelity copies of these discs. The trend toward progressive easeof disc copying has spawned a burgeoning parallel economy based on illegal software copying and merchandizing.Thor is a proposed security scheme to prevent unauthorized disc duplication and illegal use of the data. Thor relies on a combination of software and hardware strategies using time-tested cryptographic algorithms to provide resistance to tampering and malicious operations.