High Quality Content by WIKIPEDIA articles! The security of Advanced Access Content System has been a subject of discussion amongst security researchers, high definition video enthusiasts, and consumers at large since its inception. A successor to Content Scramble System, the digital rights management mechanism used by commercial DVDs, AACS was intended to improve upon the design of CSS by addressing flaws which had led to the total circumvention of CSS in 1999. The AACS system relies on a subset difference tree combined with a certificate revocation mechanism to ensure the security of high definition video content in the event of a compromise.