The Content Scramble System (CSS) is a digital rights management (DRM) scheme used on almost all commercially produced DVDs. It utilizes a relatively weak, proprietary 40-bit stream cipher algorithm. The system was introduced around 1996 and has subsequently been compromised.
The CSS key sets are licensed by the DVD Copy Control Association to manufacturers who incorporate them into products such as DVD movie releases, drives and players. Most DVD players are equipped with a CSS decryption module.
CSS key is a collective term for authentication key, disc keys, player keys, title keys, secured disk key set, and/or encrypted title keys. Some of the keys are stored on the lead-in area of the disk, which is generally only read by compliant drives. Keys can be passed from a DVD drive to a descrambler over a PC bus using a secure handshake protocol.
The purpose of CSS is twofold. First and foremost, it prevents byte-for-byte copies of an MPEG stream from being playable since such copies will not include the keys that are hidden on the lead-in area of the protected DVD disk. Second, it provides a reason for manufacturers to make compliant devices, since CSS scrambled disks will not play on noncompliant devices. Anyone wishing to build compliant devices must obtain a license, which contains the requirement that the rest of the copy-protection system be implemented.
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