Content Scramble System

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Content Scramble System (CSS) is a Digital Rights Management (DRM) scheme used on almost all commercially produced DVD-Video discs. 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 to manufacturers who incorporate them into products such as DVD drives, DVD players and DVD movie releases. Most DVD players are equipped with a CSS decryption module. CSS key is a collective term for authentication key, disc key, player key, title key, second disk key set, and/or encrypted key.

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. ^[1]

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.^[1]

In October 1999, Jon Lech Johansen and two people who remained anonymous reverse engineered the algorithm and DeCSS was released. The CSS algorithm was soon revealed to be easily susceptible to a brute force attack, apart from being an example of the trusted client problem. The weakness of the protection is primarily due to US government crypto-export regulations, which, at the time, forbade the export of cryptosystems employing keys in excess of 40 bits - a key length that had already been proven to be wholly inadequate in the face of increasing processing power by the time DVD was released (see DES). In addition, structural flaws in the algorithm reduced the effective key length to only around 16 bits, which could be brute-forced by a 450Mhz processor in less than a minute[1]. As a 450MHz processor was the stated minimum necessary to decompress a DVD-compliant MPEG-2 videostream in realtime, it effectively meant that any computer that could play a DVD could also crack one.

The CSS algorithm has been superseded by the Cryptomeria cipher in newer DRM schemes such as CPRM/CPPM, or by AES in the AACS copy-protection scheme used by HD DVD and Blu-ray Disc.

[edit] Specification

Authentication is a process for a DVD drive and CSS Decryption module to recognize (or authenticate) each other. It is necessary before reading data from DVDs. Authentication keys are used for this process.

Title keys are used for scrambling and descrambling actual data on DVDs called titles. A title could be a complete motion picture, a trailer or similar self-contained unit.

Disc keys are used for decrypting title keys on DVDs.

Player keys are used for decrypting disc keys on DVDs. Each DVD player manufacturer is allocated one of approximately 400 player keys to incorporate in its players.

While most CSS decrypting software is used to play DVD films, other non-playing software (such as DVD Decrypter, AnyDVD, DVD43, Smartripper and DVD Shrink) allows a region-specific DVD to be copied as an all-region DVD. Such software is also able to remove Macrovision, Content Scrambling System (CSS), region codes, and disabled user operations (UOPs).