Content Scramble System

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Content Scramble System (CSS) is a Digital Rights Management (DRM) system employed on almost all commercially produced DVD-Video discs. It utilizes a 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.

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 restricted 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 DRM 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 algorithm's weakness 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 450 MHz processor in less than a minute.^[2] As a 450 MHz processor was the stated minimum necessary to decompress a DVD-compliant MPEG-2 videostream in realtime, it effectively meant that any computer that could decode a DVD entirely in software could also crack one.

The CSS algorithm has been superseded by the Cryptomeria cipher in newer DRM schemes such as CPRM, or by AES in the AACS DRM scheme used by HD DVD and Blu-ray Disc.

Specification[]

Authentication

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

Title keys

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

used for decrypting title keys on DVDs.

Player keys

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 (UOP).

See also[]

• DVD Copy Control Association
• Advanced Access Content System
• AnyDVD, DVD43, DeCSS, DVD Decrypter, Smartripper, and libdvdcss
• Content Protection for Prerecorded Media (CPPM)
• Fair use
• ARccOS: an additional form of DVD copy protection
• libdvdcss, a free software library for accessing CSS-encypted DVDs, thus enabling playback of such discs on opensource players.

References[]

External links[]

• Content Scramble System (CSS) (official website)
• The Content Scrambling System
• Cryptography in Home Entertainment: A look at content scrambling in DVDs

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