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Content Scramble System
The Content Scrambling System (or Content Scramble System) is a system to encrypt and copy-protect DVDs. Such systems are generally called Digital Rights Management (or DRM) systems. CSS uses symmetric cryptography. It was introduced in 1996; today, it is considered ineffective.
In October 1999, Jon Lech Johansen and two other people (who remained anonymous) broke the system. CSS uses 40-bit cryptographic keys; because of different design problems, the effective key length is only about 16 bits. A 450 MHz processor, current at the time could brute force crack this in less than a minute. A modern processor such as the Core i7 or Athlon 64 can crack it in a few seconds.
CSS requires special information in the lead-in (a special area) of the disc. Since the lead-in cannot be changed on recordable DVDs these cannot have CSS protection (with a few exceptions, some professional drives allow writing CSS keys to recordable discs).
Open source implementations
Libdvdcss is an open-source implementation used by many DVD software players, including VLC Media Player. Libdvdcss is region-free; it can read foreign-region DVDs even when the keys can't be read from the drive, by using a brute-force attack.