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DeCSS was a program that was able to decrypt the content on retail DVDs.
The program was released by Norwegian programmer Jon Lech Johansen (aka DVD Jon) in 1999, instantly making the then-teenager an international phenomenon.
DeCSS has its name because it breaks the Content-Scrambling System (CSS) protection used by DVD publishers.
Software like DVD Decrypter became mainstream and the DVD CCA launched many lawsuits in the U.S. to block its distribution. They were unsuccessful, for the most part, until 2004-2005.
DVD Jon was just one of three developers who created DeCSS but the other two remained anonymous.
DeCSS became a reality in 1999 when a Xing DVD Player was released without an encrypted DVD decryption key. With the player, the devs were able to easily reverse engineer 170 new decryption keys.
Before DeCSS was released, however, there was another decrypter called DoD DVD Speed Ripper from the group Drink or Die. However, their software was not open and did not include the source code, nor did it work on all discs.
DeCSS had many great uses outside of piracy, including allow Linux users to finally view DVDs on their computers, which did not have any way to access CSS content beforehand.