DeCSS is a small program that unscrambles movies on DVDs, which are encrypted with an algorithm called the Content Scrambling System (CSS).
The Motion Picture Association of America (MPAA) has been trying to suppress this software.
The MPAA's characterization of CSS as copy protection is a lie. DVDs have no copy protection. None. You can copy a cassette tape onto a blank tape, and the copy can be played on any cassette player. In the same way, you can burn a copy of a DVD - without decrypting it - and the copy can be played in any DVD player. CSS encryption does not prevent bit-for-bit copying.
Less expensively, one could easily copy a film from DVD onto videotape by simply connecting a DVD player to a VCR. CSS encryption does not prevent that, either. (However, some recent model VCRs are crippled not to record digital video inputs, which also prevent recording from digital camcorders.)
Despite the groundlessness of the MPAA's claims, New York District Judge Lewis commerce trumps the first amendment Kaplan has unconstitutionally ruled that it is illegal in New York for 2600 Magazine to even link to DeCSS source code. Rejecting free speech arguments, he wrote in his opinion, Computer code is not purely expressive any more than the assassination of a political figure is purely a political statement.
When the MPAA had the original site shut down, mirror sites immediately sprung up. When the MPAA tried to suppress them, even more sprung up. Now the code is mirrored all around the globe. Some say that the MPAA is playing the world's biggest game of Whac-a-Mole.