DVDs are protected by CSS (Content Scrambling System) to encrypt their contents and prevent copying. The CSS code is licensed to makers of DVD players. Code to decrypt DVDs, called DeCSS, was written by a Norwegian teenager and posted on-line. The Motion Picture Association of America took legal action and succeeded in having the DeCSS code removed from some sites. As a result, it is now illegal under US law for any site to display the DeCSS code or to link to it.
Under the terms of the US court’s order, the illegal DeCSS code was defined by the court as:
"any computer program, file or device that may be used to decrypt or unscramble the contents of DVDs that are protected, or otherwise to circumvent the protection afforded, by CSS and that permits the copying of the contents or any portion thereof."
On the basis that any software can be represented as a sequence of binary bits, mathematician Phil Carmody has represented DeCSS as a prime number. This presents the challenge of whether a number can constitute a program, file or device.