CSS stands for Content Scrambling System.
In DVD-Video, an encryption scheme designed to protect copyrighted material that resides on a disc by periodically scrambling the data using encryption keys. A tool named Decss can allow users to circumvent it. Although Decss didn't exactly crack the CSS, but instead used leaked decryption keys.
Content-Scrambling System (CSS) is an encryption system used on some DVDs. It uses a weak, proprietary 40-bit encryption algorithm. The system was introduced circa 1996.
The CSS key sets are licensed to manufactors who incorporate them into products such as DVD drives, DVD players and DVD movie releases. Most DVD players are equipped with a CSS Decryption module. CSS key is a collective term for authentication key, disc key, player key, title key, second disk key set, and/or encrypted key.
In October 1999, the algorithm was reverse engineered by Jon Johansen and DeCSS was released. The CSS algorithm was soon revealed to be easily susceptible to a brute force attack. The weakness of the protection is due to US government crypto-export regulations, apart from being an example of the trusted client problem.