While the DVD-Video format does not require copy protection, the ease of making and distributing digital copies led the movie studios to develop an encryption-based system of access control, the Content Scrambling System or CSS. The MPEG-2 video stream of a CSS-protected DVD is encrypted using a proprietary 40-bit cipher with the key stashed in a separately-encrypted header region. This header is encrypted with a variety of different keys, which are held as a trade secret by the DVD Copy Control Association (DVD-CCA). In principle, to play a CSS-encrypted DVD you must first license one of these keys from the DVD-CCA, signing a long licensing agreement which restricts how your player may handle the decrypted content and requiring absolute confidentiality of the key.
Since the vast majority of DVDs have CSS encryption, this allows the DVD-CCA to control the implementation of the DVD format. Since the license is expensive and requires non-disclosure of the encryption keys, this prevents any free software/open source DVD player. This, in turn, prevents the community from adding DVD playback to any 'alternative' operating systems, including GNU/Linux. Beyond this, the license agreement enforces a number of the consumer-unfriendly features of the DVD standard, including region codes and the overused and frustrating 'no-skip' mode.
Fortunately for the DVD owner, the CSS cryptosystem was found to be extremely weak, in part due to US government cryptography regulations at the time the standard was established. Besides the small 40-bit key size, flaws in the algorithm decrease the strength of the encryption to a level easily brute forced with the computers available in the late 1990s. Through reverse engineering of a software player, both the algorithm and a player key were recovered in 1999, and DeCSS was released. Source code for DeCSS was soon posted online, leading to widespread analysis of the CSS algorithm.
A major flaw of DeCSS was its reliance on a player key ripped from the Xing DVD decoder software, which could be revoked by the DVD-CCA and removed from future DVD headers. In addition, DeCSS was a Windows-only program without a proper free software license, so it was not immediately useful for development of Linux-based DVD players. The VideoLAN project fixed these flaws with the development of libdvdcss. This library is a simple, highly portable DVD decryptor requiring no external input besides the contents of the DVD itself. The weakness of the CSS algorithm allows libdvdcss to directly brute force the encryption on the video data in a process that completes in roughly a minute on 1999-era hardware and in significantly less time using the current state of the art.
While the technical aspects of DVD decryption with libdvdcss are quite straightforward, it is in the legal arena that the use of the library is made difficult. Laws such as the US Digital Millennium Copyright Act (DMCA) criminalize the distribution and ownership of tools for circumventing any access or copy protection system, no matter how technically ineffective. Furthermore, the circumvention of these systems is criminalized under these laws regardless of whether the decrypted data is used in a lawful fashion. As such, most Linux distributions refuse to include any CSS decryptor or make it available from their servers.
The simplicity and modularity of libdvdcss provides a useful loophole for distributors of free video players. These players are almost universally distributed without libdvdcss, but with the capability to use it if it is present. As such, an end user may download libdvdcss at their own risk and install it, and all video players will gain the ability to play encrypted DVDs. This process is often cited as a factor in Linux's lack of user-friendliness, but the legal situation has tied the hands of the community in smoothing this issue.
Overall, libdvdcss is a useful library for the use of encrypted DVDs in a variety of circumstances. In addition to its use in video players under alternative operating systems, it allows users of all operating systems to play imported DVDs, back up movies onto their computers, and rip and convert video from DVDs for use on portable media players including the iPod and Playstation Portable. It is included in the cross-platform VLC player and widely available from a variety of sites, including the original developers' website at http://www.videolan.org/ .