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Linux and DVD

The big film studios' desire to protect their assets, prompted in particular by the possibility that digital formats would make it easy to produce very high quality lossless copies, led to their demanding that the manufacturers of playback hardware and software adopt active methods of copy protection.

The first attempt to protect the movie studios' assets was in 1996 when the Content Scrambling System (CSS) was introduced for DVD. This relies on disc to drive authentication and, in the case of computer playback, drive to player software authentication, in addition to encryption of the video content. Commercial pressed DVDs have a hidden area, which can only be read by a device that is authenticated by valid licensed keys. The hidden area contains a table of encrypted disc keys, a title key, a disc key hash and a region code.

The proprietary CSS 40-bit stream cipher algorithm is relatively weak, partly because US security regulations in force at the time the scheme was drafted prohibited the export of strong ciphers with greater than 40 bit keys and partly because a structural weakness in the CSS encryption algorithm reduced the effective key length to only 16 bits. CSS was cracked fairly early on and the open source program DeCSS has since been widely used to create encryption-free hard disk images from commercial protected DVDs, which can then be copied to writeable DVD media. One previous limitation on copying was that commercial DVDs tended to be dual layer, where writeable DVDs were only single layer. This meant that to copy a DVD required re-compressing the video content to fit which is a time consuming process and would reduce the quality of the video. Now, with cheap dual layer writeable DVDs, that step is no longer necessary.

DeCSS, perhaps the most notorious CSS cracking software, was created by a group called Masters of Reverse Engineering (MoRE) while developing a DVD player for Linux. Since the introduction of the DMCA a US court has ruled that even providing a link to a distribution site for the DeCSS code is illegal.

There are several DVD players, or rather multimedia players for Linux – for example Xine the official multimedia player of the GNOME desktop environment based on xine-lib or Gstreamer, VideoLAN Client, MPlayer and Ogle – but all of the free open source players require either an 'illegal' cracked key, or at least an unofficial key, to be able to play CSS encrypted discs. These are not usually included within the player software but will be stored in a separate file, for example, Xine requires a third party plug-in: xine_d4d_plugin.

Other players such as the VideoLAN Client and MPlayer use the libdvdcss library for decoding, which does not use cracked keys, but rather a generated list of possible keys. If this fails it uses a brute force algorithm ignoring any region coding. This approach has not so far been legally challenged.

Compared to Windows DVD players, the software available for Linux may seem incomplete and often requires several plug-ins to approximate the feature count and functionality of the Windows players.

The main providers of DVD playback software for Windows have developed proprietary Linux DVD playback software with licensed keys and are therefore DRM compliant, but at present these products are only be available to OEMs – CyberLink – PowerDVD Linux, PowerCinema Linux and InterVideo (Corel) LinDVD. This may be because these proprietary software companies do not feel confident they can sell a proprietary product to the niche open source users' market where users do not expect to pay for software. Providing support to customers who might be using any one of the hundreds of Linux distributions could also seem unattractive. Selling only to OEMs allows these proprietary developers to monetise their product.

Only recently, in July of this year, Fluendo, the Spanish company that sells the bundle of licensed CODEC plug-ins, released The Fluendo DVD player for Linux at a cost of 19.99 euros. Although this player is based on the GStreamer framework open source code, it is proprietary, since the user is paying a licence fee for the legal use of the associated CODECs. According to the companies website the Fluendo DVD Player only supports MPEG 2 and Dolby Digital Audio, the minimum required for playing the majority of DVD titles.

Next: HDCP and HDMI

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