Once a CD has been ripped and converted to an MP3 file the content owner rapidly loses control over the file – it can easily be shared out on P2P networks, downloaded to portable MP3 players, etc.

To allow copyright owners to retain control of their files several companies (Real, Microsoft and others) have developed a number of copy-prevention solutions. These solutions include:

• preventing the ripping of the file from CD (usually done by encoding CDs with non-standard markings),
• preventing the copying of the file to another computer (usually done by linking the media file to a hidden file on the hard drive, which won’t be present if the media file is copied to another computer),
• allowing free copying of files, but only allowing the file to be played by users who have permission to do so.

It is this last solution that’s of interest to us – you want P2P users to freely distribute your files to all their friends, but you don’t want their friends to be able to play the files until they’ve obtained your permission to do so, which might involve them signing up to your email list, allowing you to tell them about your upcoming CD, or having them pay you a few dollars for the ability to play your music track.

In brief, the Microsoft Windows Media DRM solution, which we use for our audio and video files, works as follows:

1. You create a DRM-protected Windows Media files using software tools available from Microsoft.
2. You freely distribute these DRM-protected files on web sites, P2P networks, etc.
3. The files are encrypted, and the only way to gain access to the contents of the file (i.e. to play it) is to supply Windows Media Player with a License Key (really just a big number) that unlocks the file.
4. The License Key that you need to supply to Windows Media Player is locked to your computer – this means that if you email the license key to a friend it won’t work on the friend’s PC. The only way for the friend to be able to play the file is to go back to the web site which issued the license key and obtain a new license key that’s keyed to their PC.
5. People are less likely pay to play a file if they haven’t had some taste of what’s in the file, so Microsoft have thoughtfully provided a very flexible license key system – for instance you can create files that you can play for free once only, or that you can play the first minute for free, or that can be played free for 30 days, etc. For more on this see DRM License Options
6. If you click to play a Windows Media file that needs a license, where does Windows Media Player know to go to get a license? Well, when you package the file (i.e. when you DRM-protect it), you encode in the file a URL, and Windows Media Player simply launches this URL whenever you click to play a Windows Media file that you don’t have a license to play. In order to be able to issue a license key, the web server at this URL needs to be running Microsoft’s License Server software, which can then issue a license key according to whatever rules the content owner stipulates. Some content owners might want payment for a license key, others might give out the license key for free but try to sign up users for other premium content that’s available on the site – the choice is yours. See our Licensing Examples for some ideas.