When Apple opened the iTunes Music Store, they licensed a technology called "FairPlay" from a company called "Veridisc". FairPlay is a Digital Rights Management (DRM) system that limits a users rights on a digital media file that they've purchased and presumably downloaded. In the case of Apple's iTunes Music Store, when a user downloads an audio track from iTMS, it is a "Protected AAC Audio File". When used as intended, these files can only be played through the iTunes program itself. Furthermore, a particular computer must first be "authorized" to play the given file. FairPlay allows up to three computers and unlimited Apple iPods to be authorized to play the file. As DRM schemes go, FairPlay is only moderately offensive.

So what will playfair do for you? The playfair program is quite simple. It takes one of the iTMS Protected AAC Audio Files, decodes it using a key obtained from your iPod or Microsoft Windows system and then writes the new, decoded version to disk as a regular AAC Audio File. It then optionally copies the metadata tags that describe the song, including the cover art, to the new file.

Most of the heavy lifting for this program is done by the mp4v2 and mp4ff libraries. More information about these libraries can be obtained from the FAAD project website.