The Hymn Project exists to break the iTunes mp4 copy-protection scheme, so you can hear the music you bought on any machine you want.
The purpose of the Hymn Project is to allow you to exercise your fair-use rights under copyright law. The various software provided on this web site allows you to free your iTunes Music Store purchases (protected AAC / .m4p) from their DRM restrictions with no loss of sound quality. These songs can then be played outside of the iTunes environment, even on operating systems not supported by iTunes and on hardware not supported by Apple.
Initially, the software recovered your iTunes password (your key, basically) from your hard drive. In response, Apple obfuscated the format and no one has yet figured out how to recover the keys cleanly. To get around this, they developed a program called FairKeys that impersonates iTunes and contacts the server. Since the iTunes client can still get your password, this works.
FairKeys … pretends to be a copy of iTunes running on an imaginary computer, one of the five computers that you’re currently allowed to authorize for playing your iTMS purchases. FairKeys logs into Apple’s web servers to get your keys the same way iTunes does when it needs to get new keys. At least for now, at this stage of the cat-and-mouse game, FairKeys knows how to request your keys and how to decode the response which contains your keys, and once it has those keys it can store them for immediate or future use by JHymn.
More security by inconvenience, and yet another illustration of the neverending arms race between attacker and defender.