FairPlay is the DRM scheme used for iTunes from Apple. It is designed to limit both the use of the file to specific machines and limit the number of copies that can be made. It is used on iBooks and on music (AAC) and video formats.
 How it works
- Prior to buying content from the iTunes Store, a user has to create an account with Apple's servers and then authorize a PC or Mac running iTunes.
- During authorization, iTunes creates a globally unique ID number for the computer it is running on, then sends it to Apple's servers, where it is assigned to the user's iTunes account. Five different machines can be authorized.
- When a user buys a song from the iTunes Store, a user key is created for the purchased file. The AAC song itself is scrambled using a separate master key, which is then included into the protected AAC song file. The master key is locked using the user key, which is both held by iTunes and also sent to Apple’s servers.
- Protected, purchased content is locked within iTunes; songs are not scrambled on Apple's server. This speeds and simplifies the transaction by delegating that work to iTunes on the local computer.
- The result is an authorization system that does not require iTunes to verify each song with Apple as it plays. Instead, iTunes maintains a collection of user keys for all the purchased tracks in its library.
- To play a protected AAC song, iTunes uses the matching user key to unlock the master key stored within the song file, when is then used to unscramble the song data.
- Every time a new track is purchased, a new user key may be created; those keys are all encrypted and stored on the authorized iTunes computer, as well as being copied to Apple's servers.
- When a new computer is authorized, it also generates a globally unique ID number for itself and sends it to Apple, which stores it as one of the five authorizations in the user account.
- Apple's server sends the newly authorized machine the entire set of user keys for all the tracks purchased under the account, so all authorized systems will be able to play all purchased songs.
It only works with items purchased at iTunes so third parties cannot use it. Customers cannot use other DRM based products.