JHymn

All the music you buy on iTunes is protected by Digital Rights Management (DRM), which makes it impossible to play it with other software applications or to transfer it to portable devices and listen to it. Basically, it locks the songs on your computer only, so you can't share and distribute it, which would be a violation of copyright law.
JHymn is a small utility designed to decrypt protected music, so it can be processed on operating systems on which iTunes does not function, such as Linux-based computers. Removing DRM protections will also allow you to play the music in any AAC-capable software tool and to make backups of the songs you purchased.

Portable tool with a user-friendly interface

This small program requires no installation and its ease of use is suggested by the simple and classic look. It allows you to add files individually or entire folders by drag and drop or via a custom dialogue. There are three input file types supported by the application, namely: M4P, M4B and M4A and MP3 and WAV for the output. Unless you specify an encoder, iTunes or QuickTime are used by default for converting the audio files and removing the DRM protections.

Easily configure settings

To spice things up, JHymn lets you add a custom signature to your songs, which can be personalized from the Preferences window of the application. This is also the place you need to access in order to customize the MP3 quality, which can vary from 24 kbps (mono) to 320 kbps stereo. There is the possibility to configure your own preset.
JHymn also has the means to download authorization keys from the Apple Store (you need to log in to the Music Store for this), but the application claims that it automatically detects new keys and downloads them for you.

Conclusion

Even though removing DRM protections potentially entitles you to all sorts of activities, JHymn should be used for legal purposes only. It was built to allow you to play the music you bought on any device and to unlock it from the limitations that prevent you from fully enjoying it.