TrueCrypt is a free software application used for transparent real-time on-the-fly encryption. It can create a virtual encrypted disk within a file, or a device-hosted encrypted volume on either an individual partition or an entire storage device. It can encrypt the Windows boot partition or entire boot drive, and has the ability to create and run a hidden encrypted operating system whose existence is deniable. It is open source software, distributed under the TrueCrypt Collective License. TrueCrypt natively runs under Microsoft Windows, with support for Mac OS X and Linux.