TrueCrypt is a software application used for on-the-fly encryption (OTFE). It is free (in terms

Gratis versus Libre

Gratis versus libre is the distinction between two meanings of the English adjective "free"; namely, "for zero price" and "with little or no restriction"...

 of cost and commercial usage) and open source. It can create a virtual encrypted disk within a file or encrypt a partition

Disk partitioning

Disk partitioning is the act of dividing a hard disk drive into multiple logical storage units referred to as partitions, to treat one physical disk drive as if it were multiple disks. Partitions are also termed "slices" for operating systems based on BSD, Solaris or GNU Hurd...

 or (under MS Windows except Windows 2000

Windows 2000

Windows 2000 is a line of operating systems produced by Microsoft for use on personal computers, business desktops, laptops, and servers. Windows 2000 was released to manufacturing on 15 December 1999 and launched to retail on 17 February 2000. It is the successor to Windows NT 4.0, and is the...

) the entire storage device

Data storage device

thumb|200px|right|A reel-to-reel tape recorder .The magnetic tape is a data storage medium. The recorder is data storage equipment using a portable medium to store the data....

 (pre-boot authentication

Pre-boot authentication

Pre-Boot Authentication or Power-On Authentication serves as an extension of the BIOS or boot firmware and guarantees a secure, tamper-proof environment external to the operating system as a trusted authentication layer...

).
TrueCrypt supports Microsoft Windows

Microsoft Windows

Microsoft Windows is a series of operating systems produced by Microsoft.Microsoft introduced an operating environment named Windows on November 20, 1985 as an add-on to MS-DOS in response to the growing interest in graphical user interfaces . Microsoft Windows came to dominate the world's personal...

, Mac OS X

Mac OS X

Mac OS X is a series of Unix-based operating systems and graphical user interfaces developed, marketed, and sold by Apple Inc. Since 2002, has been included with all new Macintosh computer systems...

, and Linux

Linux

Linux is a Unix-like computer operating system assembled under the model of free and open source software development and distribution. The defining component of any Linux system is the Linux kernel, an operating system kernel first released October 5, 1991 by Linus Torvalds...

 operating systems (using FUSE

Filesystem in Userspace

Filesystem in Userspace is a loadable kernel module for Unix-like computer operating systems that lets non-privileged users create their own file systems without editing kernel code...

). Both 32-bit

32-bit

The range of integer values that can be stored in 32 bits is 0 through 4,294,967,295. Hence, a processor with 32-bit memory addresses can directly access 4 GB of byte-addressable memory....

 and 64-bit

64-bit

64-bit is a word size that defines certain classes of computer architecture, buses, memory and CPUs, and by extension the software that runs on them. 64-bit CPUs have existed in supercomputers since the 1970s and in RISC-based workstations and servers since the early 1990s...

 versions of these operating systems are supported, except for Windows IA-64

Itanium

Itanium is a family of 64-bit Intel microprocessors that implement the Intel Itanium architecture . Intel markets the processors for enterprise servers and high-performance computing systems...

 (not supported) and Mac OS X 10.6 Snow Leopard (runs as a 32-bit process).