WARNING: It is no longer secure to use TrueCrypt as it may contain security vulnerabilities!
“The development of TrueCrypt was ended in 5/2014 after Microsoft terminated support of Windows XP. Windows 8/7/Vista and later offer integrated support for encrypted disks and virtual disk images. Such integrated support is also available on other platforms. You should migrate any data encrypted by TrueCrypt to encrypted disks or virtual disk images supported on your platform.” - the TrueCrypt team.
TrueCrypt is an open source and cross platform software project designed from the ground up to be used as a universal solution for encrypting hard disk drives (HDD) or solid state drives (SSD) on-the-fly (the encryption and decryption process is done automatically by the software, without any user intervention).
The application allows you to easily create a virtual encrypted disk within a file, which will be mounted as a real disk drive, encrypt any local or portable devices, including HDD (Hard Disk Drive), SSD (Solid State Drive) or USB flash drives.
It supports state-of-the-art encryption algorithms, such as AES-256, Serpent and Twofish, and uses the XTS (XEX-based tweaked-codebook mode with ciphertext stealing) disk encryption mode.
The entire encryption/decryption process is automatic, takes place in real-time (on-the-fly) and it’s completely transparent. Two different levels of plausible deniability are provided to the user in case he or she is forced to reveal the password for the encrypted device, hidden volume (steganography), and missing identification.
TrueCrypt is a multi-platform software, which means that it will work the same on all mainstream operating systems, including GNU/Linux, Microsoft Windows and Mac OS X, allowing users to manage their encrypted drives independent of an operating system.
You can download the latest version of TrueCrypt as binary packages, or a source archive. It supports both 32-bit and 64-bit hardware platforms and can be used in any Linux-based operating system.