TrueCrypt is a free open-source disk encryption software for Windows Vista/XP, Mac OS X, and Linux.
Main features as quoted from the website:
- Creates a virtual encrypted disk within a file and mounts it as a real disk.
- Encrypts an entire partition or storage device such as USB flash drive or hard drive.
- Encrypts a partition or drive where Windows is installed (pre-boot authentication).
- Encryption is automatic, real-time (on-the-fly) and transparent.
- Provides two levels of plausible deniability, in case an adversary forces you to reveal the password:
- Hidden volume (steganography) and hidden operating system.
- No TrueCrypt volume can be identified (volumes cannot be distinguished from random data).
- Encryption algorithms: AES-256, Serpent, and Twofish. Mode of operation: XTS.