TrueCrypt - Free Open-Source disk encryption software

Features

• Creates a virtual encrypted disk within a file and mounts it as a real disk.
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• 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.
  
  
• Parallelization and pipelining allow data to be read and written as fast as if the drive was not encrypted.
  
  
• Provides plausible deniability, in case an adversary forces you to reveal the password:
  
  Hidden volume (steganography) and hidden operating system.
  
• Encryption algorithms: AES-256, Serpent, and Twofish. Mode of operation: XTS.
  
  Further information regarding features of the software may be found in the documentation.