TrueCrypt 5.1

TrueCrypt is a software system for establishing and maintaining an on-the-fly-encrypted volume (data storage device). On-the-fly encryption means that data are automatically encrypted or decrypted right before they are loaded or saved, without any user intervention. No data stored on an encrypted volume can be read (decrypted) without using the correct password/keyfile(s) or correct encryption keys. Entire file system is encrypted (e.g.., file names, folder names, contents of every file, free space, meta data, etc). Files can be copied to and from a mounted TrueCrypt volume just like they are copied to/from any normal disk (for example, by simple drag-and-drop operations). Files are automatically being decrypted on-the-fly (in memory/RAM) while they are being read or copied from an encrypted TrueCrypt volume. Similarly, files that are being written or copied to the TrueCrypt volume are automatically being encrypted on-the-fly (right before they are written to the disk) in RAM.

What's New in version 5.1:

New features:

Support for hibernation on computers where the system partition is encrypted (previous versions of TrueCrypt prevented the system from hibernating when the system partition was encrypted). (Windows Vista/XP/2008/2003)
Ability to mount a partition that is within the key scope of system encryption without pre-boot authentication (for example, a partition located on the encrypted system drive of another operating system that is not running). (Windows Vista/XP/2008/2003)

Note: This can be useful e.g. when there is a need to back up or repair an operating system encrypted by TrueCrypt (from within another operating system).
Command line options for creating new volumes. (Linux and Mac OS X)

Improvements:

Increased speed of AES encryption/decryption (depending on the hardware platform, by 30-90%). (Windows)
Faster booting when the system partition is encrypted. (Windows Vista/XP/2008/2003)
When the system partition/drive is encrypted, the TrueCrypt Boot Loader is now stored in a compressed form and is, therefore, smaller. If a non-cascade encryption algorithm is used (i.e., AES, Serpent, or Twofish), the TrueCrypt Boot Loader is now small enough so that a backup of the TrueCrypt Boot Loader can be (and is) stored in first drive cylinder. Whenever the TrueCrypt Boot Loader is damaged, its backup copy is run automatically instead.

As a result of this improvement, the following problem will no longer occur: Certain inappropriately designed activation software (used for activation of some third-party software) writes data to the first drive cylinder, thus damaging the TrueCrypt Boot Loader. The affected users had to use the TrueCrypt Rescue Disk to repair the TrueCrypt Boot Loader. This will no longer be necessary after upgrading to this version of TrueCrypt (provided that the system partition/drive is encrypted using a non-cascade encryption algorithm, i.e., AES, Serpent, or Twofish).

Note: If your system partition/drive is currently encrypted using a non-cascade encryption algorithm (i.e., AES, Serpent, or Twofish), a backup copy of the TrueCrypt Boot Loader will be automatically stored in the first drive cylinder when you upgrade to this version of TrueCrypt.
The minimum memory requirements for the TrueCrypt Boot Loader have been reduced from 42 KB to 27 KB (twenty-seven kilobytes). This allows users to encrypt system partitions/drives on computers where the BIOS reserves a large amount of memory. (Windows Vista/XP/2008/2003)
Many other minor improvements. (Windows, Mac OS X, and Linux)

Resolved incompatibilities:

On some computers, when performing the system encryption pretest, Windows failed to display the log-on screen. This will no longer occur. (Windows Vista/XP/2008/2003)

Bug fixes:

On some systems, drive letters were not correctly assigned to newly mounted non-system volumes. This will no longer occur. (Windows)
Many other minor bug fixes. (Windows, Mac OS X, and Linux)