Disk encryption software
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Disk encryption software is computer security software that is designed to protect the confidentiality of the data stored on a computer disk by utilizing a technique called disk encryption. Compared to access restrictions commonly enforced by an OS, this technique allows data be to be protected even when the OS is not active, for example, if data are read directly from the hardware. Such software encrypts data stored on a computer's mass storage and transparently decrypts the information when an authorized user requests it: no special action by the user (except supplying a password or passphrase at the beginning of a session) is required. Some also provide plausible deniability with deniable encryption techniques.
The volume-level encryption is particularly suited to portable devices such as laptop computers and thumb drives. If used properly, someone finding a lost device will have access only to inaccessible encrypted files. A strong passphrase (e.g. five or more diceware words) is essential for full security.
While disk encryption software can transparently operate on an entire disk volume, a directory, or even a single file, non-transparent file encryption software only encrypts or decrypts whole, individual files and stores the decrypted file in a temporary file. Some examples of file encryption software are GNU Privacy Guard and PGP). File archivers, and even some text editors such as emacs or vi can encrypt files.
Common features 
This section lists features commonly found in disk encryption software, though not all functions may be provided in any given system.
Plausible deniability 
Some disk encryption systems offer two levels of Plausible Deniability, which might be useful in case a user is required to reveal the password of an encrypted volume.
Hidden volumes 
Hidden volumes are a steganographic feature that allows a second, "hidden", volume to be created within the free space of another "container" volume. The hidden volume can have its own separate file system and uses a different password, encryption algorithm, and encryption key from that of the host volume, referred to as the "outer" volume. Which volume is actually mounted depends on the password provided; if the password to the outer volume is provided, the outer volume is mounted, and likewise for the inner volume.
Once a hidden volume has been created inside another volume, the user will store important-looking information (but which the user does not actually mind revealing) on the outer volume, whereas sensitive information is stored within the hidden volume.
If the user is forced to reveal a password, the user can reveal the password to the outer volume, without disclosing the existence of the hidden volume. The hidden volume will not be compromised, if the user takes certain precautions in overwriting the free areas of the "host" disk.
No identifying features 
Volumes, be they stored in a file or a device/partition, may intentionally not contain any discernible "signatures" or unencrypted headers. As cipher algorithms are designed to be indistinguishable from a pseudorandom permutation without knowing the key, the presence of data on the encrypted volume is also undetectable unless there are known weaknesses in the cipher. This means that it is impossible to prove that any file or partition is an encrypted volume (rather than random data) without having the password to mount it. This characteristic also makes it impossible to determine if a volume contains another hidden volume.
A file hosted volume (as opposed to partitions) may look out of place in some cases since it will be entirely random data placed in a file intentionally. However, a partition or device hosted volume will look no different than a partition or device that has been wiped with a common disk wiping tool such as Darik's Boot and Nuke. One can plausibly claim that such a device or partition has been wiped to clear personal data.
Portable or "traveller mode" means the encryption software can be run without installation to the system hard drive. In this mode, the software typically installs a temporary driver from the portable media. Since it is installing a driver (albeit temporarily), administrative privileges are still required.
Resizable volumes 
Encrypted volumes contain "header" (or "CDB") data, which may be backed up. Overwriting these data will destroy the volume, so the ability to back them up is useful.
Restoring the backup copy of these data may reset the volume's password to what it was when the backup was taken.
Recovery of keys from computer memory 
A 2008 study found data remanence in dynamic random access memory (DRAM), again with data retention of seconds to minutes at room temperature and much longer times when memory chips were cooled to low temperature. The study authors were able to demonstrate a cold boot attack to recover cryptographic keys for several popular disk encryption systems despite some memory degradation, by taking advantage of redundancy in the way keys are stored after they have been expanded for efficient use. The authors recommend that computers be powered down, rather than be left in a "sleep" state, when not under physical control by the computer's legitimate owner. This method of key recovery however, is suited for controlled laboratory settings and is extremely impractical for "field" use due to the equipment and cooling systems required.
- MBR Master Boot record is typically left unencrypted by Encryption Software
See also 
- Disk encryption hardware
- Comparison of disk encryption software
- Data remanence
- Disk & Full disk encryption
- On-The-Fly Encryption
- Cold boot attack
- Single sign-on
- United States v. Boucher
- Plausible Deniability - FreeOTFE instructions for initializing an encrypted disk such that the presence of a hidden disk cannot be detected
- This is a design criterion of modern ciphers; in other words, ciphers are considered broken if their output is discernible from random.
Mihir Bellare, Phillip Rogaway (2005-09-20). "Chapter 3: Pseudorandom functions". Introduction to Modern Cryptography. p. 7. Retrieved 2007-09-30.
- J. Alex Halderman, et al. (February 2008). Lest We Remember: Cold Boot Attacks on Encryption Keys.
- Hard Disk Encryption at the Open Directory Project
- Detailed wiki-style article on disk encryption at infoanarchy.org
- Inside NetBSDs CGD
- Gradian Systems Ltd - Independent security consultants specialising in encryption, authentication, e-mail and enterprise compliance.
- Encrypting filesystems under Linux - HOWTO
- On-The-Fly Encryption: A Comparison - Reviews and lists the different features of many disk encryption systems
- Buyer's Guide to Full Disk Encryption - Overview of full-disk encryption, how it works, and how it differs from file-level encryption -- plus an overview of leading full-disk encryption software.