Disk encryption software
Disk encryption generally refers to wholesale encryption that operates on an entire volume mostly transparently to the user, the system, and applications. This is generally distinguished from file-level encryption that operates by user invocation on a single file or group of files, and which requires the user to decide which specific files should be encrypted. Disk encryption usually includes all aspects of the disk, including directories, so that an adversary cannot determine content, name or size of any file. It is well suited to portable devices such as laptop computers and thumb drives which are particularly susceptible to being lost or stolen. If used properly, someone finding a lost device cannot penetrate actual data, or even know what files might be present.
Block cipher
A block cipher consists of two paired algorithms, one for encryption, "E", and the other for decryption, "D". Both algorithms accept two inputs: an input block of size "n" bits and a key of size "k" bits; and both yield an "n"-bit output block. The decryption algorithm "D" is defined to be the inverse function of encryption, i.e., "D" = "E". More formally, a block cipher is specified by an encryption function
which takes as input a key "K" of bit length "k", called the "key size", and a bit string "P" of length "n", called the "block size", and returns a string "C" of "n" bits. "P" is called the plaintext, and "C" is termed the ciphertext. For each "K", the function "E"("P") is required to be an invertible mapping on {0,1}. The inverse for "E" is defined as a function
taking a key "K" and a ciphertext "C" to return a plaintext value "P", such that