### Question 54. What is a Block Cipher?

A *block cipher* transforms a fixed-length block of plaintext data
into a block of ciphertext data of the same length. This transformation
takes place under the action of a user-provided secret key. Decryption
is performed by applying the reverse transformation to the ciphertext block
using the same secret key. The fixed length is called the *block size*,
and for many block ciphers, the block size is 64 bits.

Since different plaintext blocks are mapped to different ciphertext
blocks (to allow *unique* decryption), a block cipher effectively
provides a permutation of the set of all possible messages. The permutation
effected during any particular encryption is of course secret, since it
is a function of the secret key.

More information about block ciphers and the various available algorithms
can be found in almost any book on contemporary cryptography and in RSA
Laboratories Technical Report [Rob95a].