Block Cipher

In cryptography, a block cipher is a deterministic algorithm operating on fixed-length groups of bits, called blocks, with an unvarying transformation that is specified by a symmetric key. Block ciphers are important elementary components in the design of many cryptographic protocols, and are widely used to implement encryption of bulk data.

The modern design of block ciphers is based on the concept of an iterated product cipher. Product ciphers were suggested and analyzed by Claude Shannon in his seminal 1949 publication Communication Theory of Secrecy Systems as a means to effectively improve security by combining simple operations such as substitutions and permutations. Iterated product ciphers carry out encryption in multiple rounds, each which uses a different subkey derived from the original key. A widespread implementation of such ciphers is called a Feistel network, named after Horst Feistel, and notably implemented in the DES cipher. Many other realizations of block ciphers, such as the AES, are classified as substitution-permutation networks.

The publication of the DES cipher by the U.S. National Bureau of Standards (now National Institute of Standards and Technology, NIST) in 1977 was fundamental in the public understanding of modern block cipher design. In the same way, it influenced the academic development of cryptanalytic attacks. Both differential and linear cryptanalysis arose out of studies on the DES design. Today, there is a palette of attack techniques that a block cipher must be secure against, in addition to being robust against brute force attacks.

Even a secure block cipher is suitable only for the encryption of a single block under a fixed key. A multitude of modes of operations have been designed to allow their repeated use in a secure way, commonly to achieve the security goals of encryption and authentication. However, block ciphers may also be used as building blocks in other cryptographic protocols, such as universal hash functions and pseudo-random number generators.