Block cipher

In cryptography, block ciphers are one of the two main types of symmetric cipher; they operate on fixed-size blocks of plaintext, giving a block of ciphertext for each. The other main type are stream ciphers, which generate a continuous stream of keying material to be mixed with messages.

The basic function of block ciphers is to keep messages or stored data secret; the intent is that an unauthorised person be completely unable to read the enciphered material. Block ciphers therefore use a key and are designed to be hard to read without that key. Of course an attacker's intent is exactly the opposite; he wants to read the material without authorisation, and often without the key. See cryptanalysis for his methods.

Among the best-known and most widely used block ciphers are two US government standards. The Data Encryption Standard (DES) from the 1970s is now considered obsolete; the Advanced Encryption Standard (AES) replaced it in 2002. To choose the new standard, the National Institute of Standards and Technology ran an AES competition. Fifteen ciphers were entered, five finalists selected, and eventually AES chosen. Text below gives an overview; for details of the process and the criteria, and descriptions of all fifteen candidates, see the AES competition article.

These standards greatly influenced the design of other block ciphers, and the latter part of this article is divided into sections based on that. DES and alternatives describes 20th century block ciphers, all with the 64-bit block size of DES. The AES generation describes the next generation, the first 21st century ciphers, all with the 128-bit block size of AES. Large-block ciphers covers a few special cases that do not fit in the other sections.