Block cipher

A block cipher is a symmetric cipher that operates on fixed-size blocks of plaintext, giving a block of ciphertext for each. The other main type of cipher is a stream cipher, which generates a stream of keying material to be mixed with messages. Block ciphers can be used in various modes when multiple block are to be encrypted.

The Data Encryption Standard, DES, is among the the best known and most thoroughly analysed block ciphers. It was invented by IBM, and was made a US government standard for non-classified government data and for regulated industries such as banking, in the late 70s. From then until about the turn of the century, it was very widely used. However, it is now considered obsolete; its 56-bit key size makes it highly vulnerable to a brute force attack, given modern computers. Some applications still use Triple DES, a variant which applies DES three times with two or three different keys.

The generation of block ciphers which followed DES in the 80s and 90s -- such as Blowfish, CAST-128 and IDEA -- nearly all used 64-bit blocks, like DES, but all used 128-bit or longer keys for better resistance to brute force. Some of their design principles came from analysis of DES.

The Advanced Encryption Standard, AES, is the block cipher which replaced DES as a US government standard in 2000. It uses 128-bit blocks and supports key sizes up to 256 bits. NIST, the National Institute of Standards and Technology, ran a contest to find the right cipher for their new standard; there were entries from all over the world and an extensive analysis process. The winner, Rijndael, from two Belgian designers, became AES.

The Block Cipher Lounge [1] web site has more information.