In 1997, the National Institute of Standards and Technology put out a call for candidates to replace DES. Among the requirements were that the new algorithm should allow key sizes of 128, 192, and 256 bits, it should operate on blocks of 128 input bits, and it should work on a variety of different hardware, for example, eight-bit processors that could be used in smart cards and the 32-bit architecture commonly used in personal computers. Speed and cryptographic strength were also important considerations. In 1998, the cryptographic community was asked to comment on 15 candidate algorithms. Five finalists were chosen: MARS (from IBM), RC6 (from RSA Laboratories), Rijndael (from Joan Daemen and Vincent Rijmen), Serpent (from Ross Anderson, Eli Biham, and Lars Knudsen), and Twofish (from Bruce Schneier, John Kelsey, Doug Whiting, David Wagner, Chris Hall, and Niels Ferguson). Eventually, Rijndael was chosen as the Advanced Encryption Standard. The other four algorithms are also very strong, and it is likely that they will used in many future cryptosystems.

As with other block ciphers, Rijndael can be used in several modes, for example, ECB, CBC, CFB, OFB, and CTR (see Section 6.3).

Before proceeding to the algorithm, we answer a very basic question: How do you pronounce Rijndael? We quote from their Web page:

If you’re Dutch, Flemish, Indonesian, Surinamer or South-African, it’s pronounced like you think it should be. Otherwise, you could pronounce it like “Reign Dahl,” “Rain Doll,” “Rhine Dahl”. We’re not picky. As long as you make it sound different from “Region Deal.”