Rijndael Encryption

Overview

About the Advanced Encryption Standard (AES)

The DES algorithm has become obsolete and is in need of replacement. To this end the National Institute of Standards and Technology (NIST) has been holding a competition to develop the Advanced Encryption Standard (AES) as a replacement for DES. Triple DES has been endorsed by NIST as a temporary standard to be used until the AES is finished sometime in 2001.

NIST has been working very closely with industry and the cryptographic community during the development of the Advanced Encryption Standard. The overall goal is to develop a Federal Information Processing Standard (FIPS) that specifies an encryption algorithm (or algorithms) capable of protecting sensitive government information well into the next century. The algorithm(s) is expected to be used by the U.S. Government and, on a voluntary basis, by the private sector.

On January 2, 1997, NIST announced the initiation of the AES development effort and made a formal call for algorithms on September 12 of that year. The call stipulated that the AES would specify an unclassified, publicly disclosed encryption algorithm(s), available royalty-free, worldwide. In addition, the algorithm(s) must implement symmetric key cryptography as a block cipher and (at a minimum) support block sizes of 128 bits and key sizes of 128, 192, and 256 bits.

On August 20, 1998, NIST announced a group of fifteen AES candidate algorithms at the First AES Candidate Conference (AES1). These algorithms had been submitted by members of the cryptographic community from around the world. At that conference and in a simultaneously published Federal Register notice, NIST solicited public comments on the candidates. A Second AES Candidate Conference (AES2) was held in March 1999 to discuss the results of the analysis conducted by the global cryptographic community on the candidate algorithms. The public comment period on the initial review of the algorithms closed on April 15, 1999. Using the analyses and comments received, NIST selected five algorithms from the original fifteen submissions.

The AES finalist candidate algorithms are MARS, RC6, Rijndael, Serpent, and Twofish. Four of the algorithms (MARS, Rijndael, Serpent, and Twofish) are supported by Private Encryptor. NIST has developed a Round 1 Report describing the selection of the finalists.

These finalist algorithms received further analysis during a second, more in-depth review period prior to the selection of the final algorithm(s) for the AES. The comment period on the remaining algorithms ended on May 15, 2000. Comments and analysis were actively sought by NIST on any aspect of the candidate algorithms, including, but not limited to, the following topics: cryptanalysis, intellectual property, comparative analyses of all of the AES finalists, and overall recommendations and implementation issues. An informal AES discussion forum was also provided by NIST for interested parties to discuss the AES finalists and relevant AES issues.

Near the end of Round 2, NIST sponsored the Third AES Candidate Conference (AES3), which was an open, public forum for discussion of the analyses of the AES finalists. AES3 was held April 13-14, 2000 in New York. Submitters of the AES finalists were invited to attend and engage in discussions regarding comments on their algorithms.

At the time this document is being written, the Round 2 public analysis period is just ending. Over the next few months NIST intends to study all available information from the Round 2 analysis and make a selection for the AES from among one or more of the finalists. Currently, NIST anticipates that it will announce the AES selection by late summer or early fall of 2000. No date has yet been set for this announcement. Following the announcement, NIST intends to publish a Round 2 Report that will summarize information from Round 2 and explain the algorithm selection.

Shortly thereafter, a draft Federal Information Processing Standard (FIPS) for the AES will be published for public review and comment. Following the comment period, the standard will be revised by NIST in response to those comments. A review and approval process will then follow. If all steps of the AES development process proceed as planned, it is anticipated that the standard will be completed by the summer of 2001.

About Rijndael

The block cipher Rijndael was designed by Joan Daemen and Vincent Rijmen as a candidate for the Advanced Encryption Standard. It is one of the five finalists chosen by NIST from a field of 15 candidates as explained above. The design of Rijndael was strongly influenced by the design of the block cipher Square, which was also created by Joan Daemen and Vincent Rijmen. The name of the algorithm is a combination of the names of its two creators. The Rijndael web page jokes that the name Rijndael was used "because we were both fed up with people mutilating the pronunciation of the names 'Daemen' and 'Rijmen'". The algorithm can be implemented very efficiently on a wide range of processors and in hardware. Like all AES candidates, Rijndael is very secure and has no known weaknesses.

In Depth

Rijndael's key length is defined to be either 128, 192, or 256 bits in accordance with the requirements of the AES. Note that unlike Serpent and Twofish, the key size must be one of these values; it is not allowed to be arbitrary. Also unlike other AES candidates, Rijndael has a variable block length of either 128, 192, or 256 bits. All nine combinations of key length and block length are possible, although the official AES block size is 128 bits. Both block length and key length can be extended very easily to multiples of 32 bits. The number of rounds, or iterations of the main algorithm, can vary from 10 to 14 and is dependent on the block size and key length. The low number of rounds has been one of the main criticisms of Rijndael, but if this ever becomes a problem the number of rounds can easily be increased at little extra cost by increasing the block size and key length.

Private Encryptor's implementation of Rijndael uses a 256 bit key and a 128-bit block size. We decided to use the largest possible key size to ensure that the user always enjoys the best possible security. Our design philosophy is that security always comes before speed. If a shorter key is provided by the user, Private Encryptor pads the key in a special, seemingly random, way to make it 256 bits long. And although block sizes greater than 128 bits are supported, there is no compelling reason to use a larger block size. We feel that it is best to stick to the standard size used by the other AES algorithms, which will be much more widely used than the other, non-standard, sizes.

The detailed description of the actual algorithm is contained in the official Rijndael paper submitted for the AES by the algorithm's designers. The paper is rather technical and a certain degree of mathematical proficiency is required of the reader in order to understand it.

Download the Rijndael algorithm specification: Rijndael.pdf

Download the MARS algorithm specification: MARS.pdf

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