AES
In cryptography, the Advanced Encryption Standard (AES), also known as Rijndael, is a block cipher adopted as an encryption standard by the US government, and is expected to be used worldwide and analysed extensively, as was the case with its predecessor, the Data Encryption Standard (DES). It was adopted by National Institute of Standards and Technology (NIST) as US FIPS PUB 197 in November 2001 after a 5-year standardisation process (see Advanced Encryption Standard process for more details).
The cipher was developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen, and submitted to the AES selection process under the name "Rijndael", a portmanteau comprised of the names of the inventors. Rijndael can be pronounced "Rhine dahl", a long "i" and a silent "e" ( IPA: [ɹaindal] ). In the wav below, it is pronounced [rʰaindau].
Development
Rijndael was a refinement of an earlier design by Daemen and Rijmen, Square; Square was a development from Shark.
Unlike its predecessor DES, it is not a Feistel network, but a substitution-permutation network. AES is fast in both software and hardware, is relatively easy to implement, and requires little memory. As a new encryption standard, it is currently being deployed on a large scale.
Description of the cipher
Strictly speaking, AES is not precisely Rijndael (although in practice they are used interchangeably) as Rijndael supports a larger range of block and key sizes; AES has a fixed block size of 128 bits and a key size of 128, 192 or 256 bits, whereas Rijndael can be specified with key and block sizes in any multiple of 32 bits, with a minimum of 128 bits and a maximum of 256 bits.
Along with the cipher itself, a document concerning "modes of operation" is also expected to be made an official standard. For a general article on that topic (not specific to AES) see Block cipher modes of operation.
Security
As of 2004, no successful attacks against AES have been recognised. The National Security Agency (NSA) reviewed all the AES finalists, including Rijndael, and stated that all of them were secure enough for US Government non-classified data. In June 2003, the US Government announced that AES may be used for classified information:
- "The design and strength of all key lengths of the AES algorithm (i.e., 128, 192 and 256) are sufficient to protect classified information up to the SECRET level. TOP SECRET information will require use of either the 192 or 256 key lengths. The implementation of AES in products intended to protect national security systems and/or information must be reviewed and certified by NSA prior to their acquisition and use." —
This marks the first time that the public has had access to a cipher approved by NSA for TOP SECRET information.
The most common way to attack block ciphers is to try various attacks on versions of the cipher with a reduced number of rounds. AES has 10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys. As of 2004, the best known attacks are on 7 rounds for 128-bit keys, 8 rounds for 192-bit keys, and 9 rounds for 256-bit keys (Ferguson et al, 2000).
Some cryptographers worry about the security of AES. They feel that the margin between the number of rounds specified in the cipher and the best known attacks is too small for comfort. The risk is that some way to improve these attacks might be found and that, if so, the cipher could be broken. In this meaning, a crytographic "break" is anything faster than an exhaustive search, so an attack against 128-bit key AES requiring 'only' 2120 operations would be considered a break even though it would be, now, quite infeasible. In practical application, any break of AES which is only this 'good' would be irrelevant. For the moment, such concerns can be ignored.
Another concern is the mathematical structure of AES. Unlike most other block ciphers, AES has a very neat mathematical description , . This has not yet led to any attacks, but some researchers are worried that future attacks may find a way to exploit this structure.
In 2002, a theoretical attack, termed the "XSL attack", was announced by Nicolas Courtois and Josef Pieprzyk, showing a potential weakness in the AES algorithm. It seems that the attack, if the mathematics is correct, is not currently practical as it would have a prohibitively high "work factor". There have been claims of considerable work factor improvement, however, so the attack technique might become practical in the future. On the other hand, several cryptography experts have found problems in the underlying mathematics of the proposed attack, suggesting that the authors have made a mistake in their estimates. Whether this line of attack can be made to work against AES remains an open question. For the moment, as far as is publicly known, the XSL attack against AES is speculative; it is unlikely that anyone could carry out the current attack in practice.
See also
