Advanced Encryption Standard

AES redirects here. For other meanings, see AES (disambiguation).

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 sound file linked 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, Rijndael is a substitution-permutation network, not a Feistel 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.

AES operates on a 4×4 array of bytes, termed the state (versions of Rijndael with a larger block size have additional columns in the state). For encryption, each round of AES (except the last round) consists of four stages:

1. SubBytes — a non-linear substitution step where each byte is replaced with another according to a lookup table.
2. ShiftRows — a transposition step where each row of the state is shifted cyclically a certain number of steps.
3. MixColumns — a mixing operation which operates on the columns of the state, combining the four bytes in each column using a linear transformation.
4. AddRoundKey — each byte of the state is combined with the round key; each round key is derived from the cipher key using a key schedule.

The final round omits the MixColumns stage.

The SubBytes step

In the SubBytes step, each byte in the array is updated using an 8-bit S-box. This operation provides the non-linearity in the cipher. The S-box used is derived from the inverse function over GF(2⁸), known to have good non-linearity properties. To avoid attacks based on simple algebraic properties, the S-box is constructed by combining the inverse function with an invertible affine transformation. The S-box is also chosen to avoid any fixed points (and so is a derangement), and also any opposite fixed points.

The ShiftRows step

The ShiftRows step operates on the rows of the state; it cyclically shifts the bytes in each row by a certain offset. For AES, the first row is left unchanged. Each byte of the second row is shifted one to the left. Similarly, the third and fourth rows are shifted by offsets of two and three respectively. In this way, each column of the output state of the ShiftRows step is composed of bytes from each column of the input state. (Rijndael variants with a larger block size have slightly different offsets).

The MixColumns step

In the MixColumns step, the four bytes of each column of the state are combined using an invertible linear transformation. Together with ShiftRows, MixColumns provides diffusion in the cipher. Each column is treated as a polynomial over GF(2⁸) and is then multiplied modulo <math>x^4+1<math> with a fixed polynomial <math>c(x)<math>.

The AddRoundKey step

In the AddRoundKey step, the subkey is combined with the state. For each round, a subkey is derived from the main key using the key schedule; each subkey is the same size as the state. The subkey is added by combining each byte of the state with the corresponding byte of the subkey using bitwise XOR.

Security

As of 2005, 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." — [1]

This marks the first time that the public has had access to a cipher approved by NSA for TOP SECRET information. It is interesting to note that many public products use 128-bit secret keys by default; it is possible that NSA suspects a fundamental weakness in keys this short, or they may simply prefer a safety margin for top secret documents (which may require security decades into the future).

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 cryptographic "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. The largest publically-known brute-force attack has been against a 64 bit RC5 key by distributed.net.

Another concern is the mathematical structure of AES. Unlike most other block ciphers, AES has a very neat mathematical description [2], [3]. 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.

In April 2005, D.J. Bernstein announced a cache timing attack that he claims breaks most real-world AES implementations, and applied it to break a custom server using the OpenSSL library's AES encryption routines. The attack appears to require a large number of packets of data to perform; in the paper, D.J. Bernstein sent over 200 million packets to the victim server. The attack may not be practical against real-world implementations. [4] D.J. Bernstein claims that the attack is hard to defend against because of the AES structure.

See also

• Advanced Encryption Standard process

External links

• The Rijndael Page
• Literature survey on AES
• Recordings of the pronunciation of "Rijndael" (85 KB wav file)
• The archive of the old official AES website
• FIPS PUB 197: the official AES standard (PDF file)
• AES4 — the fourth AES conference
• John Savard's description of the AES algorithm

Implementations

• A Javascript AES calculator showing intermediate values
• Brian Gladman's BSD licensed implementations of AES
• Pablo Barreto's public domain C implementation of AES
• D.J. Bernstein's public-domain implementation of AES
• GPL-licensed optimized Rijndael source code in C
• The GPL-licensed Nettle library also includes an AES implementation

Notes

^  The Rijndael algorithm supports block sizes of 128, 160, 192, 224, and 256 bits. Only the 128-bit block size is specified in the AES standard, however. ^  The Rijndael algorithm supports key sizes of 128, 160, 192, 224, and 256 bits. Only the 128, 192, and 256 bit key sizes are specified in the AES standard, however.

References

• Nicolas Courtois, Josef Pieprzyk, "Cryptanalysis of Block Ciphers with Overdefined Systems of Equations". pp267–287, ASIACRYPT 2002.
• Joan Daemen and Vincent Rijmen, "The Design of Rijndael: AES – The Advanced Encryption Standard." Springer-Verlag, 2002. ISBN 3540425802.
• Niels Ferguson, John Kelsey, Stefan Lucks, Bruce Schneier, Michael Stay, David Wagner and Doug Whiting: Improved Cryptanalysis of Rijndael. FSE 2000, pp213–230