Wikipedia - Advanced Encryption Standard, Security:
Until May 2009, the only successful published attacks against the full AES were side-channel attacks on specific implementations. 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 to protect classified information:
Citat:
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."[8]
AES has 10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys. By 2006, the best known attacks were on 7 rounds for 128-bit keys, 8 rounds for 192-bit keys, and 9 rounds for 256-bit keys.[9]
For cryptographers, a cryptographic "break" is anything faster than an exhaustive search. Thus, an XSL attack against a 128-bit-key AES requiring 2100 operations (compared to 2128 possible keys) would be considered a break. The largest successful publicly-known brute force attack has been against a 64-bit RC5 key by distributed.net.
Unlike most other block ciphers, AES has a very neat algebraic description.[10] In 2002, a theoretical attack, termed the "XSL attack", was announced by Nicolas Courtois and Josef Pieprzyk, purporting to show a weakness in the AES algorithm due to its simple description.[11] Since then, other papers have shown that the attack as originally presented is unworkable; see XSL attack on block ciphers.
During the AES process, developers of competing algorithms wrote of Rijndael, "...we are concerned about [its] use...in security-critical applications."[12] However, at the end of the AES process, Bruce Schneier, a developer of the competing algorithm Twofish, wrote that while he thought successful academic attacks on Rijndael would be developed someday, "I do not believe that anyone will ever discover an attack that will allow someone to read Rijndael traffic."[13]
On July 1, 2009, Bruce Schneier blogged about a related-key attack on the 192-bit and 256-bit versions of AES discovered by Alex Biryukov and Dmitry Khovratovich; the related key attack on the 256-bit version of AES exploits AES' somewhat simple key schedule and has a complexity of 2119. This is a follow-up to an attack discovered earlier in 2009 by Alex Biryukov, Dmitry Khovratovich, and Ivica Nikolic, with a complexity of 296 for one out of every 235 keys.
Another attack was blogged by Bruce Schneier on July 30, 2009 and published on August 3, 2009. This new attack, by Alex Biryukov, Orr Dunkelman, Nathan Keller, Dmitry Khovratovich, and Adi Shamir, is against AES-256 that uses only two related keys and 239 time to recover the complete 256-bit key of a 9-round version, or 245 time for a 10-round version with a stronger type of related subkey attack, or 270 time for a 11-round version. 256-bit AES uses 14 rounds, so these attacks aren't effective against full AES.
In November 2009, there exists the first published attack against the 8-round version of AES-128. This known-key distinguishing attack is an improvement of the rebound or the start-from-the-middle attacks for AES-like permutations, which view two consecutive rounds of permutation as the application of a so-called Super-Box. It works on the 8-round version of AES-128, with a computation complexity of 2^48, and a memory complexity of 2^32.[14]