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Boomerang attack against AES better than blind chance

Pesky algorithm not invulnerable

Internet Security Threat Report 2014

Updated Cryptographic researchers have uncovered a chink in the armour of the widely used AES algorithm.

The attacks pose no immediate threat to the security of AES, but they do illustrate a technique for extracting keys that is better than simply trying every possible key combination.

Instead of such a brute force approach, the researchers have derived a technique based on "finding local collisions in block ciphers and enhanced with the boomerang switching techniques to gain free rounds in the middle". Collisions in cryptographic happen when two different inputs produce the same output.

The approach, in this case, can be used to infer clues about the key used by the AES encryption cypher. AES is an encryption standard recently adopted by the US government, and widely used commercially as a result.

The paper, Related-key Cryptanalysis of the Full AES-192 and AES-256, by Alex Biryukov and Dmitry Khovratovich of the University of Luxembourg, can be found here.

Discovery of the assault is interesting to cryptographers but poses no immediate problems in practice, said noted cryptographer Bruce Schneier.

"While this attack is better than brute force - and some cryptographers will describe the algorithm as "broken" because of it - it is still far, far beyond our capabilities of computation," Schneier writes. "The attack is, and probably forever will be, theoretical. But remember: attacks always get better, they never get worse.

"While there's no reason to panic, no reason to stop using AES, no reason to insist that NIST choose another encryption standard, this will certainly be a problem for some of the AES-based SHA-3 candidate hash functions."

SHA-3 is the topic of an ongoing competition to develop a next generation hash function, which is important in digital signatures and other cryptographic functions. One candidate for the SHA-3 function, developed by Ron Rivest (the R in RSA), and dubbed MD6, has been effectively taken out of consideration by its developers.

In order to adapt MD6 to work quickly enough on lower power systems compromises might have been made that meant it was less than mathematically bulletproof, as the developers explain (extract below). Contrary to our initial report MD6 was not withdrawn from the competition, though its developers are suggesting that better alternatives might exist among the other candidates.

We are not withdrawing our submission; NIST is free to select MD6 for further consideration in the next round if it wishes. But at this point MD6 doesn't meet our own standards for what we believe should be required of a SHA-3 candidate, and we suggest that NIST might do better looking elsewhere. In particular, we feel that a minimum "ticket of admission" for SHA-3 consideration should be a proof of resistance to basic differential attacks, and we don't know how to make such a proof for a reduced-round MD6.

Rivest and his team say that MD6 might yet be revived via some clever tweak, but admit this is a bit of a long-shot. @reg;

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