Another New AES Attack

    Date03 Aug 2009
    CategoryCryptography
    4215
    Posted ByAnthony Pell
    Read Bruce Schneier's always on-target analysis of cryptography, this time with information on the new attack against AES.A new and very impressive attack against AES has just been announced.

    Over the past couple of months, there have been two (the second blogged about here) new cryptanalysis papers on AES. The attacks presented in the paper are not practical -- they're far too complex, they're related-key attacks, and they're against larger-key versions and not the 128-bit version that most implementations use -- but they are impressive pieces of work all the same.

    This new attack, by Alex Biryukov, Orr Dunkelman, Nathan Keller, Dmitry Khovratovich, and Adi Shamir, is much more devastating. It is a completely practical attack against ten-round AES-256: Abstract.
    AES is the best known and most widely used block cipher. Its three versions (AES-128, AES-192, and AES-256) differ in their key sizes (128 bits, 192 bits and 256 bits) and in their number of rounds (10, 12, and 14, respectively). In the case of AES-128, there is no known attack which is faster than the 2128 complexity of exhaustive search. However, AES-192 and AES-256 were recently shown to be breakable by attacks which require 2176 and 2119 time, respectively. While these complexities are much faster than exhaustive search, they are completely non-practical, and do not seem to pose any real threat to the security of AES-based systems.

    In this paper we describe several attacks which can break with practical complexity variants of AES-256 whose number of rounds are comparable to that of AES-128. One of our attacks uses only two related keys and 239 time to recover the complete 256-bit key of a 9-round version of AES-256 (the best previous attack on this variant required 4 related keys and 2120 time). Another attack can break a 10 round version of AES-256 in 245 time, but it uses a stronger type of related subkey attack (the best previous attack on this variant required 64 related keys and 2172 time).

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