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Title:
The Effects of the Omission of Last Round’s MixColumns on AES
Authors: Orr Dunkelman, Nathan Keller
Abstract:The Advanced Encryption Standard (AES) is the most widely deployed block cipher. It follows the modern iterated block cipher approach, iterating a simple round function multiple times. The last round of AES slightly differs from the others, as a linear mixing operation (called MixColumns) is omitted from it. Following a statement of the designers, it is widely believed that the omission of the last round MixColumns has no security implications. As a result, the majority of attacks on reduced-round variants of AES assume that the last round of the reduced-round version is free of the MixColumns operation. In this note we refute this belief, showing that the omission of MixColumns does affect the security of (reduced-round) AES. First, we consider a simple example of 1-round AES, where we show that the omission reduces the time complexity of an attack with a single known plaintext from 2^{48} to 2^{16}. Then, we examine several previously known attacks on 7-round AES-192 and show that the omission reduces their time complexities by a factor of 2^{16}.
ePrint: https://eprint.iacr.org/2010/041
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