Cryptology ePrint Archive: Report 2010/041

The Effects of the Omission of Last Round's MixColumns on AES

Orr Dunkelman and 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}.

Category / Keywords: secret-key cryptography / AES, MixColumns, Impossible Differential Cryptanalysis

Publication Info: Submitted to a journal

Date: received 27 Jan 2010

Contact author: orr dunkelman at weizmann ac il

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Version: 20100129:153203 (All versions of this report)

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