The effects of the omission of last round's MixColumns on AES

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 letter we challenge this belief, showing evidence that the omission of MixColumns affects 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 2⁴⁸ to 2¹⁶. 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 2¹⁶.