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In the "Introduction to Cryptography: With Coding Theory" by Trappe and Washington, in the chapter about AES algorithm the authors say that in the last round of the encryption algorithm the MixColumn layer isn't applied, since it only slows down the decryption algorithm. But this doesn't decrease the security of the algorithm ?

I cite:

The preceding shows why the MixColumn is omitted in the last round of the encryption algorithm.Suppose it had been left in.[...]This means the decryption would have an unnecessary "Inverted Mix Column" at the beginning of the decryption algorithm, and this would have the effect of slowing down the algorithm.

ps: it's also said that the elements of the Inverted MixColum matrix are more complex in respect of the elements of the MixColum matrix (i cite "and this is enough to make decryption take around 30% longer than encryption for certain types of processors"). Also this isn't explained. Do you know why ?

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  • $\begingroup$ the first question is clear. The MixColumn is just a permutation that has no effect on the eye of the attackers! Second, If you look at the two matrices you can see by the required arithmetic is increased. $\endgroup$ – kelalaka Oct 19 '19 at 17:52
  • $\begingroup$ If you asked only one question (and these two questions are not completely related!), I'll call this as a dupe of this Why is MixColumns omitted from the last round of AES? $\endgroup$ – kelalaka Oct 19 '19 at 17:56
  • $\begingroup$ "But this doesn't decrease the security of the algorithm" yes, but not enough to matter given the targeted security level $\endgroup$ – Richie Frame Oct 20 '19 at 1:50

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