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My prof said that the last round of the AES algorithm doesn't use the MixedColumns layer, yet this doesn't decrease security. If so, why the MC layer is used in the other rounds?

AleWolf
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  • indeed, it's amazing how people don't even do a basic search on keywords, before asking the same question. – kodlu Dec 05 '19 at 17:11
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    @kodlu actually the question here is a little different, namely "why is MC used in the other rounds" – Richie Frame Dec 06 '19 at 00:53
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    Richie is right – AleWolf Dec 06 '19 at 09:32
  • @AleQuercia Do you understand why MixColumns is important in the first place? – forest Dec 08 '19 at 03:23
  • No, can you explain ? – AleWolf Dec 08 '19 at 09:08
  • @AleQuercia There are questions here which explains its purpose. MixColumns is the linear layer of AES (although it needs ShiftRows in order to not just mix the same columns over and over in subsequent rounds), just like SubBytes is the non-linear layer. – forest Dec 08 '19 at 09:39
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    First I thought this wasn't a dupe, because the question in the body of the post is perpendicular to the one in the title, but as the dupe pointed to by kelalaka also includes a description why it is used in the first place, I'll close it none the less. If there are any remaining questions about it please ask them separately after checking if it has been asked before. – Maarten Bodewes Dec 08 '19 at 17:16

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