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I understand how work the multiplication but I'm confused if the matrix to use is fixed or variable. The site where I read how mixcolums work is this https://math.stackexchange.com/questions/248456/matrix-multiplication-in-aes-mixcolumns-step Thanks in advance for the replies.

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It is fixed.

In the forward direction, it is always:

$$ \begin{bmatrix} 02 & 03 & 01 & 01 \\ 01 & 02 & 03 & 03 \\ 01 & 01 & 02 & 03 \\ 03 & 01 & 01 & 02 \end{bmatrix} $$

In the reverse direction, it is always:

$$ \begin{bmatrix} 0e & 0b & 0d & 09 \\ 09 & 0e & 0b & 0d \\ 0d & 09 & 0e & 0b \\ 0b & 0d & 09 & 0e \end{bmatrix} $$

Note that the multiplication (and additions) done during the matrix multiplication are done using $GF(2^8)$ operations, which are not the same as $\mathbb{Z}_{256}$ (which is what was confusing Frenzy Li)

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  • $\begingroup$ Tank you , now it's clear $\endgroup$ – Dragonfighter Oct 23 '17 at 16:44

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