Does the AES Mixcolum Matrix use a fixed or random matrix? 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.
It is fixed.

In the forward direction, it is always:

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

In the reverse direction, it is always:

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

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)