AES MixColumn Polynomial Transformation I just read about the AES MixColumn step, which can be calculated as:
$$b(x)=c(x)a(x) \pmod{X^4 +1}$$
$a(x)$ is the column to encode and $c(x)=3x^3+x^2+x+2$
My question is: how did they calculate $c(x)$?
Each column of 4x4 byte array S is regarded as column vector over the Rijndael field F.
It is then updated by multiplying the column vector by a specified 4x4 matrix over F.
Thus muxColumns modifies the state array by hte matrix multiplication (0&lt;=j&lt;=3),
Hence, in your example you are encoding ${S_{(3,3)}}$ which is last row of Matrix and represented as polynomial ${3x^3+x^2+x+2}$