Rijndael mix columns

The MixColumns operation performed by the Rijndael cipher, along with the ShiftRows step, is the primary source of diffusion in Rijndael. Each column is treated as a four-term polynomial ${\displaystyle b(x)=b_{3}x^{3}+b_{2}x^{2}+b_{1}x+b_{0}}$, where the coefficients are element over ${\displaystyle \operatorname {GF} (2^{8})}$, and is then multiplied modulo ${\displaystyle x^{4}+1}$ with a fixed polynomial ${\displaystyle a(x)=3x^{3}+x^{2}+x+2}$; the inverse of this polynomial is ${\displaystyle a^{-1}(x)=11x^{3}+13x^{2}+9x+14}$.

...
Wikipedia