inv_mix_columns
inv_rndkeys[i] = inv_mix_columns(rndkeys[j]);
inv_rndkeys[i + 1] = inv_mix_columns(rndkeys[j + 1]);
inv_rndkeys[i + 2] = inv_mix_columns(rndkeys[j + 2]);
inv_rndkeys[i + 3] = inv_mix_columns(rndkeys[j + 3]);
k0 = inv_mix_columns(k0);
k1 = inv_mix_columns(k1);
k2 = inv_mix_columns(k2);
k3 = inv_mix_columns(k3);