gf28Inverse.cry

irreducible = <| x^^8 + x^^4 + x^^3 + x + 1 |>

gf28Mult (x, y) = pmod (pmult x y) irreducible

gf28Pow (n, k) = pow k
    where
        sq x  = gf28Mult (x, x)
        odd x = x ! 0
        pow i = if i == 0 then 1
                else if odd i
                     then gf28Mult (n, sq (pow (i >> 1)))
                     else sq (pow (i >> 1))

gf28Inverse x = gf28Pow (x, 254)