public static BigInteger InverseGf(this BigInteger value, BigInteger modulus)
{
BigInteger b = 1, c = 0, u = value, v = modulus;
while (u.GetBitCount() > 1)
{
var j = u.GetBitCount() - v.GetBitCount();
if (j < 0)
{
(u, v) = (v, u);
(c, b) = (b, c);
j = -j;
}
u = u.AddGf(v << j);
b = b.AddGf(c << j);
}
return(b);
}