private static void EncodePoint(PointExt p, byte[] r, int rOff)
{
uint[] x = X448Field.Create();
uint[] y = X448Field.Create();
X448Field.Inv(p.z, y);
X448Field.Mul(p.x, y, x);
X448Field.Mul(p.y, y, y);
X448Field.Normalize(x);
X448Field.Normalize(y);
X448Field.Encode(y, r, rOff);
r[rOff + PointBytes - 1] = (byte)((x[0] & 1) << 7);
}