pub fn x448(scalar: &[u8; 56], u: &[u8; 56]) -> [u8; 56]Expand description
RFC 7748 §5 X448(scalar, u).
Takes a 56-byte little-endian scalar and a 56-byte little-endian
u-coordinate, and returns the 56-byte little-endian u-coordinate
of scalar * (u, v) on Curve448.
The ladder runs 448 iterations (bits 447..0 of the clamped scalar); the clamp sets bit 447 unconditionally, so the first cswap initializes (x_2, x_3) = (u, 1), (z_2, z_3) = (1, u).