To implement the X25519(k, u) and X448(k, u) functions (where k is the scalar and u is the u-coordinate), first decode k and u and then perform the following procedure

For X25519, in order to decode 32 random bytes as an integer scalar, set the three least significant bits of the first byte and the most significant bit of the last to zero, set the second most significant bit of the last byte to 1 and, finally, decode as little-endian.