* Computes the secret key z = y' ^ x mod p, and returns the key to the
* `z' pointer sent as argument, unless the `z' is NULL. Returns FALSE if
* the computation could not be performed. The returned `z' must not be
* Computes the secret key z = y' ^ x mod p, and returns the key to the
* `z' pointer sent as argument, unless the `z' is NULL. Returns FALSE if
* the computation could not be performed. The returned `z' must not be