Function auth_challenge 

pub unsafe extern "C" fn auth_challenge(
    _as: *mut auth_session_t,
) -> *mut c_char

Get a challenge string for the BSD Authentication session

From auth_subr(3):

The `auth_challenge()` function queries the login script defined by the current `style` for a
challenge for the user specified by `name`. It internally uses the `auth_call()` function.
The generated challenge is returned. NULL is returned on error or if no challenge was
generated. The challenge can also be extracted by the `auth_getchallenge()` function, which
simply returns the last challenge generated for this session.