'Set the passphrase callback
:param callback: The Python callback to use
:param userdata: (optional) A Python object which will be given as
argument to the callback
:return: None'

def set_passwd_cb(self, callback, userdata=None):

    if (not callable(callback)):
        raise TypeError('callback    must    be    callable')
    self._passphrase_helper = self._wrap_callback(callback)
    self._passphrase_callback = self._passphrase_helper.callback
    _lib.SSL_CTX_set_default_passwd_cb(self._context, self._passphrase_callback)
    self._passphrase_userdata = userdata
