'Try to authenticate against the identity backend.'

def authenticate(self, request, auth_payload):

    response_data = {}
    user_info = auth_plugins.UserAuthInfo.create(auth_payload, METHOD_NAME)
    try:
        self.identity_api.authenticate(request, user_id=user_info.user_id, password=user_info.password)
    except AssertionError:
        msg = _('Invalid    username    or    password')
        raise exception.Unauthorized(msg)
    response_data['user_id'] = user_info.user_id
    return base.AuthHandlerResponse(status=True, response_body=None, response_data=response_data)
