def client_key_loader(self, f):
        """Registers a function to be called to find a client key.

        Function you set has to take a client id and return a client key::

            @hawk.client_key_loader
            def get_client_key(client_id):
                if client_id == 'Alice':
                    return 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn'
                else:
                    raise LookupError()

        :param f: The callback for retrieving a client key.

        """
        @wraps(f)
        def wrapped_f(client_id):
            client_key = f(client_id)
            return {
                'id': client_id,
                'key': client_key,
                'algorithm': current_app.config['HAWK_ALGORITHM']
            }

        self._client_key_loader_func = wrapped_f
        return wrapped_f
Registers a function to be called to find a client key.

        Function you set has to take a client id and return a client key::

            @hawk.client_key_loader
            def get_client_key(client_id):
                if client_id == 'Alice':
                    return 'werxhqb98rpaxn39848xrunpaw3489ruxnpa98w4rxn'
                else:
                    raise LookupError()

        :param f: The callback for retrieving a client key.