def authenticate_multi(self, msg: Dict, signatures: Dict[str, str],
                           threshold: Optional[int] = None):
        """
        :param msg:
        :param signatures: A mapping from identifiers to signatures.
        :param threshold: The number of successful signature verification
        required. By default all signatures are required to be verified.
        :return: returns the identifiers whose signature was matched and
        correct; a SigningException is raised if threshold was not met
        """
:param msg:
        :param signatures: A mapping from identifiers to signatures.
        :param threshold: The number of successful signature verification
        required. By default all signatures are required to be verified.
        :return: returns the identifiers whose signature was matched and
        correct; a SigningException is raised if threshold was not met