def _get_principal(self, principal_arn):
        """
        raise ResourceNotFoundException
        """
        if ':cert/' in principal_arn:
            certs = [_ for _ in self.certificates.values() if _.arn == principal_arn]
            if len(certs) == 0:
                raise ResourceNotFoundException()
            principal = certs[0]
            return principal
        else:
            # TODO: search for cognito_ids
            pass
        raise ResourceNotFoundException()
raise ResourceNotFoundException