def retrieve_by_key_identifier(self, key_identifier):
        """
        Retrieves a cert via its key identifier

        :param key_identifier:
            A byte string of the key identifier

        :return:
            None or an asn1crypto.x509.Certificate object
        """

        if not isinstance(key_identifier, byte_cls):
            raise TypeError(pretty_message(
                '''
                key_identifier must be a byte string, not %s
                ''',
                type_name(key_identifier)
            ))

        return self._key_identifier_map.get(key_identifier)
Retrieves a cert via its key identifier

        :param key_identifier:
            A byte string of the key identifier

        :return:
            None or an asn1crypto.x509.Certificate object