def challenge(self, challenge):
<EOL>if not challenge:<EOL><INDENT>logger.debug("<STR_LIT>")<EOL>return Failure("<STR_LIT>")<EOL><DEDENT>challenge = challenge.split(b'<STR_LIT:\x00>')[<NUM_LIT:0>]<EOL>if self.response_auth:<EOL><INDENT>return self._final_challenge(challenge)<EOL><DEDENT>realms = []<EOL>nonce = None<EOL>charset = "<STR_LIT>"<EOL>while challenge:<EOL><INDENT>match = PARAM_RE.match(challenge)<EOL>if not match:<EOL><INDENT>logger.debug("<STR_LIT>".format(challenge))<EOL>return Failure("<STR_LIT>")<EOL><DEDENT>challenge = match.group("<STR_LIT>")<EOL>var = match.group("<STR_LIT>")<EOL>val = match.group("<STR_LIT>")<EOL>logger.debug("<STR_LIT>".format(var, val))<EOL>if var == b"<STR_LIT>":<EOL><INDENT>realms.append(_unquote(val))<EOL><DEDENT>elif var == b"<STR_LIT>":<EOL><INDENT>if nonce:<EOL><INDENT>logger.debug("<STR_LIT>")<EOL>return Failure("<STR_LIT>")<EOL><DEDENT>nonce = _unquote(val)<EOL><DEDENT>elif var == b"<STR_LIT>":<EOL><INDENT>qopl = _unquote(val).split(b"<STR_LIT:U+002C>")<EOL>if b"<STR_LIT>" not in qopl:<EOL><INDENT>logger.debug("<STR_LIT>")<EOL>return Failure("<STR_LIT>")<EOL><DEDENT><DEDENT>elif var == b"<STR_LIT>":<EOL><INDENT>if val != b"<STR_LIT:utf-8>":<EOL><INDENT>logger.debug("<STR_LIT>")<EOL>return Failure("<STR_LIT>")<EOL><DEDENT>charset = "<STR_LIT:utf-8>"<EOL><DEDENT>elif var == b"<STR_LIT>":<EOL><INDENT>if val != b"<STR_LIT>":<EOL><INDENT>logger.debug("<STR_LIT>")<EOL>return Failure("<STR_LIT>")<EOL><DEDENT><DEDENT><DEDENT>if not nonce:<EOL><INDENT>logger.debug("<STR_LIT>")<EOL>return Failure("<STR_LIT>")<EOL><DEDENT>return self._make_response(charset, realms, nonce)<EOL>
Process a challenge and return the response.

        :Parameters:
            - `challenge`: the challenge from server.
        :Types:
            - `challenge`: `bytes`

        :return: the response or a failure indicator.
        :returntype: `sasl.Response` or `sasl.Failure`