def decode(self, response, method, host):
        """
        Decode the given response and attempt to generate a
        L{DigestedCredentials} from it.

        @type response: L{bytes}
        @param response: A string of comma separated key=value pairs

        @type method: L{bytes}
        @param method: The action requested to which this response is addressed
            (GET, POST, INVITE, OPTIONS, etc).

        @type host: L{bytes}
        @param host: The address the request was sent from.

        @raise error.LoginFailed: If the response does not contain a username,
            a nonce, an opaque, or if the opaque is invalid.

        @return: L{DigestedCredentials}
        """
        response = b' '.join(response.splitlines())
        parts = self._parseparts.findall(response)
        auth = {}
        for (key, bare, quoted) in parts:
            value = (quoted or bare).strip()
            auth[nativeString(key.strip())] = value

        username = auth.get('username')
        if not username:
            raise error.LoginFailed('Invalid response, no username given.')

        if 'opaque' not in auth:
            raise error.LoginFailed('Invalid response, no opaque given.')

        if 'nonce' not in auth:
            raise error.LoginFailed('Invalid response, no nonce given.')

        # Now verify the nonce/opaque values for this client
        if self._verifyOpaque(auth.get('opaque'), auth.get('nonce'), host):
            return DigestedCredentials(username,
                                       method,
                                       self.authenticationRealm,
                                       auth)

        Decode the given response and attempt to generate a
        L{DigestedCredentials} from it.

        @type response: L{bytes}
        @param response: A string of comma separated key=value pairs

        @type method: L{bytes}
        @param method: The action requested to which this response is addressed
            (GET, POST, INVITE, OPTIONS, etc).

        @type host: L{bytes}
        @param host: The address the request was sent from.

        @raise error.LoginFailed: If the response does not contain a username,
            a nonce, an opaque, or if the opaque is invalid.

        @return: L{DigestedCredentials}
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/cred/credentials.py