class ClientXMPP(sleekxmpp.ClientXMPP):
    # an hacked version of the base class which supports
    # digest-md5 authentication (for facebook)

    def __init__(self, *args, **kwargs):
        sleekxmpp.ClientXMPP.__init__(self, *args, **kwargs)
        sasl_ns = 'urn:ietf:params:xml:ns:xmpp-sasl'
        self.add_handler("<challenge xmlns='%s' />"  % sasl_ns,
                                 self._handle_sasl_digest_md5_auth,
                                 instream=True)

    def _handle_sasl_auth(self, xml):
        if '{urn:ietf:params:xml:ns:xmpp-tls}starttls' in self.features:
            return False

        logger.debug("Starting SASL Auth")
        sasl_ns = 'urn:ietf:params:xml:ns:xmpp-sasl'
        self.add_handler("<success xmlns='%s' />" % sasl_ns,
                         self._handle_auth_success,
                         name='SASL Sucess',
                         instream=True)
        self.add_handler("<failure xmlns='%s' />" % sasl_ns,
                         self._handle_auth_fail,
                         name='SASL Failure',
                         instream=True)

        sasl_mechs = xml.findall('{%s}mechanism' % sasl_ns)
        if sasl_mechs:
            for sasl_mech in sasl_mechs:
                self.features.append("sasl:%s" % sasl_mech.text)
            if 'sasl:PLAIN' in self.features and self.boundjid.user:
                user = bytes(self.boundjid.user)
                password = bytes(self.password)
                auth = base64.b64encode('\x00' + user + \
                                        '\x00' + password).decode('utf-8')

                self.send("<auth xmlns='%s' mechanism='PLAIN'>%s</auth>" \
                          % (sasl_ns, auth))
            elif 'sasl:ANONYMOUS' in self.features and not self.boundjid.user:
                self.send("<auth xmlns='%s' mechanism='%s' />" % \
                          (sasl_ns, 'ANONYMOUS'))
            elif 'sasl:DIGEST-MD5' in self.features:
                self.send("<auth xmlns='%s' mechanism='DIGEST-MD5'/>" % sasl_ns)
            else:
                logger.error("no appropriate login method.")
                self.disconnect()
        return True

    def _handle_sasl_digest_md5_auth(self, xml):
        sasl_ns = 'urn:ietf:params:xml:ns:xmpp-sasl'
        self.add_handler("<success xmlns='%s' />" % sasl_ns,
                         self._handle_auth_success,
                         name='SASL Sucess',
                         instream=True)
        self.add_handler("<failure xmlns='%s' />" % sasl_ns,
                         self._handle_auth_fail,
                         name='SASL Failure',
                         instream=True)

        challenge = [item.split('=', 1) for item in base64.b64decode(xml.text).replace("\"", "").split(',', 6) ]
        challenge = dict(challenge)
        logger.debug("MD5 auth challenge: %s", challenge)

        # succesfully authenticated, send response
        if challenge.get('rspauth'):
            self.send("<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>")
            return
        # TODO: use realm if supplied by server, use default qop unless
        # supplied by server; realm, nonce, qop should all be present.
        if not challenge.get('qop') or not challenge.get('nonce'):
            logger.error("error during digest-md5 authentication. "
                         "challenge missing critical information. "
                         "(challenge: %s" % base64.b64decode(xml.text))
            self._handle_auth_fail(xml)
            return

        # TODO: charset can be either UTF-8 or if not present use ISO
        # 8859-1 defaulting for UTF-8 for now.
        # compute the cnonce - a unique hex string only used in this request
        cnonce = ""
        for i in range(7):
            cnonce += hex(int(random.random() * 65536 * 4096))[2:]
        cnonce = base64.encodestring(cnonce)[0:-1]
        a1 = b"%s:%s:%s" %(md5("%s:%s:%s" % (self.boundjid.user,
                                             self.boundjid.host,
                                             self.password)),
                           challenge["nonce"].encode("UTF-8"),
                           cnonce.encode("UTF-8"))
        a2 = "AUTHENTICATE:xmpp/%s" % self.boundjid.host
        resp_hash = md5digest("%s:%s:00000001:%s:auth:%s" \
                                                     %(md5digest(a1),
                                                       challenge["nonce"],
                                                       cnonce, md5digest(a2)))
        response = 'charset=utf-8,username="%s",realm="%s",nonce="%s",nc=00000001,cnonce="%s",digest-uri="%s",response=%s,qop=%s,' \
            % (self.boundjid.user,
               self.boundjid.host,
               challenge["nonce"],
               cnonce,
               "xmpp/%s" % self.boundjid.host, resp_hash, challenge["qop"])
        self.send("<response xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>%s</response>" \
                    % base64.encodestring(response)[:-1])