Callback fired when client wants to attach session.

def onHello(self, realm, details):
        """
        Callback fired when client wants to attach session.
        """
        log.msg("onHello: {} {}".format(realm, details))

        self._pending_auth = None

        if details.authmethods:
            for authmethod in details.authmethods:
                if authmethod == u"wampcra":

                    ## lookup user in user DB
                    salt, key, role, uid = yield self.factory.userdb.get(details.authid)
                    log.msg("salt, key, role: {} {} {} {}".format(salt, key, role, uid))

                    ## if user found ..
                    if key:

                        log.msg("found key")

                        ## setup pending auth
                        self._pending_auth = PendingAuth(key, details.pending_session,
                            details.authid, role, authmethod, u"userdb", uid)

                        log.msg("setting challenge")
                        ## send challenge to client
                        extra = {
                            u'challenge': self._pending_auth.challenge
                        }

                        ## when using salted passwords, provide the client with
                        ## the salt and then PBKDF2 parameters used
                        if salt:
                            extra[u'salt'] = salt
                            extra[u'iterations'] = 1000
                            extra[u'keylen'] = 32

                        defer.returnValue(types.Challenge(u'wampcra', extra))

        ## deny client
        defer.returnValue(types.Deny())