Returns new signature challenge

def get_signature_challenge(self):
        """Returns new signature challenge"""

        devices = [DeviceRegistration.wrap(device) for device in self.__get_u2f_devices()]

        if devices == []:
            return {
                'status' : 'failed', 
                'error'  : 'No devices been associated with the account!'
            }

        challenge = start_authenticate(devices)
        challenge['status'] = 'ok'

        session['_u2f_challenge_'] = challenge.json

        return challenge