code: def get_signature_challenge(self):
          
  
          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
docs: Returns new signature challenge