code: func (s *session) MasterKey() (*base64.Value, error) {
  	s.mutex.Lock()
  	defer s.mutex.Unlock()
  
  	if s.Type() == apitypes.NotLoggedIn {
  		return nil, createNotLoggedInError()
  	}
  
  	if s.Type() == apitypes.UserSession {
  		return s.auth.(envelope.UserInf).Master().Value, nil
  	}
  
  	return s.auth.(*envelope.MachineToken).Body.Master.Value, nil
  }
docs: // Returns the base64 representation of the identities encrypted master key