code: def _add_secret_hash(self, parameters, key):
          
          if self.client_secret is not None:
              secret_hash = AWSSRP.get_secret_hash(self.username, self.client_id,
                                                   self.client_secret)
              parameters[key] = secret_hash
docs: Helper function that computes SecretHash and adds it
          to a parameters dictionary at a specified key