code: def get_xml_string_with_self_contained_assertion_within_encrypted_assertion(
              self, assertion_tag):
          
          prefix_map = self.get_prefix_map(
              [self.encrypted_assertion._to_element_tree().find(assertion_tag)])
  
          tree = self._to_element_tree()
  
          self.set_prefixes(
              tree.find(
                  self.encrypted_assertion._to_element_tree().tag).find(
                  assertion_tag), prefix_map)
  
          return ElementTree.tostring(tree, encoding="UTF-8").decode('utf-8')
docs: Makes a encrypted assertion only containing self contained
          namespaces.
  
          :param assertion_tag: Tag for the assertion to be transformed.
          :return: A new samlp.Resonse in string representation.