code: def match_owner(self, url):
          
          for owner in self.issuer_keys.keys():
              if owner.startswith(url):
                  return owner
  
          raise KeyError("No keys for '{}' in this keyjar".format(url))
docs: Finds the first entity, with keys in the key jar, with an
          identifier that matches the given URL. The match is a leading
          substring match.
  
          :param url: A URL
          :return: An issue entity ID that exists in the Key jar