code: def retrieve_by_key_identifier(self, key_identifier):
          
  
          if not isinstance(key_identifier, byte_cls):
              raise TypeError(pretty_message(
                  '''
                  key_identifier must be a byte string, not %s
                  ''',
                  type_name(key_identifier)
              ))
  
          return self._key_identifier_map.get(key_identifier)
docs: Retrieves a cert via its key identifier
  
          :param key_identifier:
              A byte string of the key identifier
  
          :return:
              None or an asn1crypto.x509.Certificate object