code: def sha1_hexdigest(self):
          # type: () -> str
          
          if self._sha1_hexdigest is None:
              self._sha1_hexdigest = hashlib.sha1(self._pem_bytes).hexdigest()
  
          return self._sha1_hexdigest
docs: A SHA-1 digest of the whole object for easy differentiation.
  
          .. versionadded:: 18.1.0