code: def subject_key_identifier(self):
          
          try:
              ext = self.x509.extensions.get_extension_for_oid(ExtensionOID.SUBJECT_KEY_IDENTIFIER)
          except x509.ExtensionNotFound:
              return None
          return SubjectKeyIdentifier(ext)
docs: The :py:class:`~django_ca.extensions.SubjectKeyIdentifier` extension, or ``None`` if it doesn't
          exist.