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