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