public function verifySignedJwt($id_token, $cert_location, $audience, $issuer, $max_expiry = null)
  {
    $auth = new Google_Auth_OAuth2($this);
    $certs = $auth->retrieveCertsFromLocation($cert_location);
    return $auth->verifySignedJwtWithCerts($id_token, $certs, $audience, $issuer, $max_expiry);
  }