public function setAuthSubPrivateKeyFile($file, $passphrase = null,
                                             $useIncludePath = false) {
        $fp = @fopen($file, "r", $useIncludePath);
        if (!$fp) {
            require_once 'Zend/Gdata/App/InvalidArgumentException.php';
            throw new Zend_Gdata_App_InvalidArgumentException('Failed to open private key file for AuthSub.');
        }

        $key = '';
        while (!feof($fp)) {
            $key .= fread($fp, 8192);
        }
        $this->setAuthSubPrivateKey($key, $passphrase);
        fclose($fp);
    }