protected static function _signature($data)
{
$string = static::$_classes['string'];
if (empty(static::$_secret)) {
$message = 'Form signature requires a secret key. ';
$message .= 'Please see documentation on how to configure a key.';
throw new ConfigException($message);
}
$key = 'li3,1' . static::$_secret;
$key = $string::hash(date('YMD'), array('key' => $key, 'raw' => true));
$key = $string::hash('li3,1_form', array('key' => $key, 'raw' => true));
return $string::hash($data, array('key' => $key));
}