protected static function _signature($data) {
		$hash = static::$_classes['hash'];

		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 = $hash::calculate(date('YMD'), ['key' => $key, 'raw' => true]);
		$key = $hash::calculate('li3,1_form', ['key' => $key, 'raw' => true]);

		return $hash::calculate($data, ['key' => $key]);
	}