Kohana CSRF validation rule

security.php

<?php defined('SYSPATH') or die('No direct script access.');
/**
 * Extension of Kohana_Security class
 *
 * @package     Misc
 * @category    Security
 * @author      Miodrag Tokić <mtokic@gmail.com>
 * @copyright   (c) 2011, Miodrag Tokić
 */
class Security extends Kohana_Security {

	/**
	 * Checks if CSRF token is valid
	 *
	 * This is a shortcut combination of Valid::not_empty and Security::check rules
	 *
	 *     // Set CSRF validation rule
	 *     Validation::factory($array)->rule('csrf', 'Security::csrf');
	 *
	 * @uses    Valid::not_empty
	 * @uses    Security::check
	 * @param   string  CSRF token
	 * @return  bool
	 */
	public static function csrf($token)
	{
		return Valid::not_empty($token) AND Security::check($token);
	}
}

validation.php

<?php defined('SYSPATH') or die('No direct script access.');

return array(
	'Security::csrf' => 'Form has expired.',
);