private function _encrypt($plaintext) {
        $td = mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB);
        $iv = mcrypt_create_iv($td, MCRYPT_DEV_RANDOM);
        mcrypt_generic_init($this->td, self::$key, $iv);
        $crypttext = mcrypt_generic($this->td, $plaintext);
        mcrypt_generic_deinit($this->td);
        return $crypttext;
    }
    private function _decrypt($crypttext) {
        $ivsize = mcrypt_get_iv_size(MCRYPT_CAST_256, MCRYPT_MODE_CFB);
        $iv = substr($crypttext, 0, $ivsize);
        $crypttext = substr($crypttext, $ivsize);
        mcrypt_generic_init($this->td, self::$key, $iv);
        $plaintext = mdecrypt_generic($this->td, $crypttext);
        mcrypt_generic_deinit($this->td);
        return $plaintext;
    }