Super secret decoder ring... in php

It's time for a wee bit of code. The following is a simple php class to encrypt/decrypt strings. This is useful when you need to pass around secret documents but are out of microfiche and fake lipstick tubes.

To use:

<?php
require_once('secretdecoderring.php');

$dr = new SecretDecoderRing();
$enc = $dr->encode($argv[1]);
$dec = $dr->decode($enc);
var_dump($argv[1]);
var_dump($enc);
var_dump($dec);
?>

<?php

class SecretDecoderRing
{
    private $key = "supersecret";
    private $td;

    function __construct()
    {
        $this->td = mcrypt_module_open('tripledes', '', 'ecb', '');
        $iv = mcrypt_create_iv(mcrypt_enc_get_iv_size($this->td), MCRYPT_RAND);
        mcrypt_generic_init($this->td, $this->key, $iv);
    }

    function __deconstruct()
    {
        mcrypt_generic_deinit($this->td);
        mcrypt_module_close($this->td);
    }

    function encode($string)
    {
        $enc = mcrypt_generic($this->td, $string);
        return base64_encode($enc);
    }

    function decode($string)
    {
        $enc = base64_decode($string);
        $dec = mdecrypt_generic($this->td, $enc);
        // It's padded. Really. Security is such a waste of time.
        return rtrim($dec, ((ord(substr($dec, strlen($dec)-1, 1)) >= 0 and ord(substr($dec, strlen($dec)-1, 1)) <= 16) ? c
hr(ord(substr($dec, strlen($dec)-1, 1))): null));
    }
}

?>