mcrypt not decrypting to same length - php

Question

I am trying to mcrypt some data using a class I created (methods below). This is how you mcrypt data then using pack, then you can use unpack to get the data back.
$packed = $server->cache->pack("packed", array(123,123,123), "Password");
if(!$packed){
echo "Could not encrypt data\n";
}
$server->cache->unpack("packed", "Password");
when I pack it, I do a var_dump on the json_encode() data, and get this:
string(13) "[123,123,123]"
When I unpack it, I do a var_dump on the mcrypt_decode() string, and get this:
string(32) "[123,123,123]"
Why are the lengths different? When I do a json_decode() on the mcrypt_decode() string, I get null back, and this is the reason. If I trim the data it works, but I shouldn't have to trim it.
Here are the methods:
<?php
public function put($key, $value, $life = 0)
{
$this->cache[$key] = $value;
$life = (int)$life;
if($life > 0)
{
$life = strtotime("now + $life seconds");
}
$this->life[$key] = $life;
}
public function get($key)
{
return $this->cache[$key];
}
public function pack($key, $value, $secret, $life = 0)
{
if(!function_exists("mcrypt_encrypt"))
{
$this->put($key, $value, $life);
return false;
}
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$value = json_encode($value);
var_dump($value);
$cryptdata = mcrypt_encrypt(MCRYPT_RIJNDAEL_256, $secret, $value, MCRYPT_MODE_ECB, $iv);
$this->put($key, $cryptdata, $life);
return true;
}
public function unpack($key, $secret)
{
if(!function_exists("mcrypt_decrypt"))
{
return json_decode($this->get($key), true);
}
$cryptdata = $this->get($key);
$iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_256, MCRYPT_MODE_ECB);
$iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
$data = mcrypt_decrypt(MCRYPT_RIJNDAEL_256, $secret, $cryptdata, MCRYPT_MODE_ECB, $iv);
//$data = json_decode($data, true);
var_dump($data);
}

Answer 1

When using a block cipher mode like ECB (you shouldn't be using that one btw), MCrypt will NUL-pad the data, so that its length is dividable by the encryption algorithm's block size.
If you must know, for Rijndael-256 the block size is 256 bits or 32 bytes.
Considering that you're encrypting JSON data, you can just rtrim() the data and not worry about it. There's no way around that unless you switch to a counter mode like CTR.