title: CodeIgniter 2.x set_cipher and set_flashdata cannot be used together
body: <p>When I try to use <code>set_cipher</code> and <code>set_flashdata</code> together in CodeIgniter, <code>flashdata</code> is not set and a php error is logged.</p>
  
  <p>To reproduce this on a <strong><em>fresh</em></strong> CI:</p>
  
  <p>Change the following <code>config.php</code> lines (changes from default CI config):</p>
  
  <pre><code>$config['sess_encrypt_cookie'] = TRUE;
  $config['log_threshold'] = 1; // To log the error 
  // Encryption key should be set, just set anything, for example:
  $config['encryption_key'] = 'HjyePR4FPF70vHKaHTl8jZ0hSMgOu5bW';
  </code></pre>
  
  <p>Controller (Simplified as much as I could):</p>
  
  <pre><code>&lt;?php
  
  class Welcome extends CI_Controller {
  
   function __construct(){parent::__construct();}
  
   function index()
   {
   $this-&gt;load-&gt;library('session');
   $this-&gt;load-&gt;library('encrypt');
  
   $this-&gt;encrypt-&gt;set_cipher(MCRYPT_BLOWFISH);
   $this-&gt;encrypt-&gt;encode('message');
  
   $this-&gt;session-&gt;set_flashdata('item', 'value');
   }
  
  }
  </code></pre>
  
  <p>Then simply open the controller in browser and refresh page, you will see <code>flashdata</code> is not set, and the following error is logged in CI application log folder if it has the right permission:</p>
  
  <blockquote>
    <p>Severity: Notice  --> unserialize(): Error at offset 0 of 286 bytes /Applications/MAMP/htdocs/final/system/libraries/Session.php 741</p>
  </blockquote>
  
  <p><strong>1. Is this a CodeIgniter bug or am i doing anything wrong?</strong></p>
  
  <p><strong>2. What should I do for this to work?</strong></p>
  
  <p><code>P.S. had to change cipher because default encryption result was too long for a message I needed to encrypt, but I wanted to use default method for anything else CI uses</code></p>