sudo apt-get install php5-mcrypt
sudo php5enmod mcrypt
sudo service apache2 reload
$config['encryption_key'] = "YOUR KEY";
$this->load->library('encrypt');
$msg = 'My secret message';
$key = 'super-secret-key';
$encrypted_string = $this->encrypt->encode($msg, $key);
$encrypted = 'APANtByIGI1BpVXZTJgcsAG8GZl8pdwwa84';
$key = 'super-secret-key';
$encrypted_string = $this->encrypt->decode($encrypted, $key);