// using codeigniter's encryption library, which uses mcrypt and the AES cypher
$this->load->library('encrypt');
$pathandname = $config['upload_path'].$output['content'][$a]['file_name']; 
$theFile = file_get_contents($pathandname);
$fh = fopen($pathandname,'w');
fwrite($fh,$this->encrypt->encode($theFile));
fclose($fh); 

$this->load->library('encrypt');
$pathandname = $filelocation.$results[0]['encryptedfile']; 
$theFile = file_get_contents($pathandname);
$decrypted = $this->encrypt->decode($theFile);
force_download($filename, $decrypted); // a codeigniter function to force download via headers