CodeIgniter: Encryption is Not Authentication

After encrypting a string, the CodeIgniter PHP framework applies this function to the ciphertext. The function is a shift cipher using a hash of the encryption key.

The comment says they are doing it "to protect against Man-in-the-middle attacks on CBC mode ciphers." I have no idea what that means. I can only guess that they're slightly aware of the fact that encryption is not authentication, and that attackers can modify a CBC mode ciphertext to decrypt the way they want.

 /**  
  * Adds permuted noise to the IV + encrypted data to protect  
  * against Man-in-the-middle attacks on CBC mode ciphers  
  * http://www.ciphersbyritter.com/GLOSSARY.HTM#IV  
  *  
  * @param     string  
  * @param     string  
  * @return     string  
  */  
 protected function _add_cipher_noise($data, $key)  
 {  
      $key = $this->hash($key);  
      $str = '';  
   
      for ($i = 0, $j = 0, $ld = strlen($data), $lk = strlen($key); $i < $ld; ++$i, ++$j)  
      {  
           if ($j >= $lk)  
           {  
                $j = 0;  
           }  
   
           $str .= chr((ord($data[$i]) + ord($key[$j])) % 256);  
      }  
     
   return $str;  
 }  

Lessons Learned:

• If you need to detect malicious changes to ciphertexts, use HMAC over the IV and ciphertext, or an authenticating mode like OCB.
• If you find a vulnerability, don't come up with your own way to solve it. Find out how cryptographers have already solved it, and do it the right way.