[Mono-dev] Question about TransformFinalBlock with SymmetricAlgorithms and CBC

[Sebastien Pouliot]

On Fri, 2009-01-30 at 12:27 -0500, David Wolinsky wrote:
> Hey guys,
> 
> Just wanted to let you know a behavioral difference between .Net and 
> Mono and potentially get advice from you.  The method 
> SA.CreateEncryptor.TransformFinalBlock() differs on the two platforms.  
> Specifically, Mono appears to continue from where the last one ended, 
> whereas .Net repeats itself (i.e. the IV is the same IV you inserted 
> when the ICryptoTransform was created).
> 
> Below is a sample.
> 
> using System;
> using System.Security.Cryptography;
> 
> public class tdes_test {
>   public static void Main() {
>     RijndaelManaged rm = new RijndaelManaged();
>     byte[] key = new byte[rm.KeySize / 8];
>     for(int i = 0; i < key.Length; i++) {
>       key[i] = (byte) i;
>     }
>     byte[] iv = new byte[rm.BlockSize / 8];
>     for(int i = 0; i < iv.Length; i++) {
>       iv[i] = (byte) i;
>     }
>     ICryptoTransform encryptor = rm.CreateEncryptor(key, iv);
>     byte[] data = new byte[111];
>     for(int i = 0; i < data.Length; i++) {
>       data[i] = (byte) i;
>     }
> 
>     byte[] encrypted_data = encryptor.TransformFinalBlock(data, 0, 
> data.Length);
>     for(int i = 0; i < encrypted_data.Length; i++) {
>       Console.Write(encrypted_data[i]);
>     }
>     Console.WriteLine("\n");
>     encrypted_data = encryptor.TransformFinalBlock(data, 0, data.Length);
>     for(int i = 0; i < encrypted_data.Length; i++) {
>       Console.Write(encrypted_data[i]);
>     }
>   }
> }
> 
> 
> We're currently using this on a datagram security system and on Mono 
> (not sure if .Net is the same) creation of Encryptors and Decryptors is 
> expensive.  Any thoughts or suggestions?

Your code should always* look at ICryptoTransform.CanReuseTransform
before reusing a transform. If false then it cannot be reused.

* since you should be using the factory methods to create ciphers
(e.g. Rijndael.Create) and you can't be sure what exact class the
runtime will instantiate (nor it's behavior).

Sebastien