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
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