Rijndael crypto and TransformBlock?

From: William Stacey [MVP] (staceywREMOVE_at_mvps.org)
Date: 10/20/04 

Date: Wed, 20 Oct 2004 11:53:12 -0400

The Decypt2() method below does not work. It completes, but does not do the
right thing. The first transform request returns 0 bytes. The first
Decypt() method works as we work on a stream instead of blocks. I would
like to know how to get the block method working. TIA.

using System;
using System.IO;
using System.Security.Cryptography;

namespace SocketServers.Crypto
{
 /// <summary>
 /// Summary description for MyCrypto.
 /// </summary>
 public class MyCrypto
 {
  const string s_plaintext = @"c:\mytext.txt";
  const string s_ciphertext = @"c:\mytext.bin";
  const string s_decrypted = @"c:\mytext2.txt";

  public void RunTest()
  {
   using(SymmetricAlgorithm algo = SymmetricAlgorithm.Create("Rijndael"))
   {
    algo.Padding = PaddingMode.PKCS7;
    algo.BlockSize = 256; // 32 bytes;
    Console.WriteLine("Block size:"+algo.BlockSize);
    KeySizes[] blockSizes = algo.LegalBlockSizes;
    foreach(KeySizes keysize in blockSizes)
    {
     Console.WriteLine("Max size:"+keysize.MaxSize);
     Console.WriteLine("Min size:"+keysize.MinSize);
    }
    Encrypt(algo);
    Decrypt2(algo);
   }
  }

  public void Encrypt(SymmetricAlgorithm algo)
  {
   using( Stream cipherText = GetWriteableFileStream(s_ciphertext) )
   using( ICryptoTransform enc = algo.CreateEncryptor() )
   using( CryptoStream crypt = GetWriteCryptoStream(cipherText, enc) )
   using( Stream input = GetReadOnlyFileStream(s_plaintext) )
   {
    Pump(input, crypt);
    crypt.Close(); //have to call, not called by Dispose
   }
  }

  public void Decrypt(SymmetricAlgorithm algo)
  {
   using( Stream cipherText = GetReadOnlyFileStream(s_ciphertext) )
   using( ICryptoTransform enc = algo.CreateDecryptor() )
   using( CryptoStream decrypt = GetReadCryptoStream(cipherText, enc) )
   using( Stream output = GetWriteableFileStream(s_decrypted) )
   {
    Pump(decrypt, output);
    Console.WriteLine("In File Stream len:"+cipherText.Length);
    Console.WriteLine("In Stream Blocks:"+
((double)((double)cipherText.Length / 32)));
    decrypt.Close(); // have to call, not called by Dispose
   }
  }

  public void Decrypt2(SymmetricAlgorithm algo)
  {
   ICryptoTransform dec = algo.CreateDecryptor();
   Stream inStream = GetReadOnlyFileStream(s_ciphertext);
   Stream outStream = GetWriteableFileStream(s_decrypted);
   try
   {
    byte[] inBlock = new byte[32];
    byte[] outBlock = new byte[32];
    // Read input stream in blocks (32 bytes) until end, and transform the
blocks
    // and write to outStream.
    int count = 0;
    int outCount = 0;
    while ( (count = inStream.Read(inBlock, 0, inBlock.Length)) != 0 )
    {
     outCount = dec.TransformBlock(inBlock, 0, count, outBlock, 0);
     outStream.Write(outBlock, 0, outCount);
     Console.WriteLine("OutCount:"+outCount);
    }
    outStream.Flush();
   }
   finally
   {
    outStream.Close();
    inStream.Close();
   }
  }

  private static void Pump(Stream input, Stream output)
  {
   byte[] buffer = new byte[10];
   int count = 0;

   while( (count = input.Read(buffer, 0, 10)) != 0 )
   {
    output.Write(buffer, 0, count);
   }
  }

  private static FileStream GetReadOnlyFileStream(string path)
  {
   return new FileStream(path, FileMode.Open, FileAccess.Read);
  }

  private static FileStream GetWriteableFileStream(string path)
  {
   return new FileStream(path, FileMode.Create, FileAccess.Write);
  }

  private static CryptoStream GetWriteCryptoStream(Stream stream,
ICryptoTransform transform)
  {
   return new CryptoStream(stream, transform, CryptoStreamMode.Write);
  }

  private static CryptoStream GetReadCryptoStream(Stream stream,
ICryptoTransform transform)
  {
   return new CryptoStream(stream, transform, CryptoStreamMode.Read);
  }
 }
} 

-- 
William Stacey, MVP

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