TripleDES.cs :  » Network-Clients » SharpPrivacyLibrary » SharpPrivacy » Cipher » C# / CSharp Open Source

//
// TripleDES.cs: Handles TripleDES (abstract class)
//
// Author:
//  Sebastien Pouliot (spouliot@motus.com)
//
// (C) 2002, 2003 Motus Technologies Inc. (http://www.motus.com)
//
// Modified by Daniel Fabian to fit SharpPrivacy's needs.
// This file is part of the SharpPrivacy source code contribution.
// Get get the original SymmetricAlgorithm class, please visit the
// mono project at http://www.go-mono.com.
//
using System;
using System.Security.Cryptography;

namespace SharpPrivacy.Cipher{

// References:
// a.  FIPS PUB 46-3: TripleDES
//  http://csrc.nist.gov/publications/fips/fips46-3/fips46-3.pdf
// b.  ANSI X9.52
//  not free :-(
//  http://webstore.ansi.org/ansidocstore/product.asp?sku=ANSI+X9%2E52%2D1998

public abstract class TripleDES : SymmetricAlgorithm {

  public TripleDES() {
    // from SymmetricAlgorithm
    KeySizeValue = 192;
    BlockSizeValue = 64;
    FeedbackSizeValue = 64;

    LegalKeySizesValue = new KeySizes [1];
    LegalKeySizesValue [0] = new KeySizes (128, 192, 64);

    LegalBlockSizesValue = new KeySizes [1];
    LegalBlockSizesValue [0] = new KeySizes (64, 64, 0);
  }

  public override byte[] Key {
    get {
      if (KeyValue == null) {
        // generate keys as long as we get weak keys
        GenerateKey ();
        while (IsWeakKey (KeyValue))
          GenerateKey ();
      }
      return KeyValue;
    }
    set { 
      if (value == null)
        throw new ArgumentNullException ();
      // this will check for both key size and weak keys
      if (IsWeakKey (value))
        throw new CryptographicException ();
      KeyValue = (byte[]) value.Clone (); 
    }
  }

  // Triple DES is DES in EDE = Encrypt - Decrypt - Encrypt
  // with 2 keys (a,b)
  //  EDE = Encrypt (a) - Decrypt (b) - Encrypt (a)
        //  if a == b then TripleDES == DES(a) (hence weak key)
        // with 3 keys (a,b,c)
  //  EDE = Encrypt (a) - Decrypt (b) - Encrypt (c)
  //  if ( a == b ) then TripleDES == DES(c) (hence weak key)
  //  if ( b == c ) then TripleDES == DES(a) (hence weak key)
  public static bool IsWeakKey (byte[] rgbKey) {
    // 128 bits (16 bytes) is 3 DES with 2 keys
    if (rgbKey.Length == 16) {
      // weak if first half == second half
      for (int i = 0; i < 8; i++)
        if (rgbKey [i] != rgbKey [i+8])
          return false;
    }
    // 192 bits (24 bytes) is 3 DES with 3 keys
    else if (rgbKey.Length == 24) {
      bool bFirstCase = true;  
      // weak if first third == second third
      for (int i = 0; i < 8; i++) {
        if (rgbKey [i] != rgbKey [i+8]) {
          bFirstCase = false;
          break;
        }
      }
      // weak if second third == third third 
      if (!bFirstCase) {
        for (int i = 8; i < 16; i++)
          if (rgbKey [i] != rgbKey [i+8])
            return false;
      }
    } else
      throw new CryptographicException ();

    return true;
  }

  public static new TripleDES Create() {
    return (TripleDES)Activator.CreateInstance(Type.GetType("SharpPrivacy.Cipher.TripleDESCryptoServiceProvider"), null);
  }

}

}