CBC.java

package gnu.crypto.mode;

// ----------------------------------------------------------------------------
// $Id: CBC.java,v 1.3 2005/10/06 04:24:16 rsdio Exp $
//
// Copyright (C) 2002 Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2, or (at your option)
// any later version.
//
// GNU Crypto is distributed in the hope that it will be useful, but
// WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
// General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program; see the file COPYING.  If not, write to the
//
//    Free Software Foundation Inc.,
//    51 Franklin Street, Fifth Floor,
//    Boston, MA 02110-1301
//    USA
//
// Linking this library statically or dynamically with other modules is
// making a combined work based on this library.  Thus, the terms and
// conditions of the GNU General Public License cover the whole
// combination.
//
// As a special exception, the copyright holders of this library give
// you permission to link this library with independent modules to
// produce an executable, regardless of the license terms of these
// independent modules, and to copy and distribute the resulting
// executable under terms of your choice, provided that you also meet,
// for each linked independent module, the terms and conditions of the
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// library, you may extend this exception to your version of the
// library, but you are not obligated to do so.  If you do not wish to
// do so, delete this exception statement from your version.
// ----------------------------------------------------------------------------

import gnu.crypto.Registry;
import gnu.crypto.cipher.IBlockCipher;

/**
 * The Cipher Block Chaining mode. This mode introduces feedback into
 * the cipher by XORing the previous ciphertext block with the plaintext
 * block before encipherment. That is, encrypting looks like this:</p>
 *
 * <blockquote><p>C<sub>i</sub> = E<sub>K</sub>(P<sub>i</sub> ^
 * C<sub>i-1</sub></p></blockquote>
 *
 * <p>Similarly, decrypting is:</p>
 *
 * <blockquote><p>P<sub>i</sub> = C<sub>i-1</sub> ^
 * D<sub>K</sub>(C<sub>i</sub>)</p></blockquote>
 *
 * @version $Revision: 1.3 $
 */
public class CBC extends BaseMode implements Cloneable {

   // Constants and Variables
   //------------------------------------------------------------------

   /** The last (de|en)crypted block */
   private byte[] lastBlock;

   /** An intermediate buffer. */
   private byte[] scratch;

   // Constructors
   // -----------------------------------------------------------------

   /**
    * Package-private constructor for the factory class.
    *
    * @param underlyingCipher The cipher implementation.
    * @param cipherBlockSize The cipher's block size.
    */
   CBC(IBlockCipher underlyingCipher, int cipherBlockSize) {
      super(Registry.CBC_MODE, underlyingCipher, cipherBlockSize);
   }

   /** Our constructor for cloning. */
   private CBC(CBC that) {
      this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
   }

   // Cloneable interface implementation
   // -----------------------------------------------------------------

   public Object clone() {
      return new CBC(this);
   }

   // Implementation of abstract methods in BaseMode
   // -----------------------------------------------------------------

   public void setup() {
      if (modeBlockSize != cipherBlockSize) {
         throw new IllegalArgumentException();
      }
      scratch = new byte[cipherBlockSize];
      lastBlock = new byte[cipherBlockSize];

      // lastBlock gets initialized to the initialization vector.
      for (int i = 0; i < lastBlock.length && i < iv.length; i++) {
         lastBlock[i] = iv[i];
      }
   }

   public void teardown() {
      lastBlock = null;
      scratch = null;
   }

   public void encryptBlock(byte[] in, int i, byte[] out, int o) {
      for (int k = 0; k < scratch.length; k++) {
         scratch[k] = (byte) (lastBlock[k] ^ in[k+i]);
      }
      cipher.encryptBlock(scratch, 0, out, o);
      System.arraycopy(out, o, lastBlock, 0, cipherBlockSize);
   }

   public void decryptBlock(byte[] in, int i, byte[] out, int o) {
      byte[] buf = new byte[cipherBlockSize];
      System.arraycopy(in, i, buf, 0, cipherBlockSize);
      cipher.decryptBlock(in, i, scratch, 0);
      for (int k = 0; k < scratch.length; k++) {
         out[o+k] = (byte) (lastBlock[k] ^ scratch[k]);
      }
      System.arraycopy(buf, 0, lastBlock, 0, cipherBlockSize);
   }
}