package gnu.crypto.mode;

// --------------------------------------------------------------------------
// $Id: CFB.java,v 1.2 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.,
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//    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
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// 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
// license of that module.  An independent module is a module which is
// not derived from or based on this library.  If you modify this
// 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 feedback mode. CFB mode is a stream mode that operates on
 * <i>s</i> bit blocks, where 1 &lt;= <i>s</i> &lt;= <i>b</i>, if
 * <i>b</i> is the underlying cipher's block size. Encryption is:
 *
<pre>
   I[1] = IV
   I[j] = LSB(b-s, I[j-1]) | C[j-1]   for j = 2...n
   O[j] = CIPH(K, I[j])               for j = 1,2...n
   C[j] = P[j] ^ MSB(s, O[j])         for j = 1,2...n
</pre>
 *
 * <p>And decryption is:</p>
 * 
<pre>
   I[1] = IV
   I[j] = LSB(b-s, I[j-1]) | C[j-1]   for j = 2...n
   O[j] = CIPH(K, I[j])               for j = 1,2...n
   P[j] = C[j] ^ MSB(s, O[j])         for j = 1,2...n
</pre>
 *
 * <p>CFB mode requires an initialization vector, which need not be kept
 * secret.</p>
 *
 * <p>References:</p>
 * <ol>
 * <li>Bruce Schneier, <i>Applied Cryptography: Protocols, Algorithms,
 * and Source Code in C, Second Edition</i>. (1996 John Wiley and Sons)
 * ISBN 0-471-11709-9.</li>
 *
 * <li><a href="http://csrc.nist.gov/encryption/modes/Recommendation/Modes01.pdf">
 * Recommendation for Block Cipher Modes of Operation Methods and Techniques</a>,
 * Morris Dworkin.</li>
 * </ol>
 *
 * @version $Revision: 1.2 $
 */
public class CFB extends BaseMode {

   // Constants and variables.
   // -----------------------------------------------------------------------

   /** The shift register, the input block to the block cipher. */
   private byte[] shiftRegister;

   /** The output block from the block cipher. */
   private byte[] scratch;

   // Constructors.
   // -----------------------------------------------------------------------

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

   /**
    * Cloneing constructor.
    *
    * @param that The instance being cloned.
    */
   private CFB(CFB that) {
      this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
   }

   // Instance methods implementing BaseMode.
   // -----------------------------------------------------------------------

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

   public void setup() {
      if (modeBlockSize > cipherBlockSize) {
         throw new IllegalArgumentException(
            "CFB block size cannot be larger than the cipher block size");
      }
      shiftRegister = new byte[cipherBlockSize];
      scratch = new byte[cipherBlockSize];
      System.arraycopy(iv, 0, shiftRegister, 0,
         Math.min(iv.length, cipherBlockSize));
   }

   public void teardown() {
      if (shiftRegister != null) {
         for (int i = 0; i < shiftRegister.length; i++) {
            shiftRegister[i] = 0;
         }
      }
      shiftRegister = null;
   }

   public void
   encryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
   {
      cipher.encryptBlock(shiftRegister, 0, scratch, 0);
      for (int i = 0; i < modeBlockSize; i++) {
         out[outOffset+i] = (byte) (in[inOffset+i] ^ scratch[i]);
      }
      System.arraycopy(shiftRegister, modeBlockSize, shiftRegister, 0,
         cipherBlockSize - modeBlockSize);
      System.arraycopy(out, outOffset, shiftRegister,
         cipherBlockSize - modeBlockSize, modeBlockSize);
   }

   public void
   decryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
   {
      cipher.encryptBlock(shiftRegister, 0, scratch, 0);
      for (int i = 0; i < modeBlockSize; i++) {
         out[outOffset+i] = (byte) (in[inOffset+i] ^ scratch[i]);
      }
      System.arraycopy(shiftRegister, modeBlockSize, shiftRegister, 0,
         cipherBlockSize - modeBlockSize);
      System.arraycopy(in, inOffset, shiftRegister,
         cipherBlockSize - modeBlockSize, modeBlockSize);
   }
}
