package gnu.crypto.mode;

// ----------------------------------------------------------------------------
// $Id: CTR.java,v 1.7 2005/10/06 04:24:16 rsdio Exp $
//
// Copyright (C) 2001, 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
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// you permission to link this library with independent modules to
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// 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;
import gnu.crypto.util.Sequence;

import java.math.BigInteger;
import java.util.Arrays;
import java.util.Iterator;

/**
 * <p>The implementation of the Counter Mode.</p>
 *
 * <p>The algorithm steps are formally described as follows:</p>
 *
 * <pre>
 *    CTR Encryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
 *                    C[j] = P[j] ^ O[j]; for j = 1, 2...n.
 *    CTR Decryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
 *                    P[j] = C[j] ^ O[j]; for j = 1, 2...n.
 * </pre>
 *
 * <p>where <code>P</code> is the plaintext, <code>C</code> is the ciphertext,
 * <code>E(K)</code> is the underlying block cipher encryption function
 * parametrised with the session key <code>K</code>, and <code>T</code> is the
 * <i>Counter</i>.</p>
 *
 * <p>This implementation, uses a standard incrementing function with a step of
 * 1, and an initial value similar to that described in the NIST document.</p>
 *
 * <p>References:</p>
 *
 * <ol>
 *    <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: 1.7 $
 */
public class CTR extends BaseMode implements Cloneable {

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

   /** The current counter. */
//    private BigInteger T;

   private int off;
   private byte[] counter, enc;

   // Constructor(s)
   // -------------------------------------------------------------------------

   /**
    * <p>Trivial package-private constructor for use by the Factory class.</p>
    *
    * @param underlyingCipher the underlying cipher implementation.
    * @param cipherBlockSize the underlying cipher block size to use.
    */
   CTR(IBlockCipher underlyingCipher, int cipherBlockSize) {
      super(Registry.CTR_MODE, underlyingCipher, cipherBlockSize);
   }

   /**
    * <p>Private constructor for cloning purposes.</p>
    *
    * @param that the instance to clone.
    */
   private CTR(CTR that) {
      this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
   }

   // Class methods
   // -------------------------------------------------------------------------

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

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

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

   public void setup() {
      if (modeBlockSize > cipherBlockSize) {
         throw new IllegalArgumentException("mode size exceeds cipher block size");
      }
      off = 0;
      counter = new byte[cipherBlockSize];
      int i = cipherBlockSize - 1;
      int j = iv.length - 1;
      while (i >= 0 && j >= 0) {
         counter[i--] = iv[j--];
      }
      enc = new byte[cipherBlockSize];
      cipher.encryptBlock(counter, 0, enc, 0);
//       if (modeBlockSize != cipherBlockSize) {
//          throw new IllegalArgumentException();
//       }

//       byte[] tBytes = new byte[modeBlockSize+1];
//       tBytes[0] = (byte) 0x80;
//       for (int i = 0; i < modeBlockSize; i++) {
//          tBytes[i+1] = (byte)(256 - modeBlockSize + i);
//       }

//       T = new BigInteger(1, tBytes);
   }

   public void teardown() {
      if (counter != null) {
         Arrays.fill(counter, (byte) 0);
      }
      if (enc != null) {
         Arrays.fill(enc, (byte) 0);
      }
//       T = null;
   }

   public void encryptBlock(byte[] in, int i, byte[] out, int o) {
      ctr(in, i, out, o);
   }

   public void decryptBlock(byte[] in, int i, byte[] out, int o) {
      ctr(in, i, out, o);
   }

   public Iterator blockSizes() {
      return new Sequence(1, cipherBlockSize).iterator();
   }

   // own methods
   // -------------------------------------------------------------------------

   private void ctr(byte[] in, int inOffset, byte[] out, int outOffset) {
//       T = T.add(BigInteger.ONE);
//       byte[] O = T.toByteArray();
//       int ndx = O.length - modeBlockSize;
//       cipher.encryptBlock(O, ndx, O, ndx);
//       for (int i = 0; i < modeBlockSize; i++) {
//          out[outOffset++] = (byte)(in[inOffset++] ^ O[ndx++]);
//       }
      for (int i = 0; i < modeBlockSize; i++) {
         out[outOffset++] = (byte) (in[inOffset++] ^ enc[off++]);
         if (off == cipherBlockSize) {
            int j;
            for (j = cipherBlockSize - 1; j >= 0; j--) {
               counter[j]++;
               if ((counter[j] & 0xFF) != 0) {
                  break;
               }
            }
            if (j == 0) {
               counter[cipherBlockSize - 1]++;
            }
            off = 0;
            cipher.encryptBlock(counter, 0, enc, 0);
         }
      }
   }
}
