/* CTR.java -- 
   Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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General Public License for more details.

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USA

Linking this library statically or dynamically with other modules is
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package gnu.javax.crypto.mode;

import gnu.java.security.Registry;
import gnu.java.security.util.Sequence;
import gnu.javax.crypto.cipher.IBlockCipher;

import java.util.Arrays;
import java.util.Iterator;

/**
 * The implementation of the Counter Mode.
 * <p>
 * The algorithm steps are formally described as follows:
 * 
 * <pre>
 *     CTR Encryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
 *                     C[j] = P[j] ˆ O[j]; for j = 1, 2...n.
 *     CTR Decryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
 *                     P[j] = C[j] ˆ 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>
 * 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>
 * References:
 * <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>
 */
public class CTR
    extends BaseMode
    implements Cloneable
{
  private int off;
  private byte[] counter, enc;

  /**
   * Trivial package-private constructor for use by the Factory class.
   * 
   * @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);
  }

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

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

  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);
  }

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

  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();
  }

  private void ctr(byte[] in, int inOffset, byte[] out, int outOffset)
  {
    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);
          }
      }
  }
}