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/* ICM.java --

   Copyright (C) 2001, 2002, 2003, 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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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301

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Linking this library statically or dynamically with other modules is

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permission to link this library with independent modules to produce an

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exception statement from your version.  */

package gnu.javax.crypto.mode;

import gnu.java.security.Registry;

import gnu.javax.crypto.cipher.IBlockCipher;

import java.math.BigInteger;

/**

 * An implementation of <i>David McGrew</i> Integer Counter Mode (ICM) as an

 * {@link IMode}.

 * <p>

 * ICM is a way to define a pseudorandom keystream generator using a block

 * cipher. The keystream can be used for additive encryption, key derivation, or

 * any other application requiring pseudorandom data. In the case of this class,

 * it is used as additive encryption, XOR-ing the keystream with the input text

 * --for both encryption and decryption.

 * <p>

 * In ICM, the keystream is logically broken into segments. Each segment is

 * identified with a segment index, and the segments have equal lengths. This

 * segmentation makes ICM especially appropriate for securing packet-based

 * protocols. ICM also allows a variety of configurations based, among other

 * things, on two parameters: the <i>block index length</i> and the <i>segment

 * index length</i>. A constraint on those two values exists: The sum of

 * <i>segment index length</i> and <i>block index length</i> <b>must not</b>

 * half the <i>block size</i> of the underlying cipher. This requirement

 * protects the ICM keystream generator from potentially failing to be

 * pseudorandom.

 * <p>

 * For simplicity, this implementation, fixes these two values to the following:

 * <ul>

 * <li>block index length: is half the underlying cipher block size, and</li>

 * <li>segment index length: is zero.</li>

 * </ul>

 * <p>

 * For a 128-bit block cipher, the above values imply a maximum keystream length

 * of 295,147,905,179,352,825,856 octets, since in ICM, each segment must not

 * exceed the value

 * <code>(256 ^ <i>block index length</i>) * <i>block length</i></code>

 * octets.

 * <p>

 * Finally, for this implementation of the ICM, the IV placeholder will be used

 * to pass the value of the <i>Offset</i> in the keystream segment.

 * <p>

 * References:

 * <ol>

 * <li><a

 * href="http://www.ietf.org/internet-drafts/draft-mcgrew-saag-icm-00.txt">

 * Integer Counter Mode</a>, David A. McGrew.</li>

 * </ol>

 */

public class ICM

    extends BaseMode

    implements Cloneable

{

  /** The integer value 256 as a BigInteger. */

  private static final BigInteger TWO_FIFTY_SIX = new BigInteger("256");

  /** Maximum number of blocks per segment. */

  private BigInteger maxBlocksPerSegment;

  /** A work constant. */

  private BigInteger counterRange;

  /** The initial counter for a given keystream segment. */

  private BigInteger C0;

  /** The index of the next block for a given keystream segment. */

  private BigInteger blockNdx;

  /**

   * 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.

   */

  ICM(IBlockCipher underlyingCipher, int cipherBlockSize)

  {

    super(Registry.ICM_MODE, underlyingCipher, cipherBlockSize);

  }

  /**

   * Private constructor for cloning purposes.

   *

   * @param that the instance to clone.

   */

  private ICM(ICM that)

  {

    this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);

  }

  public Object clone()

  {

    return new ICM(this);

  }

  public void setup()

  {

    if (modeBlockSize != cipherBlockSize)

      throw new IllegalArgumentException();

    counterRange = TWO_FIFTY_SIX.pow(cipherBlockSize);

    maxBlocksPerSegment = TWO_FIFTY_SIX.pow(cipherBlockSize / 2);

    BigInteger r = new BigInteger(1, iv);

    C0 = maxBlocksPerSegment.add(r).modPow(BigInteger.ONE, counterRange);

    blockNdx = BigInteger.ZERO;

  }

  public void teardown()

  {

    counterRange = null;

    maxBlocksPerSegment = null;

    C0 = null;

    blockNdx = null;

  }

  public void encryptBlock(byte[] in, int i, byte[] out, int o)

  {

    icm(in, i, out, o);

  }

  public void decryptBlock(byte[] in, int i, byte[] out, int o)

  {

    icm(in, i, out, o);

  }

  private void icm(byte[] in, int inOffset, byte[] out, int outOffset)

  {

    if (blockNdx.compareTo(maxBlocksPerSegment) >= 0)

      throw new RuntimeException("Maximum blocks for segment reached");

    BigInteger Ci = C0.add(blockNdx).modPow(BigInteger.ONE, counterRange);

    byte[] result = Ci.toByteArray();

    int limit = result.length;

    int ndx = 0;

    if (limit < cipherBlockSize)

      {

        byte[] data = new byte[cipherBlockSize];

        System.arraycopy(result, 0, data, cipherBlockSize - limit, limit);

        result = data;

      }

    else if (limit > cipherBlockSize)

      ndx = limit - cipherBlockSize;

    cipher.encryptBlock(result, ndx, result, ndx);

    blockNdx = blockNdx.add(BigInteger.ONE); // increment blockNdx

    for (int i = 0; i < modeBlockSize; i++) // xor result with input block

      out[outOffset++] = (byte)(in[inOffset++] ^ result[ndx++]);

  }

}