/* ICM.java -- 
   Copyright (C) 2001, 2002, 2003, 2006 Free Software Foundation, Inc.

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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++]);
  }
}