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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
+the Free Software Foundation; either version 2 of the License, or (at
+your option) any later version.
+
+GNU Classpath 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 GNU Classpath; if not, write to the Free Software
+Foundation, Inc., 51 Franklin St, 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
+combination.
+
+As a special exception, the copyright holders of this library give you
+permission to link this library with independent modules to produce an
+executable, regardless of the license terms of these 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.  */
+
+
+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++]);
+  }
+}