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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++]);
}
}
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