/* ICM.java --
Copyright (C) 2001, 2002, 2003, 2006 Free Software Foundation, Inc.
This file is a part of GNU Classpath.
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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 David McGrew Integer Counter Mode (ICM) as an
* {@link IMode}.
*
* 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.
*
* 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 block index length and the segment
* index length. A constraint on those two values exists: The sum of
* segment index length and block index length must not
* half the block size of the underlying cipher. This requirement
* protects the ICM keystream generator from potentially failing to be
* pseudorandom.
*
* For simplicity, this implementation, fixes these two values to the following:
*
* - block index length: is half the underlying cipher block size, and
* - segment index length: is zero.
*
*
* 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
* (256 ^ block index length) * block length
* octets.
*
* Finally, for this implementation of the ICM, the IV placeholder will be used
* to pass the value of the Offset in the keystream segment.
*
* References:
*
* -
* Integer Counter Mode, David A. McGrew.
*
*/
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++]);
}
}