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/* CTR.java --
Copyright (C) 2001, 2002, 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
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GNU Classpath is distributed in the hope that it will be useful, but
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
General Public License for more details.
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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
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permission to link this library with independent modules to produce an
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independent module, the terms and conditions of the license of that
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package gnu.javax.crypto.mode;
import gnu.java.security.Registry;
import gnu.java.security.util.Sequence;
import gnu.javax.crypto.cipher.IBlockCipher;
import java.util.Arrays;
import java.util.Iterator;
/**
* The implementation of the Counter Mode.
* <p>
* The algorithm steps are formally described as follows:
*
* <pre>
* CTR Encryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
* C[j] = P[j] &circ; O[j]; for j = 1, 2...n.
* CTR Decryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
* P[j] = C[j] &circ; O[j]; for j = 1, 2...n.
* </pre>
*
* <p>
* where <code>P</code> is the plaintext, <code>C</code> is the ciphertext,
* <code>E(K)</code> is the underlying block cipher encryption function
* parametrised with the session key <code>K</code>, and <code>T</code> is
* the <i>Counter</i>.
* <p>
* This implementation, uses a standard incrementing function with a step of 1,
* and an initial value similar to that described in the NIST document.
* <p>
* References:
* <ol>
* <li><a
* href="http://csrc.nist.gov/encryption/modes/Recommendation/Modes01.pdf">
* Recommendation for Block Cipher Modes of Operation Methods and Techniques</a>,
* Morris Dworkin.</li>
* </ol>
*/
public class CTR
extends BaseMode
implements Cloneable
{
private int off;
private byte[] counter, enc;
/**
* 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.
*/
CTR(IBlockCipher underlyingCipher, int cipherBlockSize)
{
super(Registry.CTR_MODE, underlyingCipher, cipherBlockSize);
}
/**
* Private constructor for cloning purposes.
*
* @param that the instance to clone.
*/
private CTR(CTR that)
{
this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
}
public Object clone()
{
return new CTR(this);
}
public void setup()
{
if (modeBlockSize > cipherBlockSize)
throw new IllegalArgumentException("mode size exceeds cipher block size");
off = 0;
counter = new byte[cipherBlockSize];
int i = cipherBlockSize - 1;
int j = iv.length - 1;
while (i >= 0 && j >= 0)
counter[i--] = iv[j--];
enc = new byte[cipherBlockSize];
cipher.encryptBlock(counter, 0, enc, 0);
}
public void teardown()
{
if (counter != null)
Arrays.fill(counter, (byte) 0);
if (enc != null)
Arrays.fill(enc, (byte) 0);
}
public void encryptBlock(byte[] in, int i, byte[] out, int o)
{
ctr(in, i, out, o);
}
public void decryptBlock(byte[] in, int i, byte[] out, int o)
{
ctr(in, i, out, o);
}
public Iterator blockSizes()
{
return new Sequence(1, cipherBlockSize).iterator();
}
private void ctr(byte[] in, int inOffset, byte[] out, int outOffset)
{
for (int i = 0; i < modeBlockSize; i++)
{
out[outOffset++] = (byte)(in[inOffset++] ^ enc[off++]);
if (off == cipherBlockSize)
{
int j;
for (j = cipherBlockSize - 1; j >= 0; j--)
{
counter[j]++;
if ((counter[j] & 0xFF) != 0)
break;
}
if (j == 0)
counter[cipherBlockSize - 1]++;
off = 0;
cipher.encryptBlock(counter, 0, enc, 0);
}
}
}
}