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package gnu.crypto.mode;
// ----------------------------------------------------------------------------
// $Id: CTR.java,v 1.5 2002/11/07 17:17:45 raif Exp $
//
// Copyright (C) 2001, 2002, Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto 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, or (at your option)
// any later version.
//
// GNU Crypto 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 this program; see the file COPYING. If not, write to the
//
// Free Software Foundation Inc.,
// 59 Temple Place - Suite 330,
// Boston, MA 02111-1307
// 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.
// ----------------------------------------------------------------------------
import gnu.crypto.Registry;
import gnu.crypto.cipher.IBlockCipher;
import java.math.BigInteger;
/**
* The implementation of the Counter Mode.
*
* The algorithm steps are formally described as follows:
*
*
* CTR Encryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
* C[j] = P[j] ^ O[j]; for j = 1, 2...n.
* CTR Decryption: O[j] = E(K)(T[j]); for j = 1, 2...n;
* P[j] = C[j] ^ O[j]; for j = 1, 2...n.
*
*
* where P is the plaintext, C is the ciphertext,
* E(K) is the underlying block cipher encryption function
* parametrised with the session key K, and T is the
* Counter.
*
* This implementation, uses a standard incrementing function with a step of
* 1, and an initial value similar to that described in the NIST document.
*
* References:
*
*
* -
* Recommendation for Block Cipher Modes of Operation Methods and Techniques,
* Morris Dworkin.
*
*
* @version $Revision: 1.5 $
*/
public class CTR extends BaseMode implements Cloneable {
// Constants and variables
// -------------------------------------------------------------------------
/** The current counter. */
private BigInteger T;
// Constructor(s)
// -------------------------------------------------------------------------
/**
* 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);
}
// Class methods
// -------------------------------------------------------------------------
// Cloneable interface implementation
// -------------------------------------------------------------------------
public Object clone() {
return new CTR(this);
}
// Implementation of abstract methods in BaseMode
// -------------------------------------------------------------------------
public void setup() {
if (modeBlockSize != cipherBlockSize) {
throw new IllegalArgumentException();
}
byte[] tBytes = new byte[modeBlockSize+1];
tBytes[0] = (byte) 0x80;
for (int i = 0; i < modeBlockSize; i++) {
tBytes[i+1] = (byte)(256 - modeBlockSize + i);
}
T = new BigInteger(1, tBytes);
}
public void teardown() {
T = null;
}
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);
}
// own methods
// -------------------------------------------------------------------------
private void ctr(byte[] in, int inOffset, byte[] out, int outOffset) {
T = T.add(BigInteger.ONE);
byte[] O = T.toByteArray();
int ndx = O.length - modeBlockSize;
cipher.encryptBlock(O, ndx, O, ndx);
for (int i = 0; i < modeBlockSize; i++) {
out[outOffset++] = (byte)(in[inOffset++] ^ O[ndx++]);
}
}
}