package gnu.crypto.mode;
// --------------------------------------------------------------------------
// $Id: CFB.java,v 1.2 2005/10/06 04:24:16 rsdio Exp $
//
// Copyright (C) 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.,
// 51 Franklin Street, 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
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// 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;
/**
* The cipher feedback mode. CFB mode is a stream mode that operates on
* <i>s</i> bit blocks, where 1 <= <i>s</i> <= <i>b</i>, if
* <i>b</i> is the underlying cipher's block size. Encryption is:
*
<pre>
I[1] = IV
I[j] = LSB(b-s, I[j-1]) | C[j-1] for j = 2...n
O[j] = CIPH(K, I[j]) for j = 1,2...n
C[j] = P[j] ^ MSB(s, O[j]) for j = 1,2...n
</pre>
*
* <p>And decryption is:</p>
*
<pre>
I[1] = IV
I[j] = LSB(b-s, I[j-1]) | C[j-1] for j = 2...n
O[j] = CIPH(K, I[j]) for j = 1,2...n
P[j] = C[j] ^ MSB(s, O[j]) for j = 1,2...n
</pre>
*
* <p>CFB mode requires an initialization vector, which need not be kept
* secret.</p>
*
* <p>References:</p>
* <ol>
* <li>Bruce Schneier, <i>Applied Cryptography: Protocols, Algorithms,
* and Source Code in C, Second Edition</i>. (1996 John Wiley and Sons)
* ISBN 0-471-11709-9.</li>
*
* <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>
*
* @version $Revision: 1.2 $
*/
public class CFB extends BaseMode {
// Constants and variables.
// -----------------------------------------------------------------------
/** The shift register, the input block to the block cipher. */
private byte[] shiftRegister;
/** The output block from the block cipher. */
private byte[] scratch;
// Constructors.
// -----------------------------------------------------------------------
/**
* Package-private constructor for the factory class.
*
* @param underlyingCipher The cipher implementation.
* @param cipherBlockSize The cipher's block size.
*/
CFB(IBlockCipher underlyingCipher, int cipherBlockSize) {
super(Registry.CFB_MODE, underlyingCipher, cipherBlockSize);
}
/**
* Cloneing constructor.
*
* @param that The instance being cloned.
*/
private CFB(CFB that) {
this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
}
// Instance methods implementing BaseMode.
// -----------------------------------------------------------------------
public Object clone() {
return new CFB(this);
}
public void setup() {
if (modeBlockSize > cipherBlockSize) {
throw new IllegalArgumentException(
"CFB block size cannot be larger than the cipher block size");
}
shiftRegister = new byte[cipherBlockSize];
scratch = new byte[cipherBlockSize];
System.arraycopy(iv, 0, shiftRegister, 0,
Math.min(iv.length, cipherBlockSize));
}
public void teardown() {
if (shiftRegister != null) {
for (int i = 0; i < shiftRegister.length; i++) {
shiftRegister[i] = 0;
}
}
shiftRegister = null;
}
public void
encryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
{
cipher.encryptBlock(shiftRegister, 0, scratch, 0);
for (int i = 0; i < modeBlockSize; i++) {
out[outOffset+i] = (byte) (in[inOffset+i] ^ scratch[i]);
}
System.arraycopy(shiftRegister, modeBlockSize, shiftRegister, 0,
cipherBlockSize - modeBlockSize);
System.arraycopy(out, outOffset, shiftRegister,
cipherBlockSize - modeBlockSize, modeBlockSize);
}
public void
decryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
{
cipher.encryptBlock(shiftRegister, 0, scratch, 0);
for (int i = 0; i < modeBlockSize; i++) {
out[outOffset+i] = (byte) (in[inOffset+i] ^ scratch[i]);
}
System.arraycopy(shiftRegister, modeBlockSize, shiftRegister, 0,
cipherBlockSize - modeBlockSize);
System.arraycopy(in, inOffset, shiftRegister,
cipherBlockSize - modeBlockSize, modeBlockSize);
}
}