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/* CFB.java --

   Copyright (C) 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

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.

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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

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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

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exception statement from your version.  */

package gnu.javax.crypto.mode;

import gnu.java.security.Registry;

import gnu.javax.crypto.cipher.IBlockCipher;

/**

 * The cipher feedback mode. CFB mode is a stream mode that operates on <i>s</i>

 * bit blocks, where 1 &lt;= <i>s</i> &lt;= <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] ˆ MSB(s, O[j])         for j = 1,2...n

 * </pre>

 * <p>

 * And decryption 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

 *  P[j] = C[j] ˆ MSB(s, O[j])         for j = 1,2...n

 * </pre>

 * <p>

 * CFB mode requires an initialization vector, which need not be kept secret.

 * <p>

 * References:

 * <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>

 */

public class CFB

    extends BaseMode

{

  /** The shift register, the input block to the block cipher. */

  private byte[] shiftRegister;

  /** The output block from the block cipher. */

  private byte[] scratch;

  /**

   * 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);

  }

  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);

  }

}