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/* TBC.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

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.

You should have received a copy of the GNU General Public License

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

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.  */

package gnu.javax.crypto.pad;

import gnu.java.security.Configuration;

import gnu.java.security.Registry;

import gnu.java.security.util.Util;

import java.util.logging.Logger;

/**

 * The implementation of the Trailing Bit Complement (TBC) padding algorithm.

 * <p>

 * In this mode, "...the data string is padded at the trailing end with the

 * complement of the trailing bit of the unpadded message: if the trailing bit

 * is <tt>1</tt>, then <tt>0</tt> bits are appended, and if the trailing

 * bit is <tt>0</tt>, then <tt>1</tt> bits are appended. As few bits are

 * added as are necessary to meet the formatting size requirement."

 * <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 final class TBC

    extends BasePad

{

  private static final Logger log = Logger.getLogger(TBC.class.getName());

  /**

   * Trivial package-private constructor for use by the <i>Factory</i> class.

   *

   * @see PadFactory

   */

  TBC()

  {

    super(Registry.TBC_PAD);

  }

  public void setup()

  {

    if (blockSize < 1 || blockSize > 256)

      throw new IllegalArgumentException();

  }

  public byte[] pad(byte[] in, int offset, int length)

  {

    int padLength = blockSize;

    if (length % blockSize != 0)

      padLength = blockSize - length % blockSize;

    byte[] result = new byte[padLength];

    int lastBit = in[offset + length - 1] & 0x01;

    if (lastBit == 0)

      for (int i = 0; i < padLength;)

        result[i++] = 0x01;

    // else it's already set to zeroes by virtue of initialisation

    if (Configuration.DEBUG)

      log.fine("padding: 0x" + Util.toString(result));

    return result;

  }

  public int unpad(byte[] in, int offset, int length)

      throws WrongPaddingException

  {

    int limit = offset + length - 1;

    int lastBit = in[limit] & 0xFF;

    int result = 0;

    while (lastBit == (in[limit] & 0xFF))

      {

        result++;

        limit--;

      }

    if (result > length)

      throw new WrongPaddingException();

    if (Configuration.DEBUG)

      log.fine("padding length: " + result);

    return result;

  }

}