/* 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.

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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
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permission to link this library with independent modules to produce an
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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;
  }
}