/* CFB.java -- 
   Copyright (C) 2002, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

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it under the terms of the GNU General Public License as published by
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Linking this library statically or dynamically with other modules is
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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] ^ 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>
 */
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);
  }
}