cbcblockcipher.java

java高级使用教程 全书一共分六章

// CbcBlockCipher - use a block cipher in CBC mode
//
// Copyright (C) 1996 by Jef Poskanzer <[e-mail]>.  All rights reserved.
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions
// are met:
// 1. Redistributions of source code must retain the above copyright
//    notice, this list of conditions and the following disclaimer.
// 2. Redistributions in binary form must reproduce the above copyright
//    notice, this list of conditions and the following disclaimer in the
//    documentation and/or other materials provided with the distribution.
//
// THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
// ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
// IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
// ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
// FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
// DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
// OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
// HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
// LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
// OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
// SUCH DAMAGE.
//
// Visit the ACME Labs Java page for up-to-date versions of this and other
// fine Java utilities: http://www.acme.com/java/

package Acme.Crypto;

import java.io.*;

/// Use a block cipher in CBC mode.
// <P>
// A plain old block cipher, key and cleartext-block in, ciphertext-block
// out, is said to be in Electronic Code Book (ECB) mode.  A given block
// of plaintext always encrypts to the same block of ciphertext.  This
// makes it somewhat vulnerable to known plaintext attacks, block replay
// attacks, etc.
// <P>
// A fairly cheap alternative is to use it in Cipher Block Chaining (CBC)
// mode.  All this does is XOR each plaintext block with the previous
// ciphertext block before encryption.  For the first block, where there
// is no previous ciphertext block, a caller-specified Initialization
// Vector (IV) is used for the XOR.  This makes each block's encryption
// depend on all the previous blocks
// <P>
// This class lets you use any given block cipher in CBC mode.
// <P>
// <A HREF="/resources/classes/Acme/Crypto/CbcBlockCipher.java">Fetch the software.</A><BR>
// <A HREF="/resources/classes/Acme.tar.gz">Fetch the entire Acme package.</A>
// <P>
// @see Cipher
// @see BlockCipher
// @see StreamCipher
// @see EncryptedOutputStream
// @see EncryptedInputStream

public class CbcBlockCipher extends BlockCipher
    {

    private BlockCipher blockCipher;
    private byte[] iv;
    private byte[] temp;

    /// Constructor.
    public CbcBlockCipher( BlockCipher blockCipher )
	{
	super( blockCipher.keySize(), blockCipher.blockSize() );
	this.blockCipher = blockCipher;
	iv = new byte[blockSize()];
	zeroBlock( iv );
	temp = new byte[blockSize()];
	}


    // Key routines.

    // Set the key.
    public void setKey( byte[] key )
	{
	blockCipher.setKey( key );
	}
    

    // IV routines.

    /// Set the Initialization Vector.
    public void setIv( byte[] iv )
	{
	copyBlock( iv, this.iv );
	}
    
    /// Set and return a random IV.
    // In CBC mode, the IV does not have to be kept secret.
    // Typical usage is for the caller to set a random IV and then transmit
    // it as the first block of the message.
    public byte[] setRandomIv()
	{
	byte[] riv = new byte[blockSize()];
	randomBlock( riv );
	setIv( riv );
	return riv;
	}


    // Block encryption routines.

    /// Encrypt a block of bytes.
    public void encrypt( byte[] clearText, int clearOff, byte[] cipherText, int cipherOff )
	{
	xorBlock( clearText, clearOff, iv, 0, temp, 0, blockSize );
	blockCipher.encrypt( temp, 0, cipherText, cipherOff );
	copyBlock( cipherText, cipherOff, iv, 0, blockSize );
	}

    /// Decrypt a block of bytes.
    public void decrypt( byte[] cipherText, int cipherOff, byte[] clearText, int clearOff )
	{
	blockCipher.decrypt( cipherText, cipherOff, temp, 0 );
	xorBlock( temp, 0, iv, 0, clearText, clearOff, blockSize );
	copyBlock( cipherText, cipherOff, iv, 0, blockSize );
	}

    }