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CbcBlockCipher.java

00001 // CbcBlockCipher - use a block cipher in CBC mode
00002 //
00003 // Copyright (C) 1996 by Jef Poskanzer <jef@acme.com>.  All rights reserved.
00004 //
00005 // Redistribution and use in source and binary forms, with or without
00006 // modification, are permitted provided that the following conditions
00007 // are met:
00008 // 1. Redistributions of source code must retain the above copyright
00009 //    notice, this list of conditions and the following disclaimer.
00010 // 2. Redistributions in binary form must reproduce the above copyright
00011 //    notice, this list of conditions and the following disclaimer in the
00012 //    documentation and/or other materials provided with the distribution.
00013 //
00014 // THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
00015 // ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
00016 // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
00017 // ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
00018 // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
00019 // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
00020 // OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
00021 // HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
00022 // LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
00023 // OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
00024 // SUCH DAMAGE.
00025 //
00026 // Visit the ACME Labs Java page for up-to-date versions of this and other
00027 // fine Java utilities: http://www.acme.com/java/
00028 
00029 package Acme.Crypto;
00030 
00031 import java.io.*;
00032 
00034 // <P>
00035 // A plain old block cipher, key and cleartext-block in, ciphertext-block
00036 // out, is said to be in Electronic Code Book (ECB) mode.  A given block
00037 // of plaintext always encrypts to the same block of ciphertext.  This
00038 // makes it somewhat vulnerable to known plaintext attacks, block replay
00039 // attacks, etc.
00040 // <P>
00041 // A fairly cheap alternative is to use it in Cipher Block Chaining (CBC)
00042 // mode.  All this does is XOR each plaintext block with the previous
00043 // ciphertext block before encryption.  For the first block, where there
00044 // is no previous ciphertext block, a caller-specified Initialization
00045 // Vector (IV) is used for the XOR.  This makes each block's encryption
00046 // depend on all the previous blocks
00047 // <P>
00048 // This class lets you use any given block cipher in CBC mode.
00049 // <P>
00050 // <A HREF="/resources/classes/Acme/Crypto/CbcBlockCipher.java">Fetch the software.</A><BR>
00051 // <A HREF="/resources/classes/Acme.tar.gz">Fetch the entire Acme package.</A>
00052 // <P>
00053 // @see Cipher
00054 // @see BlockCipher
00055 // @see StreamCipher
00056 // @see EncryptedOutputStream
00057 // @see EncryptedInputStream
00058 
00059 public class CbcBlockCipher extends BlockCipher
00060     {
00061 
00062     private BlockCipher blockCipher;
00063     private byte[] iv;
00064     private byte[] temp;
00065 
00067     public CbcBlockCipher( BlockCipher blockCipher )
00068         {
00069         super( blockCipher.keySize(), blockCipher.blockSize() );
00070         this.blockCipher = blockCipher;
00071         iv = new byte[blockSize()];
00072         zeroBlock( iv );
00073         temp = new byte[blockSize()];
00074         }
00075 
00076 
00077     // Key routines.
00078 
00079     // Set the key.
00080     public void setKey( byte[] key )
00081         {
00082         blockCipher.setKey( key );
00083         }
00084     
00085 
00086     // IV routines.
00087 
00089     public void setIv( byte[] iv )
00090         {
00091         copyBlock( iv, this.iv );
00092         }
00093     
00095     // In CBC mode, the IV does not have to be kept secret.
00096     // Typical usage is for the caller to set a random IV and then transmit
00097     // it as the first block of the message.
00098     public byte[] setRandomIv()
00099         {
00100         byte[] riv = new byte[blockSize()];
00101         randomBlock( riv );
00102         setIv( riv );
00103         return riv;
00104         }
00105 
00106 
00107     // Block encryption routines.
00108 
00110     public void encrypt( byte[] clearText, int clearOff, byte[] cipherText, int cipherOff )
00111         {
00112         xorBlock( clearText, clearOff, iv, 0, temp, 0, blockSize );
00113         blockCipher.encrypt( temp, 0, cipherText, cipherOff );
00114         copyBlock( cipherText, cipherOff, iv, 0, blockSize );
00115         }
00116 
00118     public void decrypt( byte[] cipherText, int cipherOff, byte[] clearText, int clearOff )
00119         {
00120         blockCipher.decrypt( cipherText, cipherOff, temp, 0 );
00121         xorBlock( temp, 0, iv, 0, clearText, clearOff, blockSize );
00122         copyBlock( cipherText, cipherOff, iv, 0, blockSize );
00123         }
00124 
00125     }

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