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Java > Open Source Codes > Acme > Crypto > CbcBlockCipher


1 // CbcBlockCipher - use a block cipher in CBC mode
2 //
3 // Copyright (C) 1996 by Jef Poskanzer <jef@acme.com>. All rights reserved.
4 //
5 // Redistribution and use in source and binary forms, with or without
6 // modification, are permitted provided that the following conditions
7 // are met:
8 // 1. Redistributions of source code must retain the above copyright
9 // notice, this list of conditions and the following disclaimer.
10 // 2. Redistributions in binary form must reproduce the above copyright
11 // notice, this list of conditions and the following disclaimer in the
12 // documentation and/or other materials provided with the distribution.
13 //
14 // THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
15 // ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
16 // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
17 // ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
18 // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
19 // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
20 // OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
21 // HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
22 // LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
23 // OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
24 // SUCH DAMAGE.
25 //
26 // Visit the ACME Labs Java page for up-to-date versions of this and other
27 // fine Java utilities: http://www.acme.com/java/
28
29 package Acme.Crypto;
30
31 import java.io.*;
32
33 /// Use a block cipher in CBC mode.
34 // <P>
35 // A plain old block cipher, key and cleartext-block in, ciphertext-block
36 // out, is said to be in Electronic Code Book (ECB) mode. A given block
37 // of plaintext always encrypts to the same block of ciphertext. This
38 // makes it somewhat vulnerable to known plaintext attacks, block replay
39 // attacks, etc.
40 // <P>
41 // A fairly cheap alternative is to use it in Cipher Block Chaining (CBC)
42 // mode. All this does is XOR each plaintext block with the previous
43 // ciphertext block before encryption. For the first block, where there
44 // is no previous ciphertext block, a caller-specified Initialization
45 // Vector (IV) is used for the XOR. This makes each block's encryption
46 // depend on all the previous blocks
47 // <P>
48 // This class lets you use any given block cipher in CBC mode.
49 // <P>
50 // <A HREF="../../../resources/classes/Acme/Crypto/CbcBlockCipher.java">Fetch the software.</A><BR>
51 // <A HREF="../../../resources/classes/Acme.tar.gz">Fetch the entire Acme package.</A>
52 // <P>
53 // @see Cipher
54 // @see BlockCipher
55 // @see StreamCipher
56 // @see EncryptedOutputStream
57 // @see EncryptedInputStream
58
59 public class CbcBlockCipher extends BlockCipher
60     {
61
62     private BlockCipher blockCipher;
63     private byte[] iv;
64     private byte[] temp;
65
66     /// Constructor.
67 public CbcBlockCipher( BlockCipher blockCipher )
68     {
69     super( blockCipher.keySize(), blockCipher.blockSize() );
70     this.blockCipher = blockCipher;
71     iv = new byte[blockSize()];
72     zeroBlock( iv );
73     temp = new byte[blockSize()];
74     }
75
76
77     // Key routines.
78
79     // Set the key.
80 public void setKey( byte[] key )
81     {
82     blockCipher.setKey( key );
83     }
84     
85
86     // IV routines.
87
88     /// Set the Initialization Vector.
89 public void setIv( byte[] iv )
90     {
91     copyBlock( iv, this.iv );
92     }
93     
94     /// Set and return a random IV.
95 // In CBC mode, the IV does not have to be kept secret.
96 // Typical usage is for the caller to set a random IV and then transmit
97 // it as the first block of the message.
98 public byte[] setRandomIv()
99     {
100     byte[] riv = new byte[blockSize()];
101     randomBlock( riv );
102     setIv( riv );
103     return riv;
104     }
105
106
107     // Block encryption routines.
108
109     /// Encrypt a block of bytes.
110 public void encrypt( byte[] clearText, int clearOff, byte[] cipherText, int cipherOff )
111     {
112     xorBlock( clearText, clearOff, iv, 0, temp, 0, blockSize );
113     blockCipher.encrypt( temp, 0, cipherText, cipherOff );
114     copyBlock( cipherText, cipherOff, iv, 0, blockSize );
115     }
116
117     /// Decrypt a block of bytes.
118 public void decrypt( byte[] cipherText, int cipherOff, byte[] clearText, int clearOff )
119     {
120     blockCipher.decrypt( cipherText, cipherOff, temp, 0 );
121     xorBlock( temp, 0, iv, 0, clearText, clearOff, blockSize );
122     copyBlock( cipherText, cipherOff, iv, 0, blockSize );
123     }
124
125     }
126
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