1 /* CFB.java -- 2 Copyright (C) 2002, 2006 Free Software Foundation, Inc. 3 4 This file is a part of GNU Classpath. 5 6 GNU Classpath is free software; you can redistribute it and/or modify 7 it under the terms of the GNU General Public License as published by 8 the Free Software Foundation; either version 2 of the License, or (at 9 your option) any later version. 10 11 GNU Classpath is distributed in the hope that it will be useful, but 12 WITHOUT ANY WARRANTY; without even the implied warranty of 13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14 General Public License for more details. 15 16 You should have received a copy of the GNU General Public License 17 along with GNU Classpath; if not, write to the Free Software 18 Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 19 USA 20 21 Linking this library statically or dynamically with other modules is 22 making a combined work based on this library. Thus, the terms and 23 conditions of the GNU General Public License cover the whole 24 combination. 25 26 As a special exception, the copyright holders of this library give you 27 permission to link this library with independent modules to produce an 28 executable, regardless of the license terms of these independent 29 modules, and to copy and distribute the resulting executable under 30 terms of your choice, provided that you also meet, for each linked 31 independent module, the terms and conditions of the license of that 32 module. An independent module is a module which is not derived from 33 or based on this library. If you modify this library, you may extend 34 this exception to your version of the library, but you are not 35 obligated to do so. If you do not wish to do so, delete this 36 exception statement from your version. */ 37 38 39 package gnu.javax.crypto.mode; 40 41 import gnu.java.security.Registry; 42 import gnu.javax.crypto.cipher.IBlockCipher; 43 44 /** 45 * The cipher feedback mode. CFB mode is a stream mode that operates on <i>s</i> 46 * bit blocks, where 1 <= <i>s</i> <= <i>b</i>, if <i>b</i> is the 47 * underlying cipher's block size. Encryption is: 48 * <pre> 49 * I[1] = IV 50 * I[j] = LSB(b-s, I[j-1]) | C[j-1] for j = 2...n 51 * O[j] = CIPH(K, I[j]) for j = 1,2...n 52 * C[j] = P[j] ˆ MSB(s, O[j]) for j = 1,2...n 53 * </pre> 54 * <p> 55 * And decryption is: 56 * <pre> 57 * I[1] = IV 58 * I[j] = LSB(b-s, I[j-1]) | C[j-1] for j = 2...n 59 * O[j] = CIPH(K, I[j]) for j = 1,2...n 60 * P[j] = C[j] ˆ MSB(s, O[j]) for j = 1,2...n 61 * </pre> 62 * <p> 63 * CFB mode requires an initialization vector, which need not be kept secret. 64 * <p> 65 * References: 66 * <ol> 67 * <li>Bruce Schneier, <i>Applied Cryptography: Protocols, Algorithms, and 68 * Source Code in C, Second Edition</i>. (1996 John Wiley and Sons) ISBN 69 * 0-471-11709-9.</li> 70 * <li><a 71 * href="http://csrc.nist.gov/encryption/modes/Recommendation/Modes01.pdf"> 72 * Recommendation for Block Cipher Modes of Operation Methods and Techniques</a>, 73 * Morris Dworkin.</li> 74 * </ol> 75 */ 76 public class CFB 77 extends BaseMode 78 { 79 /** The shift register, the input block to the block cipher. */ 80 private byte[] shiftRegister; 81 /** The output block from the block cipher. */ 82 private byte[] scratch; 83 84 /** 85 * Package-private constructor for the factory class. 86 * 87 * @param underlyingCipher The cipher implementation. 88 * @param cipherBlockSize The cipher's block size. 89 */ CFB(IBlockCipher underlyingCipher, int cipherBlockSize)90 CFB(IBlockCipher underlyingCipher, int cipherBlockSize) 91 { 92 super(Registry.CFB_MODE, underlyingCipher, cipherBlockSize); 93 } 94 95 /** 96 * Cloneing constructor. 97 * 98 * @param that The instance being cloned. 99 */ CFB(CFB that)100 private CFB(CFB that) 101 { 102 this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize); 103 } 104 clone()105 public Object clone() 106 { 107 return new CFB(this); 108 } 109 setup()110 public void setup() 111 { 112 if (modeBlockSize > cipherBlockSize) 113 throw new IllegalArgumentException( 114 "CFB block size cannot be larger than the cipher block size"); 115 shiftRegister = new byte[cipherBlockSize]; 116 scratch = new byte[cipherBlockSize]; 117 System.arraycopy(iv, 0, 118 shiftRegister, 0, 119 Math.min(iv.length, cipherBlockSize)); 120 } 121 teardown()122 public void teardown() 123 { 124 if (shiftRegister != null) 125 for (int i = 0; i < shiftRegister.length; i++) 126 shiftRegister[i] = 0; 127 shiftRegister = null; 128 } 129 encryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)130 public void encryptBlock(byte[] in, int inOffset, byte[] out, int outOffset) 131 { 132 cipher.encryptBlock(shiftRegister, 0, scratch, 0); 133 for (int i = 0; i < modeBlockSize; i++) 134 out[outOffset + i] = (byte)(in[inOffset + i] ^ scratch[i]); 135 System.arraycopy(shiftRegister, modeBlockSize, 136 shiftRegister, 0, 137 cipherBlockSize - modeBlockSize); 138 System.arraycopy(out, outOffset, 139 shiftRegister, cipherBlockSize - modeBlockSize, 140 modeBlockSize); 141 } 142 decryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)143 public void decryptBlock(byte[] in, int inOffset, byte[] out, int outOffset) 144 { 145 cipher.encryptBlock(shiftRegister, 0, scratch, 0); 146 for (int i = 0; i < modeBlockSize; i++) 147 out[outOffset + i] = (byte)(in[inOffset + i] ^ scratch[i]); 148 System.arraycopy(shiftRegister, modeBlockSize, 149 shiftRegister, 0, 150 cipherBlockSize - modeBlockSize); 151 System.arraycopy(in, inOffset, 152 shiftRegister, cipherBlockSize - modeBlockSize, 153 modeBlockSize); 154 } 155 } 156