/* OFB.java -- Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc. This file is a part of GNU Classpath. GNU Classpath is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. GNU Classpath is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with GNU Classpath; if not, write to the Free Software Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA Linking this library statically or dynamically with other modules is making a combined work based on this library. Thus, the terms and conditions of the GNU General Public License cover the whole combination. As a special exception, the copyright holders of this library give you permission to link this library with independent modules to produce an executable, regardless of the license terms of these independent modules, and to copy and distribute the resulting executable under terms of your choice, provided that you also meet, for each linked independent module, the terms and conditions of the license of that module. An independent module is a module which is not derived from or based on this library. If you modify this library, you may extend this exception to your version of the library, but you are not obligated to do so. If you do not wish to do so, delete this exception statement from your version. */ package gnu.javax.crypto.mode; import gnu.java.security.Registry; import gnu.javax.crypto.cipher.IBlockCipher; /** * The Output Feedback (OFB) mode is a confidentiality mode that requires a * unique IV for every message that is ever encrypted under the * given key. The OFB mode is defined as follows: * *

* In OFB encryption, the IV is transformed by the forward cipher * function to produce the first output block. The first output block is * exclusive-ORed with the first plaintext block to produce the first ciphertext * block. The first output block is then transformed by the forward cipher * function to produce the second output block. The second output block is * exclusive-ORed with the second plaintext block to produce the second * ciphertext block, and the second output block is transformed by the forward * cipher function to produce the third output block. Thus, the successive * output blocks are produced from enciphering the previous output blocks, and * the output blocks are exclusive-ORed with the corresponding plaintext blocks * to produce the ciphertext blocks. *

* In OFB decryption, the IV is transformed by the forward cipher * function to produce the first output block. The first output block is * exclusive-ORed with the first ciphertext block to recover the first plaintext * block. The first output block is then transformed by the forward cipher * function to produce the second output block. The second output block is * exclusive-ORed with the second ciphertext block to produce the second * plaintext block, and the second output block is also transformed by the * forward cipher function to produce the third output block. Thus, the * successive output blocks are produced from enciphering the previous output * blocks, and the output blocks are exclusive-ORed with the corresponding * ciphertext blocks to recover the plaintext blocks. *

* In both OFB encryption and OFB decryption, each forward cipher function * (except the first) depends on the results of the previous forward cipher * function; therefore, multiple forward cipher functions cannot be performed in * parallel. However, if the IV is known, the output blocks can * be generated prior to the availability of the plaintext or ciphertext data. *

* The OFB mode requires a unique IV for every message that is * ever encrypted under the given key. If, contrary to this requirement, the * same IV is used for the encryption of more than one message, * then the confidentiality of those messages may be compromised. In particular, * if a plaintext block of any of these messages is known, say, the jth * plaintext block, then the jth output of the forward cipher * function can be determined easily from the jth ciphertext block * of the message. This information allows the jth plaintext block * of any other message that is encrypted using the same IV to be * easily recovered from the jth ciphertext block of that message. *

* Confidentiality may similarly be compromised if any of the input blocks to * the forward cipher function for the encryption of a message is used as the * IV for the encryption of another message under the given key. *

* References: *

    *
  1. * Recommendation for Block Cipher Modes of Operation Methods and Techniques, * Morris Dworkin.
  2. *
*/ public class OFB extends BaseMode implements Cloneable { private byte[] outputBlock; /** * Trivial package-private constructor for use by the Factory class. * * @param underlyingCipher the underlying cipher implementation. * @param cipherBlockSize the underlying cipher block size to use. */ OFB(IBlockCipher underlyingCipher, int cipherBlockSize) { super(Registry.OFB_MODE, underlyingCipher, cipherBlockSize); } /** * Private constructor for cloning purposes. * * @param that the mode to clone. */ private OFB(OFB that) { this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize); } public Object clone() { return new OFB(this); } public void setup() { if (modeBlockSize != cipherBlockSize) throw new IllegalArgumentException(IMode.MODE_BLOCK_SIZE); outputBlock = (byte[]) iv.clone(); } public void teardown() { } public void encryptBlock(byte[] in, int i, byte[] out, int o) { cipher.encryptBlock(outputBlock, 0, outputBlock, 0); for (int j = 0; j < cipherBlockSize;) out[o++] = (byte)(in[i++] ^ outputBlock[j++]); } public void decryptBlock(byte[] in, int i, byte[] out, int o) { this.encryptBlock(in, i, out, o); } }