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jeremybenn |
/* OFB.java --
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Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.
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This file is a part of GNU Classpath.
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GNU Classpath is free software; you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 2 of the License, or (at
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your option) any later version.
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GNU Classpath is distributed in the hope that it will be useful, but
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WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
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General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with GNU Classpath; if not, write to the Free Software
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
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USA
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Linking this library statically or dynamically with other modules is
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making a combined work based on this library. Thus, the terms and
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conditions of the GNU General Public License cover the whole
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combination.
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As a special exception, the copyright holders of this library give you
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permission to link this library with independent modules to produce an
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executable, regardless of the license terms of these independent
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modules, and to copy and distribute the resulting executable under
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terms of your choice, provided that you also meet, for each linked
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independent module, the terms and conditions of the license of that
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module. An independent module is a module which is not derived from
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or based on this library. If you modify this library, you may extend
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this exception to your version of the library, but you are not
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obligated to do so. If you do not wish to do so, delete this
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exception statement from your version. */
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package gnu.javax.crypto.mode;
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import gnu.java.security.Registry;
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import gnu.javax.crypto.cipher.IBlockCipher;
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/**
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* The Output Feedback (OFB) mode is a confidentiality mode that requires a
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* unique <code>IV</code> for every message that is ever encrypted under the
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* given key. The OFB mode is defined as follows:
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* <ul>
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* <li>OFB Encryption:
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* <ul>
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* <li>I<sub>1</sub> = IV;</li>
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* <li>I<sub>j</sub> = O<sub>j -1</sub> for j = 2...n;</li>
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* <li>O<sub>j</sub> = CIPH<sub>K</sub>(I<sub>j</sub>) for j = 1, 2...n;</li>
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* <li>C<sub>j</sub> = P<sub>j</sub> XOR O<sub>j</sub> for j = 1, 2...n.</li>
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* </ul>
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* </li>
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* <li>OFB Decryption:
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* <ul>
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* <li>I<sub>1</sub> = IV;</li>
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* <li>I<sub>j</sub> = O<sub>j -1</sub> for j = 2...n;</li>
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* <li>O<sub>j</sub> = CIPH<sub>K</sub>(I<sub>j</sub>) for j = 1, 2...n;</li>
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* <li>P<sub>j</sub> = C<sub>j</sub> XOR O<sub>j</sub> for j = 1, 2...n.</li>
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* </ul>
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* </li>
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* </ul>
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* <p>
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* In OFB encryption, the <code>IV</code> is transformed by the forward cipher
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* function to produce the first output block. The first output block is
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* exclusive-ORed with the first plaintext block to produce the first ciphertext
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* block. The first output block is then transformed by the forward cipher
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* function to produce the second output block. The second output block is
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* exclusive-ORed with the second plaintext block to produce the second
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* ciphertext block, and the second output block is transformed by the forward
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* cipher function to produce the third output block. Thus, the successive
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* output blocks are produced from enciphering the previous output blocks, and
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* the output blocks are exclusive-ORed with the corresponding plaintext blocks
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* to produce the ciphertext blocks.
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* <p>
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* In OFB decryption, the <code>IV</code> is transformed by the forward cipher
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* function to produce the first output block. The first output block is
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* exclusive-ORed with the first ciphertext block to recover the first plaintext
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* block. The first output block is then transformed by the forward cipher
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* function to produce the second output block. The second output block is
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* exclusive-ORed with the second ciphertext block to produce the second
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* plaintext block, and the second output block is also transformed by the
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* forward cipher function to produce the third output block. Thus, the
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* successive output blocks are produced from enciphering the previous output
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* blocks, and the output blocks are exclusive-ORed with the corresponding
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* ciphertext blocks to recover the plaintext blocks.
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* <p>
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* In both OFB encryption and OFB decryption, each forward cipher function
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* (except the first) depends on the results of the previous forward cipher
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* function; therefore, multiple forward cipher functions cannot be performed in
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* parallel. However, if the <code>IV</code> is known, the output blocks can
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* be generated prior to the availability of the plaintext or ciphertext data.
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* <p>
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* The OFB mode requires a unique <code>IV</code> for every message that is
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* ever encrypted under the given key. If, contrary to this requirement, the
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* same <code>IV</code> is used for the encryption of more than one message,
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* then the confidentiality of those messages may be compromised. In particular,
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* if a plaintext block of any of these messages is known, say, the j<sup>th</sup>
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* plaintext block, then the j<sup>th</sup> output of the forward cipher
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* function can be determined easily from the j<sup>th</sup> ciphertext block
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* of the message. This information allows the j<sup>th</sup> plaintext block
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* of any other message that is encrypted using the same <code>IV</code> to be
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* easily recovered from the jth ciphertext block of that message.
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* <p>
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* Confidentiality may similarly be compromised if any of the input blocks to
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* the forward cipher function for the encryption of a message is used as the
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* <code>IV</code> for the encryption of another message under the given key.
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* <p>
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* References:
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* <ol>
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* <li><a
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* href="http://csrc.nist.gov/encryption/modes/Recommendation/Modes01.pdf">
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* Recommendation for Block Cipher Modes of Operation Methods and Techniques</a>,
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* Morris Dworkin.</li>
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* </ol>
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*/
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public class OFB
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extends BaseMode
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implements Cloneable
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{
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private byte[] outputBlock;
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/**
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* Trivial package-private constructor for use by the Factory class.
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*
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* @param underlyingCipher the underlying cipher implementation.
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* @param cipherBlockSize the underlying cipher block size to use.
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*/
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OFB(IBlockCipher underlyingCipher, int cipherBlockSize)
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{
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super(Registry.OFB_MODE, underlyingCipher, cipherBlockSize);
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}
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/**
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* Private constructor for cloning purposes.
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*
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* @param that the mode to clone.
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*/
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private OFB(OFB that)
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{
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this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
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}
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public Object clone()
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{
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return new OFB(this);
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}
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public void setup()
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{
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if (modeBlockSize != cipherBlockSize)
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throw new IllegalArgumentException(IMode.MODE_BLOCK_SIZE);
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outputBlock = (byte[]) iv.clone();
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}
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public void teardown()
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{
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}
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public void encryptBlock(byte[] in, int i, byte[] out, int o)
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{
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cipher.encryptBlock(outputBlock, 0, outputBlock, 0);
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for (int j = 0; j < cipherBlockSize;)
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out[o++] = (byte)(in[i++] ^ outputBlock[j++]);
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}
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public void decryptBlock(byte[] in, int i, byte[] out, int o)
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{
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this.encryptBlock(in, i, out, o);
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}
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}
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