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+/* 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 <code>IV</code> for every message that is ever encrypted under the
+ * given key. The OFB mode is defined as follows:
+ * <ul>
+ * <li>OFB Encryption:
+ * <ul>
+ * <li>I<sub>1</sub> = IV;</li>
+ * <li>I<sub>j</sub> = O<sub>j -1</sub> for j = 2...n;</li>
+ * <li>O<sub>j</sub> = CIPH<sub>K</sub>(I<sub>j</sub>) for j = 1, 2...n;</li>
+ * <li>C<sub>j</sub> = P<sub>j</sub> XOR O<sub>j</sub> for j = 1, 2...n.</li>
+ * </ul>
+ * </li>
+ * <li>OFB Decryption:
+ * <ul>
+ * <li>I<sub>1</sub> = IV;</li>
+ * <li>I<sub>j</sub> = O<sub>j -1</sub> for j = 2...n;</li>
+ * <li>O<sub>j</sub> = CIPH<sub>K</sub>(I<sub>j</sub>) for j = 1, 2...n;</li>
+ * <li>P<sub>j</sub> = C<sub>j</sub> XOR O<sub>j</sub> for j = 1, 2...n.</li>
+ * </ul>
+ * </li>
+ * </ul>
+ * <p>
+ * In OFB encryption, the <code>IV</code> 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.
+ * <p>
+ * In OFB decryption, the <code>IV</code> 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.
+ * <p>
+ * 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 <code>IV</code> is known, the output blocks can
+ * be generated prior to the availability of the plaintext or ciphertext data.
+ * <p>
+ * The OFB mode requires a unique <code>IV</code> for every message that is
+ * ever encrypted under the given key. If, contrary to this requirement, the
+ * same <code>IV</code> 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 j<sup>th</sup>
+ * plaintext block, then the j<sup>th</sup> output of the forward cipher
+ * function can be determined easily from the j<sup>th</sup> ciphertext block
+ * of the message. This information allows the j<sup>th</sup> plaintext block
+ * of any other message that is encrypted using the same <code>IV</code> to be
+ * easily recovered from the jth ciphertext block of that message.
+ * <p>
+ * 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
+ * <code>IV</code> for the encryption of another message under the given key.
+ * <p>
+ * References:
+ * <ol>
+ * <li><a
+ * href="http://csrc.nist.gov/encryption/modes/Recommendation/Modes01.pdf">
+ * Recommendation for Block Cipher Modes of Operation Methods and Techniques</a>,
+ * Morris Dworkin.</li>
+ * </ol>
+ */
+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);
+  }
+}