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+/* CFB.java --
+ Copyright (C) 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 cipher feedback mode. CFB mode is a stream mode that operates on <i>s</i>
+ * bit blocks, where 1 &lt;= <i>s</i> &lt;= <i>b</i>, if <i>b</i> is the
+ * underlying cipher's block size. Encryption is:
+ * <pre>
+ * I[1] = IV
+ * I[j] = LSB(b-s, I[j-1]) | C[j-1] for j = 2...n
+ * O[j] = CIPH(K, I[j]) for j = 1,2...n
+ * C[j] = P[j] &circ; MSB(s, O[j]) for j = 1,2...n
+ * </pre>
+ * <p>
+ * And decryption is:
+ * <pre>
+ * I[1] = IV
+ * I[j] = LSB(b-s, I[j-1]) | C[j-1] for j = 2...n
+ * O[j] = CIPH(K, I[j]) for j = 1,2...n
+ * P[j] = C[j] &circ; MSB(s, O[j]) for j = 1,2...n
+ * </pre>
+ * <p>
+ * CFB mode requires an initialization vector, which need not be kept secret.
+ * <p>
+ * References:
+ * <ol>
+ * <li>Bruce Schneier, <i>Applied Cryptography: Protocols, Algorithms, and
+ * Source Code in C, Second Edition</i>. (1996 John Wiley and Sons) ISBN
+ * 0-471-11709-9.</li>
+ * <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 CFB
+ extends BaseMode
+{
+ /** The shift register, the input block to the block cipher. */
+ private byte[] shiftRegister;
+ /** The output block from the block cipher. */
+ private byte[] scratch;
+
+ /**
+ * Package-private constructor for the factory class.
+ *
+ * @param underlyingCipher The cipher implementation.
+ * @param cipherBlockSize The cipher's block size.
+ */
+ CFB(IBlockCipher underlyingCipher, int cipherBlockSize)
+ {
+ super(Registry.CFB_MODE, underlyingCipher, cipherBlockSize);
+ }
+
+ /**
+ * Cloneing constructor.
+ *
+ * @param that The instance being cloned.
+ */
+ private CFB(CFB that)
+ {
+ this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
+ }
+
+ public Object clone()
+ {
+ return new CFB(this);
+ }
+
+ public void setup()
+ {
+ if (modeBlockSize > cipherBlockSize)
+ throw new IllegalArgumentException(
+ "CFB block size cannot be larger than the cipher block size");
+ shiftRegister = new byte[cipherBlockSize];
+ scratch = new byte[cipherBlockSize];
+ System.arraycopy(iv, 0,
+ shiftRegister, 0,
+ Math.min(iv.length, cipherBlockSize));
+ }
+
+ public void teardown()
+ {
+ if (shiftRegister != null)
+ for (int i = 0; i < shiftRegister.length; i++)
+ shiftRegister[i] = 0;
+ shiftRegister = null;
+ }
+
+ public void encryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
+ {
+ cipher.encryptBlock(shiftRegister, 0, scratch, 0);
+ for (int i = 0; i < modeBlockSize; i++)
+ out[outOffset + i] = (byte)(in[inOffset + i] ^ scratch[i]);
+ System.arraycopy(shiftRegister, modeBlockSize,
+ shiftRegister, 0,
+ cipherBlockSize - modeBlockSize);
+ System.arraycopy(out, outOffset,
+ shiftRegister, cipherBlockSize - modeBlockSize,
+ modeBlockSize);
+ }
+
+ public void decryptBlock(byte[] in, int inOffset, byte[] out, int outOffset)
+ {
+ cipher.encryptBlock(shiftRegister, 0, scratch, 0);
+ for (int i = 0; i < modeBlockSize; i++)
+ out[outOffset + i] = (byte)(in[inOffset + i] ^ scratch[i]);
+ System.arraycopy(shiftRegister, modeBlockSize,
+ shiftRegister, 0,
+ cipherBlockSize - modeBlockSize);
+ System.arraycopy(in, inOffset,
+ shiftRegister, cipherBlockSize - modeBlockSize,
+ modeBlockSize);
+ }
+}