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/* OFB.java --
   Copyright (C) 2001, 2002, 2006 Free Software Foundation, Inc.
 
This file is a part of GNU Classpath.
 
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it under the terms of the GNU General Public License as published by
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General Public License for more details.
 
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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
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Linking this library statically or dynamically with other modules is
making a combined work based on this library.  Thus, the terms and
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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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);
  }
}