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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

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WITHOUT ANY WARRANTY; without even the implied warranty of

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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

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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);

  }

}