package gnu.crypto.prng;

// ----------------------------------------------------------------------------
// $Id: ARCFour.java,v 1.4 2005/10/06 04:24:17 rsdio Exp $
//
// Copyright (C) 2002, 2003 Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto 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, or (at your option)
// any later version.
//
// GNU Crypto 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 this program; see the file COPYING.  If not, write to the
//
//    Free Software Foundation Inc.,
//    51 Franklin Street, 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.
// ----------------------------------------------------------------------------

import gnu.crypto.Registry;

import java.util.Map;

/**
 * RC4 is a stream cipher developed by Ron Rivest. Until 1994 RC4 was a
 * trade secret of RSA Data Security, Inc., when it was released
 * anonymously to a mailing list. This version is a descendent of that
 * code, and since there is no proof that the leaked version was in fact
 * RC4 and because "RC4" is a trademark, it is called "ARCFOUR", short for
 * "Allegedly RC4".
 *
 * <p>This class only implements the <i>keystream</i> of ARCFOUR. To use
 * this as a stream cipher, one would say:</p>
 *
 * <pre>    out = in ^ arcfour.nextByte();</pre>
 *
 * <p>This operation works for encryption and decryption.</p>
 *
 * <p>References:</p>
 *
 * <ol>
 * <li>Schneier, Bruce: <i>Applied Cryptography: Protocols, Algorithms,
 * and Source Code in C, Second Edition.</i> (1996 John Wiley and Sons),
 * pp. 397--398. ISBN 0-471-11709-9</li>
 * <li>K. Kaukonen and R. Thayer, "A Stream Cipher Encryption Algorithm
 * 'Arcfour'", Internet Draft (expired), <a
 * href="http://www.mozilla.org/projects/security/pki/nss/draft-kaukonen-cipher-arcfour-03.txt">draft-kaukonen-cipher-arcfour-03.txt</a></li>
 * </ol>
 *
 * @version $Revision: 1.4 $
 */
public class ARCFour extends BasePRNG implements Cloneable {

   // Constants and variables.
   // -----------------------------------------------------------------------

   /** The attributes property name for the key bytes. */
   public static final String ARCFOUR_KEY_MATERIAL =
      "gnu.crypto.prng.arcfour.key-material";

   /** The size of the internal S-box. */
   public static final int ARCFOUR_SBOX_SIZE = 256;

   /** The S-box. */
   private byte[] s;

   private byte m, n;

   // Constructors.
   // -----------------------------------------------------------------------

   /** Default 0-arguments constructor. */
   public ARCFour() {
      super(Registry.ARCFOUR_PRNG);
   }

   // Methods implementing BasePRNG.
   // -----------------------------------------------------------------------

   public void setup(Map attributes) {
      byte[] kb = (byte[]) attributes.get(ARCFOUR_KEY_MATERIAL);

      if (kb == null) {
         throw new IllegalArgumentException("ARCFOUR needs a key");
      }

      s = new byte[ARCFOUR_SBOX_SIZE];
      m = n = 0;
      byte[] k = new byte[ARCFOUR_SBOX_SIZE];

      for (int i = 0; i < ARCFOUR_SBOX_SIZE; i++) {
         s[i] = (byte) i;
      }

      if (kb.length > 0) {
         for (int i = 0, j = 0; i < ARCFOUR_SBOX_SIZE; i++) {
            k[i] = kb[j++];
            if (j >= kb.length) j = 0;
         }
      }

      for (int i = 0, j = 0; i < ARCFOUR_SBOX_SIZE; i++) {
         j = j + s[i] + k[i];
         byte temp = s[i];
         s[i] = s[j & 0xff];
         s[j & 0xff] = temp;
      }

      buffer = new byte[ARCFOUR_SBOX_SIZE];
      try {
         fillBlock();
      } catch (LimitReachedException wontHappen) { }
   }

   public void fillBlock() throws LimitReachedException {
      for (int i = 0; i < buffer.length; i++) {
         m++;
         n = (byte) (n + s[m & 0xff]);
         byte temp = s[m & 0xff];
         s[m & 0xff] = s[n & 0xff];
         s[n & 0xff] = temp;
         temp = (byte) (s[m & 0xff] + s[n & 0xff]);
         buffer[i] = s[temp & 0xff];
      }
   }
}
