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ElGamalSender.java

package gnu.crypto.key.dh;

// ----------------------------------------------------------------------------
// $Id: ElGamalSender.java,v 1.1 2003/09/26 23:50:48 raif Exp $
//
// Copyright (C) 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.,
//    59 Temple Place - Suite 330,
//    Boston, MA 02111-1307
//    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.key.KeyAgreementException;
import gnu.crypto.key.IncomingMessage;
import gnu.crypto.key.OutgoingMessage;

import java.math.BigInteger;
import java.security.SecureRandom;
import java.util.Map;

import javax.crypto.interfaces.DHPublicKey;

/**
 * <p>This implementation is the sender's part of the ElGamal key agreement
 * exchange (A in [HAC]).</p>
 *
 * @see ElGamalKeyAgreement
 * @version $Revision: 1.1 $
 */
00063 public class ElGamalSender extends ElGamalKeyAgreement {

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

   /** The recipient's public key. */
00069    private DHPublicKey B;

   // Constructor(s)
   // -------------------------------------------------------------------------

   // default 0-arguments constructor

   // Class methods
   // -------------------------------------------------------------------------

   // Instance methods
   // -------------------------------------------------------------------------

   // implementation of abstract methods in base class ------------------------

   protected void engineInit(Map attributes) throws KeyAgreementException {
      rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);
      // One-time setup (key generation and publication). Each user B generates
      // a keypair and publishes its public key
      B = (DHPublicKey) attributes.get(KA_ELGAMAL_RECIPIENT_PUBLIC_KEY);
      if (B == null) {
         throw new KeyAgreementException("missing recipient public key");
      }
   }

   protected OutgoingMessage engineProcessMessage(IncomingMessage in)
   throws KeyAgreementException {
      switch (step) {
      case 0:  return computeSharedSecret(in);
      default: throw new IllegalStateException("unexpected state");
      }
   }

   // own methods -------------------------------------------------------------

   private OutgoingMessage computeSharedSecret(IncomingMessage in)
   throws KeyAgreementException {
      BigInteger p =  B.getParams().getP();
      BigInteger g =  B.getParams().getG();
      BigInteger yb = B.getY();

      // A chooses a random integer x, 1 <= x <= p-2
      // rfc-2631 restricts x to only be in [2, p-1]
      BigInteger p_minus_2 = p.subtract(TWO);
      byte[] xBytes = new byte[(p_minus_2.bitLength() + 7) / 8];
      BigInteger x;
      do {
         nextRandomBytes(xBytes);
         x = new BigInteger(1, xBytes);
      } while (x.compareTo(TWO) >= 0 && x.compareTo(p_minus_2) <= 0);

      // A sends B the message: g^x mod p
      OutgoingMessage result = new OutgoingMessage();
      result.writeMPI(g.modPow(x, p));

      // A computes the key as K = (yb)^x mod p
      ZZ = yb.modPow(x, p); // ZZ = (yb ^ xa) mod p

      complete = true;
      return result;
   }
}

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