package gnu.crypto.key.dh;
// ----------------------------------------------------------------------------
// $Id: ElGamalSender.java,v 1.2 2005/10/06 04:24:16 rsdio 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.,
// 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
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// 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: 1.2 $
*/
public class ElGamalSender extends ElGamalKeyAgreement {
// Constants and variables
// -------------------------------------------------------------------------
/** The recipient's public key. */
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;
}
}