| /* ElGamalSender.java -- |
| Copyright (C) 2003, 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 |
| the Free Software Foundation; either version 2 of the License, or (at |
| your option) any later version. |
| |
| GNU Classpath 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 GNU Classpath; if not, write to the Free Software |
| Foundation, Inc., 51 Franklin St, 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. */ |
| |
| |
| package gnu.javax.crypto.key.dh; |
| |
| import gnu.javax.crypto.key.KeyAgreementException; |
| import gnu.javax.crypto.key.IncomingMessage; |
| import gnu.javax.crypto.key.OutgoingMessage; |
| |
| import java.math.BigInteger; |
| import java.security.SecureRandom; |
| import java.util.Map; |
| |
| import javax.crypto.interfaces.DHPublicKey; |
| |
| /** |
| * This implementation is the sender's part of the ElGamal key agreement |
| * exchange (A in [HAC]). |
| * |
| * @see ElGamalKeyAgreement |
| */ |
| public class ElGamalSender |
| extends ElGamalKeyAgreement |
| { |
| /** The recipient's public key. */ |
| private DHPublicKey B; |
| |
| // default 0-arguments constructor |
| |
| 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"); |
| } |
| } |
| |
| 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; |
| } |
| } |