Source for gnu.javax.crypto.key.dh.ElGamalSender

   1: /* ElGamalSender.java --
   2:    Copyright (C) 2003, 2006 Free Software Foundation, Inc.
   3: 
   4: This file is a part of GNU Classpath.
   5: 
   6: GNU Classpath is free software; you can redistribute it and/or modify
   7: it under the terms of the GNU General Public License as published by
   8: the Free Software Foundation; either version 2 of the License, or (at
   9: your option) any later version.
  10: 
  11: GNU Classpath is distributed in the hope that it will be useful, but
  12: WITHOUT ANY WARRANTY; without even the implied warranty of
  13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14: General Public License for more details.
  15: 
  16: You should have received a copy of the GNU General Public License
  17: along with GNU Classpath; if not, write to the Free Software
  18: Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
  19: USA
  20: 
  21: Linking this library statically or dynamically with other modules is
  22: making a combined work based on this library.  Thus, the terms and
  23: conditions of the GNU General Public License cover the whole
  24: combination.
  25: 
  26: As a special exception, the copyright holders of this library give you
  27: permission to link this library with independent modules to produce an
  28: executable, regardless of the license terms of these independent
  29: modules, and to copy and distribute the resulting executable under
  30: terms of your choice, provided that you also meet, for each linked
  31: independent module, the terms and conditions of the license of that
  32: module.  An independent module is a module which is not derived from
  33: or based on this library.  If you modify this library, you may extend
  34: this exception to your version of the library, but you are not
  35: obligated to do so.  If you do not wish to do so, delete this
  36: exception statement from your version.  */
  37: 
  38: 
  39: package gnu.javax.crypto.key.dh;
  40: 
  41: import gnu.javax.crypto.key.KeyAgreementException;
  42: import gnu.javax.crypto.key.IncomingMessage;
  43: import gnu.javax.crypto.key.OutgoingMessage;
  44: 
  45: import java.math.BigInteger;
  46: import java.security.SecureRandom;
  47: import java.util.Map;
  48: 
  49: import javax.crypto.interfaces.DHPublicKey;
  50: 
  51: /**
  52:  * This implementation is the sender's part of the ElGamal key agreement
  53:  * exchange (A in [HAC]).
  54:  *
  55:  * @see ElGamalKeyAgreement
  56:  */
  57: public class ElGamalSender
  58:     extends ElGamalKeyAgreement
  59: {
  60:   /** The recipient's public key. */
  61:   private DHPublicKey B;
  62: 
  63:   // default 0-arguments constructor
  64: 
  65:   protected void engineInit(Map attributes) throws KeyAgreementException
  66:   {
  67:     rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);
  68:     // One-time setup (key generation and publication). Each user B generates
  69:     // a keypair and publishes its public key
  70:     B = (DHPublicKey) attributes.get(KA_ELGAMAL_RECIPIENT_PUBLIC_KEY);
  71:     if (B == null)
  72:       throw new KeyAgreementException("missing recipient public key");
  73:   }
  74: 
  75:   protected OutgoingMessage engineProcessMessage(IncomingMessage in)
  76:       throws KeyAgreementException
  77:   {
  78:     switch (step)
  79:       {
  80:       case 0:
  81:         return computeSharedSecret(in);
  82:       default:
  83:         throw new IllegalStateException("unexpected state");
  84:       }
  85:   }
  86: 
  87:   private OutgoingMessage computeSharedSecret(IncomingMessage in)
  88:       throws KeyAgreementException
  89:   {
  90:     BigInteger p = B.getParams().getP();
  91:     BigInteger g = B.getParams().getG();
  92:     BigInteger yb = B.getY();
  93:     // A chooses a random integer x, 1 <= x <= p-2
  94:     // rfc-2631 restricts x to only be in [2, p-1]
  95:     BigInteger p_minus_2 = p.subtract(TWO);
  96:     byte[] xBytes = new byte[(p_minus_2.bitLength() + 7) / 8];
  97:     BigInteger x;
  98:     do
  99:       {
 100:         nextRandomBytes(xBytes);
 101:         x = new BigInteger(1, xBytes);
 102:       }
 103:     while (x.compareTo(TWO) >= 0 && x.compareTo(p_minus_2) <= 0);
 104:     // A sends B the message: g^x mod p
 105:     OutgoingMessage result = new OutgoingMessage();
 106:     result.writeMPI(g.modPow(x, p));
 107:     // A computes the key as K = (yb)^x mod p
 108:     ZZ = yb.modPow(x, p); // ZZ = (yb ^ xa) mod p
 109:     complete = true;
 110:     return result;
 111:   }
 112: }