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SRP6TLSClient.java

package gnu.crypto.key.srp6;

// ----------------------------------------------------------------------------
// $Id: SRP6TLSClient.java,v 1.3 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
// 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 gnu.crypto.sasl.srp.SRP;
import gnu.crypto.util.Util;

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

/**
 * <p>A variation of the SRP6 key agreement protocol, for the client-side as
 * proposed in
 * <a href="http://www.ietf.org/internet-drafts/draft-ietf-tls-srp-05.txt">Using
 * SRP for TLS Authentication</a>. The only difference between it and the SASL
 * variant is that the shared secret is the entity <code>S</code> and not
 * <code>H(S)</code>.</p>
 *
 * @version $Revision: 1.3 $
 */
00068 public class SRP6TLSClient extends SRP6KeyAgreement {

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

   /** The user's identity. */
00074    private String I;

   /** The user's cleartext password. */
00077    private byte[] p;

   /** The user's ephemeral key pair. */
00080    private KeyPair userKeyPair;

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

   // default 0-arguments constructor

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

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

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

   protected void engineInit(final Map attributes) throws KeyAgreementException {
      rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);

      final String md = (String) attributes.get(HASH_FUNCTION);
      if (md == null || "".equals(md.trim())) {
         throw new KeyAgreementException("missing hash function");
      }
      srp = SRP.instance(md);

      I = (String) attributes.get(USER_IDENTITY);
      if (I == null) {
         throw new KeyAgreementException("missing user identity");
      }
      p = (byte[]) attributes.get(USER_PASSWORD);
      if (p == null) {
         throw new KeyAgreementException("missing user password");
      }
   }

   protected OutgoingMessage engineProcessMessage(final IncomingMessage in)
   throws KeyAgreementException {
      switch (step) {
      case 0:  return sendIdentity(in);
      case 1:  return computeSharedSecret(in);
      default: throw new IllegalStateException("unexpected state");
      }
   }

   protected void engineReset() {
      I = null;
      p = null;
      userKeyPair = null;
      super.engineReset();
   }

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

   private OutgoingMessage sendIdentity(final IncomingMessage in)
   throws KeyAgreementException {
      final OutgoingMessage result = new OutgoingMessage();
      result.writeString(I);

      return result;
   }

   protected OutgoingMessage computeSharedSecret(final IncomingMessage in)
   throws KeyAgreementException {
      N = in.readMPI();
      g = in.readMPI();
      final BigInteger s = in.readMPI();
      final BigInteger B = in.readMPI();

      // generate an ephemeral keypair
      final SRPKeyPairGenerator kpg = new SRPKeyPairGenerator();
      final Map attributes = new HashMap();
      if (rnd != null) {
         attributes.put(SRPKeyPairGenerator.SOURCE_OF_RANDOMNESS, rnd);
      }
      attributes.put(SRPKeyPairGenerator.SHARED_MODULUS, N);
      attributes.put(SRPKeyPairGenerator.GENERATOR, g);
      kpg.setup(attributes);
      userKeyPair = kpg.generate();

      final BigInteger A = ((SRPPublicKey)  userKeyPair.getPublic()).getY();
      final BigInteger u = uValue(A, B); // u = H(A | B)
      final BigInteger x;
      try {
         x = new BigInteger(1, srp.computeX(Util.trim(s), I, p));
      } catch (Exception e) {
         throw new KeyAgreementException("computeSharedSecret()", e);
      }

      // compute S = (B - 3g^x) ^ (a + ux)
      final BigInteger a = ((SRPPrivateKey) userKeyPair.getPrivate()).getX();
      final BigInteger S = B.subtract(THREE.multiply(g.modPow(x, N)))
            .modPow(a.add(u.multiply(x)), N);

      K = S;

      final OutgoingMessage result = new OutgoingMessage();
      result.writeMPI(A);

      complete = true;
      return result;
   }
}

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