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/* SRP6User.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

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exception statement from your version.  */

package gnu.javax.crypto.key.srp6;

import gnu.java.security.hash.IMessageDigest;

import gnu.java.security.util.Util;

import gnu.javax.crypto.key.KeyAgreementException;

import gnu.javax.crypto.key.IncomingMessage;

import gnu.javax.crypto.key.OutgoingMessage;

import gnu.javax.crypto.sasl.srp.SRP;

import java.math.BigInteger;

import java.security.KeyPair;

import java.security.SecureRandom;

import java.util.HashMap;

import java.util.Map;

/**

 * The implementation of the User in the SRP-6 protocol.

 * <p>

 * Reference:

 * <ol>

 * <li><a href="http://srp.stanford.edu/design.html">SRP Protocol Design</a><br>

 * Thomas J. Wu.</li>

 * </ol>

 */

public class SRP6User

    extends SRP6KeyAgreement

{

  /** The user's identity. */

  private String I;

  /** The user's cleartext password. */

  private byte[] p;

  /** The user's ephemeral key pair. */

  private KeyPair userKeyPair;

  // default 0-arguments constructor

  protected void engineInit(final Map attributes) throws KeyAgreementException

  {

    rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);

    N = (BigInteger) attributes.get(SHARED_MODULUS);

    if (N == null)

      throw new KeyAgreementException("missing shared modulus");

    g = (BigInteger) attributes.get(GENERATOR);

    if (g == null)

      throw new KeyAgreementException("missing generator");

    final String md = (String) attributes.get(HASH_FUNCTION);

    if (md == null || md.trim().length() == 0)

      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();

  }

  private OutgoingMessage sendIdentity(final IncomingMessage in)

      throws KeyAgreementException

  {

    // 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 OutgoingMessage result = new OutgoingMessage();

    result.writeString(I);

    result.writeMPI(((SRPPublicKey) userKeyPair.getPublic()).getY());

    return result;

  }

  private OutgoingMessage computeSharedSecret(final IncomingMessage in)

      throws KeyAgreementException

  {

    final BigInteger s = in.readMPI();

    final BigInteger B = in.readMPI();

    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);

    final byte[] sBytes = Util.trim(S);

    final IMessageDigest hash = srp.newDigest();

    hash.update(sBytes, 0, sBytes.length);

    K = new BigInteger(1, hash.digest());

    complete = true;

    return null;

  }

}