/* 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.

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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.

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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
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permission to link this library with independent modules to produce an
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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;
  }
}