/* SRP6Host.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
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General Public License for more details.

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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.Registry;
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 gnu.javax.crypto.sasl.srp.SRPAuthInfoProvider;
import gnu.javax.crypto.sasl.srp.SRPRegistry;

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

/**
 * The implementation of the Host in the SRP-6 key agreement 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 SRP6Host
    extends SRP6KeyAgreement
{
  /** The user's ephemeral key pair. */
  private KeyPair hostKeyPair;

  /** The SRP password database. */
  private SRPAuthInfoProvider passwordDB;

  // 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);
    passwordDB = (SRPAuthInfoProvider) attributes.get(HOST_PASSWORD_DB);
    if (passwordDB == null)
      throw new KeyAgreementException("missing SRP password database");
  }

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

  protected void engineReset()
  {
    hostKeyPair = null;
    super.engineReset();
  }

  private OutgoingMessage computeSharedSecret(final IncomingMessage in)
      throws KeyAgreementException
  {
    final String I = in.readString();
    final BigInteger A = in.readMPI();
    // get s and v for user identified by I
    // ----------------------------------------------------------------------
    final Map credentials;
    try
      {
        final Map userID = new HashMap();
        userID.put(Registry.SASL_USERNAME, I);
        userID.put(SRPRegistry.MD_NAME_FIELD, srp.getAlgorithm());
        credentials = passwordDB.lookup(userID);
      }
    catch (IOException x)
      {
        throw new KeyAgreementException("computeSharedSecret()", x);
      }
    final BigInteger s = new BigInteger(
        1,Util.fromBase64((String) credentials.get(SRPRegistry.SALT_FIELD)));
    final BigInteger v = new BigInteger(
        1, Util.fromBase64((String) credentials.get(SRPRegistry.USER_VERIFIER_FIELD)));
    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);
    attributes.put(SRPKeyPairGenerator.USER_VERIFIER, v);
    kpg.setup(attributes);
    hostKeyPair = kpg.generate();
    final BigInteger B = ((SRPPublicKey) hostKeyPair.getPublic()).getY();
    final BigInteger u = uValue(A, B); // u = H(A | B)
    // compute S = (Av^u) ^ b
    final BigInteger b = ((SRPPrivateKey) hostKeyPair.getPrivate()).getX();
    final BigInteger S = A.multiply(v.modPow(u, N)).modPow(b, N);
    final byte[] sBytes = Util.trim(S);
    final IMessageDigest hash = srp.newDigest();
    hash.update(sBytes, 0, sBytes.length);
    K = new BigInteger(1, hash.digest());
    final OutgoingMessage result = new OutgoingMessage();
    result.writeMPI(s);
    result.writeMPI(B);
    complete = true;
    return result;
  }
}