DiffieHellmanSender.java

/* DiffieHellmanSender.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.

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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
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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
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package gnu.javax.crypto.key.dh;

import gnu.java.security.prng.IRandom;

import gnu.javax.crypto.key.KeyAgreementException;
import gnu.javax.crypto.key.IncomingMessage;
import gnu.javax.crypto.key.OutgoingMessage;

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

import javax.crypto.interfaces.DHPrivateKey;

/**
 * This implementation is the sender's part of the basic version of the
 * Diffie-Hellman key agreement exchange (A in [HAC]).
 * 
 * @see DiffieHellmanKeyAgreement
 */
public class DiffieHellmanSender
    extends DiffieHellmanKeyAgreement
{
  private BigInteger x; // the sender's random secret

  // default 0-arguments constructor

  protected void engineInit(Map attributes) throws KeyAgreementException
  {
    Object random = attributes.get(SOURCE_OF_RANDOMNESS);
    rnd = null;
    irnd = null;
    if (random instanceof SecureRandom)
      rnd = (SecureRandom) random;
    else if (random instanceof IRandom)
      irnd = (IRandom) random;
    ownerKey = (DHPrivateKey) attributes.get(KA_DIFFIE_HELLMAN_OWNER_PRIVATE_KEY);
    if (ownerKey == null)
      throw new KeyAgreementException("missing owner's private key");
  }

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

  private OutgoingMessage sendRandomSecret(IncomingMessage in)
      throws KeyAgreementException
  {
    BigInteger p = ownerKey.getParams().getP();
    BigInteger g = ownerKey.getParams().getG();
    // A chooses a random integer x, 1 <= x <= p-2
    // rfc-2631 restricts x to only be in [2, p-1]
    BigInteger p_minus_2 = p.subtract(TWO);
    byte[] xBytes = new byte[(p_minus_2.bitLength() + 7) / 8];
    do
      {
        nextRandomBytes(xBytes);
        x = new BigInteger(1, xBytes);
      }
    while (! (x.compareTo(TWO) >= 0 && x.compareTo(p_minus_2) <= 0));
    // A sends B the message: g^x mod p
    OutgoingMessage result = new OutgoingMessage();
    result.writeMPI(g.modPow(x, p));
    return result;
  }

  private OutgoingMessage computeSharedSecret(IncomingMessage in)
      throws KeyAgreementException
  {
    BigInteger m1 = in.readMPI();
    if (m1 == null)
      throw new KeyAgreementException("missing message (2)");
    BigInteger p = ownerKey.getParams().getP();
    ZZ = m1.modPow(x, p); // ZZ = (yb ^ xa) mod p
    complete = true;
    return null;
  }
}