/* DiffieHellmanReceiver.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
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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;

/**
 * <p>This implementation is the receiver's part of the basic version of the
 * Diffie-Hellman key agreement exchange (B in [HAC]).</p>
 *
 * @see DiffieHellmanKeyAgreement
 */
public class DiffieHellmanReceiver extends DiffieHellmanKeyAgreement
{

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

  private BigInteger y; // the receiver's random secret

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

  // default 0-arguments constructor

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

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

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

  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 computeSharedSecret(in);
      default:
        throw new IllegalStateException("unexpected state");
      }
  }

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

  private OutgoingMessage computeSharedSecret(IncomingMessage in)
      throws KeyAgreementException
  {
    BigInteger m1 = in.readMPI();
    if (m1 == null)
      {
        throw new KeyAgreementException("missing message (1)");
      }

    BigInteger p = ownerKey.getParams().getP();
    BigInteger g = ownerKey.getParams().getG();

    // B chooses a random integer y, 1 <= y <= p-2
    // rfc-2631 restricts y 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);
        y = new BigInteger(1, xBytes);
      }
    while (!(y.compareTo(TWO) >= 0 && y.compareTo(p_minus_2) <= 0));

    ZZ = m1.modPow(y, p); // ZZ = (yb ^ xa) mod p

    complete = true;

    // B sends A the message: g^y mod p
    OutgoingMessage result = new OutgoingMessage();
    result.writeMPI(g.modPow(y, p)); // message (2)

    return result;
  }
}
