/* ElGamalSender.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.dh;

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.DHPublicKey;

/**
 * This implementation is the sender's part of the ElGamal key agreement
 * exchange (A in [HAC]).
 * 
 * @see ElGamalKeyAgreement
 */
public class ElGamalSender
    extends ElGamalKeyAgreement
{
  /** The recipient's public key. */
  private DHPublicKey B;

  // default 0-arguments constructor

  protected void engineInit(Map attributes) throws KeyAgreementException
  {
    rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);
    // One-time setup (key generation and publication). Each user B generates
    // a keypair and publishes its public key
    B = (DHPublicKey) attributes.get(KA_ELGAMAL_RECIPIENT_PUBLIC_KEY);
    if (B == null)
      throw new KeyAgreementException("missing recipient public key");
  }

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

  private OutgoingMessage computeSharedSecret(IncomingMessage in)
      throws KeyAgreementException
  {
    BigInteger p = B.getParams().getP();
    BigInteger g = B.getParams().getG();
    BigInteger yb = B.getY();
    // 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];
    BigInteger x;
    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));
    // A computes the key as K = (yb)^x mod p
    ZZ = yb.modPow(x, p); // ZZ = (yb ^ xa) mod p
    complete = true;
    return result;
  }
}