package gnu.crypto.key.dh;

// ----------------------------------------------------------------------------
// $Id: DiffieHellmanKeyAgreement.java,v 1.2 2005/10/06 04:24:16 rsdio Exp $
//
// Copyright (C) 2003 Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto 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, or (at your option)
// any later version.
//
// GNU Crypto 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 this program; see the file COPYING.  If not, write to the
//
//    Free Software Foundation Inc.,
//    51 Franklin Street, 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
// combination.
//
// 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 modules, and to copy and distribute the resulting
// executable under terms of your choice, provided that you also meet,
// for each linked independent module, the terms and conditions of the
// license of that module.  An independent module is a module which is
// not derived from or based on this library.  If you modify this
// library, you may extend this exception to your version of the
// library, but you are not obligated to do so.  If you do not wish to
// do so, delete this exception statement from your version.
// ----------------------------------------------------------------------------

import gnu.crypto.Registry;
import gnu.crypto.key.BaseKeyAgreementParty;
import gnu.crypto.key.KeyAgreementException;
import gnu.crypto.util.Util;

import java.math.BigInteger;

import javax.crypto.interfaces.DHPrivateKey;

/**
 * <p>The basic version of the Diffie-Hellman key agreement is described in the
 * Handbook of Applied Cryptography [HAC] as follows:</p>
 * <ul>
 *    <li>An appropriate prime p and generator g of Z<sub>p</sub><sup>*</sup>
 *    (2 &lt;= g &lt;= p-2) are selected and published.</li>
 *    <li>A and B each send the other one message over an open channel; as a
 *    result, they both can then compute a shared secret key K which they can
 *    use to protect their future communication.</li>
 *    <li>A chooses a random secret x, 1 &lt;= x &lt;= p-2, and sends B message
 *    (1) which is g^x mod p.</li>
 *    <li>B chooses a random secret y, 1 &lt;= y &lt;= p-2, and sends A message
 *    (2) which is g^y mod p.</li>
 *    <li>B receives message (1) and computes the shared key as K = (g^x)^y mod
 *    p.</li>
 *    <li>A receives message (2) and computes the shared key as K = (g^y)^x mod
 *    p.</li>
 * </ul>
 *
 * <p>RFC-2631 describes a <i>Static-Static Mode</i> of operations with
 * Diffie-Hellman keypairs as follows:</p>
 * <pre>
 * "In Static-Static mode, both the sender and the recipient have a
   static (and certified) key pair. Since the sender's and recipient's
   keys are therefore the same for each message, ZZ will be the same for
   each message. Thus, partyAInfo MUST be used (and different for each
   message) in order to ensure that different messages use different
   KEKs. Implementations MAY implement Static-Static mode."
 * </pre>
 *
 * <p>Reference:</p>
 * <ol>
 *    <li><a href="http://www.ietf.org/rfc/rfc2631.txt">Diffie-Hellman Key
 *    Agreement Method</a><br>
 *    Eric Rescorla.</li>
 *    <li><a href="http://www.cacr.math.uwaterloo.ca/hac">[HAC]</a>: Handbook of
 *    Applied Cryptography.<br>
 *    CRC Press, Inc. ISBN 0-8493-8523-7, 1997<br>
 *    Menezes, A., van Oorschot, P. and S. Vanstone.</li>
 * </ol>
 *
 * @version $Revision: 1.2 $
 */
public abstract class DiffieHellmanKeyAgreement extends BaseKeyAgreementParty {

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

   public static final String SOURCE_OF_RANDOMNESS = "gnu.crypto.dh.ka.prng";

   public static final String KA_DIFFIE_HELLMAN_OWNER_PRIVATE_KEY =
         "gnu.crypto.dh.ka.owner.private.key";

   /** The key agreement party's private key. */
   protected DHPrivateKey ownerKey;

   /** The shared secret key. */
   protected BigInteger ZZ;

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

   protected DiffieHellmanKeyAgreement() {
      super(Registry.DH_KA);
   }

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

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

   // implementation of common abstract methods in BaseKeyAGreementParty ------

   protected byte[] engineSharedSecret() throws KeyAgreementException {
      return Util.trim(ZZ);
   }

   protected void engineReset() {
      ownerKey = null;
      ZZ = null;
   }
}
