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/* ElGamalKeyAgreement.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

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

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exception statement from your version.  */

package gnu.javax.crypto.key.dh;

import gnu.java.security.Registry;

import gnu.java.security.util.Util;

import gnu.javax.crypto.key.BaseKeyAgreementParty;

import gnu.javax.crypto.key.KeyAgreementException;

import java.math.BigInteger;

/**

 * The ElGamal key agreement, also known as the half-certified Diffie-Hellman

 * key agreement, is described in the Handbook of Applied Cryptography [HAC] as

 * follows:

 * <ul>

 * <li>A sends to B a single message allowing one-pass key agreement.</li>

 * <li>A obtains an authentic copy of B's public key (p, g, yb), where yb =

 * g**xb.</li>

 * <li>A chooses a random integer x, 1 &lt;= x &lt;= p-2, and sends B the

 * message g**x. A computes the shared secret key K as yb**x.</li>

 * <li>B computes the same key K on receipt of the previous message as

 * (g**x)**xb.</li>

 * </ul>

 * <p>

 * RFC-2631 describes an <i>Ephemeral-Static Mode</i> of operations with

 * Diffie-Hellman keypairs as follows:

 * <pre>

 *  "In Ephemeral-Static mode, the recipient has a static (and certified)

 *  key pair, but the sender generates a new key pair for each message

 *  and sends it using the originatorKey production. If the sender's key

 *  is freshly generated for each message, the shared secret ZZ will be

 *  similarly different for each message and partyAInfo MAY be omitted,

 *  since it serves merely to decouple multiple KEKs generated by the

 *  same set of pairwise keys. If, however, the same ephemeral sender key

 *  is used for multiple messages (e.g. it is cached as a performance

 *  optimization) then a separate partyAInfo MUST be used for each

 *  message. All implementations of this standard MUST implement

 *  Ephemeral-Static mode."

 * </pre>

 * <p>

 * Reference:

 * <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>

 */

public abstract class ElGamalKeyAgreement

    extends BaseKeyAgreementParty

{

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

  public static final String KA_ELGAMAL_RECIPIENT_PRIVATE_KEY =

      "gnu.crypto.elgamal.ka.recipient.private.key";

  public static final String KA_ELGAMAL_RECIPIENT_PUBLIC_KEY =

      "gnu.crypto.elgamal.ka.recipient.public.key";

  /** The shared secret key. */

  protected BigInteger ZZ;

  protected ElGamalKeyAgreement()

  {

    super(Registry.ELGAMAL_KA);

  }

  protected byte[] engineSharedSecret() throws KeyAgreementException

  {

    return Util.trim(ZZ);

  }

  protected void engineReset()

  {

    ZZ = null;

  }

}