/* 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
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Linking this library statically or dynamically with other modules is
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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>
 *  &quot;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.&quot;
 * </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;
  }
}
