/* DiffieHellmanKeyAgreement.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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Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301
USA

Linking this library statically or dynamically with other modules is
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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.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;

import javax.crypto.interfaces.DHPrivateKey;

/**
 * The basic version of the Diffie-Hellman key agreement is described in the
 * Handbook of Applied Cryptography [HAC] as follows:
 * <ul>
 * <li>An appropriate prime p and generator g of Z<sub>p</sub><sup>*</sup>
 * (2 <= g <= 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 <= x <= p-2, and sends B message
 * (1) which is g^x mod p.</li>
 * <li>B chooses a random secret y, 1 <= y <= 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:
 * <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:
 * <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 DiffieHellmanKeyAgreement
    extends BaseKeyAgreementParty
{
  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;

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

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

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