/* DiffieHellmanKeyAgreement.java -- 
   Copyright (C) 2003, 2006 Free Software Foundation, Inc.

This file is a part of GNU Classpath.

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it under the terms of the GNU General Public License as published by
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WITHOUT ANY WARRANTY; without even the implied warranty of
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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;

/**
 * <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>
 */
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;
  }
}
