/* ElGamalReceiver.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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your option) any later version.

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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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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
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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.javax.crypto.key.KeyAgreementException;
import gnu.javax.crypto.key.IncomingMessage;
import gnu.javax.crypto.key.OutgoingMessage;

import java.math.BigInteger;
import java.security.SecureRandom;
import java.util.Map;

import javax.crypto.interfaces.DHPrivateKey;

/**
 * This implementation is the receiver's part of the ElGamal key agreement
 * exchange (B in [HAC]).
 * 
 * @see ElGamalKeyAgreement
 */
public class ElGamalReceiver
    extends ElGamalKeyAgreement
{
  /** The recipient's private key. */
  private DHPrivateKey B;

  // default 0-arguments constructor

  protected void engineInit(Map attributes) throws KeyAgreementException
  {
    rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS);
    // One-time setup (key generation and publication). Each user B generates
    // a keypair and publishes its public key
    B = (DHPrivateKey) attributes.get(KA_ELGAMAL_RECIPIENT_PRIVATE_KEY);
    if (B == null)
      throw new KeyAgreementException("missing recipient private key");
  }

  protected OutgoingMessage engineProcessMessage(IncomingMessage in)
      throws KeyAgreementException
  {
    switch (step)
      {
      case 0:
        return computeSharedSecret(in);
      default:
        throw new IllegalStateException("unexpected state");
      }
  }

  private OutgoingMessage computeSharedSecret(IncomingMessage in)
      throws KeyAgreementException
  {
    // (b) B computes the same key on receipt of message (1) as
    // K = (g^x)^xb mod p
    BigInteger m1 = in.readMPI();
    if (m1 == null)
      throw new KeyAgreementException("missing message (1)");
    ZZ = m1.modPow(B.getX(), B.getParams().getP()); // ZZ = (ya ^ xb) mod p
    complete = true;
    return null;
  }
}