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+/* ElGamalSender.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.
+
+You should have received a copy of the GNU General Public License
+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
+obligated to do so. If you do not wish to do so, delete this
+exception statement from your version. */
+
+
+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.DHPublicKey;
+
+/**
+ * This implementation is the sender's part of the ElGamal key agreement
+ * exchange (A in [HAC]).
+ *
+ * @see ElGamalKeyAgreement
+ */
+public class ElGamalSender
+ extends ElGamalKeyAgreement
+{
+ /** The recipient's public key. */
+ private DHPublicKey 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 = (DHPublicKey) attributes.get(KA_ELGAMAL_RECIPIENT_PUBLIC_KEY);
+ if (B == null)
+ throw new KeyAgreementException("missing recipient public 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
+ {
+ BigInteger p = B.getParams().getP();
+ BigInteger g = B.getParams().getG();
+ BigInteger yb = B.getY();
+ // A chooses a random integer x, 1 <= x <= p-2
+ // rfc-2631 restricts x to only be in [2, p-1]
+ BigInteger p_minus_2 = p.subtract(TWO);
+ byte[] xBytes = new byte[(p_minus_2.bitLength() + 7) / 8];
+ BigInteger x;
+ do
+ {
+ nextRandomBytes(xBytes);
+ x = new BigInteger(1, xBytes);
+ }
+ while (x.compareTo(TWO) >= 0 && x.compareTo(p_minus_2) <= 0);
+ // A sends B the message: g^x mod p
+ OutgoingMessage result = new OutgoingMessage();
+ result.writeMPI(g.modPow(x, p));
+ // A computes the key as K = (yb)^x mod p
+ ZZ = yb.modPow(x, p); // ZZ = (yb ^ xa) mod p
+ complete = true;
+ return result;
+ }
+}