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1: /* ElGamalSender.java -- 2: Copyright (C) 2003, 2006 Free Software Foundation, Inc. 3: 4: This file is a part of GNU Classpath. 5: 6: GNU Classpath is free software; you can redistribute it and/or modify 7: it under the terms of the GNU General Public License as published by 8: the Free Software Foundation; either version 2 of the License, or (at 9: your option) any later version. 10: 11: GNU Classpath is distributed in the hope that it will be useful, but 12: WITHOUT ANY WARRANTY; without even the implied warranty of 13: MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU 14: General Public License for more details. 15: 16: You should have received a copy of the GNU General Public License 17: along with GNU Classpath; if not, write to the Free Software 18: Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 19: USA 20: 21: Linking this library statically or dynamically with other modules is 22: making a combined work based on this library. Thus, the terms and 23: conditions of the GNU General Public License cover the whole 24: combination. 25: 26: As a special exception, the copyright holders of this library give you 27: permission to link this library with independent modules to produce an 28: executable, regardless of the license terms of these independent 29: modules, and to copy and distribute the resulting executable under 30: terms of your choice, provided that you also meet, for each linked 31: independent module, the terms and conditions of the license of that 32: module. An independent module is a module which is not derived from 33: or based on this library. If you modify this library, you may extend 34: this exception to your version of the library, but you are not 35: obligated to do so. If you do not wish to do so, delete this 36: exception statement from your version. */ 37: 38: 39: package gnu.javax.crypto.key.dh; 40: 41: import gnu.javax.crypto.key.KeyAgreementException; 42: import gnu.javax.crypto.key.IncomingMessage; 43: import gnu.javax.crypto.key.OutgoingMessage; 44: 45: import java.math.BigInteger; 46: import java.security.SecureRandom; 47: import java.util.Map; 48: 49: import javax.crypto.interfaces.DHPublicKey; 50: 51: /** 52: * This implementation is the sender's part of the ElGamal key agreement 53: * exchange (A in [HAC]). 54: * 55: * @see ElGamalKeyAgreement 56: */ 57: public class ElGamalSender 58: extends ElGamalKeyAgreement 59: { 60: /** The recipient's public key. */ 61: private DHPublicKey B; 62: 63: // default 0-arguments constructor 64: 65: protected void engineInit(Map attributes) throws KeyAgreementException 66: { 67: rnd = (SecureRandom) attributes.get(SOURCE_OF_RANDOMNESS); 68: // One-time setup (key generation and publication). Each user B generates 69: // a keypair and publishes its public key 70: B = (DHPublicKey) attributes.get(KA_ELGAMAL_RECIPIENT_PUBLIC_KEY); 71: if (B == null) 72: throw new KeyAgreementException("missing recipient public key"); 73: } 74: 75: protected OutgoingMessage engineProcessMessage(IncomingMessage in) 76: throws KeyAgreementException 77: { 78: switch (step) 79: { 80: case 0: 81: return computeSharedSecret(in); 82: default: 83: throw new IllegalStateException("unexpected state"); 84: } 85: } 86: 87: private OutgoingMessage computeSharedSecret(IncomingMessage in) 88: throws KeyAgreementException 89: { 90: BigInteger p = B.getParams().getP(); 91: BigInteger g = B.getParams().getG(); 92: BigInteger yb = B.getY(); 93: // A chooses a random integer x, 1 <= x <= p-2 94: // rfc-2631 restricts x to only be in [2, p-1] 95: BigInteger p_minus_2 = p.subtract(TWO); 96: byte[] xBytes = new byte[(p_minus_2.bitLength() + 7) / 8]; 97: BigInteger x; 98: do 99: { 100: nextRandomBytes(xBytes); 101: x = new BigInteger(1, xBytes); 102: } 103: while (x.compareTo(TWO) >= 0 && x.compareTo(p_minus_2) <= 0); 104: // A sends B the message: g^x mod p 105: OutgoingMessage result = new OutgoingMessage(); 106: result.writeMPI(g.modPow(x, p)); 107: // A computes the key as K = (yb)^x mod p 108: ZZ = yb.modPow(x, p); // ZZ = (yb ^ xa) mod p 109: complete = true; 110: return result; 111: } 112: }