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The Bouncy Castle Crypto package is a Java implementation of cryptographic algorithms. This jar contains JCE provider and lightweight API for the Bouncy Castle Cryptography APIs for JDK 1.5 to JDK 1.8. Note: this package includes the NTRU encryption algorithms.

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package org.bouncycastle.crypto.agreement;

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

import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.CipherParameters;
import org.bouncycastle.crypto.CryptoServicesRegistrar;
import org.bouncycastle.crypto.generators.DHKeyPairGenerator;
import org.bouncycastle.crypto.params.AsymmetricKeyParameter;
import org.bouncycastle.crypto.params.DHKeyGenerationParameters;
import org.bouncycastle.crypto.params.DHParameters;
import org.bouncycastle.crypto.params.DHPrivateKeyParameters;
import org.bouncycastle.crypto.params.DHPublicKeyParameters;
import org.bouncycastle.crypto.params.ParametersWithRandom;

/**
 * a Diffie-Hellman key exchange engine.
 * 

* note: This uses MTI/A0 key agreement in order to make the key agreement * secure against passive attacks. If you're doing Diffie-Hellman and both * parties have long term public keys you should look at using this. For * further information have a look at RFC 2631. *

* It's possible to extend this to more than two parties as well, for the moment * that is left as an exercise for the reader. */ public class DHAgreement { private static final BigInteger ONE = BigInteger.valueOf(1); private DHPrivateKeyParameters key; private DHParameters dhParams; private BigInteger privateValue; private SecureRandom random; public void init( CipherParameters param) { AsymmetricKeyParameter kParam; if (param instanceof ParametersWithRandom) { ParametersWithRandom rParam = (ParametersWithRandom)param; this.random = rParam.getRandom(); kParam = (AsymmetricKeyParameter)rParam.getParameters(); } else { this.random = CryptoServicesRegistrar.getSecureRandom(); kParam = (AsymmetricKeyParameter)param; } if (!(kParam instanceof DHPrivateKeyParameters)) { throw new IllegalArgumentException("DHEngine expects DHPrivateKeyParameters"); } this.key = (DHPrivateKeyParameters)kParam; this.dhParams = key.getParameters(); } /** * calculate our initial message. */ public BigInteger calculateMessage() { DHKeyPairGenerator dhGen = new DHKeyPairGenerator(); dhGen.init(new DHKeyGenerationParameters(random, dhParams)); AsymmetricCipherKeyPair dhPair = dhGen.generateKeyPair(); this.privateValue = ((DHPrivateKeyParameters)dhPair.getPrivate()).getX(); return ((DHPublicKeyParameters)dhPair.getPublic()).getY(); } /** * given a message from a given party and the corresponding public key, * calculate the next message in the agreement sequence. In this case * this will represent the shared secret. */ public BigInteger calculateAgreement( DHPublicKeyParameters pub, BigInteger message) { if (!pub.getParameters().equals(dhParams)) { throw new IllegalArgumentException("Diffie-Hellman public key has wrong parameters."); } BigInteger p = dhParams.getP(); BigInteger peerY = pub.getY(); if (peerY == null || peerY.compareTo(ONE) <= 0 || peerY.compareTo(p.subtract(ONE)) >= 0) { throw new IllegalArgumentException("Diffie-Hellman public key is weak"); } BigInteger result = peerY.modPow(privateValue, p); if (result.equals(ONE)) { throw new IllegalStateException("Shared key can't be 1"); } return message.modPow(key.getX(), p).multiply(result).mod(p); } }





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