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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 1.4.

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package org.bouncycastle.crypto.tls;

import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.math.BigInteger;
import java.security.SecureRandom;

import org.bouncycastle.crypto.AsymmetricCipherKeyPair;
import org.bouncycastle.crypto.agreement.DHBasicAgreement;
import org.bouncycastle.crypto.generators.DHBasicKeyPairGenerator;
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.util.BigIntegers;

public class TlsDHUtils
{

    static final BigInteger ONE = BigInteger.valueOf(1);
    static final BigInteger TWO = BigInteger.valueOf(2);

    public static byte[] calculateDHBasicAgreement(DHPublicKeyParameters publicKey,
                                                   DHPrivateKeyParameters privateKey)
    {

        DHBasicAgreement basicAgreement = new DHBasicAgreement();
        basicAgreement.init(privateKey);
        BigInteger agreementValue = basicAgreement.calculateAgreement(publicKey);

        /*
         * RFC 5246 8.1.2. Leading bytes of Z that contain all zero bits are stripped before it is
         * used as the pre_master_secret.
         */
        return BigIntegers.asUnsignedByteArray(agreementValue);
    }

    public static AsymmetricCipherKeyPair generateDHKeyPair(SecureRandom random,
                                                            DHParameters dhParams)
    {
        DHBasicKeyPairGenerator dhGen = new DHBasicKeyPairGenerator();
        dhGen.init(new DHKeyGenerationParameters(random, dhParams));
        return dhGen.generateKeyPair();
    }

    public static DHPrivateKeyParameters generateEphemeralClientKeyExchange(SecureRandom random,
                                                                            DHParameters dhParams, OutputStream output)
        throws IOException
    {

        AsymmetricCipherKeyPair dhAgreeClientKeyPair = generateDHKeyPair(random, dhParams);
        DHPrivateKeyParameters dhAgreeClientPrivateKey = (DHPrivateKeyParameters)dhAgreeClientKeyPair
            .getPrivate();

        BigInteger Yc = ((DHPublicKeyParameters)dhAgreeClientKeyPair.getPublic()).getY();
        byte[] keData = BigIntegers.asUnsignedByteArray(Yc);
        TlsUtils.writeOpaque16(keData, output);

        return dhAgreeClientPrivateKey;
    }

    public static DHPublicKeyParameters validateDHPublicKey(DHPublicKeyParameters key)
        throws IOException
    {
        BigInteger Y = key.getY();
        DHParameters params = key.getParameters();
        BigInteger p = params.getP();
        BigInteger g = params.getG();

        if (!p.isProbablePrime(2))
        {
            throw new TlsFatalAlert(AlertDescription.illegal_parameter);
        }
        if (g.compareTo(TWO) < 0 || g.compareTo(p.subtract(TWO)) > 0)
        {
            throw new TlsFatalAlert(AlertDescription.illegal_parameter);
        }
        if (Y.compareTo(TWO) < 0 || Y.compareTo(p.subtract(ONE)) > 0)
        {
            throw new TlsFatalAlert(AlertDescription.illegal_parameter);
        }

        // TODO See RFC 2631 for more discussion of Diffie-Hellman validation

        return key;
    }

    public static BigInteger readDHParameter(InputStream input)
        throws IOException
    {
        return new BigInteger(1, TlsUtils.readOpaque16(input));
    }

    public static void writeDHParameter(BigInteger x, OutputStream output)
        throws IOException
    {
        TlsUtils.writeOpaque16(BigIntegers.asUnsignedByteArray(x), output);
    }
}