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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.
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package org.bouncycastle.crypto.tls;

import java.io.IOException;
import java.util.Hashtable;
import java.util.Vector;

public abstract class AbstractTlsClient
    extends AbstractTlsPeer
    implements TlsClient
{
    protected TlsCipherFactory cipherFactory;

    protected TlsClientContext context;

    protected Vector supportedSignatureAlgorithms;
    protected int[] namedCurves;
    protected short[] clientECPointFormats, serverECPointFormats;

    protected int selectedCipherSuite;
    protected short selectedCompressionMethod;

    public AbstractTlsClient()
    {
        this(new DefaultTlsCipherFactory());
    }

    public AbstractTlsClient(TlsCipherFactory cipherFactory)
    {
        this.cipherFactory = cipherFactory;
    }

    protected boolean allowUnexpectedServerExtension(Integer extensionType, byte[] extensionData)
        throws IOException
    {
        switch (extensionType.intValue())
        {
        case ExtensionType.elliptic_curves:
            /*
             * Exception added based on field reports that some servers do send this, although the
             * Supported Elliptic Curves Extension is clearly intended to be client-only. If
             * present, we still require that it is a valid EllipticCurveList.
             */
            TlsECCUtils.readSupportedEllipticCurvesExtension(extensionData);
            return true;
        default:
            return false;
        }
    }

    protected void checkForUnexpectedServerExtension(Hashtable serverExtensions, Integer extensionType)
        throws IOException
    {
        byte[] extensionData = TlsUtils.getExtensionData(serverExtensions, extensionType);
        if (extensionData != null && !allowUnexpectedServerExtension(extensionType, extensionData))
        {
            throw new TlsFatalAlert(AlertDescription.illegal_parameter);
        }
    }

    public void init(TlsClientContext context)
    {
        this.context = context;
    }

    public TlsSession getSessionToResume()
    {
        return null;
    }

    public ProtocolVersion getClientHelloRecordLayerVersion()
    {
        // "{03,00}"
        // return ProtocolVersion.SSLv3;

        // "the lowest version number supported by the client"
        // return getMinimumVersion();

        // "the value of ClientHello.client_version"
        return getClientVersion();
    }

    public ProtocolVersion getClientVersion()
    {
        return ProtocolVersion.TLSv12;
    }

    public boolean isFallback()
    {
        /*
         * RFC 7507 4. The TLS_FALLBACK_SCSV cipher suite value is meant for use by clients that
         * repeat a connection attempt with a downgraded protocol (perform a "fallback retry") in
         * order to work around interoperability problems with legacy servers.
         */
        return false;
    }

    public Hashtable getClientExtensions()
        throws IOException
    {
        Hashtable clientExtensions = null;

        ProtocolVersion clientVersion = context.getClientVersion();

        /*
         * RFC 5246 7.4.1.4.1. Note: this extension is not meaningful for TLS versions prior to 1.2.
         * Clients MUST NOT offer it if they are offering prior versions.
         */
        if (TlsUtils.isSignatureAlgorithmsExtensionAllowed(clientVersion))
        {
            // TODO Provide a way for the user to specify the acceptable hash/signature algorithms.

            this.supportedSignatureAlgorithms = TlsUtils.getDefaultSupportedSignatureAlgorithms();

            clientExtensions = TlsExtensionsUtils.ensureExtensionsInitialised(clientExtensions);

            TlsUtils.addSignatureAlgorithmsExtension(clientExtensions, supportedSignatureAlgorithms);
        }

        if (TlsECCUtils.containsECCCipherSuites(getCipherSuites()))
        {
            /*
             * RFC 4492 5.1. A client that proposes ECC cipher suites in its ClientHello message
             * appends these extensions (along with any others), enumerating the curves it supports
             * and the point formats it can parse. Clients SHOULD send both the Supported Elliptic
             * Curves Extension and the Supported Point Formats Extension.
             */
            /*
             * TODO Could just add all the curves since we support them all, but users may not want
             * to use unnecessarily large fields. Need configuration options.
             */
            this.namedCurves = new int[]{ NamedCurve.secp256r1, NamedCurve.secp384r1 };
            this.clientECPointFormats = new short[]{ ECPointFormat.uncompressed,
                ECPointFormat.ansiX962_compressed_prime, ECPointFormat.ansiX962_compressed_char2, };

            clientExtensions = TlsExtensionsUtils.ensureExtensionsInitialised(clientExtensions);

            TlsECCUtils.addSupportedEllipticCurvesExtension(clientExtensions, namedCurves);
            TlsECCUtils.addSupportedPointFormatsExtension(clientExtensions, clientECPointFormats);
        }

        return clientExtensions;
    }

    public ProtocolVersion getMinimumVersion()
    {
        return ProtocolVersion.TLSv10;
    }

    public void notifyServerVersion(ProtocolVersion serverVersion)
        throws IOException
    {
        if (!getMinimumVersion().isEqualOrEarlierVersionOf(serverVersion))
        {
            throw new TlsFatalAlert(AlertDescription.protocol_version);
        }
    }

    public short[] getCompressionMethods()
    {
        return new short[]{CompressionMethod._null};
    }

    public void notifySessionID(byte[] sessionID)
    {
        // Currently ignored
    }

    public void notifySelectedCipherSuite(int selectedCipherSuite)
    {
        this.selectedCipherSuite = selectedCipherSuite;
    }

    public void notifySelectedCompressionMethod(short selectedCompressionMethod)
    {
        this.selectedCompressionMethod = selectedCompressionMethod;
    }

    public void processServerExtensions(Hashtable serverExtensions)
        throws IOException
    {
        /*
         * TlsProtocol implementation validates that any server extensions received correspond to
         * client extensions sent. By default, we don't send any, and this method is not called.
         */
        if (serverExtensions != null)
        {
            /*
             * RFC 5246 7.4.1.4.1. Servers MUST NOT send this extension.
             */
            checkForUnexpectedServerExtension(serverExtensions, TlsUtils.EXT_signature_algorithms);

            checkForUnexpectedServerExtension(serverExtensions, TlsECCUtils.EXT_elliptic_curves);

            if (TlsECCUtils.isECCCipherSuite(this.selectedCipherSuite))
            {
                this.serverECPointFormats = TlsECCUtils.getSupportedPointFormatsExtension(serverExtensions);
            }
            else
            {
                checkForUnexpectedServerExtension(serverExtensions, TlsECCUtils.EXT_ec_point_formats);
            }

            /*
             * RFC 7685 3. The server MUST NOT echo the extension.
             */
            checkForUnexpectedServerExtension(serverExtensions, TlsExtensionsUtils.EXT_padding);
        }
    }

    public void processServerSupplementalData(Vector serverSupplementalData)
        throws IOException
    {
        if (serverSupplementalData != null)
        {
            throw new TlsFatalAlert(AlertDescription.unexpected_message);
        }
    }

    public Vector getClientSupplementalData()
        throws IOException
    {
        return null;
    }

    public TlsCompression getCompression()
        throws IOException
    {
        switch (selectedCompressionMethod)
        {
        case CompressionMethod._null:
            return new TlsNullCompression();

        default:
            /*
             * Note: internal error here; the TlsProtocol implementation verifies that the
             * server-selected compression method was in the list of client-offered compression
             * methods, so if we now can't produce an implementation, we shouldn't have offered it!
             */
            throw new TlsFatalAlert(AlertDescription.internal_error);
        }
    }

    public TlsCipher getCipher()
        throws IOException
    {
        int encryptionAlgorithm = TlsUtils.getEncryptionAlgorithm(selectedCipherSuite);
        int macAlgorithm = TlsUtils.getMACAlgorithm(selectedCipherSuite);

        return cipherFactory.createCipher(context, encryptionAlgorithm, macAlgorithm);
    }

    public void notifyNewSessionTicket(NewSessionTicket newSessionTicket)
        throws IOException
    {
    }
}