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        public static void RunClient(string machineName, string serverName)
        {
            // Create a TCP/IP client socket.
            // machineName is the host running the server application.
            TcpClient client = new TcpClient(machineName, 6000);
            Console.WriteLine("Client connected.");
            // Create an SSL stream that will close the client's stream.
            SslStream sslStream = new SslStream(client.GetStream(), false, new RemoteCertificateValidationCallback(ValidateServerCertificate), new LocalCertificateSelectionCallback(SelectLocalCertificate));
            // The server name must match the name on the server certificate.
            try
            {
                sslStream.AuthenticateAsClient(serverName); //Bei dieser Anweisung bekomme ich immer folgende FM: Das Remotezertifikat ist laut Validierungsverfahren ungültig.
            }
            catch (AuthenticationException e)
            {
                Console.WriteLine("Exception: {0}", e.Message);
                if (e.InnerException != null)
                {
                    Console.WriteLine("Inner exception: {0}", e.InnerException.Message);
                }
                Console.WriteLine("Authentication failed - closing the connection.");
                client.Close();
                return;
            }
            // Encode a test message into a byte array.
            // Signal the end of the message using the "<EOF>".
            byte[] messsage = Encoding.UTF8.GetBytes("hallo");
            // Send hello message to the server. 
            sslStream.Write(messsage);
            sslStream.Flush();
            // Read message from the server.
            string serverMessage = ReadMessage(sslStream);
            Console.WriteLine("Server says: {0}", serverMessage);
            // Close the client connection.
            client.Close();
            Console.WriteLine("Client closed.");
        }

        public static X509Certificate SelectLocalCertificate(object sender, string targetHost, X509CertificateCollection localCertificates, X509Certificate remoteCertificate, string[] acceptableIssuers)
        {
            if (acceptableIssuers != null && acceptableIssuers.Length > 0 && localCertificates != null && localCertificates.Count > 0)
            {
                // Use the first certificate that is from an acceptable issuer.
                foreach (X509Certificate certificate in localCertificates)
                {
                    string issuer = certificate.Issuer;
                    if (Array.IndexOf(acceptableIssuers, issuer) != -1)
                        return certificate;
                }
            }
            if (localCertificates != null && localCertificates.Count > 0)
                return localCertificates[0];

            return null;

        }

        public static bool ValidateServerCertificate(object sender, X509Certificate certificate, X509Chain chain, SslPolicyErrors sslPolicyErrors)
        {
            if (sslPolicyErrors == SslPolicyErrors.None)
                return true;

            Console.WriteLine("Certificate error: {0}", sslPolicyErrors);

            // Do not allow this client to communicate with unauthenticated servers.
            return false;
        }