I'm trying to knock up a simple demonstration for myself to try using
SSL to send and receive a simple message. I've based this on the code
on the MSDN site -
http://msdn2.microsoft.com/en-us/library/system.net.security.sslstream.aspx
- and I'd like to end up with something that sends this text and a
response in encrypted form, between an authenticated client and an
authenticated server.

I keep getting an error though - "The server mode SSL must use a
certificate with the associated private key." I've used MakeCert to
create a .cer file. The command I used was:

makecert -n "CN=Joe Bloggs" test.cer

The relevant code being run (as a server) is:

static void Main(string[] args) {
           X509Certificate cert =
X509Certificate.CreateFromCertFile("test.cer");
           TcpListener tcpServer = new TcpListener(IPAddress.Any,
9988);
           tcpServer.Start();

           while (true) {
               TcpClient tcpClient = tcpServer.AcceptTcpClient();
               SslStream sslStream = new
SslStream(tcpClient.GetStream(), false);
               try {
                   sslStream.AuthenticateAsServer(cert);
...

The exception is beign raised at AuthenticateAsServer(). I can inspect
the 'cert' variable, and it seems to contain my certificate (certainly,
the name 'Joe Bloggs' is in it).

I guess my problem must be in the key, but I'm really not sure what -
could someone shed some light on this.