We are hosting a .net winforms usercontrol within IE6 which makes POST and
GET calls to a webserver  using httpwebrequest. This was working fine until
we changed our authentication mechanism in IIS  to require client
certificates. I can fetch the certificate in the user control ok and this is
being attached to the request. i.e.:

ServicePointManager.CertificatePolicy = new TrustAllCertificatePolicy();

HttpWebRequest request = WebRequest.Create(remoteUri) as HttpWebRequest;
request.Headers = new WebHeaderCollection();
request.KeepAlive = false;
request.ProtocolVersion = HttpVersion.Version10;
request.Proxy = WebProxy.GetDefaultProxy();
request.AllowAutoRedirect = true;
request.MaximumAutomaticRedirections = 10;
request.Timeout = (int) new TimeSpan(0, 0, 60).TotalMilliseconds;

CertificateStore store = new CertificateStore();

X509Certificate cert = store.GetValidApplicationCertificate();
request.ClientCertificates.Add(cert);

request.Method = "GET";

WebResponse response = request.GetResponse();

When this last line is called, I get an error:

Could not establish secure channel for SSL/TLS. --->
System.Security.SecurityException: Request for the permission of type
System.Security.Permissions.SecurityPermission, mscorlib,
Version=1.0.5000.0, Culture=neutral, PublicKeyToken=b77a5c561934e089 failed.
  at System.Net.TlsStream.EndRead(IAsyncResult asyncResult)
  at System.Net.Connection.ReadCallback(IAsyncResult asyncResult)
  --- End of inner exception stack trace ---
  at System.Net.HttpWebRequest.CheckFinalStatus()
  at System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult)
  at System.Net.HttpWebRequest.GetResponse()

This looks like the error described in KB823177 but installing all the
latest patches does not help.

Does anybody have any suggestions as to how we might fix this as I'm stumped!