HTTPS Connection signed with user certificate - unexpected behavior on differrent computers

Question

There is a service. I have to connect to it via HTTPS protocol, upload a data via POST method, and download result. The connection is authenticated with users private certificate, in *.p12 format.

I have written following function to achieve it:

    static public string PostLPDA(string fileName, string URL, string certificateFile)
    {
        X509Certificate cert = new X509Certificate(certificateFile); // importing certificate from file <===== 1

        StreamReader sr = new StreamReader(fileName); 
        string postString = string.Format("Query={0}", sr.ReadToEnd());  // preparing data to send

        const string contentType = "application/x-www-form-urlencoded";
        System.Net.ServicePointManager.Expect100Continue = false;

        CookieContainer cookies = new CookieContainer();
        HttpWebRequest webRequest = WebRequest.Create(URL) as HttpWebRequest;
        webRequest.Method = "POST";
        webRequest.ContentType = contentType;
        webRequest.CookieContainer = cookies;
        webRequest.ContentLength = postString.Length;
        webRequest.UserAgent = "Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.9.0.1) Gecko/2008070208 Firefox/3.0.1";
        webRequest.Accept = "text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8";
        webRequest.ClientCertificates.Add(cert); //adding certificate 
        System.Net.ServicePointManager.ServerCertificateValidationCallback = 
            ((sender, certificate, chain, sslPolicyErrors) => true); // ignore untrusted site error <===== 2

        StreamWriter requestWriter = new StreamWriter(webRequest.GetRequestStream()); // <===== 3
        requestWriter.Write(postString);
        requestWriter.Close();

        StreamReader responseReader = new StreamReader(webRequest.GetResponse().GetResponseStream());
        string responseData = responseReader.ReadToEnd();

        responseReader.Close();
        webRequest.GetResponse().Close();
        return responseData;
    }

Everything works OK on my computer - there are no exceptions, errors, data is successfully uploading, there is proper response from service.

But on every other PC, there is exception on line marked with <====== 3, with message:

The request was aborted: Could not create SSL/TLS secure channel.

I'm using the same certificate file, same address, and same data. Result is error, and I cannot figure, what's wrong.

Somebody can help?

Answer 1

OK, i find the solution.

Certificate have to be registered in OS. Otherwise, it will not work.

Anyway, thanks for attention.