Force HttpWebRequest to send client certificate

Question

I have a p12 certificate, that I load it in this way:

X509Certificate2 certificate = new X509Certificate2(certName, password,
        X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet |
        X509KeyStorageFlags.Exportable);

It is loaded correcty, in fact If i do certificate.PrivateKey.ToXmlString(true); it returns a complete xml without errors. But If I do:

try
{
    X509Chain chain = new X509Chain();
    var chainBuilt = chain.Build(certificate);
    Console.WriteLine("Chain building status: "+ chainBuilt);

    if (chainBuilt == false)
        foreach (X509ChainStatus chainStatus in chain.ChainStatus)
            Console.WriteLine("Chain error: "+ chainStatus.Status);
}
catch (Exception ex)
{
    Console.WriteLine(ex);
}

it writes:

Chain building status: False
Chain error: RevocationStatusUnknown 
Chain error: OfflineRevocation 

so when I do:

        ServicePointManager.CheckCertificateRevocationList = false;
    ServicePointManager.ServerCertificateValidationCallback = (a, b, c, d) => true;
    ServicePointManager.Expect100Continue = true;
    Console.WriteLine("connessione a:" + host);
    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(host);
    req.PreAuthenticate = true;
    req.AllowAutoRedirect = true;
    req.ClientCertificates.Add(certificate);
    req.Method = "POST";
    req.ContentType = "application/x-www-form-urlencoded";
    string postData = "login-form-type=cert";
    byte[] postBytes = Encoding.UTF8.GetBytes(postData);
    req.ContentLength = postBytes.Length;
    Stream postStream = req.GetRequestStream();
    postStream.Write(postBytes, 0, postBytes.Length);
    postStream.Flush();
    postStream.Close();

    WebResponse resp = req.GetResponse();

the server says that the certificate is not sent/valid.

My question is:

• how can I sent the certificate even with chain build false?
• there is another class to post a certificate that does not check the certificate validation before send it?

many thanks. Antonino

Answer 1

I resolved the problem, The point is that a P12 file (as a PFX) contains more then 1 certificate, so it must be loaded in this way:

X509Certificate2Collection certificates = new X509Certificate2Collection();
certificates.Import(certName, password, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet);

and added to a HttpWebRequest in this way: request.ClientCertificates = certificates;

Thanks everybody for support.

COMPLETE SAMPLE CODE

string host = @"https://localhost/";
string certName = @"C:\temp\cert.pfx";
string password = @"password";

try
{
    X509Certificate2Collection certificates = new X509Certificate2Collection();
    certificates.Import(certName, password, X509KeyStorageFlags.MachineKeySet | X509KeyStorageFlags.PersistKeySet);

    ServicePointManager.ServerCertificateValidationCallback = (a, b, c, d) => true;
    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(host);
    req.AllowAutoRedirect = true;
    req.ClientCertificates = certificates;
    req.Method = "POST";
    req.ContentType = "application/x-www-form-urlencoded";
    string postData = "login-form-type=cert";
    byte[] postBytes = Encoding.UTF8.GetBytes(postData);
    req.ContentLength = postBytes.Length;

    Stream postStream = req.GetRequestStream();
    postStream.Write(postBytes, 0, postBytes.Length);
    postStream.Flush();
    postStream.Close();
    WebResponse resp = req.GetResponse();

    Stream stream = resp.GetResponseStream();
    using (StreamReader reader = new StreamReader(stream))
    {
        string line = reader.ReadLine();
        while (line != null)
        {
            Console.WriteLine(line);
            line = reader.ReadLine();
        }
    }

    stream.Close();
}
catch(Exception e)
{
    Console.WriteLine(e);
}

Answer 2

I created a command-line program using a modified version of your code with a pfx cert containing the private key exported from IE and I'm able to authenticate to a secure website and retrieve protected pages:

string host = @"https://localhost/";
string certName = @"C:\temp\cert.pfx";
string password = @"password";

try
{
    X509Certificate2 certificate = new X509Certificate2(certName, password);

    ServicePointManager.CheckCertificateRevocationList = false;
    ServicePointManager.ServerCertificateValidationCallback = (a, b, c, d) => true;
    ServicePointManager.Expect100Continue = true;
    HttpWebRequest req = (HttpWebRequest)WebRequest.Create(host);
    req.PreAuthenticate = true;
    req.AllowAutoRedirect = true;
    req.ClientCertificates.Add(certificate);
    req.Method = "POST";
    req.ContentType = "application/x-www-form-urlencoded";
    string postData = "login-form-type=cert";
    byte[] postBytes = Encoding.UTF8.GetBytes(postData);
    req.ContentLength = postBytes.Length;

    Stream postStream = req.GetRequestStream();
    postStream.Write(postBytes, 0, postBytes.Length);
    postStream.Flush();
    postStream.Close();
    WebResponse resp = req.GetResponse();

    Stream stream = resp.GetResponseStream();
    using (StreamReader reader = new StreamReader(stream))
    {
        string line = reader.ReadLine();
        while (line != null)
        {
            Console.WriteLine(line);
            line = reader.ReadLine();
        }
    }

    stream.Close();
}
catch(Exception e)
{
    Console.WriteLine(e);
}

Answer 3

The problem is that you install private key to machine store which is not generally allowed to use for client authentication for processes that doesn't run under local system account or have explicit private key permissions. You need to install the key in the current user store:

X509Certificate2 certificate = new X509Certificate2(certName, password,
        X509KeyStorageFlags.UserKeySet | X509KeyStorageFlags.PersistKeySet |
        X509KeyStorageFlags.Exportable);

Answer 4

I was trying to use a self-signed certificate whose only purpose was "Server certificate" as a client certificate using the following code:

        HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create("https://serverurl.com/test/certauth");
        X509Store store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
        store.Open(OpenFlags.ReadWrite);
        var certificate = store.Certificates.Find(X509FindType.FindByThumbprint, "a909502dd82ae41433e6f83886b00d4277a32a7b", true)[0];
        request.ClientCertificates.Add(certificate);
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();

What I learned was that .NET Framework will accept such a certificate as a client certificate, but .NET Core will not (see https://github.com/dotnet/runtime/issues/26531).

There are two solutions:

1. Switch to .NET Framework
2. Generate a new self-signed certificate that has a purpose / Enhanced Key Usage = Client certificate. Then the code will work in Core as well.