How do I get the X509Certificate sent from the client in web service?

Question

Apparently I was asking the wrong question in my earlier post. I have a web service secured with a X.509 certificate, running as a secure web site (https://...). I want to use the client's machine certificate (also X.509) issued by the company's root CA to verify to the server that the client machine is authorized to use the service. In order to do this, I need to inspect the certificate and look for some identifying feature and match that to a value stored in a database (maybe the Thumbprint?).

Here is the code I use to get the certificate from the local certificate store (lifted straight from http://msdn.microsoft.com/en-us/magazine/cc163454.aspx):

 public static class SecurityCertificate
{
    private static X509Certificate2 _certificate = null;

    public static X509Certificate2 Certificate
    {
        get { return _certificate; }
    }

    public static bool LoadCertificate()
    {
        // get thumbprint from app.config
        string thumbPrint = Properties.Settings.Default.Thumbprint;
        if ( string.IsNullOrEmpty( thumbPrint ) )
        {
            // if no thumbprint on file, user must select certificate to use
            _certificate = PickCertificate( StoreLocation.LocalMachine, StoreName.My );
            if ( null != _certificate )
            {
                // show certificate details dialog
                X509Certificate2UI.DisplayCertificate( _certificate );
                Properties.Settings.Default.Thumbprint = _certificate.Thumbprint;
                Properties.Settings.Default.Save();
            }
        }
        else
        {
            _certificate = FindCertificate( StoreLocation.LocalMachine, StoreName.My, X509FindType.FindByThumbprint, thumbPrint );
        }

        if ( null == _certificate )
        {
            MessageBox.Show( "You must have a valid machine certificate to use STS." );
            return false;
        }

        return true;
    }

    private static X509Certificate2 PickCertificate( StoreLocation location, StoreName name )
    {
        X509Store store = new X509Store( name, location );
        try
        {
            // create and open store for read-only access
            store.Open( OpenFlags.ReadOnly );

            X509Certificate2Collection coll = store.Certificates.Find( X509FindType.FindByIssuerName, STSClientConstants.NBCCA, true );
            if ( 0 == coll.Count )
            {
                MessageBox.Show( "No valid machine certificate found - please contact tech support." );
                return null;
            }

            // pick a certificate from the store
            coll = null;
            while ( null == coll || 0 == coll.Count )
            {
                coll = X509Certificate2UI.SelectFromCollection(
                        store.Certificates, "Local Machine Certificates",
                        "Select one", X509SelectionFlag.SingleSelection );
            }

            // return first certificate found
            return coll[ 0 ];
        }
        // always close the store
        finally { store.Close(); }
    }

    private static X509Certificate2 FindCertificate( StoreLocation location, StoreName name, X509FindType findType, string findValue )
    {
        X509Store store = new X509Store( name, location );
        try
        {
            // create and open store for read-only access
            store.Open( OpenFlags.ReadOnly );

            // search store
            X509Certificate2Collection col = store.Certificates.Find( findType, findValue, true );

            // return first certificate found
            return col[ 0 ];
        }
        // always close the store
        finally { store.Close(); }
    }

Then, I attach the certificate to the outbound stream thusly:

public static class ServiceDataAccess
{    
    private static STSWebService _stsWebService = new STSWebService();

    public static DataSet GetData(Dictionary<string,string> DictParam, string action)
    {
        // add the machine certificate here, the first web service call made by the program (only called once)
        _stsWebService.ClientCertificates.Add( SecurityCertificate.Certificate );
        // rest of web service call here...
    }
}

My question is this -- how do I "get" the certificate in the web service code? Most sample code snippets I have come across that cover how to do custom validation have a GetCertificate() call in there, apparently assuming that part is so easy everyone should know how to do it?

My main class inherits from WebService, so I can use Context.Request.ClientCertificate to get a certificate, but that's an HttpClientCertificate, not an X509Certificate2. HttpContext gives me the same result. Other approaches all use web configuration code to call pre-defined verification code, with no clue as to how to call a custom C# method to do the verification.

Solution

I recall doing something similar, its been awhile but, have you tried this in your web service:

X509Certificate2 cert = new X509Certificate2(Context.Request.ClientCertificate.Certificate);

Comments

[DaveN59]

That works! Now I can finally get to the meat of my problem -- figuring out what value I can use to match with a stored value in the database... Thanks!!

[DSO]

Thumbprint is fine to use if you are treating your database as the trusted authority, I've worked on apps that took this approach. OTOH, if you need a distributed trust model (using CAs), then you need to do additional checks (e.g. verifying cert chain).

[DaveN59]

I think I may have acted too soon. Trying to access the Thumbprint throws an exception 'System.Security.Cryptography.CryptographicException' -- so, even though the http Certificate is now of type X509Certificate2, is it really the same certificate sent from the client, or did I just create a new beast that just kinda looks like it?

[DaveN59]

OK, digging deeper I find the exception message is "m_safeCertContext is an invalid handle." Checking around for that message, it is probably caused by trying to run in the "thin" VS web server (debug mode), rather than IIS. Back to the drawing board! Hey, if it was easy they wouldn't pay us to do it, right?

[DSO]

I don't think you can debug SSL using Visual Studio web server (unless things changed since I last used in in VS 2005). You really should set yourself up to debug using IIS.