NSUrlSession: Challenge NSURLAuthenticationMethodServerTrust fails only when client certificate is also needed - ssl

Question

we want to connect our app to our IIS webservice. We use self signed certificates and also a client certificate for authentication.
When the webservice doesn't require client certificate authentication, everything works fine, NSURLAuthenticationMethodServerTrust gets called and the request continues.
But when I activate client certificate authentication on our server, after DidReceiveChallenge with NSURLAuthenticationMethodServerTrust as the challenge, DidCompleteWithError gets called. Error message is: "The certificate for this server is invalid. You might be connecting to a server that is pretending to be "192.168.221.118" which could put your confidential information at risk.
Note: "NSURLAuthenticationMethodClientCertificate" never gets called, the app crashes before that.
The client certificate is signed by the intermediate certificate, so I don't understand why the ServerTrust Challenge fails.
Also: in my opinion it should not be necessary, but I also tried adding the client certificate to the collection of AnchorCertificates of the Sectrust.
Thanks in advance for your help.
Here is my code:
private class SessionDelegate : NSUrlSessionDataDelegate, INSUrlSessionDelegate
{
private Action<bool, string> completed_callback;
private string antwortCache;
private int status_code;
public SessionDelegate(Action<bool, string> completed)
{
completed_callback = completed;
antwortCache = "";
}
public override void DidReceiveChallenge(NSUrlSession session, NSUrlSessionTask task, NSUrlAuthenticationChallenge challenge, Action<NSUrlSessionAuthChallengeDisposition, NSUrlCredential> completionHandler)
{
if (challenge.PreviousFailureCount == 0)
{
if (challenge.ProtectionSpace.AuthenticationMethod.Equals("NSURLAuthenticationMethodServerTrust"))
{
// GetParent is correct, because I'm too lazy to copy the certs into to the correct folders...
var path = Directory.GetParent(GlobaleObjekte.SSLZertifikatePath);
var caPath = Path.Combine(path.FullName, "ca.cert.der");
var caByteArray = File.ReadAllBytes(caPath);
var caCert = new X509Certificate2(caByteArray);
var interPath = Path.Combine(path.FullName, "intermediate.cert.der");
var interByteArray = File.ReadAllBytes(interPath);
var interCert = new X509Certificate2(interByteArray);
var secTrust = challenge.ProtectionSpace.ServerSecTrust;
var certCollection = new X509CertificateCollection();
certCollection.Add(caCert);
certCollection.Add(interCert);
secTrust.SetAnchorCertificates(certCollection);
var credential = new NSUrlCredential(secTrust);
completionHandler(NSUrlSessionAuthChallengeDisposition.UseCredential, credential);
return;
}
if (challenge.ProtectionSpace.AuthenticationMethod.Equals("NSURLAuthenticationMethodClientCertificate"))
{
var path = Directory.GetParent(GlobaleObjekte.SSLZertifikatePath);
var certPath = Path.Combine(path.FullName, "client.pfx");
var certByteArray = File.ReadAllBytes(certPath);
var cert = new X509Certificate2(certByteArray, Settings.WSClientCertPasswort);
var ident = SecIdentity.Import(certByteArray, Settings.WSClientCertPasswort);
var credential = new NSUrlCredential(ident, new SecCertificate[] { new SecCertificate(cert) }, NSUrlCredentialPersistence.ForSession);
completionHandler(NSUrlSessionAuthChallengeDisposition.UseCredential, credential);
return;
}
if (challenge.ProtectionSpace.AuthenticationMethod.Equals("NSURLAuthenticationMethodHTTPBasic"))
{
var credential = new NSUrlCredential(Settings.WebserviceBenutzer, Settings.WebservicePasswort, NSUrlCredentialPersistence.ForSession);
completionHandler(NSUrlSessionAuthChallengeDisposition.UseCredential, credential);
return;
}
completed_callback(false, "Unbekannte Authentifizierungsanfrage: " + challenge?.ProtectionSpace?.AuthenticationMethod);
}
else
{
completed_callback(false, "Authentifizierung fehlgeschlagen: " + challenge?.ProtectionSpace?.AuthenticationMethod);
}
}
}

Answer 1

I finally found a solution. I had to create the credential object in a different way. Instead of adding the certificates to the SecTrust and create the credential with the SecTrust as a parameter, I had to create a identity from the client certificate and then create the credential with the identity and the other certificates as parameters:
if (challenge.ProtectionSpace.AuthenticationMethod.Equals("NSURLAuthenticationMethodServerTrust"))
{
var path = Directory.GetParent(GlobaleObjekte.SSLZertifikatePath);
var caPath = Path.Combine(path.FullName, "ca.cert.der");
var caByteArray = File.ReadAllBytes(caPath);
var caCert = new SecCertificate(caByteArray);
var interPath = Path.Combine(path.FullName, "intermediate.cert.der");
var interByteArray = File.ReadAllBytes(interPath);
var interCert = new SecCertificate(interByteArray);
var clientPath = Path.Combine(path.FullName, "client.pfx");
var clientByteArray = File.ReadAllBytes(clientPath);
var clientCert = new X509Certificate2(clientByteArray, Settings.WSClientCertPasswort);
//var secTrust = challenge.ProtectionSpace.ServerSecTrust;
//var certCollection = new X509CertificateCollection();
//certCollection.Add(caCert);
//certCollection.Add(interCert);
//certCollection.Add(cert);
//secTrust.SetAnchorCertificates(certCollection);
//var credential = new NSUrlCredential(secTrust);
var identity = SecIdentity.Import(clientCert);
var credential = new NSUrlCredential(identity, new SecCertificate[] { caCert, interCert }, NSUrlCredentialPersistence.ForSession);
completionHandler(NSUrlSessionAuthChallengeDisposition.UseCredential, credential);
return;
}