I was able to resolve this. Had to make a few changes in the delegate method for didReceiveAuthChallenge() in my app.
There is no sample project for Swift/iOS in the getting started guide. Let's get together and put one out there.
Thanks for sharing with us your success in resolving the issue! Can you please let me know what were the few changes that you did in the delegate method for didReceiveAuthChallenge() in your app?
The didReceiveChallenge method is invoked twice with different authenticationMethod.
Once with NSURLAuthenticationMethodServerTrust and second time with NSURLAuthenticationMethodClientCertificate.
I needed to pass the p12 certificate bundle for the NSURLAuthenticationMethodClientCertificate challenge.
For NSURLAuthenticationMethodServerTrust, had to do the performDefaultHandling.