I'm using URLSession to make a get request with TLS 1.2 protocol and certificates (which are all self-signed) included in the main bundle. Hopefully I managed to do the pinning but server also requires a client certificate for authentication so I'm trying to respond to the AuthenticationChallenge with UrlCredential but it's not working: i keep getting NSURLErrorDomain Code=-1206 which is "The server “my_server_domain.it” requires a client certificate."Here is my request: func makeGetRequest(){ let configuration = URLSessionConfiguration.default var request = try! URLRequest(url: requestUrl, method: .get) let session = URLSession(configuration: configuration, delegate: self, delegateQueue: OperationQueue.main) let task = session.dataTask(with: request, completionHandler: { (data, response, error) in print("Data = \(data)") print("Response = \(response)") print("Error = \(error)") }) task.resume() }URLSessionDelegate, where I respond to the AuthenticationChallenge:func urlSession(_ session: URLSession, didReceive challenge: URLAuthenticationChallenge, completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void) { let authenticationMethod = challenge.protectionSpace.authenticationMethod print("authenticationMethod=\(authenticationMethod)") if authenticationMethod == NSURLAuthenticationMethodClientCertificate { completionHandler(.useCredential, getClientUrlCredential()) } else if authenticationMethod == NSURLAuthenticationMethodServerTrust { let serverCredential = getServerUrlCredential(protectionSpace: challenge.protectionSpace) guard serverCredential != nil else { completionHandler(.cancelAuthenticationChallenge, nil) return } completionHandler(.useCredential, serverCredential) } }Server certificate pinning:func getServerUrlCredential(protectionSpace:URLProtectionSpace)->URLCredential?{ if let serverTrust = protectionSpace.serverTrust { //Check if is valid var result = SecTrustResultType.invalid let status = SecTrustEvaluate(serverTrust, &result) print("SecTrustEvaluate res = \(result.rawValue)") if(status == errSecSuccess), let serverCertificate = SecTrustGetCertificateAtIndex(serverTrust, 0) { //Get Server Certificate Data let serverCertificateData = SecCertificateCopyData(serverCertificate) //Get Local Certificate NSData let localServerCertNSData = certificateHelper.getCertificateNSData(withName: "localServerCertName", andExtension: "cer") //Check if certificates are equals, otherwhise pinning fails and return nil guard serverCertificateData == localServerCertNSData else{ print("Certificates doesn't match.") return nil } //Certificates does match, so we can trust the server return URLCredential(trust: serverTrust) } } return nil }And here is where i obtain the client URLCredential from the PKCS12 (.pfx) certificate:func getClientUrlCredential()->URLCredential { let userCertificate = certificateHelper.getCertificateNSData(withName: "userCertificateName", andExtension: "pfx") let userIdentityAndTrust = certificateHelper.extractIdentityAndTrust(fromCertificateData: userCertificate, certPassword: "cert_psw") //Create URLCredential let urlCredential = URLCredential(identity: userIdentityAndTrust.identityRef, certificates: userIdentityAndTrust.certArray as [AnyObject], persistence: URLCredential.Persistence.permanent) return urlCredential }The func 'extractIdentityAndTrust' -successfully- returns a struct with pointers to identity, certificate-chain and trust extracted from the PKCS12; I know that identity and certificates should be stored in the keychain but at the moment I'm just including them in the bundle.I've also added App Transport Security Settings to my Info.plist allowing arbitrary loads and configuring an exception domain.It looks like client doesn't even try to authenticate, so I'm missing something, I guess...