OpenSSL.URLSessionDelegate.ClientAuthentication.swift

func urlSession(
    _ session: URLSession,
    task: URLSessionTask,
    didReceive challenge: URLAuthenticationChallenge,
    completionHandler: @escaping (URLSession.AuthChallengeDisposition, URLCredential?) -> Void
) {
    // SERVER TRUST
    
    if challenge.protectionSpace.authenticationMethod == NSURLAuthenticationMethodClientCertificate {
        guard
            let data = try? Data(contentsOf: localClientCertificateDERURL),
            let thePKCS12 = PKCS12(data, password: clientCertificatePFXPassword),
            let credentials = URLCredential(PKCS12: thePKCS12)
        else {
            fatalError("An error occurred while attempting to import the PKCS12 data using SecPKCS12Import.")
        }

        completionHandler(.useCredential, credentials)
        return
    }
}