iOS Swift view controller to interact with the OATH module of a YubiKey over NFC

OATH-NFC-Controller.swift

//
//  ViewController.swift
//  OATH-NFC
//
//  Created by Dennis Hills on 6/5/20.
//  Copyright © 2020 Dennis Hills. All rights reserved.
//
//  Description: This is sample app demonstrating NFC interaction with the OATH module of a YubiKey using the Yubico iOS SDK
//  The app establishes an open NFC connection with the YubiKey, reads the credentials from the OATH module and retrieves a TOTP code based on the first OATH credential retrieved.
//
//  There are three main NFC components to this class:
//  1. StartNFC Session - Line 49 -> Triggered by a button click that starts the NFC reader session
//  2. NFC Session (OBSERVER) - Line 66 -> Registers to receive callbacks sent from an NFC tag reader session.
//  3. NFC Session (HANDLER) - Line 94 -> This handles callbacks when the NFC tag reader session detects an ISO 7816 tag
//
//  The other two main functions are communicating with the OATH module of the YubiKey over the NFCReaderSession.
//  1. getOATHCredentials() - Line 113 gets the list of all OATH credentials stored on that key
//  2. getOTPCode() - Line 152 calls the YKFKeyOATHCalculateRequest which asks the YubiKey to generate a TOTP code for given credential

//  Updated Github Gist on June 5 here: https://gist.github.com/dmennis/e95f85e2d30e9e08672e2487282d549d

import UIKit
import YubiKit

class ViewController: UIViewController {
    
    // UI Stuff
    @IBOutlet weak var btnStartNFC: UIButton! // Tap this button to start the NFC Session
    @IBOutlet weak var lblCode: UILabel! // Displays the TOTP code
    
    // NFC Session observation
    private var isNFCObservingSessionStateUpdates = false
    private var nfcSesionStateObservation: NSKeyValueObservation?
    
    var oathService: YKFKeyOATHServiceProtocol? = nil
    
    override func viewWillAppear(_ animated: Bool) {
        if (YubiKitDeviceCapabilities.supportsISO7816NFCTags) {
            guard #available(iOS 13.0, *) else { fatalError() }
            observeNFCSessionStateUpdates = true
        }
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    // Initiates the NFCTagReaderSession for interfacing with an ISO 7816 tag
    @IBAction func startNFCSession(_ sender: Any) {
        guard #available(iOS 13.0, *) else {
            fatalError()
        }

        if YubiKitDeviceCapabilities.supportsISO7816NFCTags {
            if YubiKitManager.shared.nfcSession.iso7816SessionState != .closed {
                YubiKitManager.shared.nfcSession.stopIso7816Session()
            }
            YubiKitManager.shared.nfcSession.startIso7816Session()
        }
    }
    
    /********************
     NFC Session Observer
    ********************/
    @available(iOS 13.0, *)
    var observeNFCSessionStateUpdates: Bool {
        get {
            return isNFCObservingSessionStateUpdates
        }
        set {
            guard newValue != isNFCObservingSessionStateUpdates else {
                return
            }
            isNFCObservingSessionStateUpdates = newValue
            
            let nfcSession = YubiKitManager.shared.nfcSession as! YKFNFCSession

            if isNFCObservingSessionStateUpdates {
                self.nfcSesionStateObservation = nfcSession.observe(\.iso7816SessionState, changeHandler: { [weak self] session, change in
                    DispatchQueue.main.async { [weak self] in
                        self?.nfcSessionStateDidChange()
                    }
                })
            } else {
                self.nfcSesionStateObservation = nil
            }
        }
    }
    
    /*******************
     NFC Session Handler
    ********************/
    @available(iOS 13.0, *)
    func nfcSessionStateDidChange() {
        switch YubiKitManager.shared.nfcSession.iso7816SessionState {
        case .open:
            DispatchQueue.global(qos: .default).async { [weak self] in
                if (YubiKitManager.shared.nfcSession.iso7816SessionState != .closed) {
                    guard let self = self else { return }
                    // Take action here to begin communicating with the YubiKey
                    self.getOATHCredentials()
                }
            }
        case .closed:
            break
        default:
            break
        }
    }
    
    // Get the list of OATH credentials from the OATH module over the open NFC session
    // This is called by nfcSessionStateDidChange() when the NFCSession is active and open
    func getOATHCredentials() {
        
        var credCount: Int = 0
        
        // Using the oathService instance over the open NFC session
        oathService = YubiKitManager.shared.nfcSession.oathService
        
        oathService?.executeListRequest { (response, error) in
            guard error == nil else {
                print("The list request ended in error \(error!.localizedDescription)")
                return
            }
            // If the error is nil, the response cannot be empty.
            guard response != nil else {
                print("Error in getting list of OATH credentials: \(String(describing: response))")
                fatalError()
            }

            let credentials = response!.credentials
            credCount = credentials.count
            
            print("The key has \(credentials.count) stored credentials.")
            
            DispatchQueue.main.async {
                self.lblCode.text = "\(credCount) credentials found."
            }
            
            // Get the first YKFOATHCredential from the list of YKFOATHCredential's, if not empty
            if(!credentials.isEmpty) {
                let credential = credentials.first as! YKFOATHCredential
                self.getOTPCode(oathCredential: credential)
            } else {
                // Nothing to calculate, close the NFC session.
                YubiKitManager.shared.nfcSession.stopIso7816Session()
            }
        }
    }
    
    // Ask YubiKey for the TOTP of the provided OATH credential
    func getOTPCode(oathCredential: YKFOATHCredential) {
        
        // Create the CALCULATE request
        guard let calculateRequest = YKFKeyOATHCalculateRequest(credential: oathCredential) else { return }
        
        // Initialize the OATH Service
        oathService = YubiKitManager.shared.nfcSession.oathService
        
        // Execute the calculateRequest
        oathService?.execute(calculateRequest) { (response, error) in
            guard error == nil else {
                print("The calculateRequest ended in error: \(error!.localizedDescription)")
                return
            }
            
            // If the error is nil, the response cannot be empty.
            guard response != nil else { fatalError() }
            
            // Retrieve the generated TOTP code
            let otp = response!.otp
            print("The OTP value for credential [\(String(describing: oathCredential.label))] is \(otp)")
            
            // Display the TOTP code
            DispatchQueue.main.async {
                self.lblCode.text = "\(otp)"
            }
            
            // Done calculting, close the NFC session.
            YubiKitManager.shared.nfcSession.stopIso7816Session()
        }
    }
    
    override func viewWillDisappear(_ animated: Bool) {
        super.viewWillDisappear(animated)
        
        if (YubiKitDeviceCapabilities.supportsISO7816NFCTags) {
            guard #available(iOS 13.0, *) else {
                fatalError()
            }
            observeNFCSessionStateUpdates = false
        }
    }
    
    deinit {
        if #available(iOS 13.0, *) { observeNFCSessionStateUpdates = false }
    }
}