How to use Touch ID to authenticate users by fingerprint

Swift version: 5.10

Paul Hudson @twostraws May 28th 2019

Touch ID is an easy and secure way for users to authenticate themselves, so its no surprise that it's caught on so quickly among apps. Authenticating with Touch ID automatically uses the fingerprints registered by the user when they set up Touch ID, and you never have access to those fingerprints, which means it's both low-friction and extra-secure.

To get started, you need to import the LocalAuthentication framework like this:

import LocalAuthentication

The actual act of authenticating users has a number of possible results, and you need to catch them all:

The user might not have a Touch ID-capable device.
The user might have a Touch ID-capable device, but might not have configured it.
The user failed to authenticate, perhaps because they asked to enter a passcode rather than use Touch ID.

Note that Apple insists that your app provide a passcode method of authentication as a back up. More annoyingly, you need to request and store this passcode yourself – it's not even done by Apple using the system unlock code!

Asking for and setting a passcode is easy enough, so I'll leave that to you. The important bit is asking for Touch ID authentication, which is done using this code:

func authenticateUser() {
    let context = LAContext()
    var error: NSError?

    if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
        let reason = "Identify yourself!"

        context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: reason) {
            [unowned self] success, authenticationError in

            DispatchQueue.main.async {
                if success {
                    self.runSecretCode()
                } else {
                    let ac = UIAlertController(title: "Authentication failed", message: "Sorry!", preferredStyle: .alert)
                    ac.addAction(UIAlertAction(title: "OK", style: .default))
                    self.present(ac, animated: true)
                }
            }
        }
    } else {
        let ac = UIAlertController(title: "Touch ID not available", message: "Your device is not configured for Touch ID.", preferredStyle: .alert)
        ac.addAction(UIAlertAction(title: "OK", style: .default))
        present(ac, animated: true)
    }
}