Biometric authentication (Face ID, Touch ID) in iOS Swift

Introduction

Biometric authentication lets users unlock apps using their face or fingerprint. It makes logging in faster and more secure without typing passwords.

To let users quickly unlock a banking app using Face ID or Touch ID.

To protect sensitive information in a health app with fingerprint authentication.

To allow easy login to a notes app without entering a password every time.

To confirm a purchase or payment securely using biometric data.

To add an extra layer of security for user profiles in social media apps.

Syntax

iOS Swift

import LocalAuthentication

let context = LAContext()
var error: NSError?

if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
    context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Access requires authentication") { success, error in
        if success {
            // Authentication successful
        } else {
            // Authentication failed
        }
    }
} else {
    // Biometrics not available or not enrolled
}

Use LAContext to access biometric features.

Always check if biometrics are available before trying to authenticate.

Examples

This checks if the device supports Face ID or Touch ID.

iOS Swift

let context = LAContext()
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil) {
    print("Biometrics available")
} else {
    print("Biometrics not available")
}

This asks the user to authenticate using biometrics and handles success or failure.

iOS Swift

let context = LAContext()
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Log in with Face ID or Touch ID") { success, error in
    if success {
        print("User authenticated")
    } else {
        print("Authentication failed")
    }
}

Sample App

This SwiftUI app shows a button to start biometric authentication. It updates the text message based on success or failure.

iOS Swift

import SwiftUI
import LocalAuthentication

struct ContentView: View {
    @State private var message = "Please authenticate"

    var body: some View {
        VStack(spacing: 20) {
            Text(message)
                .padding()
            Button("Authenticate") {
                authenticateUser()
            }
            .padding()
            .background(Color.blue)
            .foregroundColor(.white)
            .cornerRadius(8)
        }
        .padding()
    }

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

        if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
            context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Unlock with Face ID or Touch ID") { success, authenticationError in
                DispatchQueue.main.async {
                    if success {
                        message = "Authentication successful!"
                    } else {
                        message = "Authentication failed. Try again."
                    }
                }
            }
        } else {
            DispatchQueue.main.async {
                message = "Biometric authentication not available."
            }
        }
    }
}

Important Notes

Test biometric features on a real device; simulators do not support Face ID or Touch ID.

Always provide a fallback option if biometrics are not available or fail.

Use clear messages to guide users during authentication.

Summary

Biometric authentication uses Face ID or Touch ID to secure apps easily.

Check availability before asking for authentication.

Handle success and failure to update the app UI accordingly.

Practice

1. What is the main purpose of using biometric authentication like Face ID or Touch ID in an iOS app?

easy

A. To change the app's theme automatically

B. To improve app loading speed

C. To securely verify the user's identity without typing a password

D. To send notifications to the user

Solution

1. Step 1: Understand biometric authentication purpose

   Biometric authentication is designed to verify the user's identity securely using Face ID or Touch ID.

2. Step 2: Compare options with purpose

   Only To securely verify the user's identity without typing a password describes secure identity verification without passwords, which is the main goal.

3. Final Answer:

   To securely verify the user's identity without typing a password -> Option C

4. Quick Check:

   Biometric authentication = Secure identity check [OK]

Hint: Biometrics replace passwords for secure login [OK]

Common Mistakes:

• Thinking biometrics speed up app loading
• Confusing biometric use with UI changes
• Assuming biometrics send notifications

2. Which Swift code snippet correctly checks if biometric authentication is available before using it?

easy

A. let context = LAContext()
if context.evaluatePolicy(.deviceOwnerAuthentication, error: nil) {
// proceed
}

B. let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
// proceed
}

C. let context = LAContext()
if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: nil) {
// proceed
}

D. let context = LAContext()
if context.canAuthenticate() {
// proceed
}

Solution

1. Step 1: Identify correct method to check biometric availability

   The method canEvaluatePolicy with .deviceOwnerAuthenticationWithBiometrics checks if biometrics are available.

2. Step 2: Verify code syntax and usage

   let context = LAContext() var error: NSError? if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) { // proceed } uses canEvaluatePolicy with correct policy and proper &error parameter, matching Apple's guidelines.

3. Final Answer:

   let context = LAContext()\nvar error: NSError?\nif context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {\n // proceed\n} -> Option B

4. Quick Check:

   Check biometrics availability = canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics) [OK]

Hint: Use canEvaluatePolicy with biometric policy to check availability [OK]

Common Mistakes:

• Using evaluatePolicy instead of canEvaluatePolicy for availability
• Using wrong policy like .deviceOwnerAuthentication
• Calling non-existent methods like canAuthenticate

3. Given this Swift code snippet, what will be printed if biometric authentication succeeds?

let context = LAContext()
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Access needed") { success, error in
  if success {
    print("Authentication succeeded")
  } else {
    print("Authentication failed")
  }
}

medium

A. Authentication succeeded

B. No output

C. Authentication failed

D. Syntax error

Solution

1. Step 1: Understand evaluatePolicy callback behavior

   The closure receives a success boolean indicating if authentication passed.

2. Step 2: Check the if condition for success

   If success is true, it prints "Authentication succeeded"; otherwise, it prints failure.

3. Final Answer:

   Authentication succeeded -> Option A

4. Quick Check:

   Success true prints "Authentication succeeded" [OK]

Hint: Success true means authentication passed and prints success message [OK]

Common Mistakes:

• Confusing success and failure branches
• Assuming no output from async closure
• Thinking code has syntax errors

4. Identify the error in this Swift code for biometric authentication:

let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: error) {
  context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Login") { success, error in
    if success {
      print("Access granted")
    }
  }
}

medium

A. The error parameter should be passed as &error to be mutable

B. evaluatePolicy should be called before canEvaluatePolicy

C. The localizedReason string is missing

D. LAContext must be declared as a class, not a let constant

Solution

1. Step 1: Check error parameter usage in canEvaluatePolicy

   The error parameter must be passed as an inout pointer using &error to capture errors.

2. Step 2: Verify other code parts

   evaluatePolicy is correctly called after canEvaluatePolicy, localizedReason is provided, and LAContext can be a let constant.

3. Final Answer:

   The error parameter should be passed as &error to be mutable -> Option A

4. Quick Check:

   Error param must be &error for canEvaluatePolicy [OK]

Hint: Pass error as &error to canEvaluatePolicy for error capture [OK]

Common Mistakes:

• Passing error without & causing no error info
• Calling evaluatePolicy before checking availability
• Missing localizedReason string
• Misunderstanding let vs var for LAContext

5. You want to allow users to authenticate using Face ID or Touch ID, but fallback to device passcode if biometrics fail or are unavailable. Which Swift code snippet correctly implements this behavior?

hard

A. let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error) {
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Authenticate") { success, error in
if success {
print("Biometric success")
} else {
print("Authentication failed")
}
}
} else {
print("Biometrics not available")
}

B. let context = LAContext()
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Authenticate") { success, error in
if success {
print("Authentication success")
} else {
print("Authentication failed")
}
}

C. let context = LAContext()
if context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: nil) {
context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Authenticate") { success, error in
if success {
print("Biometric success")
} else {
context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Fallback") { success, error in
if success {
print("Passcode success")
} else {
print("Authentication failed")
}
}
}
}
}

D. let context = LAContext()
var error: NSError?
if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) {
context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Authenticate") { success, error in
if success {
print("Authentication success")
} else {
print("Authentication failed")
}
}
}

Solution

1. Step 1: Understand fallback requirement

   The app should try biometrics first, then fallback to device passcode if biometrics fail or are unavailable.

2. Step 2: Analyze code options for fallback logic

   let context = LAContext() var error: NSError? if context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) { context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: "Authenticate") { success, error in if success { print("Authentication success") } else { print("Authentication failed") } } } uses .deviceOwnerAuthentication policy which automatically prefers biometrics (Face ID/Touch ID) and falls back to passcode if unavailable or fails. It also properly checks availability first per Apple's guidelines.

3. Final Answer:

   let context = LAContext()\nvar error: NSError?\nif context.canEvaluatePolicy(.deviceOwnerAuthentication, error: &error) {\n context.evaluatePolicy(.deviceOwnerAuthentication, localizedReason: \"Authenticate\") { success, error in\n if success {\n print(\"Authentication success\")\n } else {\n print(\"Authentication failed\")\n }\n }\n} -> Option D

4. Quick Check:

   .deviceOwnerAuthentication policy = biometric fallback to passcode [OK]

Hint: Use .deviceOwnerAuthentication policy with canEvaluatePolicy check for fallback [OK]

Common Mistakes:

• Using biometrics-only policy without fallback (Options A, B)
• Direct evaluation without availability check
• Syntax error with error: nil and incomplete fallback (Option D)