Biometric authentication (Face ID, Touch ID) in iOS Swift - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

What happens when biometric authentication succeeds?

In an iOS app using Face ID or Touch ID, what is the typical behavior after successful biometric authentication?

A The app unlocks the protected content and navigates to the main screen.

B The app closes immediately without any message.

C The app shows an error message and asks to try again.

D The app logs out the user automatically.

💡 Hint

Think about what the user expects after confirming their identity.

When biometric authentication succeeds, the app typically grants access to protected content or features, often by navigating to the main or secured screen.

🧠 Conceptual

intermediate

Which framework is used for biometric authentication in iOS?

Which Apple framework provides the APIs to implement Face ID and Touch ID authentication in iOS apps?

A UIKit

B CoreData

C AVFoundation

D LocalAuthentication

💡 Hint

It starts with 'Local' and deals with device security.

The LocalAuthentication framework provides classes and methods to evaluate biometric authentication policies like Face ID and Touch ID.

📝 Syntax

advanced

What is the correct way to create a LAContext and check if biometric authentication is available?

Given the following Swift code snippet, which option correctly creates a LocalAuthentication context and checks if biometric authentication can be evaluated?

iOS Swift

import LocalAuthentication

let context = LAContext()
var error: NSError?

// Check if biometric authentication is available
let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error)

A let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: &error)

B let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthentication, error: error)

C let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, error: error)

D let canEvaluate = context.canEvaluatePolicy(.deviceOwnerAuthenticationWithBiometrics)

💡 Hint

Check the method signature and parameter types carefully.

The method canEvaluatePolicy requires an UnsafeMutablePointer to an NSError? for the error parameter, so it must be passed as &error. Option A correctly uses this syntax.

❓ lifecycle

advanced

What happens if biometric authentication is interrupted by app going to background?

If the user starts biometric authentication but the app goes to the background (e.g., user presses home button), what is the expected behavior?

A The authentication succeeds immediately without user input.

B The app crashes due to lost context.

C The authentication is cancelled and the app must restart it when active again.

D The authentication continues in the background automatically.

💡 Hint

Think about security and user interaction requirements.

Biometric authentication requires user interaction and cannot continue in the background. If interrupted, it is cancelled and must be restarted when the app is active again.

🔧 Debug

expert

Why does this biometric authentication code always fail with error code -7?

Consider this Swift code snippet for biometric authentication: import LocalAuthentication let context = LAContext() context.evaluatePolicy(.deviceOwnerAuthenticationWithBiometrics, localizedReason: "Access needed") { success, error in if success { print("Authenticated") } else { print(error?.localizedDescription ?? "Unknown error") } } The error code returned is always -7 (LAError.biometryNotAvailable). What is the most likely cause?

A The LAContext instance must be created inside the closure.

B The device does not have biometric hardware or it is disabled in settings.

C The localizedReason string is missing and causes failure.

D The evaluatePolicy method requires a completion handler on the main thread.

💡 Hint

Error code -7 means biometry is not available on the device.

Error code -7 (LAError.biometryNotAvailable) means the device either lacks biometric hardware or the user has disabled it in system settings. The code is correct but the device environment prevents biometric use.

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)