Biometric authentication in Android Kotlin - Build, Publish & Deploy

Build & Publish - Biometric authentication

Performance Impact

Using biometric authentication has minimal impact on frame rate and memory because it relies on the device's secure hardware and system services. The biometric prompt is handled by the OS, so your app's UI remains smooth, targeting 60fps or higher. Battery usage is low since biometric checks are quick and hardware-accelerated.

Optimization Tips

To keep your app responsive during biometric authentication, avoid heavy processing on the main thread. Use asynchronous callbacks for authentication results. Cache authentication state securely to reduce repeated prompts. Also, handle fallback methods gracefully to avoid blocking UI.

App Size and Startup Time

Integrating biometric authentication uses Android's built-in BiometricPrompt API, so it adds no extra library size to your app bundle. This keeps your APK or AAB size small and does not affect startup time. Avoid adding third-party biometric libraries to keep the app lightweight.

iOS vs Android Differences

On Android, biometric authentication uses the BiometricPrompt API, supporting fingerprint, face, and iris depending on device. On iOS, Face ID and Touch ID are accessed via the LocalAuthentication framework. Both platforms handle UI prompts natively, but Android requires checking hardware availability and permissions explicitly. iOS handles permissions automatically.

Store Review Guidelines

• Privacy: Clearly explain why biometric data is used in your app's privacy policy.
• Security: Do not store biometric data yourself; use system APIs only.
• User Consent: Ensure users opt-in before biometric authentication.
• Fallback: Provide alternative login methods if biometrics fail or are unavailable.
• Google Play: Follow Google Play's User Data policy regarding biometric usage.
• Apple App Store: If using Face ID/Touch ID on iOS, comply with Apple's Human Interface Guidelines.

Self-Check Question

Your app takes 5 seconds to load the biometric authentication screen. What's likely wrong?

• You might be performing heavy work on the main thread before showing the biometric prompt.
• Biometric hardware availability checks or permission requests are blocking UI.
• Using a third-party biometric library that adds startup overhead.

Key Result

Biometric authentication uses native OS APIs with minimal performance impact and no added app size. Optimize by handling authentication asynchronously and providing fallback methods. Follow privacy and security guidelines for smooth app store approval.

Practice

1. What is the main purpose of using BiometricPrompt in an Android app?

easy

A. To access the device camera for scanning QR codes

B. To create a custom login screen with username and password

C. To show the system biometric authentication dialog to the user

D. To manage network connections securely

Solution

1. Step 1: Understand the role of BiometricPrompt

   BiometricPrompt is a system API that shows a standard biometric authentication dialog to the user.

2. Step 2: Identify its main use

   It is used specifically for biometric authentication like fingerprint or face recognition, not for login screens or camera access.

3. Final Answer:

   To show the system biometric authentication dialog to the user -> Option C

4. Quick Check:

   BiometricPrompt = system biometric dialog [OK]

Hint: BiometricPrompt always shows system biometric dialog [OK]

Common Mistakes:

• Confusing biometric prompt with custom login UI
• Thinking it manages network or camera
• Assuming it handles password input

2. Which of the following is the correct way to create a BiometricPrompt instance in Kotlin?

easy

A. val biometricPrompt = BiometricPrompt.create(this, executor, callback)

B. val biometricPrompt = BiometricPrompt(this, executor, callback)

C. val biometricPrompt = BiometricPrompt.newInstance(this, executor, callback)

D. val biometricPrompt = BiometricPrompt.Builder(this).build()

Solution

1. Step 1: Recall BiometricPrompt constructor usage

   In Kotlin, BiometricPrompt is created by calling its constructor with context, executor, and callback.

2. Step 2: Check each option

   Only val biometricPrompt = BiometricPrompt(this, executor, callback) uses the correct constructor syntax. Options A, C, and D use non-existent methods.

3. Final Answer:

   val biometricPrompt = BiometricPrompt(this, executor, callback) -> Option B

4. Quick Check:

   Constructor call = val biometricPrompt = BiometricPrompt(this, executor, callback) [OK]

Hint: Use constructor directly, no create() or newInstance() methods [OK]

Common Mistakes:

• Using non-existent factory methods
• Trying to use Builder pattern which BiometricPrompt doesn't have
• Confusing with other Android APIs

3. Given this Kotlin code snippet, what will be printed if the user successfully authenticates biometrically?

val callback = object : BiometricPrompt.AuthenticationCallback() {
  override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
    println("Success")
  }
  override fun onAuthenticationFailed() {
    println("Failed")
  }
}
// Assume biometricPrompt is initialized and authenticate() is called with this callback

medium

A. Failed

B. Error

C. Nothing

D. Success

Solution

1. Step 1: Understand callback methods

   The onAuthenticationSucceeded method runs when biometric authentication is successful.

2. Step 2: Identify printed output on success

   Inside onAuthenticationSucceeded, it prints "Success". So on success, "Success" is printed.

3. Final Answer:

   Success -> Option D

4. Quick Check:

   Success callback prints "Success" [OK]

Hint: Success callback prints "Success" exactly [OK]

Common Mistakes:

• Confusing onAuthenticationFailed with success
• Assuming error prints on success
• Thinking no output occurs

4. What is wrong with this Kotlin code snippet for biometric authentication?

val biometricPrompt = BiometricPrompt(this, executor, callback)
biometricPrompt.authenticate(promptInfo)

// callback object missing override for onAuthenticationError

medium

A. It will work but won't handle errors properly

B. It will crash because onAuthenticationError is not overridden

C. It will not compile due to missing method

D. It will authenticate without any callback

Solution

1. Step 1: Check callback completeness

   Overriding onAuthenticationError is recommended but not mandatory; missing it won't crash or prevent compilation.

2. Step 2: Understand effect of missing error override

   Without onAuthenticationError, errors won't be handled gracefully, but authentication still runs.

3. Final Answer:

   It will work but won't handle errors properly -> Option A

4. Quick Check:

   Missing error override = no error handling [OK]

Hint: Missing error override causes no error handling, not crash [OK]

Common Mistakes:

• Assuming missing override causes crash
• Thinking code won't compile
• Believing authentication runs without callback

5. You want to add biometric authentication to your app but only if the device supports it and the user has enrolled biometrics. Which Kotlin code snippet correctly checks these conditions before showing the prompt?

val biometricManager = BiometricManager.from(this)
val canAuthenticate = biometricManager.canAuthenticate(BiometricManager.Authenticators.BIOMETRIC_STRONG or BiometricManager.Authenticators.DEVICE_CREDENTIAL)
if (/* condition here */) {
  biometricPrompt.authenticate(promptInfo)
}

hard

A. canAuthenticate == BiometricManager.BIOMETRIC_SUCCESS

B. canAuthenticate != BiometricManager.BIOMETRIC_ERROR_NO_HARDWARE

C. canAuthenticate == BiometricManager.BIOMETRIC_ERROR_NONE_ENROLLED

D. canAuthenticate == BiometricManager.BIOMETRIC_ERROR_HW_UNAVAILABLE

Solution

1. Step 1: Understand canAuthenticate return values

   canAuthenticate returns BIOMETRIC_SUCCESS if biometrics are available and enrolled.

2. Step 2: Choose correct condition to proceed

   Only when canAuthenticate == BIOMETRIC_SUCCESS should the app show the biometric prompt.

3. Final Answer:

   canAuthenticate == BiometricManager.BIOMETRIC_SUCCESS -> Option A

4. Quick Check:

   Check for BIOMETRIC_SUCCESS before authenticating [OK]

Hint: Authenticate only if canAuthenticate equals BIOMETRIC_SUCCESS [OK]

Common Mistakes:

• Checking for error codes instead of success
• Ignoring enrollment status
• Using wrong constants for condition