Biometric authentication in Android Kotlin - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a BiometricPrompt instance.

Android Kotlin

val biometricPrompt = BiometricPrompt(this, ContextCompat.getMainExecutor(this), [1])

Drag options to blanks, or click blank then click option'

AauthenticationCallback

BpromptInfo

Cexecutor

DbiometricManager

Attempts:

3 left

💡 Hint

Common Mistakes

Using the executor or promptInfo instead of the callback.

Passing biometricManager which is unrelated here.

The BiometricPrompt constructor requires an AuthenticationCallback to handle authentication events.

2fill in blank

medium

Complete the code to build the BiometricPrompt.PromptInfo.

Android Kotlin

val promptInfo = BiometricPrompt.PromptInfo.Builder()
  .setTitle("Login")
  .setSubtitle("Use your fingerprint")
  .setNegativeButtonText([1])
  .build()

Drag options to blanks, or click blank then click option'

A"Retry"

B"Cancel"

C"OK"

D"Confirm"

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'OK' or 'Confirm' which confuse the cancel action.

Leaving the negative button text empty.

The negative button text should be a clear cancel action, usually labeled "Cancel".

3fill in blank

hard

Fix the error in the authentication callback method name.

Android Kotlin

val authenticationCallback = object : BiometricPrompt.AuthenticationCallback() {
  override fun onAuthentication[1](result: BiometricPrompt.AuthenticationResult) {
    super.onAuthenticationSucceeded(result)
    // Handle success
  }
}

Drag options to blanks, or click blank then click option'

ASuccessed

BSuccess

CSucceededed

DSucceeded

Attempts:

3 left

💡 Hint

Common Mistakes

Misspelling the method name causes it not to override properly.

Using 'Success' instead of 'Succeeded'.

The correct method name is onAuthenticationSucceeded with 'Succeeded' at the end.

4fill in blank

hard

Fill both blanks to check if biometric authentication is available and strong.

Android Kotlin

val biometricManager = BiometricManager.from(this)
if (biometricManager.canAuthenticate([1]) == [2]) {
  // Biometric authentication is available
}

Drag options to blanks, or click blank then click option'

ABiometricManager.Authenticators.BIOMETRIC_STRONG

BBiometricManager.Authenticators.DEVICE_CREDENTIAL

CBiometricManager.BIOMETRIC_SUCCESS

DBiometricManager.BIOMETRIC_ERROR_NO_HARDWARE

Attempts:

3 left

💡 Hint

Common Mistakes

Using DEVICE_CREDENTIAL instead of BIOMETRIC_STRONG.

Checking for BIOMETRIC_ERROR_NO_HARDWARE instead of BIOMETRIC_SUCCESS.

Use BIOMETRIC_STRONG to specify strong biometrics and check for BIOMETRIC_SUCCESS to confirm availability.

5fill in blank

hard

Fill all three blanks to start biometric authentication with the prompt info.

Android Kotlin

biometricPrompt.[1](
  [2],
  [3]
)

Drag options to blanks, or click blank then click option'

Aauthenticate

BpromptInfo

CCancellationSignal()

Dcancel

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'cancel' instead of 'authenticate'.

Passing wrong arguments or missing cancellation signal.

Call authenticate with the prompt info and a new cancellation signal to start authentication.

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