Biometric authentication in Android Kotlin

Introduction

Biometric authentication lets users unlock apps using their fingerprint or face. It makes apps safer and easier to use without typing passwords.

Unlocking a banking app quickly and securely.

Allowing access to private notes or files with fingerprint.

Confirming a purchase or payment with face recognition.

Logging into an app without remembering a password.

Protecting sensitive settings or information inside an app.

Syntax

Android Kotlin

val biometricPrompt = BiometricPrompt(this, ContextCompat.getMainExecutor(this),
    object : BiometricPrompt.AuthenticationCallback() {
        override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
            super.onAuthenticationSucceeded(result)
            // User authenticated successfully
        }

        override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
            super.onAuthenticationError(errorCode, errString)
            // Handle error
        }

        override fun onAuthenticationFailed() {
            super.onAuthenticationFailed()
            // Authentication failed
        }
    })

val promptInfo = BiometricPrompt.PromptInfo.Builder()
    .setTitle("Login with Biometrics")
    .setSubtitle("Use your fingerprint or face to login")
    .setNegativeButtonText("Cancel")
    .build()

biometricPrompt.authenticate(promptInfo)

Use BiometricPrompt to show the biometric dialog.

Always provide a negative button for fallback.

Examples

This example shows a basic biometric prompt with title, subtitle, and cancel button.

Android Kotlin

val biometricPrompt = BiometricPrompt(this, ContextCompat.getMainExecutor(this),
    object : BiometricPrompt.AuthenticationCallback() {
        override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
            super.onAuthenticationSucceeded(result)
            // Success action
        }

        override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
            super.onAuthenticationError(errorCode, errString)
            // Error handling
        }

        override fun onAuthenticationFailed() {
            super.onAuthenticationFailed()
            // Failed attempt
        }
    })

val promptInfo = BiometricPrompt.PromptInfo.Builder()
    .setTitle("Biometric login")
    .setSubtitle("Log in using your biometric credential")
    .setNegativeButtonText("Use password")
    .build()

biometricPrompt.authenticate(promptInfo)

Customize the prompt with a description instead of subtitle.

Android Kotlin

val promptInfo = BiometricPrompt.PromptInfo.Builder()
    .setTitle("Secure Access")
    .setDescription("Authenticate to continue")
    .setNegativeButtonText("Cancel")
    .build()

Sample App

This app shows a biometric login prompt when started. It prints messages to the console based on success, failure, or error.

Android Kotlin

import android.os.Bundle
import androidx.appcompat.app.AppCompatActivity
import androidx.biometric.BiometricPrompt
import androidx.core.content.ContextCompat

class MainActivity : AppCompatActivity() {
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)

        val biometricPrompt = BiometricPrompt(this, ContextCompat.getMainExecutor(this),
            object : BiometricPrompt.AuthenticationCallback() {
                override fun onAuthenticationSucceeded(result: BiometricPrompt.AuthenticationResult) {
                    super.onAuthenticationSucceeded(result)
                    println("Authentication succeeded!")
                }

                override fun onAuthenticationError(errorCode: Int, errString: CharSequence) {
                    super.onAuthenticationError(errorCode, errString)
                    println("Authentication error: $errString")
                }

                override fun onAuthenticationFailed() {
                    super.onAuthenticationFailed()
                    println("Authentication failed")
                }
            })

        val promptInfo = BiometricPrompt.PromptInfo.Builder()
            .setTitle("Login with Biometrics")
            .setSubtitle("Use your fingerprint or face to login")
            .setNegativeButtonText("Cancel")
            .build()

        biometricPrompt.authenticate(promptInfo)
    }
}

Important Notes

Test biometric features on a real device with fingerprint or face setup.

Handle cases when biometric hardware is not available or not enrolled.

Always provide a fallback option like PIN or password.

Summary

Biometric authentication improves app security and user convenience.

Use BiometricPrompt to show the system biometric dialog.

Always handle success, failure, and errors gracefully.

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