Adding multi-factor authentication to your Android app

This document shows you how to add SMS multi-factor authentication to your Android app.

Multi-factor authentication increases the security of your app. While attackers often compromise passwords and social accounts, intercepting a text message is more difficult.

Before you begin

1. Enable at least one provider that supports multi-factor authentication. Every provider supports MFA, except phone auth, anonymous auth, and Apple Game Center.

2. Ensure your app is verifying user emails. MFA requires email verification. This prevents malicious actors from registering for a service with an email they don't own, and then locking out the real owner by adding a second factor.

3. Register your app's SHA-1 hash in the Firebase console (your changes will automatically carry over to Google Cloud Identity Platform).

   1. Follow the steps in Authenticating your client to obtain your app's SHA-1 hash.

   2. In the Firebase console, go to the settings Settings > General tab.

   3. Under Your apps, select your Android app, and add your SHA-1 fingerprint in the SHA certificate fingerprints field.

Enabling multi-factor authentication

1. Go to the Identity Platform MFA page in the Google Cloud console.
   Go to MFA
   

2. In Multi-Factor Authentication, click Enable.

3. Enter the phone numbers you'll be testing your app with. While optional, registering test phone numbers is strongly recommended to avoid throttling during development.

4. If you haven't already authorized your app's domain, add it to the allowlist by clicking Add domain.

5. Click Save.

Choosing an enrollment pattern

You can choose whether your app requires multi-factor authentication, and how and when to enroll your users. Some common patterns include:

• Enroll the user's second factor as part of registration. Use this method if your app requires multi-factor authentication for all users.

• Offer a skippable option to enroll a second factor during registration. Apps that want to encourage, but not require, multi-factor authentication might prefer this approach.

• Provide the ability to add a second factor from the user's account or profile management page, instead of the sign up screen. This minimizes friction during the registration process, while still making multi-factor authentication available for security-sensitive users.

• Require adding a second factor incrementally when the user wants to access features with increased security requirements.

Enrolling a second factor

To enroll a new secondary factor for a user:

1. Re-authenticate the user.

2. Ask the user enter their phone number.

3. Get a multi-factor session for the user:

   Kotlin+KTX

   user.multiFactor.session.addOnCompleteListener { task ->
       if (task.isSuccessful) {
           val multiFactorSession: MultiFactorSession? = task.result
       }
   }
   

   Java

   user.getMultiFactor().getSession()
     .addOnCompleteListener(
         new OnCompleteListener<MultiFactorSession>() {
         @Override
         public void onComplete(@NonNull Task<MultiFactorSession> task) {
           if (task.isSuccessful()) {
             MultiFactorSession multiFactorSession = task.getResult();
           }
         }
         });
   

4. Build an OnVerificationStateChangedCallbacks object to handle different events in the verification process:

   Kotlin+KTX

   val callbacks = object : OnVerificationStateChangedCallbacks() {
       override fun onVerificationCompleted(credential: PhoneAuthCredential) {
           // This callback will be invoked in two situations:
           // 1) Instant verification. In some cases, the phone number can be
           //    instantly verified without needing to send or enter a verification
           //    code. You can disable this feature by calling
           //    PhoneAuthOptions.builder#requireSmsValidation(true) when building
           //    the options to pass to PhoneAuthProvider#verifyPhoneNumber().
           // 2) Auto-retrieval. On some devices, Google Play services can
           //    automatically detect the incoming verification SMS and perform
           //    verification without user action.
           this@MainActivity.credential = credential
       }
   
       override fun onVerificationFailed(e: FirebaseException) {
           // This callback is invoked in response to invalid requests for
           // verification, like an incorrect phone number.
           if (