You can use Firebase Authentication to sign in a user by sending an SMS message to the user's phone. The user signs in using a one-time code contained in the SMS message.
The easiest way to add phone number sign-in to your app is to use FirebaseUI, which includes a drop-in sign-in widget that implements sign-in flows for phone number sign-in, as well as password-based and federated sign-in. This document describes how to implement a phone number sign-in flow using the Firebase SDK.
Before you begin
- If you haven't already, add Firebase to your Android project.
-
In your module (app-level) Gradle file
(usually
<project>/<app-module>/build.gradle.ktsor<project>/<app-module>/build.gradle), add the dependency for the Firebase Authentication library for Android. We recommend using the Firebase Android BoM to control library versioning.dependencies { // Import the BoM for the Firebase platform implementation(platform("com.google.firebase:firebase-bom:33.6.0")) // Add the dependency for the Firebase Authentication library // When using the BoM, you don't specify versions in Firebase library dependencies implementation("com.google.firebase:firebase-auth") }
By using the Firebase Android BoM, your app will always use compatible versions of Firebase Android libraries.
(Alternative) Add Firebase library dependencies without using the BoM
If you choose not to use the Firebase BoM, you must specify each Firebase library version in its dependency line.
Note that if you use multiple Firebase libraries in your app, we strongly recommend using the BoM to manage library versions, which ensures that all versions are compatible.
dependencies { // Add the dependency for the Firebase Authentication library // When NOT using the BoM, you must specify versions in Firebase library dependencies implementation("com.google.firebase:firebase-auth:23.1.0") }
- If you haven't yet connected your app to your Firebase project, do so from the Firebase console.
- If you haven't already set your app's SHA-1 hash in the Firebase console, do so. See Authenticating Your Client for information about finding your app's SHA-1 hash.
Security concerns
Authentication using only a phone number, while convenient, is less secure than the other available methods, because possession of a phone number can be easily transferred between users. Also, on devices with multiple user profiles, any user that can receive SMS messages can sign in to an account using the device's phone number.
If you use phone number based sign-in in your app, you should offer it alongside more secure sign-in methods, and inform users of the security tradeoffs of using phone number sign-in.
Enable Phone Number sign-in for your Firebase project
To sign in users by SMS, you must first enable the Phone Number sign-in method for your Firebase project:
- In the Firebase console, open the Authentication section.
- On the Sign-in Method page, enable the Phone Number sign-in method.
Enable app verification
To use phone number authentication, Firebase must be able to verify that phone number sign-in requests are coming from your app. There are three ways Firebase Authentication accomplishes this:
- Play Integrity API: If a user has a device with Google Play services installed,
and Firebase Authentication can verify the device as legitimate with the
Play Integrity API,
phone number sign-in can proceed. The Play Integrity API is enabled on a Google-owned project by
Firebase Authentication, not on your project. This does not contribute to any Play Integrity API
quotas on your project. Play Integrity Support is available with the
Authentication SDK v21.2.0+
(Firebase BoM v31.4.0+).
To use Play Integrity, if you haven't yet specified your app's SHA-256 fingerprint, do so from the Project settings of the Firebase console. Refer to Authenticating Your Client for details on how to get your app's SHA-256 fingerprint.
- reCAPTCHA verification: In the event that Play Integrity cannot be used,
such as when a user has a device without Google Play services installed, Firebase Authentication
uses a reCAPTCHA verification to complete the phone sign-in flow. The reCAPTCHA challenge can
often be completed without the user having to solve anything. Note that this flow requires that
a SHA-1 is associated with your application. This flow also requires your API Key to be
unrestricted or allowlisted for
PROJECT_ID.firebaseapp.com.Some scenarios where reCAPTCHA is triggered:
- If the end-user's device does not have Google Play services installed.
- If the app is not distributed through Google Play Store (on Authentication SDK v21.2.0+).
- If the obtained SafetyNet token was not valid (on Authentication SDK versions < v21.2.0).
When SafetyNet or Play Integrity is used for App verification, the
%APP_NAME%field in the SMS template is populated with the app name determined from Google Play Store. In the scenarios where reCAPTCHA is triggered,%APP_NAME%is populated asPROJECT_ID.firebaseapp.com.
forceRecaptchaFlowForTesting
You can disable app verification (when using fictional phone numbers) using
setAppVerificationDisabledForTesting.
Troubleshooting
"Missing initial state" error when using reCAPTCHA for app verification
This can occur when the reCAPTCHA flow completes successfully but does not redirect the user back to the native application. If this occurs, the user is redirected to the fallback URL
PROJECT_ID.firebaseapp.com/__/auth/handler. On Firefox browsers, opening native app links is disabled by default. If you see the above error on Firefox, follow the steps in Set Firefox for Android to open links in native apps to enable opening app links.
Send a verification code to the user's phone
To initiate phone number sign-in, present the user an interface that prompts them to type their phone number. Legal requirements vary, but as a best practice and to set expectations for your users, you should inform them that if they use phone sign-in, they might receive an SMS message for verification and standard rates apply.
Then, pass their phone number to the
PhoneAuthProvider.verifyPhoneNumber method to request that Firebase
verify the user's phone number. For example:
Kotlin+KTX
val options = PhoneAuthOptions.newBuilder(auth) .setPhoneNumber(phoneNumber) // Phone number to verify .setTimeout(60L, TimeUnit.SECONDS) // Timeout and unit .setActivity(this) // Activity (for callback binding) .setCallbacks(callbacks) // OnVerificationStateChangedCallbacks .build() PhoneAuthProvider.verifyPhoneNumber(options)
Java
PhoneAuthOptions options = PhoneAuthOptions.newBuilder(mAuth) .setPhoneNumber(phoneNumber) // Phone number to verify .setTimeout(60L, TimeUnit.SECONDS) // Timeout and unit .setActivity(this) // (optional) Activity for callback binding // If no activity is passed, reCAPTCHA verification can not be used. .setCallbacks(mCallbacks) // OnVerificationStateChangedCallbacks .build(); PhoneAuthProvider.verifyPhoneNumber(options);
The verifyPhoneNumber method is reentrant: if you call it
multiple times, such as in an activity's onStart method, the
verifyPhoneNumber method will not send a second SMS unless the
original request has timed out.
You can use this behavior to resume the phone number sign in process if your
app closes before the user can sign in (for example, while the user is using
their SMS app). After you call verifyPhoneNumber, set a flag that
indicates verification is in progress. Then, save the flag in your Activity's
onSaveInstanceState method and restore the flag in
onRestoreInstanceState. Finally, in your Activity's
onStart method, check if verification is already in progress, and
if so, call verifyPhoneNumber again. Be sure to clear the flag when
verification completes or fails (see
Verification callbacks).
To easily handle screen rotation and other instances of Activity restarts,
pass your Activity to the verifyPhoneNumber method. The callbacks
will be auto-detached when the Activity stops, so you can freely write UI
transition code in the callback methods.
The SMS message sent by Firebase can also be localized by specifying the
auth language via the setLanguageCode method on your Auth
instance.
Kotlin+KTX
auth.setLanguageCode("fr") // To apply the default app language instead of explicitly setting it. // auth.useAppLanguage()
Java
auth.setLanguageCode("fr"); // To apply the default app language instead of explicitly setting it. // auth.useAppLanguage();
When you call PhoneAuthProvider.verifyPhoneNumber, you must also
provide an instance of OnVerificationStateChangedCallbacks, which
contains implementations of the callback functions that handle the results of
the request. For example:
Kotlin+KTX
callbacks = object : PhoneAuthProvider.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. // 2 - Auto-retrieval. On some devices Google Play services can automatically // detect the incoming verification SMS and perform verification without // user action. Log.d(TAG, "onVerificationCompleted:$credential") signInWithPhoneAuthCredential(credential) } override fun onVerificationFailed(e: FirebaseException) { // This callback is invoked in an invalid request for verification is made, // for instance if the the phone number format is not valid. Log.w(TAG, "onVerificationFailed", e) if (e is FirebaseAuthInvalidCredentialsException) { // Invalid request } else if (e is FirebaseTooManyRequestsException) { // The SMS quota for the project has been exceeded } else if (e is FirebaseAuthMissingActivityForRecaptchaException) { // reCAPTCHA verification attempted with null Activity } // Show a message and update the UI } override fun onCodeSent( verificationId: String, token: PhoneAuthProvider.ForceResendingToken, ) { // The SMS verification code has been sent to the provided phone number, we // now need to ask the user to enter the code and then construct a credential // by combining the code with a verification ID. Log.d(TAG, "onCodeSent:$verificationId") // Save verification ID and resending token so we can use them later storedVerificationId = verificationId resendToken = token } }
Java
mCallbacks = new PhoneAuthProvider.OnVerificationStateChangedCallbacks() { @Override public void onVerificationCompleted(@NonNull PhoneAuthCredential credential) { // 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. // 2 - Auto-retrieval. On some devices Google Play services can automatically // detect the incoming verification SMS and perform verification without // user action. Log.d(TAG, "onVerificationCompleted:" + credential); signInWithPhoneAuthCredential(credential); } @Override public void onVerificationFailed(@NonNull FirebaseException e) { // This callback is invoked in an invalid request for verification is made, // for instance if the the phone number format is not valid. Log.w(TAG, "onVerificationFailed", e); if (e instanceof FirebaseAuthInvalidCredentialsException) { // Invalid request } else if (e instanceof FirebaseTooManyRequestsException) { // The SMS quota for the project has been exceeded } else if (e instanceof FirebaseAuthMissingActivityForRecaptchaException) { // reCAPTCHA verification attempted with null Activity } // Show a message and update the UI } @Override public void onCodeSent(@NonNull String verificationId, @NonNull PhoneAuthProvider.ForceResendingToken token) { // The SMS verification code has been sent to the provided phone number, we // now need to ask the user to enter the code and then construct a credential // by combining the code with a verification ID. Log.d(TAG, "onCodeSent:" + verificationId); // Save verification ID and resending token so we can use them later mVerificationId = verificationId; mResendToken = token; } };
Verification callbacks
In most apps, you implement the onVerificationCompleted,
onVerificationFailed, and onCodeSent callbacks. You
might also implement onCodeAutoRetrievalTimeOut, depending on your
app's requirements.
onVerificationCompleted(PhoneAuthCredential)
This method is called in two situations:
- Instant verification: in some cases the phone number can be instantly verified without needing to send or enter a verification code.
- Auto-retrieval: on some devices, Google Play services can automatically detect the incoming verification SMS and perform verification without user action. (This capability might be unavailable with some carriers.) This uses the SMS Retriever API, which includes an 11 character hash at the end of the SMS message.
PhoneAuthCredential object that's passed to the
callback to sign in the user.
onVerificationFailed(FirebaseException)
This method is called in response to an invalid verification request, such as a request that specifies an invalid phone number or verification code.
onCodeSent(String verificationId, PhoneAuthProvider.ForceResendingToken)
Optional. This method is called after the verification code has been sent by SMS to the provided phone number.
When this method is called, most apps display a UI that prompts the user
to type the verification code from the SMS message. (At the same time,
auto-verification might be proceeding in the background.) Then, after the user
types the verification code, you can use the verification code and the
verification ID that was passed to the method to create a
PhoneAuthCredential object, which you can in turn use to sign in
the user. However, some apps might wait until
onCodeAutoRetrievalTimeOut is called before displaying the
verification code UI (not recommended).
onCodeAutoRetrievalTimeOut(String verificationId)
Optional. This method is called after the timeout duration specified to
verifyPhoneNumber has passed without
onVerificationCompleted triggering first. On devices without SIM
cards, this method is called immediately because SMS auto-retrieval isn't
possible.
Some apps block user input until the auto-verification period has timed out, and only then display a UI that prompts the user to type the verification code from the SMS message (not recommended).
Create a PhoneAuthCredential object
After the user enters the verification code that Firebase sent to the user's
phone, create a PhoneAuthCredential object, using the verification
code and the verification ID that was passed to the onCodeSent or
onCodeAutoRetrievalTimeOut callback. (When
onVerificationCompleted is called, you get a
PhoneAuthCredential object directly, so you can skip this step.)
To create the PhoneAuthCredential object, call
PhoneAuthProvider.getCredential:
Kotlin+KTX
val credential = PhoneAuthProvider.getCredential(verificationId!!, code)
Java
PhoneAuthCredential credential = PhoneAuthProvider.getCredential(verificationId, code);
Sign in the user
After you get a PhoneAuthCredential object, whether in the
onVerificationCompleted callback or by calling
PhoneAuthProvider.getCredential, complete the sign-in flow by
passing the PhoneAuthCredential object to
FirebaseAuth.signInWithCredential:
Kotlin+KTX
private fun signInWithPhoneAuthCredential(credential: PhoneAuthCredential) { auth.signInWithCredential(credential) .addOnCompleteListener(this) { task -> if (task.isSuccessful) { // Sign in success, update UI with the signed-in user's information Log.d(TAG, "signInWithCredential:success") val user = task.result?.user } else { // Sign in failed, display a message and update the UI Log.w(TAG, "signInWithCredential:failure", task.exception) if (task.exception is FirebaseAuthInvalidCredentialsException) { // The verification code entered was invalid } // Update UI } } }
Java
private void signInWithPhoneAuthCredential(PhoneAuthCredential credential) { mAuth.signInWithCredential(credential) .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() { @Override public void onComplete(@NonNull Task<AuthResult> task) { if (task.isSuccessful()) { // Sign in success, update UI with the signed-in user's information Log.d(TAG, "signInWithCredential:success"); FirebaseUser user = task.getResult().getUser(); // Update UI } else { // Sign in failed, display a message and update the UI Log.w(TAG, "signInWithCredential:failure", task.getException()); if (task.getException() instanceof FirebaseAuthInvalidCredentialsException) { // The verification code entered was invalid } } } }); }
Test with fictional phone numbers
You can set up fictional phone numbers for development via the Firebase console. Testing with fictional phone numbers provides these benefits:
- Test phone number authentication without consuming your usage quota.
- Test phone number authentication without sending an actual SMS message.
- Run consecutive tests with the same phone number without getting throttled. This minimizes the risk of rejection during App store review process if the reviewer happens to use the same phone number for testing.
- Test readily in development environments without any additional effort, such as the ability to develop in an iOS simulator or an Android emulator without Google Play Services.
- Write integration tests without being blocked by security checks normally applied on real phone numbers in a production environment.
Fictional phone numbers must meet these requirements:
- Make sure you use phone numbers that are indeed fictional, and do not already exist. Firebase Authentication does not allow you to set existing phone numbers used by real users as test numbers. One option is to use 555 prefixed numbers as US test phone numbers, for example: +1 650-555-3434
- Phone numbers have to be correctly formatted for length and other constraints. They will still go through the same validation as a real user's phone number.
- You can add up to 10 phone numbers for development.
- Use test phone numbers/codes that are hard to guess and change those frequently.
Create fictional phone numbers and verification codes
- In the Firebase console, open the Authentication section.
- In the Sign in method tab, enable the Phone provider if you haven't already.
- Open the Phone numbers for testing accordion menu.
- Provide the phone number you want to test, for example: +1 650-555-3434.
- Provide the 6-digit verification code for that specific number, for example: 654321.
- Add the number. If there's a need, you can delete the phone number and its code by hovering over the corresponding row and clicking the trash icon.
Manual testing
You can directly start using a fictional phone number in your application. This allows you to perform manual testing during development stages without running into quota issues or throttling. You can also test directly from an iOS simulator or Android emulator without Google Play Services installed.
When you provide the fictional phone number and send the verification code, no actual SMS is sent. Instead, you need to provide the previously configured verification code to complete the sign in.
On sign-in completion, a Firebase user is created with that phone number. The user has the same behavior and properties as a real phone number user, and can access Realtime Database/Cloud Firestore and other services the same way. The ID token minted during this process has the same signature as a real phone number user.
Another option is to set a test role via custom claims on these users to differentiate them as fake users if you want to further restrict access.
To manually trigger the reCAPTCHA flow for testing, use the
forceRecaptchaFlowForTesting() method.
// Force reCAPTCHA flow FirebaseAuth.getInstance().getFirebaseAuthSettings().forceRecaptchaFlowForTesting();
Integration testing
In addition to manual testing, Firebase Authentication provides APIs to help write integration tests for phone auth testing. These APIs disable app verification by disabling the reCAPTCHA requirement in web and silent push notifications in iOS. This makes automation testing possible in these flows and easier to implement. In addition, they help provide the ability to test instant verification flows on Android.
On Android, call setAppVerificationDisabledForTesting() before the
signInWithPhoneNumber call. This disables app verification automatically,
allowing you to pass the phone number without manually solving it. Even though
Play Integrity and reCAPTCHA are disabled, using a real phone number will still fail to
complete sign in. Only fictional phone numbers can be used with this API.
// Turn off phone auth app verification. FirebaseAuth.getInstance().getFirebaseAuthSettings() .setAppVerificationDisabledForTesting();
Calling verifyPhoneNumber with a fictional number triggers the
onCodeSent callback, in which you'll need to provide the corresponding verification
code. This allows testing in Android Emulators.
Java
String phoneNum = "+16505554567"; String testVerificationCode = "123456"; // Whenever verification is triggered with the whitelisted number, // provided it is not set for auto-retrieval, onCodeSent will be triggered. FirebaseAuth auth = FirebaseAuth.getInstance(); PhoneAuthOptions options = PhoneAuthOptions.newBuilder(auth) .setPhoneNumber(phoneNum) .setTimeout(60L, TimeUnit.SECONDS) .setActivity(this) .setCallbacks(new PhoneAuthProvider.OnVerificationStateChangedCallbacks() { @Override public void onCodeSent(@NonNull String verificationId, @NonNull PhoneAuthProvider.ForceResendingToken forceResendingToken) { // Save the verification id somewhere // ... // The corresponding whitelisted code above should be used to complete sign-in. MainActivity.this.enableUserManuallyInputCode(); } @Override public void onVerificationCompleted(@NonNull PhoneAuthCredential phoneAuthCredential) { // Sign in with the credential // ... } @Override public void onVerificationFailed(@NonNull FirebaseException e) { // ... } }) .build(); PhoneAuthProvider.verifyPhoneNumber(options);
Kotlin+KTX
val phoneNum = "+16505554567" val testVerificationCode = "123456" // Whenever verification is triggered with the whitelisted number, // provided it is not set for auto-retrieval, onCodeSent will be triggered. val options = PhoneAuthOptions.newBuilder(Firebase.auth) .setPhoneNumber(phoneNum) .setTimeout(30L, TimeUnit.SECONDS) .setActivity(this) .setCallbacks(object : PhoneAuthProvider.OnVerificationStateChangedCallbacks() { override fun onCodeSent( verificationId: String, forceResendingToken: PhoneAuthProvider.ForceResendingToken, ) { // Save the verification id somewhere // ... // The corresponding whitelisted code above should be used to complete sign-in. this@MainActivity.enableUserManuallyInputCode() } override fun onVerificationCompleted(phoneAuthCredential: PhoneAuthCredential) { // Sign in with the credential // ... } override fun onVerificationFailed(e: FirebaseException) { // ... } }) .build() PhoneAuthProvider.verifyPhoneNumber(options)
Additionally, you can test auto-retrieval flows in Android by setting the fictional number and
its corresponding verification code for auto-retrieval by calling
setAutoRetrievedSmsCodeForPhoneNumber.
When verifyPhoneNumber is
called, it triggers onVerificationCompleted with the PhoneAuthCredential
directly. This works only with fictional phone numbers.
Make sure this is disabled and no fictional phone numbers are hardcoded in your app when publishing your application to the Google Play store.
Java
// The test phone number and code should be whitelisted in the console. String phoneNumber = "+16505554567"; String smsCode = "123456"; FirebaseAuth firebaseAuth = FirebaseAuth.getInstance(); FirebaseAuthSettings firebaseAuthSettings = firebaseAuth.getFirebaseAuthSettings(); // Configure faking the auto-retrieval with the whitelisted numbers. firebaseAuthSettings.setAutoRetrievedSmsCodeForPhoneNumber(phoneNumber, smsCode); PhoneAuthOptions options = PhoneAuthOptions.newBuilder(firebaseAuth) .setPhoneNumber(phoneNumber) .setTimeout(60L, TimeUnit.SECONDS) .setActivity(this) .setCallbacks(new PhoneAuthProvider.OnVerificationStateChangedCallbacks() { @Override public void onVerificationCompleted(@NonNull PhoneAuthCredential credential) { // Instant verification is applied and a credential is directly returned. // ... } // ... }) .build(); PhoneAuthProvider.verifyPhoneNumber(options);
Kotlin+KTX
// The test phone number and code should be whitelisted in the console. val phoneNumber = "+16505554567" val smsCode = "123456" val firebaseAuth = Firebase.auth val firebaseAuthSettings = firebaseAuth.firebaseAuthSettings // Configure faking the auto-retrieval with the whitelisted numbers. firebaseAuthSettings.setAutoRetrievedSmsCodeForPhoneNumber(phoneNumber, smsCode) val options = PhoneAuthOptions.newBuilder(firebaseAuth) .setPhoneNumber(phoneNumber) .setTimeout(60L, TimeUnit.SECONDS) .setActivity(this) .setCallbacks(object : PhoneAuthProvider.OnVerificationStateChangedCallbacks() { override fun onVerificationCompleted(credential: PhoneAuthCredential) { // Instant verification is applied and a credential is directly returned. // ... } // ... }) .build() PhoneAuthProvider.verifyPhoneNumber(options)
Next steps
After a user signs in for the first time, a new user account is created and linked to the credentials—that is, the user name and password, phone number, or auth provider information—the user signed in with. This new account is stored as part of your Firebase project, and can be used to identify a user across every app in your project, regardless of how the user signs in.
-
In your apps, you can get the user's basic profile information from the
FirebaseUserobject. See Manage Users. In your Firebase Realtime Database and Cloud Storage Security Rules, you can get the signed-in user's unique user ID from the
authvariable, and use it to control what data a user can access.
You can allow users to sign in to your app using multiple authentication providers by linking auth provider credentials to an existing user account.
To sign out a user, call
signOut:
Kotlin+KTX
Firebase.auth.signOut()
Java
FirebaseAuth.getInstance().signOut();