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Firebase phone authentication(Flutter).

Firebase phone authentication(Flutter).

Peter Musembi's photo
Peter Musembi
ยทMay 3, 2021ยท

3 min read


Phone authentication comes in handy when you want to verify the user's phone number. In the world of cybersecurity, this is known as two-factor authentication.

Welcome to my first blog post. I appreciate blogs, am not so much into creating them, and yes, this is a task I enjoyed doing. This article leaves out the nitty-gritty details concerning the flutter framework workflow. Since we need that OTP feature, let's dive in ๐Ÿš€.


  1. Set up a firebase project and create a new app using your flutter project package name.
    Hint: minimum SDK version in app-level Gradle should be 21 or add multidex configuration in the android files.
  2. Sign your app by creating a Keystore and add (SHA1 and SHA256 keys) as fingerprints in your Firebase project (settings>general>your apps).
    Follow the steps in the link below and add the necessary files and code snippets.
    The Keystore explorer software is useful in viewing the Keystore properties.
  3. Create a debug key by running the command below in the project directory. Use android as the password

      keytool -list -v -alias androiddebugkey -keystore %USERPROFILE%\.android\debug.keystore

    Then again copy and add (SHA1 and SHA256 keys) as fingerprints in your Firebase project (settings>general>your apps).

  4. Start coding ... ๐Ÿ‘ฉโ€๐Ÿ’ป


Firstly, we import the following libraries in the pubspec.ymal file

firebase_auth: "^1.0.1"
firebase_core: ^1.0.2

Then we create a firebase auth instance in the root file in this case main.dart

void main() async {
  await Firebase.initializeApp();

Secondly, we set up two methods namely
Hint: Documentation URL

  1. Generate OTP(one-time pin) method, this method will take the phone number as an input, create a phone verification instance and provide a verification Id as the output.
    A completer came in handy in preventing the function from returning a null in place of the verification Id.

    generateOtp(String phoneNumber) async {
     try {
       final _completer = Completer<String>();
       await _auth.verifyPhoneNumber(
           phoneNumber: phoneNumber,
           codeAutoRetrievalTimeout: (String verId) {},
           codeSent: (String verificationId, [int? forceCodeResend]) {
           timeout: const Duration(seconds: 60),
           verificationCompleted: (PhoneAuthCredential phoneAuthCredential) {
             if (Platform.isAndroid && phoneAuthCredential.smsCode != null) {
           verificationFailed: (FirebaseAuthException e) {
             if (e.code == 'invalid-phone-number') {
               throw Exception('The provided phone number is not valid');
             } else {
               throw Exception('Exception occured');
       return _completer.future;
     } catch (e) {
       throw Exception('Verfication Error');
  2. Verify OTP method, this method will take SMS code and verification Id passed by the above method, if the SMS code was correct it will provide a Firebase UserCredential object as an output.

    verifyOtp(String smsCode, String verificationId) async {
     try {
       final phoneAuthCredential = PhoneAuthProvider.credential(
         verificationId: verificationId,
         smsCode: smsCode,
       final user = await _auth.signInWithCredential(phoneAuthCredential);
       return user;
     } catch (e) {
       throw Exception('Error happened');

Finally, we set up the UI we have three screens,

  1. Phone number entry screen, (in normal setting this would be the login screen).
  2. OTP screen where we call the generate-otp method in the initState method and the verify-otp method once a user has clicked verify code button.
  3. The home page, the final screen after a successful verification

The entire source code can be found in this repository. .
Feel free to open a PR.
Happy coding ๐Ÿž.