Authenticate with Firebase on Android Using a Custom Authentication System

You can integrate Firebase Authentication with a custom authentication system by modifying your authentication server to produce custom signed tokens when a user successfully signs in. Your app receives this token and uses it to authenticate with Firebase.

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.kts or <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:34.16.0"))

    // 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:24.2.0")
}
```
Get your project's server keys:
1. In the Firebase console, go to the Settings > Service accounts tab.
2. At the bottom of the Firebase Admin SDK section, click Generate New Private Key.
3. The new service account's public/private key pair is automatically saved on your computer. Copy this file to your authentication server.

Authenticate with Firebase

In your sign-in activity's onCreate method, get the shared instance of the FirebaseAuth object:

Kotlin

private lateinit var auth: FirebaseAuth
// ...
// Initialize Firebase Auth
auth = Firebase.authCustomAuthActivity.kt

public override fun onStart() { super.onStart() // Check if user is signed in (non-null) and update UI accordingly. val currentUser = auth.currentUser updateUI(currentUser) }

CustomAuthActivity.kt

@Override public void onStart() { super.onStart(); // Check if user is signed in (non-null) and update UI accordingly. FirebaseUser currentUser = mAuth.getCurrentUser(); updateUI(currentUser); }

CustomAuthActivity.java

customToken?.let { auth.signInWithCustomToken(it) .addOnCompleteListener(this) { task -> if (task.isSuccessful) { // Sign in success, update UI with the signed-in user's information Log.d(TAG, "signInWithCustomToken:success") val user = auth.currentUser updateUI(user) } else { // If sign in fails, display a message to the user. Log.w(TAG, "signInWithCustomToken:failure", task.exception) Toast.makeText( baseContext, "Authentication failed.", Toast.LENGTH_SHORT, ).show() updateUI(null) } } }

CustomAuthActivity.kt

mAuth.signInWithCustomToken(mCustomToken) .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, "signInWithCustomToken:success"); FirebaseUser user = mAuth.getCurrentUser(); updateUI(user); } else { // If sign in fails, display a message to the user. Log.w(TAG, "signInWithCustomToken:failure", task.getException()); Toast.makeText(CustomAuthActivity.this, "Authentication failed.", Toast.LENGTH_SHORT).show(); updateUI(null); } } });

CustomAuthActivity.java

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 FirebaseUser object. 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 auth variable, and use it to control what data a user can access.

To sign out a user, call signOut:

Kotlin

Firebase.auth.signOut()MainActivity.kt

Java

FirebaseAuth.getInstance().signOut();MainActivity.java

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Last updated 2026-07-20 UTC.

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Before you begin

Authenticate with Firebase

Kotlin

Java

Kotlin

Java

Kotlin

Java

Next steps

Kotlin

Java

Authenticate with Firebase on Android Using a Custom Authentication System