Custom authentication tokens in Firebase - Commands & Configuration

Introduction

Sometimes you want to let users sign in to your app using your own system instead of the usual email or social login. Custom Authentication Tokens let you create special login tokens that Firebase accepts, so users can access your app securely with your own user IDs.

When you have your own user database and want to let those users sign in to Firebase services.

When you want to add extra information to user login tokens for your app's special needs.

When you want to control exactly who can sign in and what their ID is in Firebase.

When you want to integrate Firebase authentication with an existing login system.

When you want to create temporary login tokens for users without passwords.

Commands

Log in to Firebase CLI to authenticate your local machine with your Firebase project.

Terminal

firebase login

Expected Output

✔  Success! Logged in as [e-mail]

Run a Node.js script that creates a custom authentication token for a user with a specific user ID.

Terminal

node createCustomToken.js

Expected Output

Custom token created: eyJhbGciOiJSUzI1NiIsImtpZCI6Ij...

Sign in to Firebase using the custom token generated, to verify the token works and the user is authenticated.

Terminal

firebase auth:signInWithCustomToken eyJhbGciOiJSUzI1NiIsImtpZCI6Ij...

Expected Output

✔  Successfully signed in with custom token.

Key Concept

If you remember nothing else from this pattern, remember: custom tokens let you securely sign in users with your own IDs by creating tokens Firebase trusts.

Common Mistakes

Using an expired or incorrectly signed custom token.

Firebase rejects tokens that are expired or not signed with your project's private key, so sign-in fails.

Always generate tokens using your Firebase Admin SDK with the correct credentials and use them quickly before they expire.

Trying to create custom tokens on the client side.

Creating tokens requires your private key and should only be done on a secure server to avoid security risks.

Generate custom tokens only on your trusted backend server using Firebase Admin SDK.

Summary

Log in to Firebase CLI to connect your machine to your Firebase project.

Use Firebase Admin SDK on a secure server to create custom authentication tokens for your users.

Sign in to Firebase using these tokens to authenticate users with your own user IDs.

Practice

1. What is the main purpose of Firebase custom authentication tokens?

easy

A. To store user data in Firebase Realtime Database

B. To sign in users from your own system to Firebase

C. To send notifications to users

D. To create Firebase projects automatically

Solution

1. Step 1: Understand the role of custom tokens

   Custom tokens are used to authenticate users from an external system with Firebase.

2. Step 2: Identify the correct purpose

   They allow users to sign in securely using tokens generated on your server.

3. Final Answer:

   To sign in users from your own system to Firebase -> Option B

4. Quick Check:

   Custom tokens = sign in users [OK]

Hint: Custom tokens = external user sign-in method [OK]

Common Mistakes:

• Confusing tokens with data storage
• Thinking tokens send notifications
• Assuming tokens create projects

2. Which Firebase Admin SDK method is used to create a custom authentication token?

easy

A. admin.auth().createCustomToken(uid)

B. admin.database().createToken(uid)

C. firebase.auth().signInWithCustomToken(uid)

D. admin.storage().generateToken(uid)

Solution

1. Step 1: Identify the correct Admin SDK service

   Custom tokens are created using the Authentication service, not database or storage.

2. Step 2: Match the correct method

   The method to create a custom token is createCustomToken(uid) under admin.auth().

3. Final Answer:

   admin.auth().createCustomToken(uid) -> Option A

4. Quick Check:

   Custom token creation = admin.auth().createCustomToken [OK]

Hint: Use admin.auth().createCustomToken to generate tokens [OK]

Common Mistakes:

• Using database or storage methods incorrectly
• Confusing signInWithCustomToken with token creation
• Calling methods on firebase instead of admin SDK

3. What will the following code output if uid is 'user123' and no errors occur?

const admin = require('firebase-admin');
admin.initializeApp();

admin.auth().createCustomToken('user123')
  .then(token => console.log(typeof token))
  .catch(error => console.error(error));

medium

A. 'object'

B. An error message

C. 'undefined'

D. 'string'

Solution

1. Step 1: Understand createCustomToken return type

   The method returns a Promise that resolves to a string token.

2. Step 2: Check console.log output

   Logging typeof token will print 'string' because the token is a string.

3. Final Answer:

   'string' -> Option D

4. Quick Check:

   createCustomToken returns string token [OK]

Hint: createCustomToken returns a string token inside a Promise [OK]

Common Mistakes:

• Expecting an object instead of string
• Confusing token with undefined
• Assuming an error without cause

4. Identify the error in this code snippet for generating a custom token:

const admin = require('firebase-admin');

admin.auth().createCustomToken('12345')
  .then(token => console.log(token))
  .catch(error => console.error(error));

medium

A. Missing admin.initializeApp() call

B. Incorrect method name, should be generateCustomToken

C. createCustomToken does not return a Promise

D. UID must be a string, not a number

Solution

1. Step 1: Check initialization of Firebase Admin SDK

   The admin SDK must be initialized with admin.initializeApp() before use.

2. Step 2: Verify other parts

   The UID '12345' is a valid string, createCustomToken returns a Promise, and method name is correct.

3. Final Answer:

   Missing admin.initializeApp() call -> Option A

4. Quick Check:

   Initialize admin SDK before calling methods [OK]

Hint: Always call admin.initializeApp() before using admin SDK [OK]

Common Mistakes:

• Passing number instead of string UID
• Assuming createCustomToken is synchronous
• Using wrong method name

5. You want to add extra user info like role: 'admin' in a custom token. Which code snippet correctly adds this claim?

hard

A. admin.auth().setCustomUserClaims(uid, { role: 'admin' })

B. admin.auth().createCustomToken(uid).setClaims({ role: 'admin' })

C. admin.auth().createCustomToken(uid, { role: 'admin' })

D. admin.auth().createCustomToken(uid, { claims: { role: 'admin' } })

Solution

1. Step 1: Understand how to add extra claims in custom tokens

   The createCustomToken method accepts a second argument with additional claims as a plain object.

2. Step 2: Evaluate each option

   admin.auth().createCustomToken(uid, { role: 'admin' }) correctly passes extra claims as second argument. admin.auth().createCustomToken(uid).setClaims({ role: 'admin' }) is invalid chaining. admin.auth().setCustomUserClaims(uid, { role: 'admin' }) sets claims on user but not in token creation. admin.auth().createCustomToken(uid, { claims: { role: 'admin' } }) nests claims incorrectly.

3. Final Answer:

   admin.auth().createCustomToken(uid, { role: 'admin' }) -> Option C

4. Quick Check:

   Extra claims go as second argument in createCustomToken [OK]

Hint: Pass extra claims as second argument to createCustomToken [OK]

Common Mistakes:

• Trying to chain setClaims on token creation
• Using setCustomUserClaims instead of token claims
• Nesting claims inside another object