Authenticate with Firebase Using a Custom Authentication System and C++

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

Add Firebase to your C++ project.
In your project-level build.gradle file, make sure to include Google's Maven repository in both your buildscript and allprojects sections.
Get your project's server keys:
1. Go to the Service Accounts page in your project's settings.
2. Click Generate New Private Key at the bottom of the Firebase Admin SDK section of the Service Accounts page.
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

The Auth class is the gateway for all API calls.

Add the Auth and App header files:

#include "firebase/app.h"
#include "firebase/auth.h"

In your initialization code, create a firebase::App class.

#if defined(__ANDROID__)
  firebase::App* app =
      firebase::App::Create(firebase::AppOptions(), my_jni_env, my_activity);
#else
  firebase::App* app = firebase::App::Create(firebase::AppOptions());
#endif  // defined(__ANDROID__)

Acquire the firebase::auth::Auth class for your firebase::App. There is a one-to-one mapping between App and Auth.
```
firebase::auth::Auth* auth = firebase::auth::Auth::GetAuth(app);
```

Call Auth::SignInWithCustomToken with the token from your authentication server.

When users sign in to your app, send their sign-in credentials (for example, their username and password) to your authentication server. Your server checks the credentials and returns a custom token if they are valid.
After you receive the custom token from your authentication server, pass it to Auth::SignInWithCustomToken to sign in the user:
```
firebase::Future<firebase::auth::User*> result =
    auth->SignInWithCustomToken(custom_token);
```

If your program has an update loop that runs regularly (say at 30 or 60 times per second), you can check the results once per update with Auth::SignInWithCustomTokenLastResult:

firebase::Future<firebase::auth::User*> result =
    auth->SignInWithCustomTokenLastResult();
if (result.status() == firebase::kFutureStatusComplete) {
  if (result.error() == firebase::auth::kAuthErrorNone) {
    firebase::auth::User* user = *result.result();
    printf("Sign in succeeded for `%s`\n", user->display_name().c_str());
  } else {
    printf("Sign in failed with error '%s'\n", result.error_message());
  }
}

Or, if your program is event driven, you may prefer to register a callback on the Future.

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 firebase::auth::User object:

firebase::auth::User* user = auth->current_user();
if (user != nullptr) {
  std::string name = user->display_name();
  std::string email = user->email();
  std::string photo_url = user->photo_url();
  // The user's ID, unique to the Firebase project.
  // Do NOT use this value to authenticate with your backend server,
  // if you have one. Use firebase::auth::User::Token() instead.
  std::string uid = user->uid();
}

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.

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():

auth->SignOut();