Authenticate Using Microsoft and C++

You can let your users authenticate with Firebase using OAuth providers like Microsoft Azure Active Directory by integrating web-based generic OAuth Login into your app using the Firebase SDK to carry out the end to end sign-in flow. Since this flow requires the use of the phone-based Firebase SDKs, it is only supported on Android and Apple platforms.

Before you begin

Add Firebase to your C++ project.
In the Firebase console, open the Auth section.
On the Sign in method tab, enable the Microsoft provider.
Add the Client ID and Client Secret from that provider's developer console to the provider configuration:
1. To register a Microsoft OAuth client, follow the instructions in Quickstart: Register an app with the Azure Active Directory v2.0 endpoint. Note that this endpoint supports sign-in using Microsoft personal accounts as well as Azure Active Directory accounts. Learn more about Azure Active Directory v2.0.
2. When registering apps with these providers, be sure to register the *.firebaseapp.com domain for your project as the redirect domain for your app.
Click Save.

Access the `firebase::auth::Auth` class

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);
```

Handle the sign-in flow with the Firebase SDK

To handle the sign-in flow with the Firebase SDK, follow these steps:

Construct an instance of a FederatedOAuthProviderData configured with the provider id appropriate for Microsoft.

firebase::auth::FederatedOAuthProviderData
    provider_data(firebase::auth::MicrosoftAuthProvider::kProviderId);

Optional: Specify additional custom OAuth parameters that you want to send with the OAuth request.
```
// Prompt user to re-authenticate to Microsoft.
provider_data.custom_parameters["prompt"] = "login";

// Target specific email with login hint.
provider_data.custom_parameters["login_hint"] =
    "user@firstadd.onmicrosoft.com";
```
For the parameters Microsoft supports, see the Microsoft OAuth documentation. Note that you can't pass Firebase-required parameters with setCustomParameters(). These parameters are client_id, response_type, redirect_uri, state, scope and response_mode.

To allow only users from a particular Azure AD tenant to sign into the application, either the friendly domain name of the Azure AD tenant or the tenant's GUID identifier can be used. This can be done by specifying the "tenant" field in the custom parameters object.
```
// Optional "tenant" parameter in case you are using an Azure AD tenant.
// eg. '8eaef023-2b34-4da1-9baa-8bc8c9d6a490' or 'contoso.onmicrosoft.com'
// or "common" for tenant-independent tokens.
// The default value is "common".
provider_data.custom_parameters["tenant"] ="TENANT_ID";
```
Optional: Specify additional OAuth 2.0 scopes beyond basic profile that you want to request from the authentication provider.
```
provider_data.scopes.push_back("mail.read");
provider_data.scopes.push_back("calendars.read");
```
To learn more, refer to the Microsoft permissions and consent documentation.

Once your provider data has been configured, use it to create a FederatedOAuthProvider.

// Construct a FederatedOAuthProvider for use in Auth methods.
firebase::auth::FederatedOAuthProvider provider(provider_data);

Authenticate with Firebase using the Auth provider object. Note that unlike other FirebaseAuth operations, this will take control of your UI by popping up a web view in which the user can enter their credentials.

To start the sign in flow, call SignInWithProvider:
```
firebase::Future<firebase::auth::AuthResult> result =
  auth->SignInWithProvider(provider_data);
```
Your application may then wait or register a callback on the Future.

Using the OAuth access token, you can call the Microsoft Graph API.

Unlike other providers supported by Firebase Auth, Microsoft does not provide a photo URL and instead, the binary data for a profile photo has to be requested via Microsoft Graph API.
While the above examples focus on sign-in flows, you also have the ability to link a Microsoft Azure Active Directory provider to an existing user using LinkWithProvider. For example, you can link multiple providers to the same user allowing them to sign in with either.
```
firebase::Future<firebase::auth::AuthResult> result = user.LinkWithProvider(provider_data);
```
The same pattern can be used with ReauthenticateWithProvider which can be used to retrieve fresh credentials for sensitive operations that require recent login.
```
firebase::Future<firebase::auth::AuthResult> result =
  user.ReauthenticateWithProvider(provider_data);
```
Your application may then wait or register a callback on the Future.

Unlike other OAuth providers supported by Firebase such as Google, Facebook, and Twitter, where sign-in can directly be achieved with OAuth access token based credentials, Firebase Auth does not support the same capability for providers such as Microsoft due to the inability of the Firebase Auth server to verify the audience of Microsoft OAuth access tokens. This is a critical security requirement and could expose applications and websites to replay attacks where a Microsoft OAuth access token obtained for one project (attacker) can be used to sign in to another project (victim). Instead, Firebase Auth offers the ability to handle the entire OAuth flow and the authorization code exchange using the OAuth client ID and secret configured in the Firebase Console. As the authorization code can only be used in conjunction with a specific client ID/secret, an authorization code obtained for one project cannot be used with another.

If these providers are required to be used in unsupported environments, a third party OAuth library and Firebase custom authentication would need to be used. The former is needed to authenticate with the provider and the latter to exchange the provider's credential for a custom token.

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.is_valid()) {
  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();

Authenticate Using Microsoft and C++

Before you begin

Access the firebase::auth::Auth class

Handle the sign-in flow with the Firebase SDK

Advanced: Handle the sign-in flow manually

Next steps

Access the `firebase::auth::Auth` class