A simple authenticator to generate tokens for the Firebase Cloud Messaging API (V1) and other cloud services using the service account JSON file.

https://firebase.google.com/docs/cloud-messaging/auth-server

Requests sent to FCM from your app server or trusted environment must be authorized. Note these important differences between the deprecated legacy HTTP API and HTTP v1 API authorization:

• The FCM HTTP v1 API authorizes requests with a short-lived OAuth 2.0 access token. To mint this token, you can use Google Application Default Credentials (in Google server environments) and/or manually obtain the required credentials from a JSON private key file generated for a service account. If you are using the Firebase Admin SDK to send messages, the library handles the token for you.

Use credentials to mint access tokens

Unless you are using the Admin SDK, which handle authorization automatically, you'll need to mint the access token and add it to send requests.

Use your Firebase credentials together with the Google Auth Library to retrieve a short-lived OAuth 2.0 access token