The process of authentication for tokens in Firebase Cloud Messaging involves the following steps:
- Client app requests a registration token from FCM.
- FCM generates a unique registration token and returns to the client app.
- Client app sends the registration token to the server.
- Server authenticates the token by validating it with FCM API using the project's server key.
- If the token is valid, the server saves the token in its database for future use.
- The server can now use the registration token to send messages directly to the client device via FCM.
The authentication process ensures that only authorized servers can send messages to a particular client device using its registration token. It also ensures that the token is current and valid, preventing unauthorized access to FCM services.