What is the process of authentication for tokens in Firebase Cloud Messaging?

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The process of authentication for tokens in Firebase Cloud Messaging involves the following steps:

1. Client app requests a registration token from FCM.
2. FCM generates a unique registration token and returns to the client app.
3. Client app sends the registration token to the server.
4. Server authenticates the token by validating it with FCM API using the project's server key.
5. If the token is valid, the server saves the token in its database for future use.
6. 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.