How can I change the encoded title and the encoded body for the received firebase notification before fire it up by the system

I am trying to decode the encoded title and message body received from the FCM notification.
i could able to decrypt when the app is in the foreground but not working when app is in the background.

_firebaseMessaging.configure(onMessage: (Map<String, dynamic> message) async {
// when app is running on foreground
onBackgroundMessage: myBackgroundMessageHandler,
onLaunch: (Map<String, dynamic> message) async {
//when app is terminated or not running
onResume: (Map<String, dynamic> message) async {
//when app is running in background