Implementing an Android app using the Firebase framework by Google.
I implemented an app able to host chat rooms. A user can enter the room (the room can require a PIN to enter in it) and once in, every room works exactly like a Whatsapp chat group. Every message is encrypted with asymmetric encryption. Messages are saved on the Firebase's database in an encrypted way.
This app needs a back-end side to be configured by Firebase console. I configured my own application on my Firebase account and this Android app is linked to it but you can simply modify the source to link the app to your Firebase instance.