I want to have a hardware where it will send an occasional message or notifications to the mobile app. For example, when the hardware camera is now low on battery it should send a notification to the mobile app that it is now low on battery.
I am reading somewhere to use MQTT and TLS. I am new to IoT and would like some input. Security and cost is of utmost importance.
Use pushbullet it is a great app which let's you send data from your raspberry pi to your other devices. Also you can send messages to your raspberry pi as well from your mobile phone. it is available on both ios & android. You can encrypt your data as well. During trial service rate limitation will be there if you buy the pro version then there are no or very less limitation. it costs 40 $ a year i think. you can find more info on their website.
you still need to implement what you want to send or receive in your script. Pushbullet is just a service which can transfer data in between devices.
