Regarding security, everything falls on a spectrum between whispering in someone’s ear in the waves of a desert island with no electronic devices for miles, where you can still be photographed by satellites, and emailing your thoughts to the FBI. Don’t fall into the trap of believing your communication is totally secure. Encryption software is regularly defeated by researchers in the search for better security. If your phone has malware on it, for example from the NSA, they can read your keyboard and even remotely activate your microphone or camera when your phone is turned off. Be careful with what you say using digital devices, always.

Sources:

https://www.wired.com/2014/06/nsa-bug-iphone/

https://wikileaks.org/ciav7p1/