My friend and I were discussing cell phone security and he said that if someone backdoored your phone that they could read all your signal messages. Is this true? I would think that the only way to view signal messages is to literally open up the app and view them.
You are both sort of right. Signal’s on device storage is AFAIK only weakly encrypted with your screen-lock password (although you can enable a second weak password in Signal specifically). So if your phone is unlocked and backdoored the attacker can pretty much do what ever they want including reading your Signal messages.
Edit: to make it more clear… someone with a backdoor can just start a hidden application that does the same as the Signal client and open the on device stored messages with out your knowledge. And it is also quite trivial to install a keylogger to capture all passwords including those inside Signal.