For open source messengers, you can check whether they actually encrypt your messages and whether the server has access to your encryption keys but what about WhatsApp? Since it’s not open source, you can’t be sure that the encryption keys aren’t sent to the server, right? Has there been a case where a government was able to access WhatsApp chats without reading them from the phone itself?
Well, in my comment I describe quite a number of methods. It doesn’t matter how secure or reviewed signal is, if the feds have a keylogger at the OS or compiler level. It’s really unbelievable how much code is involved in day to day security
The keylogger and operating system (if you’re using Android) is open source as well. They can’t just put a keylogger in there.