Note for all about the Signal messaging app. While the level of stupid for someone to have a secret level government...

[usonian]

3. A few simple points.

An ordinary phone is subject to compromise (surprise)

1. A shoulder surfer can watch you type in passwords. I curse Apple every time I make an Apple Pay purchase and the phone demands that I type in a passcode (IN PUBLIC) instead of a thumbprint (I got the last iphone model that doesn't do face id)

2. Someone can just coerce you into typing in your password and opening up Signal. (an offer you cannot refuse)
This is how signal conversations were revealed in trials.

3. Malware can steal your keystrokes/taps.

4. Malware can take over your selfie camera or front-facing camera.

5. Malware can capture your screen, messages and passwords

6. Malware can open up your microphone and listen in.

Probably more of a technical nature. Suffice it to say, these common ones are plenty.

This is why secrecy bans personal phones.