If you have a private conversation to have that would risk you getting arrested, you shouldn't be using WhatsApp or Signal for it. Consider something more obscure, not connected to your phone number or name, and make messages disappear after 24h. Consider SimpleX, Briar, etc. Obviously don't leave any trail or photos on your device either. Moreover, the device shouldn't be reachable via WhatsApp, Signal, SMS, or even a phone number, as these are common vectors for attackers. Your mobile device should probably be using hardened GrapheneOS or something else with sufficient obscurity. Do not make the mistake of activating a SIM or installing any Google app on the device.