Is the model of the phone known? If it's a relatively new BlackBerry running Android, and if it can be rooted:
* the main WhatsApp msgstore database in /data is not encrypted
* the msgstore backup databases (.crypt* in /sdcard) can be decrypted easily using the key file (mentioned in the article) which is also stored in /data
One could probably reverse engineer the WhatsApp APK to figure out how the key file is generated.
I would hazard a guess that Signal messages are also not stored encrypted at the source and destination (beyond the protection offered by the operating system).
Yea this is key. It's possible to have Signal on your device but not implement any device locking passcode or passcode for unlocking Signal... That would make it trivial to recover data if you have the device.
An alternative method of protecting oneself is to set an expiry timer, which makes messages in a conversation ephemeral. 1 week seems to be a fine balance between being able to look back at old messages, while also not having those you communicate with be allowed to store data on your device for too long.
Signal won't prune the messages until you open the app (or maybe the conversation) again. There've been a few releases where the changelog notes that the app didn't expire ephemeral messages properly.
That's a good idea. Even if you enable FDE and a strong passphrase, the court can still compel you to unlock it. They cannot compel you to automagically undelete expired messages (hopefully those are expired and removed in a secure mannner...)
I would hazard a guess that Signal messages are also not stored encrypted at the source and destination (beyond the protection offered by the operating system).