You should assume all devices are compromised
If you assume every device you use is compromised, how can you possibly use any encryption?
On a similar note services and networks should be treated as compromised as well, meaning you must use encryption, authentication and in general make sure to limit attack surface.
And all of that boils down that you should make sure you should not rely on services, users, etc. don't for example access personal information they are not supposed to access.
After all the problem with things like Ransomware is exactly that this isn't assumed.