A paper-only journal would defend against the state, but not against people you live with. A digital journal can be encrypted, but an intelligence agency could potentially gain access (like, them reading your anti-government rants that may involve violence… that sort of stuff).
So… how to defend against both threats?
(Also, I just realized, paper journals cannot really be easily backed up…)
Write everything in digital form … use text only because it’s easier to archive, manage, organize and refer to in the future
Encrypt as much as you can and keep a copy locally and in as much safe keeping and off line as much as possible whenever and however you can.
I wouldn’t worry about having the state come after you and use the information against you … or worry too much about intelligence agencies cracking your encryption and reading all your writing … really … who cares because if the state does want to come after you, they wouldn’t care what evidence was available … a journal might make their case easier but in the long run, they wouldn’t care any way.
I would think about the journal and writing from an archival point of view. You’re not writing and creating all that content for yourself … you’re writing and creating it for future generations to see and read. Sure, it might get destroyed or erased … but it also might have a chance of being stored, shared, archived and rediscovered in the future. It would be like reading journals and content written by people before the first or second world wars and historians would gain a valuable insight into the minds of people during our time period. The information we produce now doesn’t matter much to anyone right now whether for good or bad reasons … the information we create now will mean so much more to people far into the future.
So go ahead … write, write often and write as much as you can … the more you write, the more you leave behind for someone in the future to find.