I’m curious about the cross platform requirement given the offline one. I’m wondering if Standard Notes or something similar can be used in a purely offline mode.
I’d hope that practically anything on Android today would suffice given the typical enforcement of either FBE/FDE. Do you specifically need an app that is password protected in addition to Android’s device security, or is this more of an ask for desktop counterparts?
That’s fair. I quickly tested standard notes with it firewalled on my phone. it can be used offline and can be set up to require a passcode or biometric verification. Most significantly, the sync server can be self hosted, in case you do decide to pursue this avenue in the future.
The client apps are available on all platforms, including linux distros, though the desktop apps use electron and feel somewhat sluggish (it uses React Native for android and iOS which feels almost equally bad).
I’ve not attempted to use this completely offline but it seems to work acceptably. Bear in mind that the client apps can have subtle nags to subscribe to extended mode, which aren’t present when self hosting.
That said, the default user flow emphasises on the use of an account with an email address, and I’m not sure that’s what you want. I’m wondering if KeePassDX could be used for secure note taking, not sure if keepass entries have some kind of character limit though.