I suggest looking at the apps and tools developed by the Guardian Project https://guardianproject.info/code/ https://guardianproject.info/code/ There was an en

I suggest looking at the apps and tools developed by the Guardian Project https://guardianproject.info/code/ https://guardianproject.info/code/ There was an encrypted note-taking app but it's not developed any more. The generic solution is to use GnuPG.