Just putting it out there that I really wish OmniGroup would put their considerable talents and design chops toward a simple, synced, secure notes app. There are many, but they all come and go. The developers never seem to stick with the project for more than a few years, and no respectable ones have yet to take up the need for an end-to-end encrypted option.
It seems to me that with a little reuse of the sync code from other Omni apps, a Markdown library and a little TLC Omni could have a hit on its hands. I for one would love this!