I should have been clearer, apologies: Not sending from Notesnook to external sources but sending from external sources to Notesnook in order to create a note. But I guess if it's encrypted by the apps before syncing, that's going to be tricky. Although a commandline helper app would work - it would have access to the local encryption configuration of the apps.
I have been brainstorming on how to make this possible. The use case in point was: creating notes via Email. I have a couple of rough ideas but I don't know how feasible they are security wise.
The most reliable way would be to send notes from external sources into an "inbox" of sorts; this inbox wouldn't be encrypted. Later on, when the user will open the app it would move and encrypt everything in the inbox. This is not secure or private at all.
Your idea seems good as well; a command-line helper app responsible for encryption.
