How to store User private data?

is there a way to store data for specific user - only user that write data to some storage can read data from storage? My app needs to perform HTTP requests on behalf of an user logged in Jira. That’s why every user needs to set his own private variable in some storage.
I checked article Storage and I don’t see any sufficient API. “App storage” is currently experimental and it looks like it can store data on application level. Other option - “Entity Properties” - there is a note in documentation that it is not secure storage either:

Users can access entity and app properties using the REST APIs. For this reason, never store private user data or configuration information using entity properties or app properties.
Does that mean I have to store such data in my own storage outside Jira?

I would be thankful for any help,