Excellent thought. I’ve been using hashed ISBNs (as note IDs) for storing ‘hidden’ book information like progress and reviews. This way the user, who keeps a list of their books (with some encrypted isbns tags in the content), can simply recreate their book data but others cannot easily see what books they hid. I like your idea a bit more though so will need to see if I am think a way around achieving something similar/better with gift wrapping.

At least with the book info it doesn’t need to be the most secure model in the world. My main thought there was a leaked isbn shouldn’t be detrimental to safety etc. (in most circumstances).