You’re right that in most languages, encapsulation isn’t strong enough to be used for security in this way. However, there are languages in which it is; scheme

You’re right that in most languages, encapsulation isn’t strong enough to be used for security in this way. However, there are languages in which it is; scheme I believe has strict enough lexical scope and encapsulation to be used like this. (And of course there are languages designed specifically for this purpose, though they’re not often used: E and Monte come to mind.) Of the popular languages, though, JavaScript is the closest to being strict enough for this style of programming, and there’s been active work for a while towards that goal: https://agoric.com/blog/technology/ses-securing-javascript/ https://agoric.com/blog/technology/ses-securing-javascript/