I'd like to specify rules in Firebase such that a path can only be accessed if it's known.

```
/root
  /messages
    /message1
    /message2
    /message3

```

If you access /messages you can receive a permission denied (or nothing).

If you explicitly access /messages/message2 you get the contents.

Update 1: Expected behaviour (iOS)

FIRDatabase
 .database()
 .reference()
 .child("invitations")
 .observeEventType(.Value, withBlock: { snapshot in

// snapshot returns nothing or permission denied.
})

```
FIRDatabase
  .database()
  .reference()
  .child("invitations/message1")
  .observeEventType(.Value, withBlock: { snapshot in

  // snapshot returns message1
})

```