Ah yes! But the shift or scramble would be at the font level. So URYYB would read HELLO. Or rather, the reader reading HELLO will have no idea the document act

Ah yes! But the shift or scramble would be at the font level. So URYYB would read HELLO. Or rather, the reader reading HELLO will have no idea the document actually says URYYB as the contents of that text because the font renders URYYB as HELLO. Presentation detached from substance. Without the "key" font it'll just read URYYB.