"When I type "a" it would show as "z" and so on". Someone mentioned http://www.rot13.com/ - is it what you loukyngue phor? :)
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.