Fill a grid with your message, reading normally, left to right. Then pick the letters back up along a path you and your friend agreed on: a plow path, a spiral, or straight up the columns. Same letters, wildly different order.
A card full of windows
Girolamo Cardano, an Italian mathematician, cut holes in a card. Lay it over a page and the secret words show through the windows; lift it off and fill the gaps with ordinary words until the page reads like a dull letter about the weather. Anyone who intercepts it sees a letter. Anyone with the matching card sees the message. German army units were still using spinning versions of this card in the First World War.
Print this page and you get a worksheet: how the cipher works, where it came from, and the puzzles to crack, with the machine and the menus left off.
Print this page as a worksheet Its sheet in the pack Try the challengesLonger, harder and written for grown-ups. Worth a look when this page runs out.