Use a five by five square. Write each letter as its row above its column, so the message becomes two rows of digits. Then read those digits out in one long line, and cut them into fresh pairs. Each new pair is a new letter.
The clever part is that a letter’s row now sits next to some other letter’s column. Every letter has been split in half and mixed with its neighbours, which is called fractionation, and it is why the answer looks nothing like the message.
Cutting letters in half
Delastelle’s cleverest idea: break every letter into two numbers, then mix those halves with the halves of other letters before putting them back together. It is called fractionation, and it means a single letter of the message has been smeared across several letters of the answer. Almost every serious cipher since, including the ones protecting this page, uses some version of the same trick.
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.