Take a date, such as October 21, 1973, and write it as 102173. Repeat those digits above your message. Each letter shifts forward by the digit sitting on top of it. Now the two E’s in FEET turn into different letters, which makes life very hard for a code-breaker.
The code nobody has ever broken
Gilbert Vernam, an American engineer, worked out that if the shift changes with every single letter, and the list of shifts is completely random, and you never use that list twice, the cipher cannot be broken. Not by a clever person, not by a computer, not ever. It is called a one-time system. The catch is the list: both people need the same one, and a fresh one for every message.
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.