Build a five by five square from a key word, then split the message into pairs. Each pair is looked up in the square and replaced by two other letters, following one of three rules. Because it works on pairs, counting single letters tells a code-breaker almost nothing.
The cipher named after the wrong man
Charles Wheatstone invented it. His friend Lyon Playfair took it to the Foreign Office, who turned it down as too complicated. Wheatstone offered to teach it to three out of four boys from a nearby school in fifteen minutes. The name that stuck was Playfair’s, not Wheatstone’s. The British Army used it in the Boer War and both world wars, because two people with a key word and no equipment could send messages a field telephone could not protect.
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.