Both sides hold the same list. HAYSTACK means the delivery is on. LANTERN means it is off. RHUBARB means we have been discovered, burn everything.
There is nothing to break. No pattern, no frequency, no key to work out. If the list is only in two heads, an enemy holding the word HAYSTACK has a word about hay. This is why the BBC could broadcast the giraffe has a long neck to occupied France in the open, and why a codebook is the oldest and quietest system on this site.
Nonsense read out after the news
Every evening the BBC broadcast strings of harmless nonsense: the giraffe has a long neck, John has a long moustache. Each phrase had been agreed in advance with one resistance circuit and meant one thing: the drop is on, the landing is tonight. Lines of Verlaine’s poetry announced the Normandy invasion. Everyone in Europe could hear them, and nobody could break them, because there was nothing to break.
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.