Write the message into a grid, but drop a null letter into agreed places first. Then read the grid along a secret path. Anyone who works out the path still gets rubbish, because they do not know which cells were never part of the message.
Used in the American Civil War. The nulls cost you nothing to add and cost a code-breaker a great deal.
The clerk who beat the generals
Anson Stager, a telegraph man rather than a soldier, built the Union route ciphers: words written into a grid, read along an agreed path, with meaningless words dropped in to spoil it for anyone else. Lincoln’s telegrams travelled this way. The Confederates captured plenty of messages and printed them in newspapers, asking readers to solve them. Almost nobody could.
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.