Agree on a book, or here, a passage. Every letter of the message becomes two numbers: which word, and which letter of that word. The message travels as numbers and nothing else.
There is no key to steal and no alphabet to analyse. The weakness is practical: both of you need the same book, on the same page, in the same edition.
The traitor and the paperback
Benedict Arnold sent his messages to the British as numbers pointing at words in a book: page, line, word. Both sides held the same edition, so there was nothing incriminating to find — only a book on a shelf and a letter of numbers. He was uncovered when his contact, Major André, was caught with papers in his boot. A book cipher is only as safe as the person carrying it.
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.