The shift code that Roman generals used, plus a way to crack it. Make a code wheel, write a message, and hand it to someone who has the key.
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Nothing sharp, nothing hot, nowhere far. Tell someone what you're up to, then go ahead and do it on your own.
6 steps. Tap one when it's done.
THE CAT IS ON THE MAT. The letter that appears most is W, so guess W stands for E... that gives a shift of 18, which doesn't work. Try the second most common, H: H as E means a shift of 3, and the whole thing reads clean.
A shift code has only 25 possible keys, so a patient person can crack it by trying them all, or faster by guessing that the most common letter is E. Real codes today use keys so big that trying them all would take longer than the universe has existed.