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The use of hidden messages—i.e., messages that are available only to people who know what to look for, where, and when—has been with us for centuries. Herodotus tells how a message about Xerxes’ hostile intentions was passed to the Greeks underneath the wax of a writing tablet. In ancient China, code ideograms were embedded at prearranged positions in dispatches. During the Middle Ages, monarchs used the grille system, where a wooden template was placed over a seemingly innocuous text, highlighting a secret message beneath.