In 1587, Mary, Queen of Scots, lost her head due to a scrap of paper. Fifty-five years later, Sir John Trevanion, a supporter of another beheaded monarch, Charles the First, had his head saved by a scrap of paper. What made the difference? Steganography.

Steganography is the time-tested practice of concealing messages so well that their existence isn’t even suspected. The name is based on the Greek words for “covered writing,” and a very literal Grecian example was to take wax-covered wooden tablets used for writing, scrape off the wax, write on the wood, and then cover the board with a new coating of smooth ...

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