Cardan grille
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In cryptography, a Cardan grille is an important tool in the reading of a message obfuscated through steganography. Introduced in 1550 by Renaissance mathematician Gerolamo Cardano (also known as Jerome Cardan), the grille is usually a card perforated with holes at selected places. To decrypt a message, the card is laid over the page of text that contains a hidden message, reading only the letters that appear through the holes in the grille.