De Furtivis Literarum Notis

De Furtivis Literarum Notis is a manuscript by Giovan Battista della Porta that describes the use of a deck of card to encode secret information by writing on the sides of the deck in a known order, then shuffling the cards to destroy the order and render the message illegible to anyone not familiar with the stack. The plot of Unshuffled later contained this idea.