One can write messages quite effectively on playing cards. It is first necessary to lay out the cards in a certain order, each one beside the next, either face up or face down.

Once you have arranged them in this way, you can write whatever message you want along the borders between cards. Then you flip the cards and shuffle them well. The message will no longer appear, and if anyone is curious enough to examine the cards closely, he will see only some disorderly markings.

But when the intended recipient wants to read the message, he will lay out the cards in the predetermined order, so that the corners and edges join and line up with each other, and it will be possible to read the message perfectly.

– Giambattista Della Porta, Della magia naturale, 1677

Image by Theodoor Rombouts, detail of Jouers de cartes 17th c.
Via Ask the Past.