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TURN Spycraft Handbook – The Cardan Grille

In “Pilot,” Abraham Woodhull’s search of Major Hewlett’s bedroom uncovers what appears to be an ordinary letter, accompanied by a thin metal object with boxes cut out of it. As he slides the letter into place behind the metal object, he is amazed as the boxes reveal a secret message regarding a rebel safe house in Connecticut. The object, a simple cryptography device called a Cardan Grille, has been a trick of the trade since the 16th century.

Sir Henry Clinton, who served as a British general during the Revolutionary War (including as the British Commander-in-Chief in North America from 1778 to 1782), often used a device known as a “grille” or “mask” with an hour glass-shaped hole cut out of the center of it when sending correspondence. Those who were versed in subterfuge knew this was the “Cardano system” of spycraft, named after Giralamo Cardano, an Italian cryptologist. Cardano was a skilled inventor and mathematician known for his contributions to modern algebra, but it was his work in encryption that would become his most famous. In 1550, Cardano developed a means for sending clandestine messages in letters written in longhand by using a grille to reveal a hidden message.

The first Cardan Grille was a rectangular stencil with irregularly placed cutouts which allowed specific letters, syllables, or words to be written by the sender and then read later by the recipient. The written fragments within the plain text could be further disguised by filling the gaps between the fragments and writing around them. The purity and beauty of the device was that the sender’s true message was in plain sight, hidden within the body of a document that otherwise seemed innocuous.

For spies using ciphers, avoiding capture was always first and foremost in their minds. The Cardan Grille was significantly useful because it was very difficult to break: If a message was intercepted by the enemy, an operative needed both the letter and the grille in order to decipher the coded message. Not only was the grille easy to use, it was a relatively safe way to deliver important information without detection.

Stenography encryption remains an important tool of the spy trade, and the Cardan Grille is one of the grandfathers of modern spycraft technology.

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