Some weeks ago, I rented the 2007 adventure film National Treasure: Book of Secrets on DVD, in which treasure hunter Benjamin Franklin Gates (played by Nicholas Cage) looks to discover the truth behind the assassination of Abraham Lincoln. In the movie’s first scene, which takes place in a tavern in Washington, D.C. five days after the end of the Civil War, Ben Gates’ great-great-grandfather Thomas Gates is approached by John Wilkes Booth and another member of the Knights of the Golden Circle, who ask him to decipher a secret message, which has obviously been encrypted using the Playfair cipher and might lead them to a mythological city of gold called Cíbola.

As the Playfair cipher was state-of-the-art at the end of the Civil War in 1865, I wondered how someone (even if portrayed as a well-known puzzle solver) would be able to perform a successful ciphertext-only attack within just one or two hours, not having any frequency tables at hand and given a ciphertext consisting of only 22 digraphs (= pairs of letters). The following article will explain the basic concepts (encryption, decryption and cryptanalysis) of the Playfair cipher using the example from National Treasure: Book of Secrets.

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