Digital Fortress

by Dan Brown

Caesar, she explained, was the first code-writer in history. When his foot-messengers started getting ambushed and his secret communiqués stolen, he devised a rudimentary way to encrypt his directives. He rearranged the text of his messages such that the correspondence looked senseless. Of course, it was not. Each message always had a letter-count that was a perfect square—sixteen, twenty-five, one hundred—depending on how much Caesar needed to say. He secretly informed his officers that when a random message arrived, they should transcribe the text into a square grid. If they did, and read top-to-bottom, a secret message would magically appear. Over time Caesar’s concept of rearranging text was adopted by others and modified to become more difficult to break. The pinnacle of non-computer-based encryption came during World War II. The Nazis built a baffling encryption machine named Enigma. The device resembled an old-fashioned typewriter with brass interlocking rotors that revolved in intricate ways and shuffled cleartext into confounding arrays of seemingly senseless character groupings. Only by having another Enigma machine, calibrated the exact same way, could the recipient break the code.