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How Code Breakers Work

Cipher Machines

One of the earliest cipher devices known is the Alberti Disc, invented by Leon Battista Alberti, in the 15th century. The device consisted of two discs, the inner one containing a scrambled alphabet and the outer one a second, truncated alphabet and the numbers 1 to 4. The outer disc rotated to match up different letters with the inner circle, which letters the cryptographer used as plaintext. The outer disc's letters then served as the cipher text.

Da Vinci Code
William West/AFP/Getty Images
Dan Brown's novel "The Da Vinci Code" follows the adventures
of a symbology professor as he solves codes and ciphers, some
of which he breaks using a Cardano Grille.

Because the inner disc's alphabet was scrambled, the recipient would need an identical copy of the disc the cryptographer used to decipher the message. To make the system more secure, the cryptographer could change the disc's alignment in the middle of a message, perhaps after three or four words. The cryptographer and recipient would know to change the disc settings after a prescribed number of words, perhaps first setting the disc so that the inner circle "A" matched with the outer circle "W" for the first four words, then with "N" for the next four, and so on. This made cracking the cipher much more difficult.

Cardano Grilles and Steganography
A clever way to hide a secret message is in plain sight. One way to do this is to use a Cardano Grille -- a piece of paper or cardboard with holes cut out of it. To cipher a message, you lay a grille on a blank sheet of paper and write out your message through the grille's holes. You fill the rest of the paper with innocent text. When your recipient receives the message, he lays an identical grille over it to see the secret text. This is a form of steganography, hiding a message within something else.

In the 19th century, Thomas Jefferson proposed a new ciphering machine. It was a cylinder of discs mounted on a spindle. On the edge of each disc were the letters of the alphabet, arranged in random sequence. A cryptographer could align the discs to spell out a short message across the cylinder. He would then look at another row across the cylinder, which would appear to be gibberish, and send that to the recipient. The recipient would use an identical cylinder to spell out the series of nonsense letters, then scan the rest of the cylinder, looking for a message spelled out in English. In 1922, the United States Army adopted a device very similar to Jefferson's; other branches of the military soon followed suit [source: Kahn].

Perhaps the most famous ciphering device was Germany's Enigma Machine from the early 20th century. The Enigma Machine resembled a typewriter, but instead of letter keys it had a series of lights with a letter stamped on each. Pressing a key caused an electric current to run through a complex system of wires and gears, resulting in a ciphered letter illuminating. For instance, you might press the key for the letter "A" and see "T" light up.

Enigma Machine
Photo Courtesy U.S. Army
German soliders using an
Enigma Machine in the field.

What made the Enigma Machine such a formidable ciphering device was that once you pressed a letter, a rotor in the machine would turn, changing the electrode contact points inside the machine. This means if you pressed "A" a second time, a different letter would light up instead of "T." Each time you typed a letter, the rotor turned, and after a certain number of letters, a second rotor engaged, then a third. The machine allowed the operator to switch how letters fed into the machine, so that when you pressed one letter, the machine would interpret it as if you had pressed a different letter.

How does a cryptanalyst crack such a difficult code? In the next section, we'll learn how codes and ciphers are broken.