PLATFORM06

Vigenere

CIPHER

WHEN

FROM

STATUS

Frequency analysis works because one cipher alphabet is used for the whole message. So use many. Take a keyword, say LAGOON, and shift the first letter by L, the second by A, the third by G, and round again. Now E comes out as a different letter almost every time, and the counting stops working.

The machine

A tabula recta and a Saint-Cyr slide

The square holds 26 alphabets, each one step further along than the last. For each letter, find the key letter in the left margin and the plain letter along the top: the cipher letter sits where the row and the column cross. Type a key and a message, step through the letters or press play. Under the square, the same job done by a sliding strip, as French army officers did it in the late 19th century.

Switch to autokey to see Vigenere's own improvement: the keyword only starts the key, and the message itself carries it on.

Break it yourself

A letter from the Doge's palace

An intercepted report, enciphered with a short keyword. You cannot count your way in, but you can guess a word that must be in it: the letter is about the new ambassador. Slide the word AMBASSADOR along the cipher. Wherever it really sits, subtracting it from the cipher gives the key, and a key repeats.

ARRIVED · CIPHER CRACKED

The story

In 1553 a small book appeared in Venice under the name of Giovan Battista Bellaso, a gentleman of Brescia who served as a secretary to a cardinal. It described a way to encipher with a short, easily remembered password: each letter of the password picks a different alphabet, and the password is used over and over along the message. Earlier writers, Alberti and the abbot Trithemius among them, had played with changing alphabets, but Bellaso's keyword made it practical. His own alphabets were laid out differently from the 26-row square shown here, but the idea is the same.

Thirty-three years later, in Paris, the diplomat Blaise de Vigenere published his Traicte des chiffres, a huge treatise on secret writing. Among much else it describes a stronger system, the autokey, in which a short primer starts the key and the message itself supplies the rest, so the key never repeats. By the 19th century, though, the simpler repeating-keyword cipher was being called by his name, and the name stuck.

Le chiffre indechiffrable

For about three hundred years it had a reputation as practically unbreakable, and it earned the nickname le chiffre indechiffrable, the undecipherable cipher. It was slow by hand, so courts and armies often preferred simpler systems, but it kept turning up. In the American Civil War the Confederacy used it with keyword phrases, and Union clerks are said to have read a good deal of that traffic, partly because the same few keys were used again and again.

The Saint-Cyr slide, named after the French military academy, turned the square into a pocket tool: a card with a fixed alphabet and a sliding strip with two more. Setting the strip once per key letter is quicker than running a finger across a 26 × 26 table.

How it was broken

The keyword's weakness is that it repeats. If the key is six letters long, then every sixth letter is a plain Caesar shift, and a Caesar shift falls to counting. The question is only how long the key is, and the answer was found twice: by Charles Babbage in London around 1854, who kept it to himself, and by Friedrich Kasiski in Berlin, who published it in 1863. That attack is the Kasiski platform.

The other way in is the one in the puzzle: a guessed word, or crib. Subtract a word you expect from the cipher and, at the right place, the key itself falls out.