Cabinet Noir>History / Vigènere cipher

Not sure which cipher? Identify it
Each letter of the message is shifted by the matching letter of a repeating key.
Typepolyalphabetic substitution cipher
Breaking ithard to break by hand
Era16th century
NameDescribed by Giovan Battista Bellaso in 1553, later misattributed to Blaise de Vigenère.
Modesencrypt · decrypt · cryptoanalysis

History

Vigènere cipher

History of the Vigenère cipher

The first polyalphabetic ideas

Using several alphabets in turn was the great leap of Renaissance cryptography. Around 1467 the Italian architect and cryptographer Leon Battista Alberti proposed a cipher disk: two stacked wheels of letters, rotated between words so that the same letter would not always encrypt the same way. A few decades later the German abbot Johannes Trithemius published the tabula recta — the square table of all 26 shift alphabets that Vigenère-style ciphers still use — in his steganography book Polygraphia (1508).

Bellaso, not Vigenère

The cipher we now call Vigenère was described in 1553 by Giovan Battista Bellaso, who paired the tabula recta with an easy-to-remember key phrase and noted that the key should be changed often. In 1586 the French diplomat Blaise de Vigenère published a stronger, autokey variant of his own. History got the attribution backwards: the simpler repeating-key cipher was named after Vigenère, and the name stuck.

"Le chiffre indéchiffrable"

For roughly three centuries the cipher was known as le chiffre indéchiffrable — the indecipherable cipher. It was widely used for diplomatic correspondence, and for a long time the reputation was deserved: as long as the key was as long as the message and never reused, the methods below do not work.

The fall

The repeating key is the flaw. Charles Babbage worked out how to recover the key from English ciphertext in 1854, likely to help crack a cipher advertisement in the press, but never published. In 1863 the Prussian officer Friedrich Kasiski published the same general method — which is why the repeated-sequence examination carries his name. In the 1920s William Friedman's index of coincidence turned key-length detection into a measurable statistic, and the cipher became a textbook exercise.

1467Leon Battista Alberti invents the cipher disk — the first idea of using several alphabets. Source
1508Johannes Trithemius publishes the tabula recta, a table of all letter shifts. Source
1553Giovan Battista Bellaso describes the repeating-key cipher with an easy-to-remember key phrase. Source
1586Blaise de Vigenère publishes a similar cipher; the repeating-key scheme is later misattributed to him. Source
1863Friedrich Kasiski publishes a general method for finding the key length (Charles Babbage had broken it privately a decade earlier). Source
1920sWilliam Friedman develops the index of coincidence, making the attack statistical and rigorous. Source

Sources