How Does The Vigenère Cipher Work?

The Vigenère cipher encrypts messages consisting of only text and no special characters or numeric digits. In the Vigenère cipher, the encryptor determines a different displacement for each character. A keyword decides the sequence of displacement, which both the encryptor and decryptor know beforehand.

Giovan Battista Bellaso discovered the technique in 1553, but people wrongly attributed it to Blaise de Vigenère, a 16th-century French diplomat and cryptographer. He published a different kind of cipher (an autokey cipher) in 1586. Later, people confused the two similar systems and incorrectly named Bellaso’s invention after Vigenère because Vigenère’s writings became very popular.

They decode the Vigenère cipher by matching the repeating keyword to the cipher text and using a Vigenère Square to reverse the letter shifts.

Researchers derived it from a much simpler and one of the oldest known encryption systems called Caesar cipher, where each letter shift depends on a key word. While they called it the “indecipherable cipher” and it remained unbroken for 300 years, mathematicians figured out how to crack it in the 19th century using basic statistical analysis and computer tools.

Vigenère cipher uses a Vigenère square, like the one below, to encrypt the message. The square contains 26 rows of Caesar ciphers starting with a zero letter shift. On each additional row, the letters’ shift increases by 1.

As Vigenère ciphers are a bit more complicated, consider the following example: we want to encrypt the message MABELEATSSPRINKLES and use GRAVITY as the key. We would repeat the key word until it matched the length of the message (in this case, GRAVITYGRAVITYGRAV).

Using the table above, we match the message letter to a row and the corresponding key letter to a column and find the letter at their intersection on the table.

For example, the row starting with M and the column starting with G intersect at S. The row starting with A and the column starting with R intersect at R. So using our message and the key, the we’d create the following cipher:

Note that any time the message is A, the cipher letter will be the same as the key letter and vice versa. Another way to think of Vigenère ciphers is that each letter of the key corresponds to a Caesar shift number (A=0, B=1, C=2, etc.) and for each letter of the message you form a Caesar shift based on the corresponding key letter.

Content for this question contributed by Jill Merisko, resident of Pittsburgh, Allegheny County, Pennsylvania, USA