What is the Vigenère Cipher?

The Vigenère cipher is a method of encrypting alphabetic text by using a series of interwoven Caesar ciphers, based on the letters of a keyword. It is a form of polyalphabetic substitution.

The Vigenère Square

The Vigenère square (or tabula recta) is a 26x26 grid of alphabets, where each row is shifted one letter to the left from the previous row. This table is used to encrypt and decrypt messages.

Encryption

To encrypt a message, you repeat a keyword over the plaintext. For each letter in the plaintext, you find the intersection of the row given by the keyword letter and the column given by the plaintext letter in the Vigenère square. For example, to encrypt "HELLO" with the keyword "KEY", you would do the following:

H (row K) -> R

E (row E) -> I

L (row Y) -> J

L (row K) -> V

O (row E) -> S

So, "HELLO" becomes "RIJVS".