Invented in 1533 by Giovan Battista Bellaso, misattributed to Blaise de Vigenère.
A variant restricts the ciphertext alphabets used to cyclic shifts of the standard alphabet, so an -alphabet Vigenère cipher has key space at the cost of just numeric values as the key.
Viewed as a stream cipher, the keystream is a repetition of the short secret key, each character replaced by its position value, and the ciphertext stream is the modulo addition of the plaintext stream with the keystream.
Attacking the Vigenère Cipher
Security depends significantly on keeping the key length secret. Once the key length is found, the cipher can be attacked in the same manner as a shift cipher, using statistical properties of the plaintext language for each block of ciphertext extracted from separate keyword positions.
The Kasiski test determines the keyword length.
Repeated short strings, often common bigrams or trigrams of the language, that align to the same keyword position produce the same ciphertext string.