Description
Leon Batista Alberti came up with the idea of using multiple cipher alpahbets. These require switching the key every few words (in our example, every two words). He owned the first european publication on frequency analysis in the 15th century and he later developed the cipher disk.
Johannes Trithemius introduced the idea of changing alphabets with each letter of plaintext. This means that the key does not repeat for two consecutive letters. The key can start from a previously given letter and then iterates through the rest of the alphabet.
Giovan Batista Belaso originally described the method in his 1553 book "La cifra", but the scheme was later misattributed to Blaise de Vigenère in the 19th century, and is now widely known as the "Vigenère cipher".
A passphrase is used ("PENNSIC WAR" in the example) to generate several alphabets with different shifts. The cipher was considered unbreakable for a long time, and was broken only in the middle of 19 century.
Blaise de Vigenère introduced the idea of "autokey ciphers" in 1585, which use the text itself as the passphrase. This technique has been forgotten until the 19th century, when it has been reinvented. In order to use it, the first letter of the key has to be given (in our example, the letter "F").
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