Vigenère's actual cipher: the key starts with a short primer, then the plaintext itself feeds the keystream.
polyalphabetic substitution cipher
hard to break by hand
Renaissance — the real 1586 Vigenère
Giovan Battista Bellaso, 1553
encrypt · decrypt · cryptanalysis
In autokey the key is the plaintext itself: one guessed word unravels the whole message. Pick a word that likely appears in the text.
Primer length
how long the secret primer was (2-12)