Vigenère cipher
A polyalphabetic cipher considered unbreakable for three centuries, until someone realised the key length can be computed from repetitions.
A Caesar cipher in which the shift changes for every character according to a repeating keyword. The method was described by Giovan Battista Bellaso in 1553 but carries the name of Blaise de Vigenère, who published a variant in 1586. For a long stretch it was known as le chiffre indéchiffrable, the unbreakable cipher.
In 1863 Friedrich Kasiski published the method that undoes it: repeated sequences in the ciphertext reveal the key's length, and once that is known the text falls apart into that many simple Caesar ciphers, each of which yields to frequency analysis. A Vigenère cipher with a truly random key as long as the message, used exactly once, is a one-time pad — and genuinely unbreakable.