Friedrich Kasiski
Prussian army officer whose 1863 method exposed the weakness of the Vigenère cipher.
Friedrich Wilhelm Kasiski (1805–1881) was a Prussian infantry officer with a serious interest in cryptology. In 1863 he published Die Geheimschriften und die Dechiffrir-Kunst, which shows how to find the key length of a polyalphabetic cipher by looking for repeated sequences in the ciphertext and measuring the distances between them.
Once the key length is known, the ciphertext splits into several simple Caesar ciphers, each of which falls to frequency analysis. Kasiski was not the first to have the idea. Charles Babbage appears to have arrived at the same method in the 1840s and 1850s without publishing it, so the credit went to Kasiski.