Friedrich W. Kasiski

Friedrich W. Kasiski was a German army officer and cryptanalyst who, in 1861, published a method for breaking Vigenère ciphers. The Vigenère cipher is a type of substitution cipher that uses multiple monoalphabetic substitution ciphers to encrypt a message, making it more secure than a simple substitution cipher.12

Kasiski's breakthrough was based on the observation that identical pairings of message and key symbols produce the same ciphertext symbols. By identifying repeated sequences in the ciphertext, cryptanalysts could deduce the length of the repeating key. This allowed them to break the cipher into smaller substitution ciphers, which could then be solved using frequency analysis. This method was a significant advancement in cryptanalysis, as the Vigenère cipher was previously considered unbreakable.12