Wahlwort

Wahlwort (nonsense word or filler) is a cryptographic term used particularly in connection with the Wehrmacht, which used wahlworts on their Enigma rotor machine in the encryption of their communication in World War II. The term describes a randomly selected word which was inserted at the beginning or end of the radiogram plaintext. The wahlwort was intended to hinder the enemy's cryptanalysis and prevent the decryption of the ciphertext.

Application

According to the secret regulations in force at that time, outlined in Der Schlüssel M - Verfahren M Allgemein (The Cipher M − M General Procedure), this procedure was used primarily to give radio messages different lengths ("Cipher M" refers to the Enigma M4, naval variant of the series of Enigma machines). Indeed, plaintexts with the same content often had to be transmitted to different receiving operators in their encrypted form. To do so they were encrypted with different keys corresponding to the different encryption networks. This resulted in ciphertexts that were different, yet had the same length. If the enemy noticed different ciphertexts of a similar length at approximately the same time, possibly from the same transmitting operator, the receiving operator would assume that a known-ciphertext-attack was taking place. The British Codebreakers from the English Bletchley Park (B. P.) were familiar with such cases and glad to receive them, naming this a kiss. Such a kiss was considered as an ideal opportunity to decipher radio messages, even better than a "Crib," a deciphered text section.