Editing Bomba (cryptography) (section)

==Background==
The German Enigma used a combination [[key (cryptography)|key]] to control the operation of the machine: rotor order, which rotors to install, which ring setting for each rotor, which initial setting for each rotor, and the settings of the ''stecker'' plugboard. The rotor settings were trigrams (for example, "NJR") to indicate the way the operator was to set the machine. German Enigma operators were issued lists of these keys, one key for each day. For added security, however, each individual message was encrypted using an additional key modification. The operator randomly selected a trigram rotor setting for each [[message]] (for example, "PDN"). This message key would be typed twice ("PDNPDN") and [[encrypt]]ed, using the daily key (all the rest of those settings). At this point each operator would reset his machine to the message key, which would then be used for the rest of the message. Because the configuration of the Enigma's rotor set changed with each depression of a key, the repetition would not be obvious in the [[ciphertext]] since the same [[plaintext]] letters would encrypt to different ciphertext letters. (For example, "PDNPDN" might become "ZRSJVL.")

This procedure, which seemed reasonably secure to the Germans, was nonetheless a [[cryptography|cryptographic]] malpractice, since the first insights into Enigma encryption could be inferred from seeing how the same character string was encrypted differently two times in a row.