Re: Explain like I'm five ..
@Walter Bishop
"Using "Heil Hitler" to close the msg must be an urban legend. I read somewhere that the cribs or clues were gotten as the initial key was repeated twice and the msgs used similar phrases for weather reports and enemy sightings."
There were lots of crypto-cockups exploited; it's very tricky to crack decently encrypted messages unless someone's made an exploitable mistake. One I recall reading about and found on-line was this:
<http://home.bt.com/tech-gadgets/cracking-the-enigma-code-how-turings-bombe-turned-the-tide-of-wwii-11363990654704>
"In one example the Atlantic weather forecast, which was written in the same format each day, was crucial. Location-detecting equipment in listening stations allowed codebreakers to find where a message was originating from and, if it matched up with the positioning of a weather station, it was likely that the word “wettervorhersage” (weather forecast) would be both present and in a similar place in every message."
IIRC, only the German navy used four rotor Enigma; other German users thought three rotor Enigma was secure, which was not the case.
"Enigma, the battle for the code" by Simon Sebag-Montefiore is a fascinating account of how much of the cracking of naval Enigma traffic was enable by "capture of ships and U-boats and their codeboks [...] and the betrayal of his German homeland by the Enigma Spy." - "Without for a moment belittling the work of Alan Turing and his team of eccentric codebreakers."
Apparently, cracking four rotor naval Enigma traffic relied on having access to cribs or other captured German material:
<https://uboat.net/technical/enigma_breaking.htm>
says that even four rotor Enigma was being generally read within 24 hours from September 1943 onwards, due to the introduction of 4 rotor bombes in June and August (plus, one assumes, the application of many clever brains and some captured material).
The above link contains lots of interesting stuff, including:
"Hut 8 suffered a massive reverse on 1 February 1942 when a new Enigma machine (M4) came into service on Triton (codenamed Shark by Hut 8), a special cipher for the Atlantic and Mediterranean U-boats. The combination of M4, Shark and a second edition of the Wetterkurzschlüssel proved devastating. Bletchley Park became blind against Shark for over 10 months. Fortunately, M4's fourth rotor (beta) was not interchangeable with rotors I to VIII. Beta increased M4's power by a factor of 26, but rotors could still only be mixed in 336 (8x7x6) different ways - not 3,024 (9x8x7x6).
At one setting of beta, M4 emulated M3, which was M4's undoing. Three members of the British destroyer HMS Petard seized the second edition of the Wetterkurzschlüssel from U-559 on 30 October 1942, before it sank near Port Said. Hut 8 once again had cribs, which it could run on three-rotor bombes, the only type available. The U-boats were using M4 in M3 mode when enciphering the short weather reports. A three-rotor bombe run on 60 rotor combinations therefore took only about 17 hours instead of the 442 hours (18 days) required if M4 had used its full potential."
<https://www.bbc.co.uk/news/magazine-17486464>
has an interesting little tale about the work by Dilly Knox a classicist who had been working on breaking ciphers since World War I) on cracking Spanish Enigma messages in 1936, which I'd never heard of before now.
Finally, I recall reading somewhere that one mistake made by the Germans in using Enigma was, when they sent a message and were asked to re-send because of a communication problem (i.e., the radio message hadn't been received clearly, or a mistake was found in the source plaintext when the message was decrypted), the message would be re-sent with the same Enigma machine settings. If the problem was in fact an error in the source plaintext (typically an operator typing error), then the second message would be the nearly-but-not-quite-the-same as the first one, and apparently that's as good as having a crib to work from.
Biting the hand that feeds IT
