16

In The Imitation Game, we see that when Alan Turing climbs into Joan's room, he has in his hands actual decrypted Enigma messages "direct from Nazi high command".

How did Alan have access to those messages?

4
  • 5
    The imitation game is inaccurate on many points. Are you looking for an in-universe answer? Or a real-world answer?
    – Taladris
    Feb 12, 2023 at 4:32
  • Both would be nice.
    – penguin99
    Feb 12, 2023 at 6:53
  • 1
    What are you really Asking, please? Isn't the whole point that Turing had access to those messages? Feb 13, 2023 at 21:07
  • I'm asking how he even add access to those messages in the first place
    – penguin99
    Feb 14, 2023 at 2:17

4 Answers 4

39

Because brute-forcing encryption was possible, just not in real time

Your mistake is thinking that Enigma couldn't be decrypted. It absolutely could — it just took a long time per message to brute-force the decryption with human beings doing the work.

The point with intercepting any encrypted real-time message stream is that you need to be able to decrypt it in real-time. If you're getting messages about troop movement, say, you need to be able to read the message before your army finds out about those troop movements by running into the aforesaid troops. The code breakers had got to the point of understanding the encryption method — what they didn't have was a way to break it quickly enough to be useful.

15
  • 4
    Do you have any source to back this up? Real-life brute-force decryption can be unreasonably long: youtube.com/watch?v=G2_Q9FoD-oQ&ab_channel=Numberphile
    – Taladris
    Feb 12, 2023 at 4:37
  • 29
    Brute Force is the wrong term for what the large teams at Bletchley park did. They used targeted attacks based on the structure of the code (helped by the fact that the Poles had smuggled an actual machine from berlin). And a lot of language analysis based on known code flaws and poor German message discipline. Those targeted attacks are far more efficient than "brute force" but still often took weeks.
    – matt_black
    Feb 12, 2023 at 16:24
  • 3
    @matt_black Oh, for sure they massively reduced the scope of the problem. Still though, brute force remains brute force, just with an attack taking weeks of effort instead of years.
    – Graham
    Feb 12, 2023 at 17:26
  • 5
    @Taladris Because knowing (or guessing) a few characters of the plaintext doesn't automatically decrypt the entire message. It reduces the attack cost, but not the fact that you still have to trial-and-error your way through configurations which would result in that hypothetical plaintext. And if you do want to fully brute-force any encryption, then the solution doesn't take a fixed time - it's an exercise in probability, and the more attempts you try, the higher your probability of success, and basic statistics then says the chance of success at some time will follow some kind of bell curve.
    – Graham
    Feb 12, 2023 at 17:33
  • 4
    @graham Indeed. But weeks is long enough for the team to have some decrypted messages, which was the point. And, the movie makes this harder to appreciate as it makes it look like the team was a handful of people: more than 10,000 people worked at Bletchley in the real world.
    – matt_black
    Feb 12, 2023 at 20:56
26

Because they have managed to decrypt some messages.

When Alan and Hugh are working together, they have this dialogue during 18:00 in the movie.

HUGH ALEXANDER: You know to pull off this irascible genius routine, one has to actually be a genius. Yet we’re the ones making progress here, aren’t we?

ALAN TURING: You have?

HUGH ALEXANDER: We’ve decrypted a number of German messages by analyzing the frequency of letter distribution.

ALAN TURING: Oh. Even a broken clock is right twice a day. That’s not progress at all, that’s just blind luck. I’m designing a machine that will allow us to break every message, every day, instantly.

Alan, being the core member of the team, didn't have any difficulty accessing those messages. What Alan smuggled out of the laboratory were some of these decrypted messages.

It should be noted that they didn't brute force and decrypt those messages, because there would be 159 million million million possibilities of setting to go through. Brute forcing wouldn't be possible. There's a difference between brute forcing and cryptanalysis. Brute forcing involves trying out all the possible combinations whereas cryptanalysis involves logical techniques such as letter frequency analysis.

4

If you want a more accurate (but, IIRC, not too technical) description of what went on, you can try either Station X: The Codebreakers of Bletchley Park, which accompanied a TV documentary series (eg part 1)...

... or maybe (but it's been a long time since I read it) The Code Book

Briefly though, IIRC, there were a few ways they got "known plain text" including

  1. "Cillis". A lazy operator might always choose the same 3(6?)-letter session key which always came at the start of the message (after his GF's name "CIL..."

  2. Some, isolated command posts would regularly send out a "nothing to report" message

  3. They'd request for the RAF to send a patrol plane to go past an observation post, and wait for the sighting report. That may have been called "gardening" but my memory is hazy.

  4. Someone was requested to send a test message, and they pressed the same key over and over again. The fact the character was not present at all in the long message made the codebreaker suspicious.

UPDATE: Watching through the second of the Channel 4 documentary (which had so much I'd forgotten) but apparently "rude" words often helped breaking in :-)

3
  • 3
    One route that is often forgotten: the Polish Cipher Bureau had intercepted an enigma machine that was accidentally sent to poland in 1928, and so they had worked out a method (en.wikipedia.org/wiki/Zygalski_sheets) that worked with the more limited 3 rotor slot 3 rotor enigmas, and when expanded with british resources, could be made to work with 3 rotor slot, 5 rotor machines used by the army. The is where the term bombe comes from. Feb 13, 2023 at 14:30
  • 2
    I think I remember something as well about the first broadcast of the day always including a weather forecast and that this would help them decipher the initial broadcast.
    – Nzall
    Feb 14, 2023 at 7:54
  • 1
    @user1937198 in the documentary they discuss how, just prior to the war, an office worker tried to make some quick cash by selling the details. He tried the French (not interested) then the Brits (again not interested) but the Poles "with the Germans breathing down their necks" were very keen
    – Simon F
    Feb 14, 2023 at 16:04
2

When you say how did he have access, I assume you mean how was he allowed access ?

I am not sure that Turing had authorized access to all messages. Message text was obviously in German (which Turing had studied to some extent though by no means a German scholar) and there was a translation section (that included Cairncross, the "fifth man" traitor whose job involved translation and military interpretation of decrypted messages) before the final English drafts were sent to the armed forces intelligence branch.

Turing and the other cryptographers were working on methods to decode the Enigma-encoded messages using intrinsic limitations of the Enigma machine itself, as well as knowledge of the structure of German military messages, a little insight into the human weaknesses of chaps in the Wehrmacht signals staff and the occasional intuitive guess (ABC) at wheel starting positions. The approach was therefore not pure brute force attack; it was rather more like sifting through the remaining uneliminated possibilities for decryption settings.

So Turing, Jeffreys et alia just worked on improving the decryption procedure, sketching out the basic design of the "bombe" machines that Flowers and his electrical engineering team would assemble for electromechanical testing of numerous decryption options.

Please be mindful of the fact that films, necessarily or not, will often alter the actual facts to make the visual narrative simpler, more exciting or cheaper to produce. Hence we get 'romances' or 'rivalries' that never really were, mis-sequencing of the order of events, composite characters and so on.

For a fuller exposé of what really happened in Bletchley Park during WW2 I recommend perusing Welchman's Hut Six Story, Turing's biography and those BBC2 documentaries (try YouTube) that covered Ultra/Enigma cracking.

2
  • 1
    An answer is not the place for questions, even if rhetorical. 1) State uncertainty as an assumption/conditional 2) Say it directly, instead of rhetorical. This is not a forum. Feb 12, 2023 at 18:39
  • Removed last paragraph.
    – Trunk
    Feb 12, 2023 at 21:17

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .