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In the movie The Imitation Game, Alan Turing has a Eureka moment when he learns from a woman that one particular German begins every message that he sends out with the term "CILLY".

From what I understand, the significant of this moment is that Turing realizes that every coded message sent by the Germans contains several words that appear in many messages; namely "weather" in some, and "Heil Hitler" in surely almost all.

But why does this one German person that this woman is listening in on through a radio tower send out every message by beginning with the word "CILLY"? And the messages that he sends out all coded in Enigma, I presume?

And also, immediately after, Turing remarks that the Germans are instructed to use five random letters at the start of every message. I guess this is intended to make it harder to crack Enigma?

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    I still haven't seen the movie yet so I can't answer in-universe, but I can tell you by now that the movie doesn't depict history or cryptology accurately. "Heil Hitler" wasn't part of military reports. There was no such Eureka moment because that was probably the very first thing the decrypt team tested for. There wasn't such "particular (stupid) German". And so on, it was all dramatization. See also this related question. But I can't answer the question in the movie reality. – his Jan 25 '15 at 5:28
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This isn’t that well explained in the movie. It’s mentioned that every message was supposed to start with a few random letters, but the movie doesn’t explain why. The movie also makes it seem like every Enigma message was encrypted with a key that the Germans changed every day. That’s not entirely right, if they would have done it that way, it would mean that each day there were quite a lot of messages that were encrypted with the exact same key. For a code breaker, the more text you have that is encrypted with the same key, the easier it becomes to find patterns that reveal something about the key.

The Germans actually used a system that came down to using two keys. There was one key that all the Enigma operators knew and which was changed every day, hence the day key. But the day key wasn’t directly used to encrypt a message. The day key was only used to encrypt a second key, the message key. The operator was supposed to simply make up a random message key for every message. The day key was then used to encrypt the message key, and the message key was used to encrypt the message. The complete encrypted message would consist of two parts: the encrypted message key, and the encrypted content part of the message. (Hence the movie’s mention that every encrypted message would start with a few random letters.) Decrypting the message would mean using the day key to decrypt the message key, and then using the message key to decrypt the content part. This way, there would be a lot less text encrypted with the same key, and the randomness of the message keys would make it harder to find patterns. At least in theory.

In practice, the Enigma operators would sometimes fail to use random message keys, instead using the same message key multiple times. Thus on a given day, there might be multiple pieces of encrypted text that would start with the same letters, being multiple uses of the same message key encrypted with the day key. As for the term “cilly”, I’ll quote Simon Singh, who is also the source for most of the above explanation: (I tried to summarize the above without getting into an explanation of an Enigma machine’s operations, more detail is given in Singh’s book)

Once they had mastered the Polish techniques, the Bletchley cryptanalysts began to invent their own shortcuts for finding the Enigma keys. For example, they cottoned on to the fact that the German Enigma operators would occasionally choose obvious message keys. For each message, the operator was supposed to select a different message key, three letters chosen at random. However, in the heat of battle, rather than straining their imaginations to pick a random key, the overworked operators would sometimes pick three consecutive letters from the Enigma keyboard, such as QWE or BNM. These predictable message keys became known as cillies. Another type of cilly was the repeated use of the same message key, perhaps the initials of the operator’s girlfriend — indeed, one such set of initials, C.I.L., may have been the origin of the term. Before cracking Enigma the hard way, it became routine for the cryptanalysts to try out the cillies, and their hunches would sometimes pay off.

Cillies were not weaknesses of the Enigma machine, rather they were weaknesses in the way the machine was being used.

Source: The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography — Simon Singh

  • Is CILLY supposed to sound like silly or is that a coincidence? – Joshua Mar 11 at 17:26
  • @Joshua: it's such an obvious bit of wordplay, “the Enigma operator made a ‘cilly’ mistake”, that I would assume it to be intentional, though I’m not sure whether it’s documented as such anywhere. – Rinzwind Mar 11 at 18:04
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The story about every message beginning with Cilly is a simple one - it's suspected it's the name of the German operator's girlfriend. This is the relevant exchange from the film:

Turing: Wh-Why do you think your German counterpart has a girlfriend?
Helen: It's just a stupid joke; don't worry.
Turing: No, no, no, no. Tell me.
Helen: Well, each of his messages begins with the same five letters: C-I-L-L-Y. So I suspect that Cilly must be the name of his amore.
Turing: But that's impossible. The Germans are instructed to use five random letters at the start of every message.
Helen: Well, this bloke doesn't.

Using this knowledge, the film depicts Turing having a eureka moment as he realises his machine doesn't have to search through every possible setting - instead, it can search for words that they know will be in the message.

Take the operator sending out the messages beginning with Cilly. Sure, they're still encrypted. But Turing has a device that cracks encryption. Rather than making it depict every possible logical combination of letters, he would be able to set it so that only messages that had the first five letters of 'Cilly' were display.

Turing and the group realise that the 6AM weather reports almost always sign off with 'Heil Hitler'. Using this knowledge, they are able to only scan messages where the machine produces that output at the end of the message, allowing them to decode the messages much more quickly (the film depicts it in a matter of minutes). This means they've got the ability to crack the code long before it is changed daily - i.e. success!

As for your final comment - yes, that was intended to make the code much, much harder to crack.

  • Okay, so CILLY really is a German operator's girlfriend's name? So that operator is just spending German resources and time basically sending messages to his girlfriend all day, encrypted with Enigma? – Gary Jan 25 '15 at 16:16
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    He is "just" lazy. He is ordered to send five different random letters but he sends always the same. It would be ok to send love letters, five characters at a time, but not this. -- Btw, can anyone quickly explain how they know it were the same letters every time? They are encoded after all, or are they plain text? But then how does it help to break the code? (Assuming the movie reality.) – his Jan 25 '15 at 17:02
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    He's not sending love letters, as far as I know. They're official messages. But instead of starting them with 5 different, random letters everyday, every one of this bloke's messages starts with "CILLY". – BrettFromLA Jan 25 '15 at 18:15
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    Okay so I guess he's just lazy. Yeah it would have been weird if he was sending love letters. And I guess the British female operator listening in on his messages probably just got bored at work and so made up the girlfriend story in her mind to keep herself busy, basically. – Gary Jan 27 '15 at 19:01
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in my opinion the operator's message is not an encrypted one he is sending normal Morse code message that is irrelevant (from the war point of view e.g. for civilian transaction market value for coffee ...). and this guy's message always begin with the same letter pretty normal thing as of dear/sir/madam/regards. But for the code breaking team this is a new feedback that assuming there will be words that for sure will be in the message (and when the check the previous decrypted messages they found “weather” and “heil” and “Hitler”). this means they machine gonna only search for those words to get the enigma setting. so Turing only take the concept that there will be a repetition in everyday message, not the other way. if the message of the guy was encrypted it will never be always Cilly.

  • Sorry but this is completely wrong. As Rinzwind explains, there was a daily key that was known by every radio operator in the German army. For each message they sent, the operator would choose a "random" message key. The would encrypt the message key using the daily key, then encrypt the message using the message key. So every message was supposed to be encrypted with its own key. The problem is that some of the message keys weren't random, because people are lazy and don't understand why a good random key is important. – David Richerby Mar 11 at 21:59

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