At a Westminster Under School Open Day not so long ago, I was delighted to find out from Miss Ellis (WUS’s Head of Mathematics) that after their final exams, Year 8 boys there spend time discovering the joys of cryptography and code-breaking: specifically, they go to Bletchley Park, find out about Enigma, and break some ciphers.

All the same, what with “The Imitation Game” and so forth, Nazi cryptography is at risk of becoming old hat (or do I mean alter Hut?). But what about Allied codes and ciphers? And – dare I ask it – what about that pesky WW2 cipher pigeon? Of course, once she found out that I knew about such wonderfully recondite (yet also historically and cryptologically rich) subjects, she set about trying to persuade me to give a presentation at the school.

What, me talk about Allied codes and ciphers, wartime pigeons, Typex machines, and D-Day history for two hours? (To be fair, I should perhaps say “for only two hours”.)

Needless to say, she didn’t have to twist my arm particularly hard. Or… at all, truth be told.

And so a few weeks ago, I pitched up at Westminster Under with a giant printout of the enciphered pigeon message, a 50-plus-slide Powerpoint presentation (heavy on nice photos, but light on text), and… a big empty space where I had originally hoped a Typex machine would be. (My cunning plan to borrow one from a friendly crypto collector fell through a few weeks beforehand, sadly. Oh well!)

To make the crypto side of the talk as ‘hands on’ as possible, I gave the boys a practical challenge using Double Transposition (The link is to a description I adapted from a genuine WW2 document Stu Rutter and I found at the National Archives in Kew.)

The boys divided into teams of three or four on a table, for each team to encipher secret messages for a different team to decipher (i.e. by supplying a pair of transposition keys with the message). Once they had got the hang of the technique, I set everyone the challenge of reliably enciphering the same short message at high speed: this was competitive and fun, yielding quite a nice balance of serotonin and adrenaline. 😉

In fact, there was a very specific Cipher Mysteries point to the exercise. Because we can tell that the cipher pigeon’s message was enciphered in only three minutes, we can deduce that it can only have been made using a machine cipher – nobody, I’m sure, could reliably encipher a message of that size in three minutes using Double Transposition, nor indeed with any of the other non-machine ciphers the Allies used in WW2. And Typex aside, the only other Allied machine cipher was the M-209, which had a ten-letter indicator (both the cipher pigeon message and Typex messages had five-letter indicators).

Incidentally, one nice crypto thing is that, having first gone through the history of the Typex machine, I decided to see how on the ball everyone was by throwing out a properly difficult crypto question to the audience:

Early in the war, German cryptanalysts noted that the probability of the letter ‘X’ appearing in the last 4 letters of a Typex message was extremely low. What did that tell them?

Naturally, when a boy (I didn’t get his name, sorry) at the far side called out the answer, I was hugely delighted, because many, many adults would not have reached that. (And so I leave it as an exercise for the reader, just as you’d expect).

As I’m sure will be no surprise, I thoroughly enjoyed myself – it was a great opportunity to give my cipher pigeon / Typex material a bit of a public airing – and I really hope that a fair few of the boys did too. They behaved in an exemplary manner, and were every bit as sharp, fast and engaged as I hoped they’d be. All credit to them for that, and a twenty-one gun salute to Miss Ellis not only for making it happen, but also for helping out on the tables for the Double Transposition exercise (which was very kind of her, and utterly necessary as it turned out).

Same again next year? I hope so! 🙂

PS: right at the end, one of the teams very kindly handed me a nice Double Transposition cipher challenge they had devised, which I thought I’d share with you here (I hope that’s OK with them). I’ve kept their original keys ( 😉 ) but modified their cipher only very slightly:

Sender: Strawberries & Cream
Primary Transposition Key: 9, 1, 10, 4, 2, 7, 6, 8, 3, 5
Secondary Transposition Key: 3, 1, 4, 5, 9, 2, 6, 8, 7, 10
Cipher Message:

Can you decipher it? Enjoy! 🙂

13 thoughts on “Westminster Under School Crypto Challenge

  1. bdid1dr on June 23, 2015 at 2:43 pm said:

    Have you ever been offered a job as a middle-school teacher? A huge part of teaching pleasure is the feedback one gets from the students. Another word would be ‘rapport’. You seem to have a lot !
    And those blessed pigeons; heroes, whether they knew themselves to be so – or not.

  2. bdid1dr on June 25, 2015 at 9:15 pm said:

    So, Nick, am I the only one of your fans who really wants you to reveal the young man’s cipher’s meaning? Did you ever consider becoming a teacher? Did your son come along for the adventure? (You know I’m not going to stop ‘nagging’ you, in the form of Q’s). I still have a lot of Q’s in reserve. 😉

  3. bdid1dr: there’s a big difference between ‘decipher’ (you have the key) and ‘decrypt’ (you don’t have the key). Anyone who wants to can decipher this message, because I included all the keys and all the knowledge you need to use that key to decipher the message. 🙂

  4. bdid1dr on June 26, 2015 at 1:21 am said:

    Having been born cross-eyed, I still learned to read (at a very early age: 4 – 5 y.o.). Surgical intervention was not timely enough for me to be able to read with both eyes. So, this old woman never did take on ‘codiology”/ “cryptology” in all its forms.
    I do appreciate the mind-work involved in deciphering/decoding. I also appreciate the efforts of those beautiful birds — even if they were totally unaware of all the importance of their efforts.
    Getting more ‘one-eyed’ than ever with cataracts and a cyst in my maxillary sinus.
    I still sm-i-i-i-e widely as ever; just can’t get the smiley icon to cooperate.

  5. GeorgeC on June 29, 2015 at 3:39 pm said:

    [redacted] is always a good [redacted] to play with…..

  6. GeorgeC: well done! You’re the first to get to the answer! 😉

  7. bdid1dr on June 30, 2015 at 5:10 pm said:

    OK, Nick and GeorgeC,
    So, now that the ‘first to get to the answer (BTW who did the redacting?) has been congratulated, are we still going to have to wait for at least 5? — 10? —- successful answerers?
    A full house is always a good hand to play with……?

  8. GeorgeC on July 1, 2015 at 11:05 am said:

    bdid1dr: 10 minutes with some squared paper and a pencil will get you the answer – everything you need is in Nicks blog, including one big clue….

    Well done to the pupils responsible for devising it.

    It’s back to my TypeX simulator and the pesky pigeon cipher – all I need is some accurate TypeX rotor wiring tables – anyone???

  9. GeorgeC: I liked the clue too, but it was 100% theirs, so all real credit to the boys. 🙂

    As far as Typex rotor wiring tables go, there aren’t any around (as I recall, Kelly Chang used Enigma rotor wiring in her Master’s Project cryptanalyzing Typex), but perhaps some will emerge before the century’s end. 😐

  10. GeorgeC on July 2, 2015 at 10:02 am said:

    Nick: Yes, I’ve seen Kelly Changs Masters paper, and, in fact based my TypeX simulator on her code, but I have also built a USB device to scan the rotor wiring from a real rotor and save it to a PC file. If only I could borrow some TypeX rotors to scan….

  11. GeorgeC: email me directly – nickpelling (at) nickpelling (dot) com [as normal]

  12. JennE on July 15, 2015 at 2:44 pm said:

    I am a little out of my league here, having stumbled upon this site during a 2 am bout of insomnia. I have zero experience with crypto, but find the idea fascinating. I have to ask, though, what is the answer to your properly difficult question? I spent another two hours looking online and reading intro primers for crypto trying to figure it out, but I obviously don’t have the background to determine what it means. If you don’t want to post the answer here, could you email it to me? Please? So it doesn’t keep me awake any more nights?

  13. GeorgeC on July 16, 2015 at 9:47 am said:

    JennE: I would be happy to email the solution to you, but I need an email address…

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