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. ðŸ˜‰

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 ( ðŸ˜‰ ) 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:

SCCJL SRMTS SNJZH SCXFS VPTCT FSSLS XHZCS DVNQC

DHTKK TLSMT RKTSS FWSPW SNMZP XWVQM CZLKS X

Can you decipher it? Enjoy!