I’ve been wondering whether the pigeon cipher might be based around bigrams, i.e. where you split the cipher into pairs. If you disregard what seems to be the ‘AOAKN’ key indicator at the start and end, there are two basic ways to split the message into pairs. What is immediately interesting is that if you start immediately after AOAKN, you get three repeated pairs in the message (rather than just one), and that four of those six repeats occur every other pair along a short stretch towards the end of the message.

AOAKN
HV PK DF NF JW YI DD CR QX SR DJ HF PG OV FN MI AP XP AB UZ WY
YN PC MP NW HJ RZ HN LX KG ME MK KO NO IB AK EE QU AO TA RB QR
HD JO FM TP ZE HL KX GH RG GH TJ RZ CQ FN KT QK LD TS GQ IR U
AOAKN

AOAK
NH VP KD FN FJ WY ID DC RQ XS RD JH FP GO VF NM IA PX PA BU ZW
YY NP CM PN WH JR ZH NL XK GM EM KK ON OI BA KE EQ UA OT AR BQ
RH DJ OF MT PZ EH LK XG HR GG HT JR ZC QF NK TQ KL DT SG QI RU
AOAKN

The WW2 “Slidex” code works by having a table of codes particular to each arm of the armed forces (i.e. Royal Engineers, Operations/Signals, etc) where each bigram pair encodes a particular word often used by that arm. To encipher numbers or words, you bracket them inside a pair of SWITCH ON and SWITCH OFF bigram codes (there are several of each), and pick bigrams that correspond to each letter or digit or digit pair.

Hence I strongly suspect that what we are looking at in “GH RG GH TJ RZ CQ FN” is a sequence of letters enciphered in the Slidex SWITCH mode, i.e. where each letter is enciphered as a bigram.

Unsurprisingly, I would really like to read the 1944 Revised Instructions for using the Slidex R/T Code, and to have copies of the various Slidex code sheets and coordinate offsets used by the Royal Engineers and Infantry Brigade on D/Day, as I suspect these will allow us to read this message (does anyone have copies of these I can see?) Fingers crossed!

9 thoughts on “Bigrams in the pigeon cipher…

  1. George Thopas on December 26, 2012 at 10:34 pm said:

    You may want to look at http://www.royalsignals.org.uk/misc.html
    (middle of the page) I suppose you want document 1719 .
    There’s a request procedure in place, if you click through.
    Good luck !

  2. George: indeed, I’m a member there already. Very sadly, however, one of the people behind that whole site (Alister J Mitchell) died just a few days ago, and so it has been suspended for a while out of respect, which I completely understand.

  3. Anne-Lise Pasch on December 27, 2012 at 2:03 am said:

    Hm. I was just looking at your second bigram set above, and the triplet: JR ZC QF matches the outer Slidex strips on http://www.duke.edu/web/isis/gessler/collections/crypto-slidex1page.jpg … which seems very coincidental. Slidex may be worth a look.

  4. Potentially another little hint towards Slidex is in the letter frequency. The bigrams contain 24 unique letters in the first position (ie no B or V) and 26 unique letters in the second position.

    This matches what would seem to be the maximum counts from common usage pattern of the 12 column *17 row Slidex grid.

    No real help but a 25 or 26 count in the 1st position would have been concerning

  5. Mike: thanks for pointing that out, I’d noticed the two blank rows in the bigram frequency, but didn’t note their sigificance for Slidex. 🙂

  6. bdid1dr on December 27, 2012 at 2:05 pm said:

    Nick,

    Does the term “Slidex” by any chance indicate some kind of perforated overlay material which can be slid across a line of the code alphabet pairs (or moved down the rows) to “highlight” the significant words or phrases?

  7. bdid1dr: no, Slidex was a very specific R/T (“radio telephone”) code used before, during, and after WW2. There are some helpful pictures here: http://www.jproc.ca/crypto/slidex.html

  8. George C on January 1, 2013 at 6:07 pm said:

    Nick: Accuracy is of the utmost importance if the code is to broken! I see in your 5th bigram pair ( 1st set ) you have JW – this should be JU, the original 5 letter sequence being FNFJU.

    All the U’s seem to be exaggerated and squared off, presumably to distinguish them definitively from V’s.

    Your theories about W. Stout and D-Day are very convincing, and Slidex appears to a good candidate for the coding method, we just need the right table and keys for the day !!

  9. Although I’ve used text from the Voynich manuscript in it, this was inspired by your post.
    http://dnodonovan.wordpress.com/2012/12/27/voynicheriana-02/

    Cheers

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