Death Ray
Page 25
The first thing Finn did was write out his key phrase in lines of five letters, each line directly underneath the previous one, to form the beginnings of a grid. Importantly, no letter gets repeated, so, for example, although the letter O appears twice in his key phrase it only appears once in the grid.
So Finn’s grid looked like this:
Finn then added all the unused letters of the alphabet in the right order to complete a 5 x 5 grid. As the alphabet contains 26 letters and only 25 are needed, the letters I and J are both treated as I. This is what Finn’s completed grid looked like:
The message Finn sent to London in Death Ray was:
URGENT … ODETTE ARRESTED … WILL TRY RESCUE … SUSPECT JACQUES IS ENEMY SPY … DEATH RAY COMPROMISED … ADVISE NEXT STEPS
Finn divided his message into pairs of letters (called bigrams):
UR … GE … NT … OD … ET … TE … etc.
Taking each pair in turn, Finn located their positions in his grid. So, for the first pair, ‘UR’, their positions are:
Now comes the clever bit! Finn had to think of these letters forming the opposite corners of a square or rectangle (in this case the square, ORUZ). He then wrote down the opposite corners to UR – that is, OZ. These are the letters he transmitted to London (there they’d do the operation in reverse to decode it).
If you try out the Playfair code, you will soon discover several complications. Quite often pairs of letters fall in the same row or column and hence don’t form a square or rectangle. For example, the fourth pair of letters (or fourth bigram) in Finn’s message is OD; both letters are in the top row of his grid. Finn would get round this by shifting the second letter one space (either left or right, or up or down). In the case of OD, Finn would shift down. So the pair OD becomes OM, enabling him to form the rectangle ODKM. Taking the opposite corners, he can now code OD as DK. Worse still, sometimes he would be faced with coding an identical pair of letters, for example DD. To make a square out of these Finn would have to shift the second letter in both directions; that is, diagonally – so DD would become DY, thus creating the square DRMY. His coded letters for DD would thus be RM.
These complications can make decoding a message quite a challenge because the person receiving Finn’s message doesn’t know that he’s had to make the above adjustments. So they would simply decode each pair of letters and try to form proper words from them. What would look like bad spelling mistakes would alert them and they’d then try out various options until the message made sense.
As you can imagine, coding and decoding was a hard task for an agent working in the field, demanding great concentration and attention to detail while always fearing possible discovery and arrest. Try and see if you can work out the rest of the coding. Then imagine you’ve received the message and have a go at decoding it using the same approach, or invent your own key phrase and grid.
The Playfair code was extremely hard for the enemy to break, although it had one major drawback. If captured, an agent might reveal his key phrase under interrogation. The enemy could then use it to send false messages. A series of ‘security checks’ were often included, e.g. deliberate mistakes – their presence or absence alerting London that the agent had probably been compromised. Eventually the SOE developed its own coding methods, including what were called WOCs (Worked Out Codes) printed on silk. The agent could easily destroy them if he or she feared capture, and with no secret key to remember, there was nothing the agent could divulge under interrogation. These later evolved into what were called One Time Pads, in which a WOC was used just once and then destroyed.
Finn has sent you an urgent message. Can you decode it?
TIPS:
1. First use the above grid to decode each pair of letters in turn and write your answers in the boxes below, underneath Finn’s coded version. The first pair is tricky to fully decode so I’ve got you started and helped you out by solving the three other really difficult pairs.
2. Now look at your results and see if you can read the message. If you can’t, it is because one or more of your pairs needs to be reversed (there are two ways of writing down each pair). For example, when you decode Finn’s second pair, FU, you will find that it forms the square EFUV. You can write down the opposite corners as either EV or VE. You can’t be sure which is correct until you try and read the message. So work along your decoding from left to right, and see if by switching round your pairs of letters you can make sense of the message.
Good luck.
(Note: Those I’ve already solved for you are in the right order.)
3. Still having problems? Hint: Finn’s message contains five words (4, 3, 4, 8 and 3 letters respectively).
A big thank you to Mum and Dad, Charlie Sheppard, Harriet Wilson and Carolyn Whitaker for all their encouragement, guidance and support.
About the Author
Craig Simpson spent his childhood in southern England. At eighteen he headed off to veterinary school in Bristol but soon realized that in the wrong hands a scalpel could do more harm than good, and switched to studying science. He spent a while juggling test tubes before realizing there had to be more to life. After fifteen years scaling the corporate ladder, and travelling widely he became an independent consultant. A keen amateur historian, inspired by true stories he then abandoned the rat-race to write adventure novels. He now lives between the New Forest and the Hampshire seaside.
Also by Craig Simpson:
SPECIAL OPERATIONS: DOGFIGHT
RESISTANCE
SPECIAL OPERATIONS: DEATH RAY
AN RHCP DIGITAL EBOOK 978 1 407 04815 4
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