Page 21 of The Dark of the Moon
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‘There are actually some more jars of Marmite in the cupboard where the spaghetti is kept, you know,’ I said.
‘Well, that’s a good thing. But you see how the basic premise works, in principle? We would transmit the coded message in groups of four letters, starting WSEL because they’re under numbers 1 and 2 in our key. See if you can write out the whole coded message.’
She handed me the pencil again and I wrote out:
Wsel Oetp Sxip Mnmu Adrs Exli
She nodded, looking at the work we’d done.
‘It’s not perfect. Any enemy cryptanalyst with an ounce of intelligence might be able to work it out fairly easily, given enough time. So the next step is to make it more complicated. We make another grid and re-transpose the message. That’s called a double transposition code, you see? And then we send that coding in groups of five letters. We also need to tell those on the receiving end which keywords we’re using – in the case of your poem it would be line two, words six, seven and eight. You could easily send that information as a cipher at the start of the message.’
She showed me how to do the double transposition and we worked on writing out a few messages that would probably have stumped even the cleverest Nazis, according to Philly.
‘So to decipher the message at the other end, you’d need to know the grid layout, with the keywords and the right number of rows, and work it backwards?’ I said.
‘Exactly. That’s one of the jobs the cryptographers at Bletchley Park were doing, day in, day out. Poem codes were a pretty effective way of sending messages from the field, especially if the agent only used the poem once. Even more so if the poem wasn’t a well-known one, or if it was something made-up.’
I took the pencil from her and wrote another coded message on the paper, using the simplest way of coding she’d shown me at the beginning.
She looked at it for a moment, then smiled and said, ‘You’re very welcome, Finn.’
‘Are you going to tell Mum some more of your story now?’ I asked.
‘Would you like to hear some more?’ she said.
