Macintyre reprints the explanation of Chapman’s code in full from the MI5 archive.
Puzzling over it, I’m reminded of my girlhood fascination with codes, secret ink, and hidden messages. But my problem, of course, always was that, not being a spy but, rather, a 10-year-old girl in Seattle in peace time, I had no message to impart other than “Hi. Can you read this message?”
Leaving the war to the side for a moment, there is something thrilling in having a message so important, so particular that you would want to take the word CONSTANTINOPLE, assign each letter in the word a number according to its place in the alphabet, then, the word (A=1, there being no B in Constantinople, C=2, the first O=9, the second O=10), multiply the resulting number by the date of the transmission, then do four or five other really complicated things so that
I HAVE ARRIVED AND IN GOOD HEALTH FRITZ [Chapman's nom de guerre]becomes
HILNO PHFYL YFZVQ VNFCR FLTOX VDMHH MYPBN RRVBB
And then, too, it’s exciting and useful to know that even spies had to send dull messages at times. I will be sure to impart this information to my children when they send each other messages in code. You can pretend, I’ll tell them, that you’ve just completed a very, very dangerous mission and your control agent needs to know that you’re ok.