[dbaron]

Having read the book in the past (though I don't have it in front of me right now), I think what it means is that he has to give Menaleus a way to identify himself in encrypted radio communications, such that if Menaleus is captured by the Germans and ordered to send certain messages (to misinform the British), he can omit his identity check without detection by the Germans so the British will know that he's transmitting under coercion. Doing this in a way that works if the Germans have decrypted his back traffic is hard -- I think the assumption was that they hadn't done so due to lack of resources; they'd instead torture the agent to get him/her to reveal the identity check.

[dbaron]

Now that I'm home: the scan is from page 508; the relevant bits are from the end of page 504 through page 508, or perhaps all of Chapter 67 (502-509).

Personally, I'm stumped by the "without anything passing in writing" part, which seems to me to make the problem unnecessarily harder, since Pandarus was carrying codes for them printed on silk (to be sewn into the lining of clothing).

And... since I haven't mentioned it already: this is a great book. It's gripping, and reads a little bit like a movie for a good reason -- its author (who is telling the story of his experiences working on cryptography during the war in his early 20s) went on to become a screenwriter.

[rottendoubt]

Thanks for the clarification. So it has to be some sort of verbally transmitted code (ie. a phrase) since it's used over the radio? That pretty much takes out almost all the guesses posted here so far.