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.