While doing some last-minute research for my PhD dissertation, I came across the above 1943 poster from the Office of the Chief Signal Officer (of the Army). I was immediately struck with the realization that this was one of those cases, common in wartime, I’m sure, where the left hand did not know what the right hand was doing.
This poster urges signal operators not to give the Germans easy “cribs,” for instance in broadcasting weather reports. What is a crib? A crib is a word which a cryptanalyst–someone trying to break into your encrypted communications–has reason to believe is in a body of enciphered text. In essence, by telling the enemy cryptanalyst what the answer is, you can help him figure out how the encryption system really works. And THAT is the really valuable thing to him.
All good stuff. What is the problem? Well, at the same time that the US was warning its people not to give away cribs in weather reports, Bletchley Park, the British codebreaking outfit was busily using weather reports as one of its most reliable sources of cribs in breaking into the various German Enigma systems. The Americans, of course, were not running Bletchley Park, but they were well aware of what was going on there. I can just imagine some American signals intelligence officer clutching his heart after seeing this poster, but knowing that objecting to it would cause an even greater security problem.