"The most valuable information to the Germans was data regarding the movement of troops and ammunition from the United States to Europe," Crossle said at the next conference with Russell. "So naturally their spies used every conceivable device to that end, with an ingenuity that increased as our counterespionage grew more and more efficient. During the final period of the war, we were advised that an airplane had been making certain, rather peculiar maneuvers off the south coast of Long Island. We sent one of our cryptographers over to make observations."

"It didn’t take him long to discover what was going on. The pilot was apparently indulging in stunt flying. But by carefully following the trajectory of one tip of the wing, our observer was able to trace a series of letters such as the following:"

RQVKM EFAUD XUGRQ AINAG LIPBC ROKJJ L

"This is a substitution cipher, as you can readily see," Crossle added. "It is made up with the Julius Caesar cipher, as were your first three ciphers. But I warn you that you will be unable to solve it unless you write down in vertical columns at least the first thirteen letters of the alphabet. Even then if you fail to search carefully, the message will escape you."

Solution