“Until they’re not,” Miss Atkins agreed flatly. “And you’re right. They keep changing the rules just when we think we have the game figured.” She rolled her cigarette between her fingers. “There is a new device that the Germans seem to favour. It’s being referred to as Tunny by the minds at Bletchley. It is not dissimilar from the encryption device that the Nazis have been using since the start of this war, the one that we broke.” Miss Atkins slid a photo from the folder beneath her hands. It was an image of what looked almost like a typewriter, a plug board of some sort fronting the device. The top surface was flat, with raised letter keys at the front and a second set of letters set flush to the surface behind. Three rotors were set into the board near the rear. The entire device was nestled neatly into a hinged wooden box. “This is the German military Enigma machine that Bletchley’s been deciphering for some time now. They think that this new device is very similar, though they believe there to be far more than three or four rotors. They are guessing that the new device is significantly larger than this one and attached in line to a teleprinter. Guessing, because, unlike the Enigma, we have not yet been able to recover individual pieces, a whole working machine, nor the operator codebooks or instructions that accompany it.” The photo disappeared back into the folder and two more appeared. “These are what the codebooks for the Enigma look like.”

This was a photo of what looked almost like a school primer. The familiar eagle and swastika were stamped front and center, Special Machines Key written in fancy lettering above it. The second photo was of the charts inside, each containing dozens of lines of letters and numbers. Sophie turned the photos over. What looked like a file number had been written in black ink next to the words Operation Overlord on the bottom corner of each. Without comment, Sophie handed them back to Miss Atkins and they, too, vanished back into the folder.

“I’ve been briefed that Bletchley is working on a new solution for this Tunny device based solely on a feat of reverse engineering,” Miss Atkins said. “That they are developing a new machine that can solve these encryptions in hours instead of weeks. But like anything new, it is taking time. And we do not have the luxury of time. Not weeks, not months, not when supplies and men are sinking to the bottom of the sea faster than they can be loaded.” She smoothed her hands over the top of the folder. “And not when there are plans being made to take back Europe at this very moment.”

Sophie leaned back in the hard chair. “What sort of plans?”

Miss Atkins gave her a long look.

Sophie felt her cheeks heat. A stupid, amateur question.