The Bletchley Riddle: Chapters 11–25

His roguish eyes turn resolute. “I’m not some tearaway. As soon as I’m old enough, I’ll be up there with him. But in the meantime, I’m signing up with the Home Guard.”

“Making the point you’re not a conscientious objector.” I nod.

“Oh,” says Jakob, surprised. “Your brother’s a pilot? How’d I miss that?”
“You’re a bit…preoccupied, Mr. Novis. Working such late hours and all,” says Colin.

She was to retrieve and protect sensitive documents per standard protocol.

He’s got this theory that any cipher written by a machine can be broken by a machine. He’s testing an electronic gadget over in Hut 1, an ingenious invention by all accounts. Only it doesn’t work. Yet.

“I never learned basic arithmetic,” Turing says.
“Your teacher taught it,” Welchman says. “You just weren’t paying attention.”
Turing smiles. “That sounds plausible.”

"...In a cipher, one letter stands for another letter.”
I tap the bowl of the pipe on the chalkboard. “This one’s known as the Caesar shift. Probably the first cipher used in war, dating back to—you guessed it— Julius Caesar. To encipher a message with the Caesar shift, you simply shift each letter of your message down the alphabet some set number of places. For a Caesar shift of one, A becomes B, B becomes C, and so on...."

“So, right…the weakness of the Caesar shift.” I tap the chalkboard. “It’s susceptible to brute force. That’s to say, you can simply check every possible solution. There are only twenty-five Caesar shifts, since a shift of twenty-six gets you right back where you started. Once people figured that out, the cipher was useless. That set off a two-thousand-year war between makers and breakers of secret writing. A twisting path that leads to this Park. Right here to this room.”

“So, um…the operator types in the German message and different letters light up on the lamp board. That’s the enciphered message. That gets sent by radio in Morse code. The receiver sets up his Enigma exactly the same way as the sender. He types in the gibberish, and, letter by letter, the original German lights up on the lamp board.”

He begins slowly. “This morning…when I asked about your mum…you looked relieved that my question was interrupted. Am I right?”
Oh my, Colin is very good at being straightforward.
“Yes. Perceptive of you to notice,” I reply. “I’ve been lied to regarding her whereabouts.” I look to Colin. “It hurts to be lied to, of course.”
He nods. “Yeah. It does.”

“You put those six letters on the top line of your message. It’s called the preamble. The operator who receives the message has got his Enigma set up according to the monthly sheet. He sets rotors to BER and types in KLW. What lights up on his lamp board?”
“THA,” Beryl says. Amazing how quick her mind is.
“Yes,” I say. “The receiver then moves his rotors to THA—and he can begin to decipher the message.”

The mansion stands before me in the distance. A hulking, labyrinth of a manor, punctuated with chimneys of all shapes and sizes.

“Hello,” says the commander, eyeing my proximity to his desk. “I didn’t see you come in.”

“The position requires quite a bit of stamina,” says the commander. “You’ll carry messages between the various departments and it often calls for a swift stride. You’ll be stationed in the center of the Park.”

She looks to me, serious. “I find Winston Churchill attractive, though. Very attractive.”
“Yes, strong orators are appealing, aren’t they?”
“Did you know that Churchill was born in America?” says Marion.