do with your future, Arthur?" I asked.
"Surgery, sir," said Arthur. "I'm very much inspired by Doctor Bell at the Edinburgh University. But I'd also like to become a writer like you, actually, and have written some stories for Reginald's children. Edgar Allan Poe's another inspiration, and I'm attempting a mystery now."
"Poe! Now there was a true Copernican. I heard he was a great cryptographer," I said.
Moriarty disagreed. "He indulged in substitution codes, barely worth the effort to solve, unlike Vigen猫re ciphers."
Arthur asked what a Vigen猫re cipher was.
"It's based on the Caesar cipher, which shifted letters by a chosen number of positions, looping back to the beginning of the alphabet if necessary," I explained. "Shift the word JABBERWOCKY four positions to the right, you'd have NEFFIVSGOC. To solve the coded message, apply the shift in reverse.
"The Vigen猫re cipher uses a keyword where each letter in the alphabet represented a different shift. For example, if ALICE were my keyword, that would mean shifts of 0, 11, 8, 2, and 5. The keyword is repeated as needed, and so JABBERWOCKY becomes JLJDIRHWEOY."
Moriarty nodded. "There are mathematical ways to solve a Vigen猫re code. Kasiski's method determines the length of the keyword by measuring distances between repeating combinations, then uses frequency analysis to break the code. Kerckhoffs' method focuses on solving the keyword itself."
"Moriarty, didn't you have a notebook at Christ Church where you jotted down your formulae?" I asked. "You used a cipher on the annotations. Was that Vigen猫re?"
Moriarty smiled. "It was, to ensure that the musings and errors of my youth were for my eyes alone. But I'm confident that my code cannot be broken by either the Kerckhoffs or Kasiski solutions. They assume the keyword would be short, and repeated. But if the keyword's significantly long, like a piece of text, then a coded message would be virtually unbreakable."
"Respectfully, Professor, the keyword may be impervious to analysis, but the codemaker is not," said Arthur. "At medical school, Professor Bell taught us to observe the person as well as the disease in our diagnoses. By analyzing the man's habits, experiences and indulgences, one can deduce the choice of text he uses for his code."
"You presume to unriddle my cipher by mere observation?" asked Moriarty.
"If I had the chance to get to know you better, Professor, I daresay I could figure out your code," said Arthur.
"I know a challenge when I hear one! Reginald, indulge me by releasing Arthur from his obligations on weekends. My university's not far, and I will pay for travel and lodging. He may observe me in my element, while I in turn will mentor him and provide him with a sample of cipher to unriddle. If he succeeds by summer's end, I will pay his tuition."
Moriarty had obviously taken a liking to Arthur Doyle, perhaps eager to guide a young man into his genius.
Reginald heartily agreed. "If he doesn't fall behind his duties here, why not?"
Arthur smiled. "It will be a pleasure learning from you, Professor."
"Before I forget, gentlemen, Samuel Haughton left these for you on his last visit," Reginald said, giving us each a copy of a treatise titled On Hanging. Reverend Haughton was a doctor and fellow Copernican from Dublin who dabbled in mathematics. "Haughton claims the mathematics of hanging can be useful in medicine. Humane versus inhumane hangings: depending on the criminal's weight and the length of drop, it could mean the difference between a quick death from snapping his spinal cord, to a long death from strangulation."
We argued awhile over the need for executions, with Moriarty maintaining ambivalence. When the topic somehow drifted onto the three-body problem in astronomy, which I had little interest in, Arthur and I excused ourselves so I could see his story manuscripts.
Arthur's room was modest, and his desk cluttered with papers on medicine, scraps of writing, and sheet music. Among his medical books were works by Burton, Dickens, Leibniz, and of course, Poe. He moved a violin case off a chair so I could sit and read his stories. I found them well-written and engaging, and encouraged Arthur to continue writing.
Arthur retrieved a book bound in red Moroccan leather, gilt in silver: a copy of Alice's Adventures in Wonderland. "Would you mind signing this, Reverend, for Herr Gleiwitz? I see him on my rounds, and feel sorry for the man. He's down on his luck, raising his children on what little income he receives from giving German lessons. Perhaps your book will give his family some joy."
I gladly signed the