The Book of Air and Shadows - Michael Gruber [103]
Crosetti rapidly filled in the ciphertext and produced
F L E E A L L I S D I S C O V E R E D
R L V C P P R U S U G H G U H E I C S
“And notice how the double L in all is disguised too,” he said. “Now you have something that can’t be broken by simple frequency analysis, and for three hundred years no one could break a cipher like that without learning the key word. That’s mainly what they tortured spies for.”
“How do you break it?”
“By finding the length of the key word, and you do that by analyzing repeating patterns in the ciphertext. It’s called the Kasiski-Kerckhoff Method. In a long enough message or set of messages, FL is going to line up with MA again, and give you RL again, and there’ll be other two-and three-letter patterns, and then you count the distance between repeats and figure out any common numerical factors. In our example, with a seven-letter key you might get repeats at seven, fourteen, and twenty-one significantly more than would be the case by chance. Obviously, nowadays you use statistical tools and computers. Then when you know our key has seven letters, it’s a piece of cake, because what you have then is seven simple substitution alphabets derived from the Vigenère tableau, and you can break those by ordinary frequency analysis to decrypt the ciphertext or reconstruct the key word. There are downloadable decrypting programs that can do it in seconds on a PC.”
“So why haven’t you cracked it?”
He ran his hand through his hair and groaned. “If I knew that, I’d know how to crack it. This thing’s not a simple Vigenère.”
“Maybe it is, but it has a really long key. From what you said, the longer the key, the harder it would be to factor out the repeating groups.”
“Good point. The problem with long keys is that they’re easy to forget and hard to transmit if you want to change them. For instance, if these guys wanted to change the key every month to make absolutely sure that no spy had discovered it, they’d want something an agent could receive with a whisper in the dark or in a totally innocent message. What they do nowadays is that the agent gets what they call a onetime pad, which is a set of preprinted segments of an infinitely long, totally random key. The agent enciphers one message and then burns the sheet. It’s totally unbreakable even by advanced computers. But that kind of method wasn’t invented in 1610.”
“So what else?”
“It could be a grille, in which case we’re screwed.” Seeing her puzzled look, he added, “A Cartan Grille, a literal piece of stiff paper with holes punched in it that reveals the message when you place it over the page. That would mean it’s not a cipher at all. For example, assume the ciphertext I wrote is just random noise, but if you slide a grille over it you can get RUG or USE or RUSE….”
“But surely, if they were using a grille, the ciphered message would look like a normal letter. ‘Dear Mom, having a great time in London, bought a new doublet, baited some bears, wish you were here, love, Dick.’ And the grille would reveal the plaintext: ‘flee, all is discovered.’ I mean the point would be to allow the concealed message to pass as innocent, no?”
Crosetti tapped his head in the what-a-jerk gesture. “Of course. Obviously, I’m losing it. Anyway, I’m stumped—I have no idea where to go from here.”
“I rest my case. Like I said, you need a break.”
“You’re right.” He rubbed his face with both hands and then asked, “What day is this?”
“October 14. Why?”
“There’s a Caribbean film festival at BAM, and I wanted to catch Of Men and Gods. Maybe if I lose myself in gay Haitian voodoo, I’ll come back to it fresh.”
“That’s a good plan, dear,” said Mary Peg.
Something about her tone and the expression on her face made him pause. He