The life of a monarch, the fate of a country, the defeat of an army—strange how the fate of the mighty could be determined by a tiny, twisting hodgepodge of letters.
Resting her elbows on the arm of the chair, she settled into a more comfortable position. “If it’s not too distracting, might you take a few minutes to explain your Square?”
“To begin with, there are all sorts of systems for creating codes,” he answered. “A common form is a cipher code—that is, where one letter is replaced by another. Here is an example.”
Placing a blank sheet of paper atop his notes, Saybrook wrote the words “The fox is in the henhouse.” Above it, he lettered the alphabet in one line. “Now, I’ll use a simple Caesar shift of three to encrypt the message, which means you take each letter of the original message and shift it over three positions.” He quickly wrote out a line that looked liked complete gibberish—wkh iua lv lq wkh khqkuvh.
“The spaces are often omitted to make the text harder to decipher. Still, an experienced code breaker knows to use frequency analysis, a concept developed by the Arabs while we Europeans were mired in the Dark Ages. This helps determine what the real letter might be. For example, ‘e,’ ‘t,’ and ‘a,’ are the most commonly used letters in English. So, one can begin by substituting a ‘t’ for whatever letter occurs the most frequently in the encrypted letter. It’s a matter of trial and error, of course. And the longer the message, the better the odds of the system working. Still, it helps one to make an educated guess.”
“Fascinating.”
“Yes, it is,” he agreed. “But that’s just the beginning. A code maker has all sorts of tricks to throw a code breaker off the scent. He—”
“Or she,” remarked Arianna.
Saybrook smiled. “Point taken. I suspect you would be frighteningly good at this.”
“Algorithms,” she mused. “I can see where mathematical concepts come into play.”
“Indeed. Mathematicians make excellent cryptographers. Oddly enough, so do poets. Chaucer was quite a good one. It has to do with imagination—which you also possess in spades.” He smiled. “But as I was saying, the code maker can use other elements to protect his text. He—or she—can insert a code word, known only to the sender and receiver of the message, which is inserted as a ‘blind’ so to speak, in order to throw the frequency off. In cryptography, we call it a key.”
Arianna made a face. “It sounds hopelessly complicated.”
“Complicated, yes. The permutations of a complex cipher defy the human brain. However, keep in mind that a code maker can’t get too clever or complicated. The receiver must also know the system being used.”
“Ah, I see what you mean,” she murmured. “And yet, what you were working on seems awfully complex.”
“I thought it safe to assume that our enemy would be too clever to use a simple text cipher, so I’m trying out a few other schemes.” He shuffled back to his original page. “A Vigenère Square seemed a good choice.”
“What, precisely, is that?”
“A grid invented in the sixteenth century by Blaise de Vigenère, a French diplomat posted to Rome. It’s a method for encrypting that offers a mind-twisting array of possibility.”
He finished lettering in the alphabet both vertically and horizontally, forming two sides of a square. “You have twenty-six letters across, and twenty-six letters down, both of which begin with ‘a.’ ”
She nodded.
“Then you begin the next row with ‘b,’ and then ‘c,’ and continue on like that until you have filled out the square. Now, you have twenty-six possible cipher alphabets. You can encrypt using two or twenty-two. Oftentimes, a code word is used to tell the receiver what rows to use. For example, say ‘pen’ is the code word. The receiver uses the row that begins with ‘p’ to decode the first letter of his secret message. For the second letter, he would use the row beginning with ‘e,’ and so forth.”
Arianna blinked. “Ingenious.”
“There are, of course, a multitude of other systems. Breaking a code requires intuition, patience, time—and most of all, luck.” He made a wry face. “The odds of stumbling upon a solution for this cipher tonight are stacked against me. However, I am familiar with the way the French cryptographers think, and if our enemy is really a man named Renard, then perhaps I shall get lucky. In any case, it is worth a try.”
“I should like to learn more about this,” she mused. “I can see where mathematics would be a helpful skill. Probability and patterns—it’s very much like gambling.”
“An apt analogy,” he commented. “As it happens, I brought along a book on the subject that was recently published by a don at Oxford. It is on my dressing table.”
Arianna went into his room, returning with not only the book but also two glasses of brandy.
“What are you going to tell Charles about this?” she asked, watching his face from over the rim of her drink. Firelight swirled within the amber liquid, the play of molten sparks dancing along the ridge of his cheekbones.
His eyes remained shadowed. “I haven’t yet decided.” He looked tired. Pensive. “But come morning, I will have to make up my mind.”
She fingered the wads of discarded paper, wishing that she could help. “Is there nothing I can do?”
Saybrook shook his head. “Not at the moment. I just want to test a few more ideas . . .”
The scratch of his pencil took up where his voice left off.
Patterns and probabilities, intertwining with deceptions and betrayals. The brandy burned a slow, sinuous trail down her throat. She had lived most of her life within the murky netherworld of secrets and lies. Which perhaps explained why the prospect of matching wits with a dangerous traitor was more tantalizing than terrifying.
I suppose that Charles Mellon is right to think me a very odd sort of female.
Taking another mouthful of the spirits, Arianna savored the heat of it against her tongue as she cracked open the book and began to read.
“Your pardon, milord.” Saybrook’s valet discreetly cleared his throat as he poked his head into the dawn-dappled sitting room. “But Mr. Henning has arrived. Shall I show him up?”
“God yes, before he wakes the house with his bellows.” The earl yawned and stretched out his long legs. “He tends to be in an ill humor when he is hungry.”
“Ouch.” Arianna winced as she sat up. Her muscles were stiff and knotted with cold. “I shall likely have a bruise on my shin, though it probably serves me right for being such a nodcock as to fall sleep on the floor.”
“You had better order up a big breakfast too, Hobbs,” added the earl. “Eggs, gammon, kippers, along with plenty of rolls and jam. Henning isn’t the only one who turns snappish when his bread box is empty.”
“Wretch,” muttered Arianna, tossing the sofa pillow at his head. “Please bring pots of coffee and chocolate as well, Hobbs.”
“Yes, milady.” The valet disappeared.
“I had better go and make myself presentable,” she said, rising and retying the sash of her wrapper.
“An excellent suggestion,” said her husband drily, waggling a brow. “You did summon Henning to make an inspection of naked flesh. However, I’d prefer it wasn’t yours.”
“As would I, seeing as most of the bodies he ogles are dead.”
She returned—fully dressed—to find their friend Basil Henning warming his hands by the rekindled fire. His frayed clothing was rumpled and the expression on his angular face looked equally out of sorts—but that was nothing unusual. Henning always looked grumpy.
As if on cue, he gestured at the steaming silver pot set on the side table. “Auch, Sandro, ye roust me from a nice warm bed and drag my carcass halfway to Hades, only to greet me with naught but a puling cup of coffee?” The outspoken Scotsman had been a surgeon in the earl’s army regiment, and the two men had formed a fast friendship during the long, brutal Peninsular campaign, despite the difference of wealth and birth. “Ye gods, man,” he groused.