“I’ve never met him personally, but of course I knowofhim. He’s been our continental counterpart for years. In code-breaking circles the man’s a legend.” Her eyes gleamed. “Major Scovell’s the one who broke the Portuguese cipher back in 1812. Thanks to him, Wellington was able to continue the siege of Badajoz secure in the knowledge that French reinforcements were too far away to threaten Ciudad Rodrigo, the fortress he’d captured a few months earlier.”
Raven noted her starry-eyed look with a sharp stab of annoyance and closed his eyes. Oh, wonderful. That’s all he needed, a major case of hero worship.
Heloise carried on, blissfully unaware of his darkening mood. “And then, of course, Major Scovell came up with the impregnable system used by our own army.”
“No doubt you’re going to tell me about that, too?”
If she was aware of his sarcasm she gave no sign. “He gave the same edition of a pocket dictionary to each of the two parties. The code is based on the location of words within these dictionaries, so ‘134A18’ translates to page 134, column A, row 18.”
“Thrilling.”
She sent him a chiding look for his obvious lack of enthusiasm. “The word ‘cryptography’ comes from the Greek word meaning hidden writing.”
He held up a hand. “No more etymology, you hear me, or I’ll throw you in the nearest river myself.”
She set her lips into a mutinous line. “Fine.”
She looked so chagrined he unbent a little. She was magnificent in a huff, but he preferred listening to her talk. He’d bet she could make even calculus sound fascinating. “How did you get into code-breaking, anyway?”
“About five years ago, just after my accident, Castlereagh came to visit Father. He happened to read a paper I’d written on translating the Rosetta Stone and challenged me to read some basic codes.” She smiled in memory. “They only took a couple of minutes—they were simple substitution cyphers—but he was impressed. He gave me more. I cracked every one. He told Father he could use a mind like mine at the Foreign Office and Father agreed, so Castlereagh started sending me messages to decode.”
“So how do you go about breaking a code?”
She shot him a suspicious look. “You’re really interested?”
“Absolutely,” he lied.
Her smile did strange things to his insides. “Well, the simplest type of code is a letter substitution code. That’s where you just swap one letter of the alphabet for another by shifting them along a certain number of places. SoAisB,BisC,and so on. Codes like that have been used for thousands of years. It’s called a Caesar shift, after the Roman emperor who often used it.”
“I’ve heard of those. I’m not a complete idiot.”
She didn’t look convinced. “Codes like that are easy to crack using frequency analysis.”
“And that is…?”
“Certain letters are used more often in each language. In English it’sE, T,andA.In French, it’sE, S,andA.If you find the most common letters in a coded message, the chances are they belong to a high-frequency letter in the original language. You try substituting them and see whether words start to make any sense. After that it’s basically a lot of educated guesswork. You use the decoded words to try to deduce the meaning of the coded words from the context.”
Raven smothered a yawn. She looked like an erotic nursery rhyme character in that hat. Mary Mary Quite Contrary. Or Little Bo Peep. Except for those damned breeches. They outlined every inch of her long, slim thighs. He imagined them wrapped around his hips and almost groaned.
“Sometimes it’s as simple as looking for a repeated phrase. Napoleon’s correspondence from his generals, for example, almost always concluded with the phrase‘Vive l’Empereur.’In the same way, you can expect military messages to be concerned with troops, location, provisions, and so on, so you look for words like ‘brigade,’ ‘division,’ ‘artillery,’ and ‘enemy’ to help you guess the rest of the code.”
Raven grunted. Would she talk this much when she was making love? He shifted uncomfortably in the saddle as his body urged him to find out.
“French codes of recent years have been much stronger than simple substitutions though,” she continued, mercifully unaware of the direction of his depraved thoughts. “Five years ago they started using a new code they called the Great Paris Cipher, which used several different methods of encrypting simultaneously. It was amazingly complicated, but Major Scovell cracked it in less than a year.” She sounded as if someone had just presented her with the Crown Jewels.
Raven rolled his eyes. “How did he crack it, if it was so damn complicated?”
Heloise frowned at his irreverence. Clearly the man was a god in her eyes.
“The code wasn’t the weakness, it was the French themselves. They were overconfident in the cipher’s security. Instead of using it properly, they’d often only encryptpartof a message, mistakenly believing the encoded parts would be strong enough to keep the full meaning a secret. But by leaving some words in common French, they provided Scovell with an invaluable foothold.”
Raven tried to pull himself together. She was looking at him expectantly, so clearly some comment was expected of him. He opted for the all-encompassing, “Hmm?”
Heloise inhaled sharply. “Oh, goodness, I just realized somethingwonderful.”
“What?”
“Major Scovell always gets sent copies of the messages we receive in London. And he sendsuscopies of the ones his men have intercepted. That way we have all our code-breaking resources working on a message at the same time. Sometimes he’s been the first to crack a code, other times its been myself or Edward who’ve managed it.”