Harris inspected the lines of seemingly unrelated letters. “Fantastic! That will save a lot of time.”

“Where do we start?”

“About two thousand years ago.”

“Pardon?”

“Julius Caesar used a substitution cipher to get a message to Cicero doing the Gallic Wars. He simply substituted Greek letters for Roman. He improved on the system later on. He’d take the plain text alphabet and create a cipher alphabet by shifting letters three places down. Put one alphabet over the other and you can substitute those letters on one row for the other.”

“Is that what we have here?”

“Not exactly. The Arabs discovered that if you figured out the frequency of a letter’s appearance in written language, you could decipher a substitution cipher. Mary, Queen of Scots, lost her head after Queen Elizabeth’s code-breakers intercepted the messages used in the Babington Plot. Jefferson developed a variation of a system known as the Vigenere method.”

“Which is an expansion of the Caesar substitution.”

“Correct. You create a batch of cipher alphabets by shifting so many letters over for each one. You stack them in rows to form a Vigenere box. Then you write a key word repetitively across the top of the box. The letters in the key word help you locate your encoded letters, something like plotting points on a graph.”

“That would mean that the letters in your clear text would be represented by different letters.”

“That’s the beauty of the system. It prevents the use of letter frequency tables.”

Harris turned to a computer and, after typing furiously for several minutes, created columns of letters arranged in a rectangular shape. “This is the standard Vigenere box. There’s only one problem. We don’t know the key word.”

“How about using artichoke?”

Harris laughed. “Poe’s ‘Purloined Letter,’ out in plain sight? Artichoke was the key word Jefferson and Meriwether Lewis used to unlock the code they agreed on for the LouisianaTerritory expedition.”

He wrote the word artichoke several times across the top of the square and tried to decipher the encrypted message revealed through the grille. He tried the plural form and shook his head.

“Maybe that was too obvious,” Angela said. They tried Adams, Washington, Franklin, and Independence, all with the same disappointing results.

“We could spend all day doing this,” Angela said.

“Actually, we could spend decades. The key word doesn’t even have to make sense.”

“So there is no way a Vigenere cipher can be broken?”

“Any cipher can be broken. This one was busted in the 1800s by a guy named Babbage, a genius who’s been called the father of the computer. His system looked for sequences of letters. Once he had those, he could figure out the key word. Something like that exceeds my skills. Fortunately, we’re within spitting distance of the greatest code breakers in the world.”

“You know someone at the NSA?”

“I’ll give my professor a jingle.”