Viral

remembering the night before.

He drove out into Maryland farm country thinking about his brother’s puzzle. He came to a gas mart, pulled off, and parked. Listened to the stillness. He opened the first sheet of paper and spread it on the seat. It was well-worn now at the folds. He studied the numbers again, knowing it had to be something simple—something that sophisticated surveillance might miss. Something that he had shared, once, with his brother.

Focus. If the numbers were substitutions for letters, they came out as this: 14672224 = ADFGBBBD. Jon wrote them down, as he had earlier. It didn’t help, although something about the sequence looked familiar.

He started the car and drove again, past cornfields, barns, occasional houses.

Try again, he thought. Stick with those letters. Just consider them in a different way. Think smarter.

Several miles ahead, he pulled over. A small country grocery. “Homemade Preserves,” a sign said. There were two pick-ups and three cars in the lot. Jon Mallory wrote the sequence out one more time: 14672224. Underneath, the corresponding letters: ADFGBBBD. Where had he seen those letters before? Somewhere, long ago. During his childhood, maybe. Codes that only the two of them would know. Let the information come to you.

Then, as he was crossing the Chesapeake Bay Bridge, something else occurred to him. In changing the numbers to letters, he had only considered single digits—one through nine. There was no reason to think that there wouldn’t be corresponding letters beyond the ninth letter in the alphabet. What if some of the numbers were paired? If 1 and 4 meant 14, say—the fourteenth letter of the alphabet: N—instead of A and D.

On the Eastern Shore, he pulled over at a Sunoco station and scribbled out all of the possible configurations involving paired numbers. Most meant nothing. But something about the letters clicked in his memory.

Then all at once he saw it. A combination that he recognized: if the final four numbers were two sets instead of four—22 and 24, instead of 2, 2, 2 and 4—then the sequence was one he knew: ADFGVX.

Yes. A combination that his brother would know that he’d recognize. ADFGVX was a famous war cipher—the code used by the German military during World War I. Jon couldn’t remember all of the specifics, but as a kid he and his brother had used it to exchange secret messages. Charlie telling him in code that he had to go away for a few hours. Never saying where.

Jon needed to find a wireless hot spot or an Internet cafe. He asked at the gas station. About five miles “that way,” the man said, was a public library. Jon pulled back into traffic and drove quickly down the two-lane roads to a small town called Stevensville. The library was on Main Street, as the man had said, across from the Stevensville Cemetery.

Four computer monitors were lined up against a wall in the back, three of them in use. He logged on the free computer and Googled “ADFGVX”: 34,200 returns. He skimmed through the details of the cipher: It was first used in the spring of 1918, as German troops advanced on Paris, to transmit attack plans to commanders on the front lines. The Allies routinely intercepted German cables but were unable to crack this cipher; for a time, military leaders, on both sides, considered it unbreakable. Then on June 2, a French cryptanalyst managed to decipher an ADFGVX- encoded cable detailing plans for a German offensive in France. The Allies sent troops to the front lines, and the German Army was turned back.

The ingenuity of the ADFGVX cipher, Jon read, was that it mixed substitution and transposition. It was made up of a simple six-by-six grid, randomly filled with 26 letters and 10 numbers, beginning with zero. It was only useful when the receiver knew the sequence of the numbers and letters.

Jon opened the sheet of paper with the letters and numbers and counted again. Yes: 36.

That was the easy part. Draw a grid, fill it with numbers and letters. He’d been given the correct number: 36. There were two possible ways of doing it: lay the numbers in vertically or lay them in horizontally.

Two grids. Then he understood: “V” circled. A directive. V for vertical. It had to be.

The rest of it, though, he wasn’t so sure about. Jon went back to the explanation on the Internet: “In its first, substitution, phase, the ADFGVX cipher could be broken by frequency analysis, so it was further scrambled by transposition, meaning the use of a seven-letter word.”

So he had the cipher, but nothing to use it with. Or was he missing something? Something obvious. The cipher required something else. A coded message to use on it—in a way that was simple but would be detected only by them. Invisible to others. This message was being conveyed in three parts, Jon realized, each useless without the other two.

He thought about Thomas Trent. The way his eyes had scanned the Mall. And the last thing he had said to him. Go back to where you’ve already been. You’ll know what that means. A method they had used before to transfer information.

Jon wondered if he should go back through the e-mails on his laptop, looking for familiar words, numbers, phrases. Or was it something else? It had to be simple, something he’d already figured out. His brother would have made it deliberately easy. Go back where he had already been.

His fingers rested on the keypad. Then it came to him. He tried a Web address he’d used before: Horticult.net.

It took him less than five minutes to find it, in a message board under an entry from D. Gude—their grade- school mathematics instructor. In a long message titled “Rampaging weeds”—what to do when wild violet or Bermuda grass takes root in your lawn, and how, through a “careful strategy of rooting, spacing, and mulching”—D. Gude was able to win “the war over rampaging weeds.” Two-thirds of the way through, he found the combination of numbers he was looking for. “My personal weeding cycles, by days of the month,” it said, then: 1,4,6,6,7,4,6,6,1,4,24,4,7,7,22,22,7,6,24,6,4,6,24,7,24,

1,7,7,1,22,6,6,7,4,22,22,24,4,7,6,24,1,24,1,7,7,

1,4,7,4,7,4,7,22,22,6,1,4, 7,6,22,24,7,7,6,22.

That had to be it.

The largest number was 24, so that would work as days of the month. The numbers all less than the number of days in a month.

But it would also work with letters in the alphabet.

If this was the coded message, it would have to be translated into letters, though. The commas made it easier, so that 22 would be V, not BB. Jon Mallory went through the numbers and jotted the corresponding letters underneath:ADFFGDFFADXDGGVVGFXFDFXGXAGGAVFFGD

VVXDGFXAXAGGADGDGDGVVFADGFVXGGFV

That was only half of the ADFGVX code, though. It also involved a transposition of letters. He called up the explanation of the cipher again on the computer. But it was a little like putting together a complicated toy or piece of furniture. Maybe he was making it too difficult for himself.

His brother would have made it easy.

If he had found the coded message, there was no point in coding it further through transposition. Charlie would have kept it simple, giving him a key and a code, from two different sources. Jon just had to put them together. Let the information come to you.

To decipher the code, then, he went back to the ADFGVX grid. At its simplest, the code used pairs of letters to form corresponding letters or numbers. There were 35 in all in this message.

So where A crossed with D on the grid was the letter R. FF became E. GD was S.

Eleven minutes later, he had it. The message in its entirety read as follows:

“Reservation. 6 Lake St, Villars, SW. Friday.”

THIRTY-FIVE

FROM THERE, JON MALLORY worked backward. But he didn’t want to do it on the same computer,