Digital Fortress

Soshi plowed into the document. She scrolled past a recipe for urea nitrate, an explosive ten times more powerful than dynamite. The information rolled by like a recipe for butterscotch brownies.

"Plutonium and uranium," Jabba repeated. "Let's focus."

"Go back," Susan ordered. "The document's too big. Find the table of contents."

Soshi scrolled backward until she found it.

I. Mechanism of an Atomic Bomb

A) Altimeter

B) Air Pressure Detonator

C) Detonating Heads

D) Explosive Charges

E) Neutron Deflector

F) Uranium Plutonium

G) Lead Shield

H) Fuses

II. Nuclear Fission/Nuclear Fusion

A) Fission (A-Bomb) Fusion (H-Bomb)

B) U-235, U-238, and Plutonium

III. History of the Atomic Weapons

A) Development (The Manhattan Project)

B) Detonation 1) Hiroshima 2) Nagasaki 3) By-products of Atomic Detonations 4) Blast Zones "Section two!" Susan cried. "Uranium and plutonium! Go!"

Everyone waited while Soshi found the right section. "This is it," she said. "Hold on." She quickly scanned the data. "There's a lot of information here. A whole chart. How do we know which difference we're looking for? One occurs naturally, one is man-made. Plutonium was first discovered by-"

"A number," Jabba reminded. "We need a number."

Susan reread Tankado's message. The prime difference between the elements… the difference between… we need a number… "Wait!" she said. "The word 'difference' has multiple meanings. We need a number-so we're talking math. It's another of Tankado's word games-'difference' means subtraction."

"Yes!" Becker agreed from the screen overhead. "Maybe the elements have different numbers of protons or something? If you subtract-"

"He's right!" Jabba said, turning to Soshi. "Are there any numbers on that chart? Proton counts? Half-lives? Anything we can subtract?"

"Three minutes!" a technician called.

"How about supercritical mass?" Soshi ventured. "It says the supercritical mass for plutonium is 35.2 pounds."

"Yes!" Jabba said. "Check uranium! What's the supercritical mass of uranium?"

Soshi searched. "Um… 110 pounds."

"One hundred ten?" Jabba looked suddenly hopeful. "What's 35.2 from 110?"

"Seventy-four point eight," Susan snapped. "But I don't think-"

"Out of my way," Jabba commanded, plowing toward the keyboard. "That's got to be the kill-code! The difference between their critical masses! Seventy-four point eight!"

"Hold on," Susan said, peering over Soshi's shoulder. "There's more here. Atomic weights. Neutron counts. Extraction techniques." She skimmed the chart. "Uranium splits into barium and krypton; plutonium does something else. Uranium has 92 protons and 146 neutrons, but-"

"We need the most obvious difference," Midge chimed in. "The clue reads 'the primary difference between the elements.' "

"Jesus Christ!" Jabba swore. "How do we know what Tankado considered the primary difference?"

David interrupted. "Actually, the clue reads prime, not primary."

The word hit Susan right between the eyes. "Prime!" she exclaimed. "Prime!" She spun to Jabba. "The kill-code is a prime number! Think about it! It makes perfect sense!"

Jabba instantly knew Susan was right. Ensei Tankado had built his career on prime numbers. Primes were the fundamental building blocks of all encryption algorithms-unique values that had no factors other than one and themselves. Primes worked well in code writing because they were impossible for computers to guess using typical number-tree factoring.

Soshi jumped in. "Yes! It's perfect! Primes are essential to Japanese culture! Haiku uses primes. Three lines and syllable counts of five, seven, five. All primes. The temples of Kyoto all have-"

"Enough!" Jabba said. "Even if the kill-code is a prime, so what! There are endless possibilities!"

Susan knew Jabba was right. Because the number line was infinite, one could always look a little farther and find another prime number. Between zero and a million, there were over 70,000 choices. It all depended on how large a prime Tankado decided to use. The bigger it was, the harder it was to guess.

"It'll be huge." Jabba groaned. "Whatever prime Tankado chose is sure to be a monster."

A call went up from the rear of the room. "Two-minute warning!"

Jabba gazed up at the VR in defeat. The final shield was starting to crumble. Technicians were rushing everywhere.

Something in Susan told her they were close. "We can do this!" she declared, taking control. "Of all the differences between uranium and plutonium, I bet only one can be represented as a prime number! That's our final clue. The number we're looking for is prime!"

Jabba eyed the uranium/plutonium chart on the monitor and threw up his arms. "There must be a hundred entries here! There's no way we can subtract them all and check for primes."

"A lot of the entries are nonnumeric," Susan encouraged. "We can ignore them. Uranium's natural, plutonium's man-made. Uranium uses a gun barrel detonator, plutonium uses implosion. They're not numbers, so they're irrelevant!"

"Do it," Fontaine ordered. On the VR, the final wall was eggshell thin.