Dan Brown: Digital Fortress

Susan smiled. "You of all people should know. It's like studying a foreign language. At first the text looks like gibberish, but as you learn the rules defining its structure, you can start to extract meaning."

Becker nodded, impressed. He wanted to know more.

With Merlutti's napkins and concert programs as her chalkboard, Susan set out to give her charming new pedagogue a mini course in cryptography. She began with Julius Caesar's "perfect square" cipher box.

Caesar, she explained, was the first code-writer in history. When his foot-messengers started getting ambushed and his secret communiques stolen, he devised a rudimentary way to encrypt this directives. He rearranged the text of his messages such that the correspondence looked senseless. Of course, it was not. Each message always had a letter-count that was a perfect square-sixteen, twenty-five, one hundred-depending on how much Caesar needed to say. He secretly informed his officers that when a random message arrived, they should transcribe the text into a square grid. If they did, and read top-to-bottom, a secret message would magically appear.

Over time Caesar's concept of rearranging text was adopted by others and modified to become more difficult to break. The pinnacle of non computer-based encryption came during World War II. The Nazis built a baffling encryption machine named Enigma. The device resembled an old-fashioned typewriter with brass interlocking rotors that revolved in intricate ways and shuffled cleartext into confounding arrays of seemingly senseless character groupings. Only by having another Enigma machine, calibrated the exact same way, could the recipient break the code.

Becker listened, spellbound. The teacher had become the student.

One night, at a university performance of The Nutcracker, Susan gave David his first basic code to break. He sat through the entire intermission, pen in hand, puzzling over the eleven-letter message:

Finally, just as the lights dimmed for the second half, he got it. To encode, Susan had simply replaced each letter of her message with the letter preceding it in the alphabet. To decrypt the code, all Becker had to do was shift each letter one space forward in the alphabet-"A" became "B," "B" became "C," and so on. He quickly shifted the remaining letters. He never imagined four little syllables could make him so happy:

He quickly scrawled his response and handed it to her:

Susan read it and beamed.

Becker had to laugh; he was thirty-five years-old, and his heart was doing back flips. He'd never been so attracted to a woman in his life. Her delicate European features and soft brown eyes reminded him of an ad for Estee Lauder. If Susan's body had been lanky and awkward as a teenager, it sure wasn't now. Somewhere along the way, she had developed a willowy grace-slender and tall with full, firm breasts and a perfectly flat abdomen. David often joked that she was the first swimsuit model he'd ever met with a doctorate in applied mathematics and number theory. As the months passed, they both started to suspect they'd found something that could last a lifetime.

They'd been together almost two years when, out of the blue, David proposed to her. It was on a weekend trip to the Smoky Mountains. They were lying on a big canopy bed at Stone Manor. He had no ring-he just blurted it out. That's what she loved about him-he was so spontaneous. She kissed him long and hard. He took her in his arms and slipped off her nightgown.

"I'll take that as a yes," he said, and they made love all night by the warmth of the fire.

That magical evening had been six months ago-before David's unexpected promotion to chairman of the Modern Language Department. Their relationship had been in a downhill slide ever since.

The crypto door beeped once, waking Susan from her depressing reverie. The door had rotated past its fully open position and would be closed again in five seconds, having made a complete 360-degree rotation. Susan gathered her thoughts and stepped through the opening. A computer made note of her entry.

Although she had practically lived in Crypto since its completion three years ago, the sight of it still amazed her. The main room was an enormous circular chamber that rose five stories. Its transparent, domed ceiling towered 120 feet at its central peak. The Plexiglas cupola was embedded with a polycarbonate mesh-a protective web capable of withstanding a two-megaton blast. The screen filtered the sunlight into delicate lacework across the walls. Tiny particles of dust drifted upward in wide unsuspecting spirals-captives of the dome's powerful deionizing system.

The room's sloping sides arched broadly at the top and then became almost vertical as they approached eye level. Then they became subtly translucent and graduated to an opaque black as they reached the floor-a shimmering expanse of polished black tile that shone with an eerie luster, giving one the unsettling sensation that the floor was transparent. Black ice.

Pushing through the center of the floor like the tip of a colossal torpedo was the machine for which the dome had been built. Its sleek black contour arched twenty-three feet in the air before plunging back into the floor below. Curved and smooth, it was as if an enormous killer whale had been frozen mid breach in a frigid sea.

This was TRANSLTR, the single most expensive piece of computing equipment in the world-a machine the NSA swore did not exist.

Like an iceberg, the machine hid 90 percent of its mass and power deep beneath the surface. Its secret was locked in a ceramic silo that went six stories straight down-a rocketlike hull surrounded by a winding maze of catwalks, cables, and hissing exhaust from the freon cooling system. The power generators at the bottom droned in a perpetual low-frequency hum that gave the acoustics in Crypto a dead, ghostlike quality.

TRANSLTR, like all great technological advancements, had been a child of necessity. During the 1980s, the NSA witnessed a revolution in telecommunications that would change the world of intelligence reconnaissance forever-public access to the Internet. More specifically, the arrival of E-mail.

Criminals, terrorists, and spies had grown tired of having their phones tapped and immediately embraced this new means of global communication. E-mail had the security of conventional mail and the speed of the telephone. Since the transfers traveled through underground fiber-optic lines and were never transmitted into the airwaves, they were entirely intercept-proof-at least that was the perception.

In reality, intercepting E-mail as it zipped across the Internet was child's play for the NSA's techno-gurus. The Internet was not the new home computer revelation that most believed. It had been created by the Department of Defense three decades earlier-an enormous network of computers designed to provide secure government communication in the event of nuclear war. The eyes and ears of the NSA were old Internet pros. People conducting illegal business via E-mail quickly learned their secrets were not as private as they'd thought. The FBI, DEA, IRS, and other U.S. law enforcement agencies-aided by the NSA's staff of wily hackers-enjoyed a tidal wave of arrests and convictions.

Of course, when the computer users of the world found out the U.S. government had open access to their E-mail communications, a cry of outrage went up. Even pen pals, using E-mail for nothing more than recreational correspondence, found the lack of privacy unsettling. Across the globe, entrepreneurial programmers began working on a way to keep E-mail more secure. They quickly found one and public-key encryption was born.