Digital Fortress

Susan explained how the intercepted transmissions often originated from dangerous governments, hostile factions, and terrorist groups, many of whom were inside U.S. borders. Their communications were usually encoded for secrecy in case they ended up in the wrong hands-which, thanks to COMINT, they usually did. Susan told David her job was to study the codes, break them by hand, and furnish the NSA with the deciphered messages. This was not entirely true.

Susan felt a pang of guilt over lying to her new love, but she had no choice. A few years ago it would have been accurate, but things had changed at the NSA. The whole world of cryptography had changed. Susan's new duties were classified, even to many in the highest echelons of power.

"Codes," Becker said, fascinated. "How do you know where to start? I mean… how do you break them?"

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:

HL FKZC VD LDS

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:

IM GLAD WE MET

He quickly scrawled his response and handed it to her:

LD SNN

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.

Chapter 4

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.