Digital Fortress

"Susan," Strathmore said quietly. "This is going to be hard to accept at first, but just listen a minute." He chewed his lip. "This code that TRANSLTR's working on-it's unique. It's like nothing we've ever seen before." Strathmore paused, as if the words were hard for him to say. "This code is unbreakable."

Susan stared at him and almost laughed. Unbreakable? What was THAT supposed to mean? There was no such thing as an unbreakable code-some took longer than others, but every code was breakable. It was mathematically guaranteed that sooner or later TRANSLTR would guess the right key. "I beg your pardon?"

"The code's unbreakable," he repeated flatly.

Unbreakable? Susan couldn't believe the word had been uttered by a man with twenty-seven years of code analysis experience.

"Unbreakable, sir?" she said uneasily. "What about the Bergofsky Principle?"

Susan had learned about the Bergofsky Principle early in her career. It was a cornerstone of brute-force technology. It was also Strathmore's inspiration for building TRANSLTR. The principle clearly stated that if a computer tried enough keys, it was mathematically guaranteed to find the right one. A code's security was not that its pass-key was unfindable but rather that most people didn't have the time or equipment to try.

Strathmore shook his head. "This code's different."

"Different?" Susan eyed him askance. An unbreakable code is a mathematical impossibility! He knows that!

Strathmore ran a hand across his sweaty scalp. "This code is the product of a brand-new encryption algorithm-one we've never seen before."

Now Susan was even more doubtful. Encryption algorithms were just mathematical formulas, recipes for scrambling text into code. Mathematicians and programmers created new algorithms every day. There were hundreds of them on the market-PGP, Diffie-Hellman, ZIP, IDEA, El Gamal. TRANSLTR broke all of their codes every day, no problem. To TRANSLTR all codes looked identical, regardless of which algorithm wrote them.

"I don't understand," she argued. "We're not talking about reverse-engineering some complex function, we're talking brute force. PGP, Lucifer, DSA-it doesn't matter. The algorithm generates a key it thinks is secure, and TRANSLTR keeps guessing until it finds it."

Strathmore's reply had the controlled patience of a good teacher. "Yes, Susan, TRANSLTR will always find the key-even if it's huge." He paused a long moment. "Unless…"

Susan wanted to speak, but it was clear Strathmore was about to drop his bomb. Unless what?

"Unless the computer doesn't know when it's broken the code."

Susan almost fell out of her chair. "What!"

"Unless the computer guesses the correct key but just keeps guessing because it doesn't realize it found the right key." Strathmore looked bleak. "I think this algorithm has got a rotating cleartext."

Susan gaped.

The notion of a rotating cleartext function was first put forth in an obscure, 1987 paper by a Hungarian mathematician, Josef Harne. Because brute-force computers broke codes by examining cleartext for identifiable word patterns, Harne proposed an encryption algorithm that, in addition to encrypting, shifted decrypted cleartext over a time variant. In theory, the perpetual mutation would ensure that the attacking computer would never locate recognizable word patterns and thus never know when it had found the proper key. The concept was somewhat like the idea of colonizing Mars-fathomable on an intellectual level, but, at present, well beyond human ability.

"Where did you get this thing?" she demanded.

The commander's response was slow. "A public sector programmer wrote it."

"What?" Susan collapsed back in her chair. "We've got the best programmers in the world downstairs! All of us working together have never even come close to writing a rotating cleartext function. Are you trying to tell me some punk with a PC figured out how to do it?"

Strathmore lowered his voice in an apparent effort to calm her. "I wouldn't call this guy a punk."

Susan wasn't listening. She was convinced there had to be some other explanation: A glitch. A virus. Anything was more likely than an unbreakable code.

Strathmore eyed her sternly. "One of the most brilliant cryptographic minds of all time wrote this algorithm."

Susan was more doubtful than ever; the most brilliant cryptographic minds of all time were in her department, and she certainly would have heard about an algorithm like this.

"Who?" she demanded.

"I'm sure you can guess." Strathmore said. "He's not too fond of the NSA."

"Well, that narrows it down!" she snapped sarcastically.

"He worked on the TRANSLTR project. He broke the rules. Almost caused an intelligence nightmare. I deported him."

Susan's face was blank only an instant before going white. "Oh my God…"

Strathmore nodded. "He's been bragging all year about his work on a brute-force-resistant algorithm."

"B-but…" Susan stammered. "I thought he was bluffing. He actually did it?"

"He did. The ultimate unbreakable code-writer."