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Carnegie Mellon University was located in the Oakland section of Pittsburgh, next to a large park. They parked on Forbes Avenue and walked to the Gates Hillman Complex. His father moved stiffly, as if he had arthritis. Along the way, Paul noticed a camera mounted to a streetlamp pole and wondered if it was the university’s or the government’s. He pulled the visor of his Mets cap lower over his brow. His father, he noticed, had done the same with his filthy fishing cap.

The Bill and Melinda Gates Building was a funky-looking structure rising from a ravine on the west side of the campus. Most of the buildings on the campus were classical looking, yellow brick with copper roofs that had turned green. But the Gates Building was modern, ten levels, each seemingly placed haphazardly atop the other, cantilevered, with black zinc shingles in a diamond pattern.

Inside, a long spiral walkway dominated the lobby.

“Stanley Brightman, in the actual flesh!” cried Professor Moss Sweetwater when they arrived. “Never thought I’d see you again!”

Professor Sweetwater was clearly a proud eccentric. For one thing, he wore turquoise-aqua socks. His office contained a big rolling whiteboard covered with numbers and strange characters and a standing desk.

The two men hugged each other.

“You’re absolutely right, Stan,” the professor said after Paul and his father had recounted their story. “This uses backdooring primes in the Diffie-Hellman key exchange algorithm. Like the backdoor discovered in the Dual EC DRBG.”

“Sorry—what’s that?” Paul asked, trying valiantly to keep up.

“The Dual Elliptic Curve Deterministic Random Bit Generator.”

“Ah, I see,” Paul said dryly.

“It’s a bug in the Diffie-Hellman algorithm introduced by the NSA,” Professor Sweetwater said. “See, they put out a crypto algorithm that contained a backdoor known only to them. I mean, they bribed RSA Security to put the backdoor in their software.”

“Much clearer,” Paul said, smiling faintly.

“Wow, what is this?” Stan said, tapping the front of a large computer standing on the floor in a rack five feet high with cables snaking out of it and coiling on the floor.

“It’s got eight Nvidia Hopper GPUs,” Professor Sweetwater said.

“I’ve only heard of that,” Stan said, marveling.

Sweetwater turned to Paul. “Whatever’s on this drive, it’s a fossil. Seven years ago, this might have been unbreakable.”

“But now?”

“A breeze. First thing we do is run a virus check on this thing.” The professor’s high-performance computer was buzzing and whirring and whining.

Apparently, the USB stick passed his test, because the next thing Paul saw was Sweetwater’s computer screen filling up with neat rows of green and red numbers like soldiers on parade. The professor said something about hexadecimal numbers and complex numbers.

Paul said, “Uh-huh.”

Professor Sweetwater said something about a lattice and elliptic curves and modular exponentiation.

Paul said, “Uh-huh.”

Yet his father was following along avidly. “That’s a defective crypto system,” he said.

Sweetwater said to Stan, “If I had a quantum computer, I’d try to factor them using Peter Shor’s algorithm.”

As both men chortled quietly, Paul surveyed the office. He saw a framed photo on the wall of Moss Sweetwater with a handsome black Labrador, a red bandana around its neck.

“That your Lab?” Paul asked.

“Yup. That’s évariste.”

Paul nodded, continued scanning. He saw a silver-framed citation, the Presidential Early Career Award for Scientists and Engineers. Framed covers of journal articles. On a regular steel desk were a vintage robot from RadioShack; a toy light saber with signatures all over it in Sharpie marker; a well-worn paperback copy of Pnin , by Vladimir Nabokov; a Japanese tea set. Next to the desk, shelves of yellow math books and the Yale Banner yearbook for 2015. A photograph of Professor Sweetwater at the White House; another of the professor, this time with Paul’s father, a younger, happier Stan Brightman—both accepting some kind of award.

Stan and his old student chatted far more and more easily than Stan had ever chatted with his son. Some twenty minutes later, the professor said, “Anyone here read Russian?”

Paul circled back to the computer screen. “I do, a little.”

He stared at the screen. It appeared to be an archive of emails, mostly in Russian, going back decades. He skimmed the emails, clicking and clacking and scrolling. He understood very little of it. The Cyrillic characters were interrupted occasionally, here and there, by stock symbols in English and occasional dollar signs. The emails were between two people: Arkady Galkin and Geraldine Dempsey. Both had Proton Mail accounts. Dempsey writing in Russian. Signing her emails “GP” in Cyrillic letters. Galkin signed his notes “AG.”

No wonder Dempsey was so desperate to retrieve the thumb drive, this secret cache of messages.

Arkady Galkin had Geraldine Dempsey, a top-ranking CIA officer, on his payroll.

“Who or what is Phantom?” the professor asked.

“It’s an operation,” Paul said. “A CIA operation.”

Then Professor Sweetwater’s landline phone rang, jarringly. He picked up the handset and pushed the button to answer. “Marge.” He listened for ten seconds. “Okay. . . . How many? . . . How do you know?” He listened, inhaled and exhaled, then hung up the phone.

He turned around to face Paul and his father. “The FBI called Marge to ask where my office was.”

“They’re coming here?” Paul asked.

“Apparently. But I think we’ve still got a little time before they get here.”

There was a loud pounding at the door. A booming male voice: “Professor Sweetwater?”

The professor turned his head. Without opening the door, he called out, “Yeah, who’s there?”

A long pause. “FBI,” the man’s voice said. “Please open the door, professor.”