Her expression soured. “It’s a temporary thing. It’s something my manager usually does, but she was unavailable.” She handed a sheet of paper across the table. It was maybe thirty rows of unreadable letters and numbers in five-character groups. Under each row was a blank line. I assumed it was an encrypted text, and the blank lines were to write the plaintext message.
“Now you’re talking,” I said.
She pressed her mouth into a line. “Look. This is the practical portion of the interview. You can try it if you want.” Her tone of voice communicated that I shouldn’t bother.
“But you’ve already made your decision, is that it?”
“I don’t make hiring decisions. I’m a software engineer, not a manager. I just report on my impressions of your technical qualifications. I’m pretty sure the only reason you got an interview at all is because my manager saw something interesting about your resume—don’t ask me what. If she wants to offer you a position, or call you back for another interview, she will.”
“Okay.”
She indicated a computer on the table to my right. “There’s a file in the home directory with the same encrypted message. When you’ve got it solved, copy it down onto the sheet of paper.” Her slim shoulders gave a slight shrug. “They still like their hardcopy around here.”
She stood, gathering her folder and handbag. “I’ll leave you to it. Good luck, Mr. Johns.”
“Do most people solve it?”
She met my gaze. “Most competent ones, yes.”
She left me alone in the room. I pressed the power button on the computer. Nothing happened. It was just as well. I felt more comfortable with a pen and paper in hand than typing numbers into a spreadsheet anyway.
I crossed to a printer on the far side of the room and ripped a sheaf of paper out of the tray. Then I started to work.
The first thing I did was to make a few deductions. First of all, they expected new college graduates to solve this thing. That meant it wasn’t encrypted with modern methods. Public key encryption could be cracked, but it required banks of high-powered computers working in tandem for hours or days or, depending on the length of the key, weeks. So this would rely on somewhat simpler methods.
I figured the most likely was a Vigenère cipher or something in the same family. It was the dominant style of cipher used during the World Wars, and although it would take some serious effort to crack, I was confident I could do it. In a Vigenère cipher, the message was encrypted by adding a repeating key phrase to it. So if the message was:
and the key was Shaunessy Brennan, then the message would be encrypted by treating each letter as a number and adding them together. ‘M’ would be added to ‘S’, ‘Y’ would be added to ‘H’, etc. If any sum went higher than Z, it would just wrap around again to A.
If you knew the key phrase, then deciphering the message was as simple as subtracting it out of the cipher text. If you didn’t know the key phrase, then figuring it out could be remarkably difficult. Fortunately, avenues of attack had been worked out for such ciphers over the years, and I knew them. Unfortunately, it eventually became clear to me that the problem in the exercise was not a Vigenère cipher.
By “eventually,” I mean that two hours had gone by, and Ms. Brennan had peeked in on me three times, in between her other interviews, to see if I was done yet. I don’t think it was concern for my well-being that had her checking in as much as a desire for me to vacate the interview room. There was a lobby full of other candidates. She must have given up on me after that, because she didn’t check again.
I was getting pretty worried. I had expected the practical part of the interview to be where I would impress them. I had a working knowledge of cryptological history, and as I told Ms. Brennan, I was pretty good at math. But I was starting to think I had underestimated the competition. The NSA was, after all, the biggest employer of mathematicians in the world, in a country that had dominated world politics for decades. These were the best of the best. Ms. Brennan had obviously expected me to solve it quickly, and the increasingly patronizing expression on her face when she peeked in let me know that I had already failed the test.
But I don’t give up. Maybe it was a genetic deficiency, or maybe my big brother had knocked me in the head once too often growing up, but I had a complete inability to let a problem go once I’d sunk my teeth in it, no matter what the consequences. So even though I’d been there for hours, my stomach was growling, and I had to pee, I kept on working.
I tried frequency analysis and the Kasiski examination and the Friedman test. I tried digraph mapping and the shotgun hill climbing method. Finally, out of options, I tried the time-honored approach of every stymied exam taker in the history of exams. I guessed.
An unknown cipher could be cracked much more easily if you knew some portion of the plaintext. It dramatically reduced the number of possibilities to be analyzed, and if you could determine which portion of the cipher text matched the portion you knew, then in most cases, you could crack the rest of the message as well. I didn’t know a portion of the plaintext, but I still had the deep tones of the male voice actor from the NSA video running through my head.
I decided to take a gamble. It wasn’t much of a risk, really, since I didn’t have any other ideas, and time was ticking away. They might let me spend all afternoon here, but I didn’t think Ms. Brennan and the NSA were going to let me spend the night working on it. It was now or never.
I wrote “GLOBAL CRYPTOLOGIC DOMINANCE” on a new sheet of paper and got to work. Twenty minutes later, I had it cracked. It was a Playfair cipher, named for the British lord who promoted its use by the British in World War I. The plaintext message wasn’t an exact transcript of the video, but it was the same sort of high-minded advertising jargon lauding the mission and vision of the NSA. I was annoyed it hadn’t occurred to me sooner.
When I emerged from my cave, the lights were turned low and the hallway was empty. I peeked into rooms until I found Shaunessy Brennan hunched wearily over a terminal, typing. “Long day?” I said.
She looked at me in surprise. “Are you still here?”
I held up the paper. “Solved it,” I said.
“The last of the candidates went home hours ago.”
My heart sank. “I guess I’m a little rusty. I did get the answer, though.”
She sighed. “Fine.” She held out a hand for the paper, which I passed over. She glanced at it briefly, then set it on the desk. “I’ll add it to your file. Did you certify completion on the web page?”
“Web page?”
“The portal, I mean. When you logged in and accessed the decryption tools, the last step was to certify that you’d successfully completed the test. Some people forget that step.”
“I didn’t use the computer.”
She stared at me, eyes hard. “The computer in the interview room. For the practical exam.”
“I never turned it on. I hit the power button and nothing happened, so I just ignored it.”
“So how did you decrypt the message?”
I shrugged. “Pen and paper.” I felt a glimmer of hope. If she had been expecting me to use the computer, then maybe she wouldn’t hold my slowness against me. Not that I type any faster than I write, but there might have been tools, Matlab or Mathematica for instance, or at least a calculator, that would have sped the process a little.
Without changing expression, she stood and walked past me. I followed her back to the interview room. She stopped short when she saw the table, strewn with pages and pages of calculations from my failed attempts. She crossed to the computer and hit the power button, as I had done. Nothing happened. She followed the power cord from the back into a snarl of cables behind the table, and found the plug dangling loose beside an outlet. The outlet was covered with a wide strip of masking tape and a sign that said, “Outlet loose. Maintenance notified. Do not use.”