Here was the situation Opsec faced. The package no longer mattered, but the hack most certainly did. Someone had emerged from the Internet, slithered into the Company’s heart, and then disappeared. The specific vulnerability the attacker had exploited was still unknown, and was likely to be used again: he had established a back door, a way in. Some back doors are permanent, but most are short-lived. Possibly this one was already for sale on the black markets that exist for such information in obscure recesses of the Internet. Until Opsec could find and lock it, the back door constituted a serious threat. Opsec reviewed the basics with the Company’s managers. He said, Look, we’re in the Internet business. We know we’re going to get hacked. We have to assume, always, that our network is already owned. It is important to go slowly and stay calm. We will soon know how and when to lock the door. We will have to decide later if we should do more.

To me he said, “Also, relax. In the long run, the chance of survival always drops to zero anyway.” He did not say this to his client. It was not an insight the Company would have valued at the time. Even in the short run, as it turned out, the news would be alarming enough.

II. Anarchist at Heart

Definitions. A vulnerability is a weakness in a network’s defenses. An exploit is a piece of software that takes advantage of a vulnerability. A zero-day exploit is a piece of software that takes advantage of a vulnerability that is known to a small group of aggressors and generally not to the defenders. “Back door” is another name for much the same. There are variations. Infinite invention is at play. Welcome to the Dark Net, a wilderness where wars are fought and hackers roam. More definitions. The Dark Net exists within the deep web, which lies beneath the surface net, which is familiar to everyone. The surface net can be roughly defined as “anything you can find through Google” or that is otherwise publicly indexed for all to see. The deep web is deep because it cannot be accessed through ordinary search engines. Its size is uncertain, but it is believed to be larger than the surface net above it. And it is mostly legitimate. It includes everything from I.R.S. and Social Security data to the internal communications of Sony and the content management system at The New York Times. It includes Hillary Clinton’s e-mails and text messages, along with everyone else’s. Almost all of it is utterly mundane.

The Dark Net occupies the basement. Its users employ anonymizing software and encryption to hide themselves as they move around. Such tools offer a measure of privacy. Whistle-blowers and political dissidents have good reason to resort to them. Criminals do, too. White fades quickly through gray and then to black in the Dark Net. Furtive sites there offer all manner of contraband for sale—narcotics, automatic weapons, contract killings, child pornography. The most famous of these sites was Silk Road—the brainchild of Ross Ulbricht, a libertarian entrepreneur who was arrested by the F.B.I. in San Francisco in 2013 and sentenced last year to life in prison without parole. New and even larger marketplaces have opened, including the current leader, AlphaBay, which is owned by a man who has been quoted as saying he resides in an “off-shore country where I am safe,” gives interviews to the press, and openly defies attempts by the authorities to shut him down. There are twists: illegal narcotics sold over the Dark Net tend to be purer, and therefore safer, than those sold on the street—this because of the importance to the sellers of online customer ratings. By comparison, it is hard to see the bright side of missile launchers or child pornography.

However noxious the illicit Web sites may be, they are merely the e-commerce versions of conventional black markets that exist in meatspace. The real action on the Dark Net is in the trade of information. Stolen credit cards and identities, industrial secrets, military secrets, and especially the fuel of the hacking trade: the zero days and back doors that give access to closed networks. A short-lived back door to the iPhone operating system may sell for a million dollars. In 2015 a black-market site called TheRealDeal, the first one to specialize exclusively in cyber-weaponry, opened for business. Several others have followed. There is something strangely circular about all this—the Dark Net chasing its tail through the Dark Net—but the stakes have turned out to be high.

HE CAN BREAK INTO ALMOST ANY SECURE NETWORK WITHOUT SETTING OFF ALARMS.

And the trade is new. So new that when Opsec looks back on recent history he can sound like an old man remembering the onset of World War II. He was born to a middle-class family in the orbit of Washington, D.C., and by the time he was in kindergarten it was obvious that he was a bright if stubborn child. This was toward the end of the 1980s, in the pre-dawn before the Internet as we know it. His mother owned an early personal computer—a big box with a keyboard, a black screen, and white letters. It had a dial-up modem for point-to-point connections to other computers. When Opsec was six, he discovered that he could play games on it. The first was a Japanese action game called Thexder, in which he could transform a robot into an airplane and bomb things on the ground. This was so gratifying that on weekends he would wake up his mother at five A.M. and get her to go through the necessary keyboard commands to access it. She grew so weary of this that she wrote out the commands for him to use. He then figured out how to write a simple program to automate the log-in.