How millennia of honed tradecraft will help us survive Thunderdome.

As long as there have been people, there have been spies. The Arthashastra, an ancient Indian text, details how a spy might take the guise of a holy man and con his way into the confidence of the king’s court. Darius I of the Achaemenid Empire employed spies called “the king’s ears” to keep tabs on provincial governors. The Aztec Empire commissioned traveling merchants as a kind of ancient State Department, charging them with diplomacy and espionage.

Today, we send Lacrosse satellites into orbit and relay signals down to Area 58. Unmanned aerial vehicles with stealth technology map out nuclear facilities. We have Gorgon Stares and biometrics, and whenever we need to know the general mood in Peshawar, we harvest data from social networks and feed it to “vengeful librarians.”

But what would happen if the satellites went dark? What would have happened if the United States and Soviet Union had unleashed their full arsenals of intercontinental ballistic missiles and pressed the reset button on humanity, tabula rasa. Chances are, survivors would keep doing what we’ve always done: Try to learn what the other tribe is doing, and how best to get the jump on him. In other words, we’d keep on spying. But what would espionage look like in the post-apocalypse (with or without zombies)? Whether in the Capital Wasteland or the Mad Max deserts, people will always spy on one another. Here’s how we’d do it.

Once the radiation reaches acceptable levels and politicians crawl out of “Area B” of the Mount Weather Emergency Operations Center, the first thing they’re going to want is a read of the new operating environment. As Sun Tzu wrote, “What enables the wise sovereign and the good general to strike and conquer, and achieve things beyond the reach of ordinary men, is foreknowledge.” Human intelligence (HUMINT) will be critical in establishing an understanding of the mutated hordes. Only with such an understanding can the “rightful” government reassert authority. The power of observation can provide the most basic understanding of an army or people. “When his troops lean on their weapons, they are famished. When drawers of water drink before carrying it to camp, his troops are suffering from thirst.”

When the spies who emerge from the ruins of Site R communicate with one another, cryptography will be essential. This has been essential to intelligence operations since at least the time of the ancient Greeks. The quality of encryption today is unrivaled and hard to grasp—even with hypothetical computers of unbridled speed, it has been estimated that to break PGP with brute force would take ten trillion years. (For comparison, the universe is only 13.75 billion years old.)

Post-apocalypse, things would throttle back a bit, and we might revert to Spartan technology, or at least a variant of it. The scytale, for example, (pronounced skittle-ly, as in: “This fruit candy tastes Skittle-ly.”) was a baton with parchment wrapped around it, running up, like you might imagine the bandages of a mummy’s arm. Unwrapped, the parchment would seem to be random letters; wrapped on a baton of exact diameter, however, letters would align and the message would reveal itself. Phil Zimmerman might not be impressed, but it’s better than plaintext.

Last year, the Central Intelligence Agency declassified the government’s oldest classified documents, dating back to World War I. Said then-Director Leon Panetta: “These documents remained classified for nearly a century until recent advancements in technology made it possible to release them.” Any tool of espionage with that kind of shelf life has to be good. In this case, the government was protecting information on steganography—that is, the use of invisible ink.

The ancient Romans discovered that print made with the milk of the tithymalus plant would dry invisible, but reveal itself if gently heated. Different formulas were developed over the centuries to employ hidden messages, though inks could be formulated from ingredients as basic as lemon juice and water. During the American Revolutionary War, General George Washington employed the use of secret ink called “sympathetic stain.” When used in combination with a code, the ink was so effective that it took 150 years before anyone learned the identities of the spies under his aegis. Once the grid is gone and we’re living in a zombie wasteland, such basic tradecraft as writing two messages on a single page—one with invisible ink and one with standard black ink—will again become a necessity. The zombies won’t know what hit them. (Take note also that lemon groves will become a vital national asset.)

Another technique Washington employed (and Sun Tzu wrote about) was the use of the double agent. Early in the Revolution, he recruited a former British soldier named John Honeyman to serve as a secret agent for the Continental Army. Honeyman plied his trade as a butcher and craftsman, and stridently trafficked with the British, all the while gathering intelligence. After collecting enough information, he arranged to be detained by the Continentals and brought directly to General Washington for “interrogation.” The information he had gathered: The British and German troops at Trenton were preoccupied with the coming Christmas holiday, and susceptible to attack. This information was valuable, but Washington raised the stakes by arranging for Honeyman to escape the stockade. Honeyman then went to Colonel Johann Rall, the Hessian commander in Trenton, with intelligence he had “gathered”: The Continental forces under Washington were in disarray and mutinous, and posed little threat.

The Christmas celebration thus went forward, and that night, General Washington and his soldiers quietly crossed the Delaware River before mounting a decisive attack on the Hessians at the Battle of Trenton. The United States of America exists today specifically because of an instance of effective tradecraft. That’s not bad considering the Continental Army didn’t have a National Geospatial-Intelligence Agency. The use of double agents will work well when Cormac McCarthy novels become nonfiction.

Therefore, we can say with confidence that espionage in the grim future (if that’s what’s in store) will look a lot like that of the shiny past. “An army without secret agents is exactly like a man without eyes or ears,” wrote Sun Tzu. In our horribly mutated future, a want for eyes and ears might well be the norm. Burgeoning spy agencies might not have quantum computers or nanotechnology, but they will have something, in a sense, more valuable—something in many ways lost in the digital age—fundamental, proven, time-tested tradecraft. To the mutants, zombies, smokers, and raiders: Bring it on.

D.B. Grady is the pseudonym of author David Brown. He is the co-author of The Command: Deep Inside the President’s Secret Army, and can be found at http://dbgrady.com or on Twitter at @dbgrady.