TWO WEEKS AGO USA Today broke the shocking news that Osama bin Laden's terrorist organization has infiltrated the world's supply of Web porn, hiding messages for its global operatives deep within the digits of pictures posted on Godless Western triple-X sites. For historically minded readers, the article afforded a moment of wonder at the depths of the national-security establishment's Cold War nostalgia and the media's willingness to indulge it. There, once again, was the old familiar intimacy of the alleged subversion, the thrilling suggestion that the enemy might lurk among us everywhere, sneaking into our bedrooms and our cubicles under cover of cultural trash. "You very well could have a photograph and image with the time and information of an attack sitting on your computer, and you would never know it," one cyberwar expert told USA Today's reporter.

I confess, though, that I got a bit nostalgic myself when I read the story. Not for the Cold War -- I was born too late to enjoy it in the fullness of its Eisenhowerian heyday -- but for its Bush-era aftermath. Specifically, I found myself looking back with melancholy fondness upon the summer of 1992, a moment perhaps not equal to the summer of '67 in its hold on the memories of a generation but one which for me, at least, holds much the same sense of freedom and promise in the bubble of its recollection. It was a moment, after all, when radical political thought was just beginning to adjust to the reality of '89, just rising to the challenge of imagining the possibilities that that reality implied. It was a moment, as well, when the Internet, long a distant, reverie-inspiring rumor known firsthand only to military contractors and computer-science majors, was just starting to enter the lives of the rest of us. But most importantly, perhaps, and certainly not at all coincidentally, it was the moment when I first learned it was possible to do with digital communications what Osama bin Laden is now reported to have done.

THE TECHNICAL NAME for it is steganography, from the Greek for "covered writing." It is the art of keeping communications undetected, and it is not to be confused with the related discipline of cryptography. Cryptography assumes that messages will be intercepted and uses codes and ciphers to make sure they can't be understood if they are. But steganography aims for a deeper sort of cover: it assumes that if the message is so much as found to exist, the game is over.

Steganographic techniques are as old, at least, as Herodotus, who documented their use among the Greeks of the fifth century B.C. In Book Seven of The Histories, he writes that when Demaratus, a Spartan living in Persia, got wind of the emperor Xerxes' plan to invade Greece, he contrived to tip his compatriots off by sending them a stegotext: he took a pair of folding wooden message tablets, scraped the wax writing surface off them, wrote his message on the wood, then covered his message back over with wax. Persian counterintelligence never suspected a thing. Nor did the Persians have a clue when Histiaeus of Miletus sent a similarly subversive letter home tattooed onto the scalp of a trusted slave. The messenger arrived safely at his destination and said no more than what he'd been instructed to say: "Shave my head and look thereon."

In contrast with cryptography, a field long given over to high math and puzzle-making abstraction, steganography was always more or less a materials science, its history florid with the range of substances and gadgetry used at one time or another to conceal communications. Simon Singh's The Code Book relates that in the first century A.D., Pliny the Elder explained how the milk of the thithymallus plant dried to transparency when applied to paper but darkened to brown when subsequently heated, thus recording one of the earliest recipes for invisible ink. The ancient Chinese wrote notes on small pieces of silk that they then wadded into little balls and coated in wax, to be swallowed by a messenger and retrieved, I guess, at the messenger's gastrointestinal convenience. The sixteenth-century Italian scientist Giovanni Porta proposed a steganographic scheme involving hard-boiled eggs: Write on the shell with a vinegar-and-alum solution and your message passes through to the surface of the egg white, where it can't be read until the shell is peeled away.

In the 1860s, the technology of microfilm was perfected, ushering in a golden age of stealth that reached its manic peak in World War II, when German spies began making heavy use of the microdot. A page of text shrunk down to a one-millimeter speck of film, the microdot could be pasted almost indetectably into any humdrum business letter, hiding out in the shallow well of a typewritten period or comma. With swarms of them passing through the mails, the U.S. government went sort of nuts and started seeing hidden messages everywhere. By the end of the war, censors had either prohibited or tampered with flower deliveries, commercial-radio song requests, broadcast weather reports, postal chess games, children's drawings on their way to Grandma, knitting instructions, and anything else that might too easily encode Axis intelligence. At one point, an entire shipment of watches was held up so officials could spin the dials and wipe out any messages hidden in the positions of the hands.

In the half century since the war, however, the sweeping digitization of communications tech has caused steganography at last to veer away from the material world, joining cryptography in the realms of math and abstraction. As ever, of course, information still reaches the mind in the form of concrete sensory stimuli -- as light and sound -- but increasingly it is the universal code of binary numbers that shapes that information, and it is in the numbers that hackers and spooks have looked for places to hide still more information.

They have found those places. Digital stego takes a number of forms, but most of them are variations on the most popular technique (the one most likely to be used by bin Laden and company, in fact, if they are really using any at all). It's called "least significant bit" steganography, and to understand how it works you have to think a little about how digital media work. Consider the dots of light that compose an image on a computer screen. Or the slivers of sound that blend together to form a song as it streams from a CD player. Each dot, each sliver, is recorded as a small number of bits -- ones and zeroes -- maybe sixteen of them, maybe twenty-four. Most of the bits specify crucial information about the color or tone of the sensory blip they represent, but a few stand for nuances the average eye or ear won't even pick up. These latter, the so-called least significant bits (LSBs), are effectively indistinguishable from noise -- from the random hiss and blur that shows up in any information channel. And since properly encrypted data is also indistinguishable from noise, it turns out that an untouched digital copy of a song or photograph is very hard to tell from a copy whose LSBs have been overwritten with a well-enciphered message. The collected LSBs of a Radiohead CD, for instance, might encode a completely undetectable blueprint of the Stealth bomber. This sentence might fit imperceptibly inside a small JPEG image embedded in a Web page somewhere.

Indeed, this sentence actually does fit inside a small JPEG image on a Web page somewhere -- the one directly to the left of this paragraph, to be precise. It's in there right now, braided into the bits with the aid of a free stego program I downloaded just the other day, a program called Jsteg.

Or maybe it isn't.

And how would you ever know?