Most of what Peter Wayner writes is aimed squarely at the behind-the-screen world of computer-code poets. Yet when he wrote a book on the practice of steganography in 1996, the term seemed so arcane, so daunting, that his publisher insisted he not use that word in the title.

That changed in the days after Sept. 11, however. Among the many rumors floating around after the attacks, one held that Osama Bin Laden's minions communicated with one another via messages embedded in digital photographs that they sent around the Internet. Or something along those lines. The story was never verified, Wayner says, although the idea brought to light the little-known and ancient art of steganography. As luck would have it, the Baltimore resident was updating Disappearing Cryptography (the 1996 version was subtitled Being and Nothingness on the Net) at the time.

Steganography has long been a sort of lonely stepchild to encryption. At its simplest, encryption is the art of scrambling a written message so that the resulting seemingly random collection of characters cannot be easily deciphered by another party. Steganography is the art of hiding of a message within another message, such that a passer-by is unaware of the concealed meaning.

Digital photographs are a particularly good medium for steganography. As Wayner writes in the new edition of the book, "Digitized photos or sounds are represented by numbers that encode the intensity at a particular moment in space and/or time." A digital photograph is made up from millions of ones and zeros. The color in each pixel of the photograph is defined for the computer by clusters of these numbers. As Wayner explains, when these images or sounds are turned into ones and zeros they are captured at resolutions that go far beyond what can be perceived by humans. So it is possible to take out some of these digits and replace them with others carrying a secret message without anyone being the wiser.

Even a small digital photograph can hide a lot of information. Disappearing Cryptography shows a before-and-after series of seemingly identical photographs of bridges, city skylines, and deserts that were encoded with messages. It takes very careful work to find the difference, if it can be seen at all. Many of the photographs contain the text of an entire chapter of the book itself, yet the most careful observer will only see a bit of loss in the picture's color as the binary digits that represent those colors are intermingled with the binary digits carrying the words.

"We want to believe that if there's anything real and true in the world, it's the meaning of numbers like zero and one," Wayner says in an e-mail interview. "But after steganography, even bits can offer multiple readings and misreadings."

After reading the newly released second edition of Disappearing Cryptography, it's hard not to look around for hidden messages that may be buried in handbills, menus, Billboard's album charts. Everything becomes a potential carrier for surreptitious information. Even Wayner's book may be suspect, though he insists this isn't the case.

"For the record, I didn't really put any [hidden codes of my own] in the book," he says. "If anything, the math and the metaphors are tricky enough the first time."

Steganography is hardly a computer-age phenomenon. As another cryptography-obsessed writer, Simon Singh, pointed out in his 1999 history of encryption, The Code Book, the practice dates back at least a few centuries, when Chinese messengers swallowed secret notes sealed in wax to carry to their intended recipients. More recently, James Bamford's profile of the National Security Agency, The Puzzle Palace, tells of how gangsters used to communicate with one another by the number of shirts they sent to Las Vegas to be dry cleaned. The basic idea behind steganography is the same--finding ways to transport data within other forms.

Wayner, 38, has long been one of the most respected writers on computer technology, having done stints as a technology correspondent for The New York Times and the newsstand version of Byte magazine, which now exists only online. He has also written a number of books for the computer trade, such as 1999's Compression Algorithms for Real Programmers and the also newly released Translucent Databases.

Judging from a recent lunch interview near his Roland Park home, Wayner seems uninterested in the usual book-promotion rigamarole. When asked how he became so entranced with steganography, Wayner says only that it started when a friend asked if it was possible to hide one message within another. Asked about where to get more information about Disappearing Cryptography's rerelease, he randomly muses about Congress and the entertainment industry's ham-fisted attempts to utilize encryption as a method of copyright control ("Hollywood Vs. the Internet" May 8). A Princeton-trained mathematician, Wayner loathes the idea of outlawing the act of circumnavigating digital copyright-protection schemes. In the heart of such legislation, he writes in the book, is an attempt "add security by prohibiting knowledge."

The field of computer-technology writing teems with writers who lose themselves and their readers in thickets of directionless arcana and technical terms. Wayner is one of those rare tech scribes who can get the details right and shape a story. While much of Disappearing Cryptography gets into the nitty-gritty of writing and using computer programs to hide information, it also revels in the philosophical and cultural underpinnings of its subject matter.

In the book, Wayner asks whether this tool can be used for good or for evil. The answer is, of course, both. He admits that steganography holds obvious allure for the child pornographers and terrorists of the world. But he also notes that the technique can be used by freedom fighters to safely communicate beneath the radar of corrupt governments, by law enforcement to pass messages back and forth to undercover agents, and by whistle-blowers to pass on secrets about corporate malfeasance.

Steganography can even be fun--sort of. Wayner's Web site (www.wayner.org) features a section on how to use a random collection of just about any object--a shopping list, a tally of favorite disco records--as a way to embed secret messages.

"Life often reduces to formulas," Wayner writes in the book. Formulas identify the redundancies, or the noise, of daily life that falls just under our perceptions, then simplifies them. Steganography exploits those redundancies, most often without us ever noticing.

With thinking like that, it's easy to imagine that Wayner sees patterns everywhere. Does he look for hidden messages in the world around him? He shrugs it off.

"In most cases, the best algorithms are good enough to be virtually unbreakable. So I don't try to look for messages encoded with the best algorithms," he e-mails. "But vanity license plates can be a real game."

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