When a Boeing 757 struck the Pentagon on September 11, 2001, a blastproof film on its windows prevented them from shattering into a swarm of flying shards. Now, a version of that same film promises to block not only projectiles but also the collective electromagnetic chatter generated by our increasingly wireless society.

Once manufactured under an exclusive contract with the U.S. government, this recently declassified window film is now available to the public. But don't expect to see it on store shelves anytime soon. Currently, it's only available directly from the manufacturer, and at prices that will likely make it prohibitive for all but the wealthiest home owners.

The coating, which in its thinnest incarnation is only two one thousandths of an inch thick, can block Wi-Fi signals, cell phone transmissions, even the near-infrared, yet is almost transparent, making it no more intrusive than conventional window treatments. It can keep signals in (preventing attempts to spy on electronic communications) or out, minimizing radio interference and even the fabled electronics-destroying electromagnetic pulse (EMP) generated by a nuclear blast.

The film has already been plastered across the windows of more than 200 government buildings, including structures operated by the departments of Defense and the Treasury, as well as in the homes of high-level members of the current administration.

"We are limited by confidentiality agreements to say exactly which buildings [the window film] is on," says Kent Davies, president of CPFilms, Inc., in Martinsville, Va., which manufacturers the protective covering dubbed LLumar Signal Defense Security Film. "But immediately after 9/11 one of the senior military officials talked about a window film that seriously protected against the damage from the plane crash. You can put two and two together and assume it was also protecting against wireless signals."

But Is It for You?

Unlike the built-in security measures present in nearly all wireless routers, Signal Defense Film doesn't come cheap. CPFilms declined to give details of their pricing structure, in part because their technology is only sold as part of an all-inclusive package (of which the film is one component).

In addition, some experts are skeptical whether there is—or should be—a market for the film outside of the government and large corporations.

"If you're military, sure, it's useful. But if you're a normal person, it's kind of dumb," says Bruce Schneier, a consultant and authority in the fields of cryptography and computer security. "The way you secure wireless is [by securing your computer]—this is the wrong point to apply the solution." Without means of encryption and software-based security, he asserts, "you're out in the open anyway." Schneier himself is so confident in his ability to secure his computers that he runs a completely open wireless network in his home.

On the other hand, despite the widespread availability of wireless security features, many organizations and individuals have failed to secure their networks. In 2005, for instance, hackers broke into the wireless network of a Marshalls department store by using an antenna to remotely intercept its transmissions. Using this access as a starting point they managed to steal more than 200 million credit card numbers from a central database of customer information. The wireless network they breached was protected by Wireless Encryption Protocol (WEP), an older standard whose weak encryption can be cracked in under two minutes.

An internal audit revealed that the chain did not move quickly enough to convert its networks to the stronger Wireless Protected Access (WPA) standard. When the plot was finally uncovered, TJX, Marshalls's parent company, hired dozens of investigators and offered to pay for fraud monitoring for customers whose data was stolen.

Lisa Winckler, global director of research and technology at CPFilms, argues that the film has applications beyond simply blocking Wi-Fi signals, which are transmitted in the same unregulated 2.4-gigahertz band that is used by many cordless handsets. By shielding against signals across a wide swath of the electromagnetic spectrum, including the near-infrared, Signal Defense film can also thwart eavesdropping technology that depends on transmissions in the near-infrared, or terahertz range.

"With the advent of laser microphones," explains Winckler, "you can pick up voice data [through a window]. It's done every day in government."

Born of the Cold War

Many of these more exotic applications—including blocking the stray transmissions of keyboards, old-style televisions and computer monitors, and even LCD flat-panel displays—originated in the military's so-called TEMPEST program. TEMPEST was a code name for an initially classified effort to determine whether foreign governments could extract useful information from recordings of the stray electromagnetic noise generated by electronic devices. (In the first unclassified paper on the subject, published in 1985, Wim van Eck, then a researcher at Neher Laboratories in the Netherlands, demonstrated that it was possible to use a wireless antenna to reproduce the image on a television screen even when they were separated by a wall.)

"There was a huge movement in the government several years ago for TEMPEST protection," says Arthur Money, who was an assistant secretary of defense from 1999 to 2001 and has since become a consultant for defense contractors, including CPFilms. As a result, according to computer security expert Schneier, the government covered the windows of the National Security Agency's headquarters with a fine metallic mesh, and sandwiched metallic shielding between its wallboards.

In contrast to these early efforts, Signal Defense Film has the advantage of being inconspicuous. It is transparent to over 50 percent of the light that shines on it, "which is a lot," says Winckler, CPFilms's chief of research and development. "Sunglasses are always less than 5 percent transmission…. You could put this on the glass in an office building and most people would come in on Monday morning and not know it was there."

CPFilms's Davies sees applications for Signal Defense Film in everything from hospitals, which now have obligations under the federal Health Insurance Portability and Accountability Act (HIPAA) to keep patients' records private, to the financial sector, where interior conference rooms might benefit from being shielded from electronic eavesdropping.

"Retailers are trying to make their environments more appealing by letting in more light," says Davies, "but in doing so they're also allowing more information to flow through the glass."

As Networks Proliferate, So Does Noise

As the number of wireless networks and devices multiplies, even individuals who aren't worried about security stand to benefit from increased electromagnetic shielding.

"We have had conversations with buildings, especially high-rise buildings where there is a lot of signal going on at the upper floors of those buildings," Winckler says. When the ambient signal from transmission towers, other networks and even microwave ovens becomes overwhelming, shielding a building can improve the performance of any wireless networks within its walls.

Starting in 2009 even some cars will have their own wireless networks. "Over time, you can imagine," former defense official Money speculates, "if you had cars stacked up at a stoplight, the amount of interference is potentially horrific. This film will probably be in every car window just to keep interference down, let alone for privacy reasons."

Winkler also notes that in the unlikely event of a nuclear terrorist attack, Signal Defense Security Film would help safeguard its users' electronics from the potentially devastating effects of an electromagnetic pulse, or EMP. Although the sensitivity of such information prevents her from providing specifics on this feature of the film, she did note that it attenuates by 35 decibels electromagnetic signals in the range of 30 megahertz to 10 gigahertz, which translates into a greater than 99 percent decrease in signal strength.