Are chip makers building electronic trapdoors in key military hardware? The Pentagon is making its biggest effort yet to find out:

So what's the best way to kill a chip? No one agrees on the most likely scenario, and in fact, there seem to be as many potential avenues of attack as there are people working on the problem. But the threats most often mentioned fall into two categories: a kill switch or a backdoor.  A kill switch is any manipulation of the chip's software or hardware that would cause the chip to die outright�to shut off an F-35's missile-launching electronics, for example. A backdoor, by contrast, lets outsiders gain access to the system through code or hardware to disable or enable a specific function. Because this method works without shutting down the whole chip, users remain unaware of the intrusion. An enemy could use it to bypass battlefield radio encryption, for instance.  Depending on the adversary's degree of sophistication, a kill switch might be controlled to go off at a set time, under certain circumstances, or at random.

A kill switch built to be triggered at will, as was allegedly incorporated into the European microprocessors, would be more difficult and expensive to pull off, but it's also the more likely threat, says David Adler, a consulting professor of electrical engineering at Stanford, who was previously funded by DARPA to develop chip-testing hardware in an unrelated project.  To create a controlled kill switch, you'd need to add extra logic to a microprocessor, which you could do either during manufacturing or during the chip's design phase. A saboteur could substitute one of the masks used to imprint the pattern of wires and transistors onto the semiconductor wafer, Adler suggests, so that the pattern for just one microchip is different from the rest. "You're printing pictures from a negative," he says. "If you change the mask, you can add extra transistors."

Or the extra circuits could be added to the design itself. Chip circuitry these days tends to be created in software modules, which can come from anywhere, notes Dean Collins, deputy director of DARPA's Microsystems Technology Office and program manager for the Trust in IC initiative. Programmers "browse many sources on the Internet for a component," he says. "They'll find a good one made by somebody in Romania, and they'll put that in their design." Up to two dozen different software tools may be used to design the chip, and the origin of that software is not always clear, he adds. "That creates two dozen entry points for malicious code."