How to Foil a Wiretap

The technology law enforcement agents have used for decades to wiretap telephones has a flaw that allows the person being wiretapped to stop the recorder remotely, The New York Times reports today.

It’s also possible to falsify the numbers dialed, experts say.

To defeat wiretapping systems, the target need only send the same “idle signal” that the tapping equipment sends to the recorder when the telephone is not in use. The target could continue to have a conversation while sending the forged signal.

The tone, also known as a C-tone, sounds like a low buzzing and is “slightly annoying but would not affect the voice quality” of the call, says Matt Blaze, an associate professor of computer and information science at the University of Pennsylvania. “It turns the recorder right off,” Blaze says.

The paper can be found at www.crypto.com/papers/wiretapping.

–David Rosenbaum