Researchers at the University of Pennsylvania say they've discovered a way to circumvent the networking technology used by law enforcement to tap phone lines in the U.S.

The flaws they've found "represent a serious threat to the accuracy and completeness of wiretap records used for both criminal investigation and as evidence in trial," the researchers say in their paper, set to be presented Thursday at a computer security conference in Chicago.

Following up on earlier work on evading analog wiretap devices called loop extenders, the Penn researchers took a deep look at the newer technical standards used to enable wiretapping on telecommunication switches. They found that while these newer devices probably don't suffer from many of the bugs they'd found in the loop extender world, they do introduce new flaws. In fact, wiretaps could probably be rendered useless if the connection between the switches and law enforcement are overwhelmed with useless data, something known as a denial of service (DOS) attack.

Four years ago, the University of Pennsylvania team made headlines after hacking an analog loop extender device they'd bought on eBay. This time, the team wanted to look at newer devices, but they couldn't get a hold of a switch. So instead they took a close look at the telecommunication industry standard -- ANSI Standard J-STD-025 -- that defines how switches should transmit wiretapped information to authorities. This standard was developed in the 1990s to spell out how telecommunications companies could comply with the 1994 Communications Assistance for Law Enforcement Act (CALEA).

"We asked ourselves the question of whether this standard is sufficient to have reliable wiretapping," said Micah Sherr, a post-doctoral researcher at the university and one of the paper's co-authors. Eventually they were able to develop some proof-of-concept attacks that would disrupt devices. According to Sherr, the standard "really didn't consider the case of a wiretap subject who is trying to thwart or confuse the wiretap itself."

It turns out that the standard sets aside very little bandwidth -- 64K bits per second -- for keeping track of information about phone calls being made on the tapped line. When a wire tap is on, the switch is supposed to set up a 64Kbps Call Data Channel to send this information between the telco and the law enforcement agency doing the wiretap. Normally this channel has more than enough bandwidth for the whole system to work, but if someone tries to flood it with information by making dozens of SMS messages or VoIP (voice over Internet protocol) phone calls simultaneously, the channel could be overwhelmed and simply drop network traffic.

That means that law enforcement could lose records of who was called and when, and possibly miss entire call recordings as well, Sherr said.

Back in 2005, the FBI downplayed the Penn team's loop extender research, saying that it applied to only about 10 percent of wire taps. The J- standard studied in this paper is much more widely used, however, Sherr said. An FBI representative did not return messages seeking comment for this story.

The researchers wrote a program that connected to a server over Sprint's 3G wireless network 40 times per second, enough to flood the Call Data Channel. They say that they could get the same results by programming a computer to make seven VoIP calls per second or to fire off 42 SMS messages per second.

These techniques would work on mobile phones or VoIP systems, but not on analog devices, Sherr said.

Because the researchers weren't able to test their techniques on real-world systems they don't know for certain that they could thwart a wiretap. But Sherr believes that "there are definitely dangers" in the way the standard is written. "Because it's a black-box system, we don't know for sure."

Of course, criminals have plenty of easier ways to dodge police surveillance. They can use cash to buy prepaid mobile phones anonymously, or reach out to their accomplices with encrypted Skype calls, said Robert Graham, CEO with Errata Security. Luckily for the cops, criminals usually don't take their communications security that seriously. "Most criminals are stupid," he said. "They just use their same cell phone."