Cell phone eavesdropping enters script-kiddie phase

Get your GSM snooping tools here

Black Hat Independent researchers have made good on a promise to release a comprehensive set of tools needed to eavesdrop on cell phone calls that use the world's most widely deployed mobile technology.

“The whole topic of GSM hacking now enters the script-kiddie stage, similar to Wi-Fi hacking a couple years ago, where people started cracking the neighbor's Wi-Fi,” said Karsten Nohl, a cryptographer with the Security Research Labs in Berlin who helped spearhead the project. “Just as with Wi-Fi, where they changed the encryption to WPA, hopefully that will happen with GSM, too.”

The suite of applications now includes Kraken, software being released at the Black Hat security conference on Thursday that can deduce the secret key encrypting SMS messages and voice conversations in as little as 30 seconds. It was developed by Frank A. Stevenson, the same Norwegian programmer who almost a decade ago developed software that cracked the CSS encryption scheme protecting DVDs.

It has been designed to work seamlessly with 1.7TB worth of rainbow tables that are used to crack A5/1, a decades-old encryption algorithm used to protect cell phone communications using GSM, which is used by about 80 percent of the world's mobile operators. A small confederation of researchers announced last year they were setting out to create the voluminous index, which exploits known weaknesses in the encryption formula.

Distributing the rainbow tables has proved to be a challenge to the project participants. Stevenson said people in Oslo, where he's located, are welcome to exchange a blank hard disk for one that contains the data. Eventually, the group expects to make the tables available as a BitTorrent.

The GSM Alliance, which represents almost 800 operators in 219 countries, pooh poohed the universal snooping plan by characterizing the attack as theoretical and saying encryption wasn't the only protection preventing eavesdropping on real-time communications.

That's where another tool, called AirProbe, comes in. An updated version of the program, also to be distributed Thursday, works with USRP radios to record digital signals as they pass from an operator's base station to a GSM handset. Combined with refinements in the open-source GNU radio, it works by pulling down voluminous amounts of data in real time as it travels to the targeted cell phone and storing only those packets that are needed to snoop on a call.

GSM insecurity is largely the result of widely known weaknesses in A5/1, the algorithm used to decrypt calls in most of the developed world. Years ago, mobile operators devised A5/3, which requires some quintillion more mathematical operations to be cracked. It has yet to be adopted as mobile operators fret that the change will be expensive and won't work on older handsets. Many countries continue to use A5/0, which uses no meaningful encryption at all.

The eavesdropping initiative is just one of the security pitfalls to hit the GSM standard. On Wednesday, a researcher calling himself “The Grugq” described several attacks that can be launched with standard handsets to disrupt network communications.

One, called RACHell, can take down nearby cellular towers by sending out a torrent of so-called RAC requests. The result is handsets in the immediate vicinity will be unable to send or receive messages. A separate attack he referred to as an IMSI detach can be used to prevent a given cell phone from receiving SMS messages and incoming calls. All that's needed is the target's phone number. ®

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