July 21, 2010, 10:45 PM — The (Global System for Mobile Communications) technology used by the majority of the world's mobile phones will get some scrutiny at next week's Black Hat security conference, and what the security researchers there have to say isn't pretty.
On Friday, an open source effort to develop GSM-cracking software released software that cracks the A5/1 encryption algorithm used by some GSM networks. Called Kraken, this software uses new, very efficient, encryption cracking tables that allow it to break A5/1 encryption much faster than before.
The software is key step toward eavesdropping on mobile phone conversations over GSM networks. Since GSM networks are the backbone of 3G, they also provide attackers with an avenue into the new generation of handsets.
In December, the group released a set of encryption tables designed to speed up the arduous process of breaking A5/1 encryption, but the software component was incomplete. Now the software is done, and the tables are much more efficient than they were seven months ago. "The speed of how fast you could crack a call is probably orders of magnitude better than anything previously," said Frank Stevenson, a developer with the A5/1 Security Project. "We know we can do it in minutes; the question is, can we do it in seconds?"
As the software becomes more polished it will make GSM call eavesdropping practical. "Our attack is so easy to carry out, and the cost of attack is lowered so significantly, that there is now a real danger of widespread intercepting of calls," Stevenson said.
Stevenson and his co-developers haven't put together all the components someone would need to listen in on a call -- that would be illegal in some countries. Someone must still develop the radio listening equipment needed to gain access to the GSM signal, but that type of technology is within reach. Stevenson believes that this could be done using an inexpensive mobile phone and a modified version of open-source software called OsmocomBB. Hackers could also use a more-expensive Universal Software Radio Peripheral (USRP) device in conjunction with another program, called Airprobe.
A5/1 Security Project leader Karsten Nohl will discuss the hardware and software setup for his project's GSM cracking tools at next week's conference.
Last year there were about 3.5 billion GSM phones in use, according to data from the GSM Association. Not all of these phones are on networks that use A5/1 encryption -- some use the more-secure A5/3 algorithm; others use no encryption -- but a sizeable percentage are.
In the U.S., both AT&T and T-Mobile operate GSM networks.
The trade group that represents GSM network operators and equipment manufacturers, the GSM Association, has said in the past that A5/1 cracking efforts such as this are interesting, but attacks are extremely difficult to pull off in the real world. Intercepting mobile phone calls is illegal in many countries, including the U.S. The GSM Association did not respond to messages seeking comment for this story.
Project developers say the point of their work is to show how easy it really would be to crack A/51 -- something they say that grey market commercial products are already doing. According to Stevenson many of these security problems are solved in next-generation mobile network technologies such as 3G and LTE (Long Term Evolution).
However, even 3G phones can be compromised because they can roll back to GSM mode when a 3G network is not available. "You can choose to operate in 3G mode only, but then you will have very limited coverage," Stevenson said. "GSM has become the Achilles Heel of 3G security."
Meanwhile, another Black Hat presenter, Chris Paget plans to demonstrate a completely different way to intercept GSM calls. He's setting up a fake cellular tower that masquerades as a legitimate GSM network.
According to Paget, using open-source tools and a US$1,500 USRP radio, he can assemble his fake tower, called an IMSI (International Mobile Subscriber Identity) catcher. In a controlled experiment, he's going to set one up at Black Hat and invite audience members to connect their mobile phones. Once a phone has connected, Paget's tower tells it to drop encryption, giving him a way of listening in on calls.
"I think there's been too much focus on the cryptographic weaknesses in GSM," he said. "People need to recognize that the cryptographic weaknesses are not the worst weaknesses in GSM. "
