Double whammy for GSM Security

Via PC World:

A researcher at the Def Con security conference in Las Vegas demonstrated that he could impersonate a GSM cell tower and intercept mobile phone calls using only $1500 worth of equipment. The cost-effective solution brings mobile phone snooping to the masses, and raises some concerns for mobile phone security.

How does the GSM snooping work?

Chris Paget was able to patch together an IMSI (International Mobile Identity Subscriber) catcher device for about $1500. The IMSI catcher can be configured to impersonate a tower from a specific carrier. To GSM-based cell phones in the immediate area--the spoofed cell tower appears to be the strongest signal, so the devices connect to it, enabling the fake tower to intercept outbound calls from the cell phone.

What happens to the calls?

Calls are intercepted, but can be routed to the intended recipient so the attacker can listen in on, and/or record the conversation. To the real carrier, the cell phone appears to no longer be connected to the network, so inbound calls go directly to voicemail. Paget did clarify, though, that it's possible for an attacker to impersonate the intercepted device to the wireless network, enabling inbound calls to be intercepted as well.

But, aren't my calls encrypted?

Generally speaking, yes. However, the hacked IMSI catcher can simply turn the encryption off. According to Paget, the GSM standard specifies that users should be warned when encryption is disabled, but that is not the case for most cell phones. Paget explained "Even though the GSM spec requires it, this is a deliberate choice on the cell phone makers."

What wireless provider networks are affected?

Good news for Sprint and Verizon customers--those networks use CDMA technology rather than GSM, so cell phones on the Sprint or Verizon networks would not connect to a spoofed GSM tower. However, AT&T and T-Mobile--as well as most major carriers outside of the United States--rely on GSM.

Does 3G protect me from this hack?

This IMSI catcher hack will not work on 3G, but Paget explained that the 3G network could be knocked offline with a noise generator and an amplifier--equipment that Paget acquired for less than $1000. With the 3G network out of the way, most cell phones will revert to 2G to find a viable signal to connect to.

Another one from CNET:

A researcher released software at the Black Hat conference on Thursday designed to let people test whether their calls on mobile phones can be eavesdropped on.

The public availability of the software - dubbed Airprobe -- means that anyone with the right hardware can snoop on other peoples' calls unless the target telecom provider has deployed a patch that was standardized about two years ago by the GSMA, the trade association representing GSM (Global System for Mobile Communications) providers, including AT&T and T-Mobile in the U.S.

Most telecom providers have not patched their systems, said cryptography expert Karsten Nohl.

"This talk will be a reminder to this industry to please implement these security measures because now customers can test whether they've patched the system or not," he told CNET in an interview shortly before his presentation. "Now you can listen in on a strangers' phone calls with very little effort."

An earlier incarnation of Airprobe was incomplete so Nohl and others worked to make it usable, he said.

Airprobe offers the ability to record and decode GSM calls. When combined with a set of cryptographic tools called Kraken, which were released last week, "even encrypted calls and text messages can be decoded," he said.

To test phones for interception capability you need: the Airprobe software and a computer; a programmable radio for the computer, which costs about $1,000; access to cryptographic rainbow tables that provide the codes for cracking GSM crypto (another Nohl project); and the Kraken tool for cracking the A5/1 crypto used in GSM, Nohl said.

More information about the tool and the privacy issues is on the Security Research Labs Web site.