Researcher Karsten Nohl, a former graduate student at the University of Virginia, revealed his decryption methods this week at the Chaos Communication Conference in Berlin, the largest hackers conference in Europe. Nohl and a team of two dozen other experts worked for five months to crack the security algorithm that protects Global System for Mobile communications.
GSM is the world's most widely used phone technology, accounting for more than 4 billion mobile phones. To prevent eavesdropping, the technology uses an encryption algorithm called A5/1 developed by the GSM Association.
To break the code, Nohl and the other researchers used networks of computers to crunch through the trillions of mathematical possibilities. The result was the development of a code book comprising 2 TB of data that's compiled into cracking tables. The tables can be used as a kind of reverse phone book to determine the encryption key used to secure a GSM mobile phone conversation or text message.
Before the latest hack, hundreds of thousands of dollars of computer equipment was needed to break the GSM code, mostly limiting hacking to government agencies. Nohl told the conference that someone with the code book could eavesdrop on GSM communications using about $30,000 worth of computer gear, making such illegal activity possible by many more criminal organizations.
On Tuesday, a GSMA statement sent to InformtationWeek by e-mail said Nohl's work "isn't something that we take lightly at all." Nevertheless, the organization said, the hack did not present an immediate danger to GSM security.
"All in all, we consider this research, which appears to be motivated in part by commercial considerations, to be a long way from being a practical attack on GSM," the organization said.
The GSMA said in a statement that over the last few years, a number of academic papers have explained, in theory, how the A5/1 algorithm, which is more than 20 years old, could be compromised. "However, none to date have led to a practical attack capability being developed against A5/1 that can be used on live, commercial GSM networks," the group said.
One area not covered by Nohl's work, according to the GSMA, is how the GSM call would be identified and recorded from the radio interface. To do that, a hacker would need a radio receiver system and the signal processing software necessary to process the raw radio data.
"So far, this aspect of the methodology has not been explained in any detail and we strongly suspect that the teams attempting to develop an intercept capability have underestimated its practical complexity," the GSMA said.
Nevertheless, the group recognizes that A5/1 needs to be replaced, and is in the process of phasing in a new security algorithm called A5/3