Mobile operators have struck back at organizers of an open-source project that plans to crack the encryption used to protect cell phone calls, saying they are a long way from devising a practical attack.
"The theoretical compromise presented at the Black Hat conference requires the construction of a large look-up table of approximately 2 Terabytes - this is equivalent to the amount of data contained in a 20 kilometre high pile of books," the group, which represents almost 800 operators in 219 countries, said in a statement issued Friday. "In theory, someone with access to the data in such a table could use it to analyse an encrypted call and recover the encryption key."
The GSMA went on to say that even if such a table were built, the researchers still would need to build a complex radio receiver to process the raw radio data.
The vast majority of world's cell phone calls are protected by an algorithm known as A5/1 that has been in existence for more than a decade, said project leader Karsten Nohl, a cryptography expert and a researcher at the University of Virginia. Because it hails from the cold-war era when export laws prohibited the exportation of strong cryptography, the cipher is relatively trivial to break using a large number of networked computers.
More recently, cell phone makers have folded a newer cipher known as A5/3 into handsets to protect internet communications. Because its key is twice as long as A5/1, it's about a quintillion times harder to break, Nohl estimates. But despite the uncontested superiority of the newer algorithm, handset manufacturers still cling to the older one to protect voice calls.
"This is cold war stuff," Nohl told The Register in explaining why he spearheaded the project, which plans to use a distributed, peer-to-peer computing system to create the table needed to crack A5/1. "I don't see why I should be using something that was intentionally weakened during the cold war."
Indeed, in many respects, the GSMA's statement reads like a relic from the 1960s, when it was still fashionable to believe that arcane and proprietary technologies were sufficient to thwart determined hackers from breaking into protected systems.
"The complex knowledge required to develop such [signal-processing] software is subject to intellectual property rights, making it difficult to turn into a commercial product," it states.
The statement said the GSMA is phasing in A5/3, but provided no timetable or other details.
The GSM rainbow table project was announced at the recent Black Hat security conference in Las Vegas.
Nohl has already proved to be adept at defeating such security-through-obscurity defenses. In late 2007, he unveiled a practical attack on the world's most popular smartcard. For decades, Netherlands-based NXP Semiconductors relied on a proprietary algorithm to protect its widely used Mifare Classic wireless card.
"I'm puzzled by the GSMA's attempt to hide behind the alleged inability of hackers to snoop GSM traffic," Nohl wrote in an email to reporters. "This is 20 years old technology that ships in billions of handsets. The GSMA should take the hacker community and its current interest in GSM technology more serious." ®