Cracking the GSM A5/1 Encryption Via Distributed Computing

Via CNET -

If you are using a GSM phone (AT&T or T-Mobile in the U.S.), you likely have a few more months before it will be easy for practically anyone to spy on your communications.

Security researcher Karsten Nohl is launching an open-source, distributed computing project designed to crack the encryption used on GSM phones and compile it into a code book that can be used to decode conversations and any data that gets sent to and from the phone.

He hopes that by doing this it will spur cellular providers into improving the security of their services and fix a weakness that has been around for 15 years and affects about 3 billion mobile users.

"We're not creating a vulnerability but publicizing a flaw that's already being exploited very widely," he said in a phone interview Monday.

"Clearly we are making the attack more practical and much cheaper, and of course there's a moral question of whether we should do that," he said. "But more importantly, we are informing (people) about a longstanding vulnerability and hopefully preventing more systems from adopting this."

This weakness in the encryption used on the phones, A5/1, has been known about for years. There are at least four commercial tools that allow for decrypting GSM communications that range in price from $100,000 to $250,000 depending on how fast you want the software to work, said Nohl, who previously has publicized weaknesses with wireless smart card chips used in transit systems.

It will take 80 high-performance computers about three months to do a brute force attack on A5/1 and create a large look-up table that will serve as the code book, said Nohl, who announced the project at the Hacking at Random conference in the Netherlands 10 days ago.

Using the code book, anyone could get the encryption key for any GSM call, SMS message, or other communication encrypted with A5/1 and listen to the call or read the data in the clear. If 160 people donate their computing resources to the project, it should only take one and a half months to complete, he said.

Participants download the software and three months later they share the files created with others, via BitTorrent, for instance, Nohl said. "We have no connection to them," he added.

Once the look-up table is created it would be available for anyone to use.

Distributed computing, which has long been used for research and academic purposes, like SETI@home, and which companies have built businesses around, not only solves the technical hurdle to cracking the A5/1 code, but it could solve the legal ones too.

A few years ago a similar GSM cracking project was embarked upon but was halted before it was completed after researchers were intimidated, possibly by a cellular provider, Nohl said. By distributing the effort among participants and not having it centralized, the new effort will be less vulnerable to outside interference, he said.

Nohl wasn't certain of the legal ramifications of the project but said it's likely that using such a look-up table is illegal but possession is legal because of the companies that openly advertise their tables for sale.

A T-Mobile spokeswoman said the company had no comment on the matter.

AT&T spokesman Mark Siegel said, "We take extraordinary care to protect the privacy of our customers and use a variety of tools, many technical and some human approaches. I can't go into the details for security reasons." He declined to elaborate or comment further.