Technology

Check out this video from last August’s CCC Camp, which describes using a Universal Software Radio Perhiperal (USRP) to record GSM messages, and then using an FPGA to defeat the A5/1 encryption that’s used to secure an encrypted GSM channel in the span of a couple weeks. By spending a couple months to precompute a 5 TB lookup table you could bring the decryption process down to just a few minutes.

First half of the talk is an introduction into GSM interception. Second half presents a new method for cracking the GSM encryption A5/1. This is a new attack that can crack any encrypted channel (SMS, Voice) within 3-5 minutes regardless of how long the conversation is (e.g. can crack a telephone conversation that only lasts 4 seconds).

Now, most of us won’t be running out right now to grab an FPGA and a software radio so we can start cracking GSM voice converstations and SMS messages, but the actual discussion of how GSM works and how the team went about putting together a real-time cracking method for A5/1 is fascinating. What’s really crazy is that for a few thousand dollars, anyone could really set up a GSM recording and cracking system. This isn’t just NSA or government-funded spy stuff.

At about the 19 minute mark, Steve talks a little about how mobile identification and position information is transmitted. If you’ve ever called the phone company to track down a stolen phone, you’ve probably been told this isn’t possible. Turns out that if you’ve had a phone lost or stolen, it actually transmits its position information _all_the_time_. So, technically, your network operator should be able to tell you the phone’s location to within 200 meters.

The A5 Cracking Project – [via] Link
GNU Radio – Link

6 thoughts on “Decrypting GSM

  1. “Turns out that if you’ve had a phone lost or stolen, it actually transmits its position information _all_the_time_. So, technically, your network operator should be able to tell you the phone’s location to within 200 meters.”

    This is not universally true. A company called TruePosition has a whole business around locating cell phones, and it doesn’t involve the handset at all. In fact it involves devices in the cell tower called location measurement units, and they don’t transmit the position information “_all_the_time_” but rather on demand from the operator. Different operators have different technologies deployed so weather they transmit the position information “_all_the_time_” depends on the technology in use by the operator, it is not the GSM protocol that provides this feature. It has nothing to do with GSM.

  2. I talked to h1kari at ToorCon and they’ve apparently run into a technical limitation. They can make a 6TB rainbow table, but the constant lookups are too much for the drives making the process too slow. It looks like now the only option is a 6TB ram disk, if that even is an option.

  3. thanks, spammb, for the clarification.

    eliot – yikes. 6TB of ram definitely pushes things out of the affordable territory. it sounded like they had looked at a few different possibilities for the lookup table and there’s supposed to be more information published in november. i’d guess this is a pretty active area of their research and optimization efforts. it’ll be cool to see what they come up with.

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