Eavesdropping on GSM Calls
It’s easy and cheap:
Speaking at the Chaos Computer Club (CCC) Congress in Berlin on Tuesday, a pair of researchers demonstrated a start-to-finish means of eavesdropping on encrypted GSM cellphone calls and text messages, using only four sub-$15 telephones as network “sniffers,” a laptop computer, and a variety of open source software.
The encryption is lousy:
Several of the individual pieces of this GSM hack have been displayed before. The ability to decrypt GSM’s 64-bit A5/1 encryption was demonstrated last year at this same event, for instance. However, network operators then responded that the difficulty of finding a specific phone, and of picking the correct encrypted radio signal out of the air, made the theoretical decryption danger minimal at best.
As part of this background communication, GSM networks send out strings of identifying information, as well as essentially empty “Are you there?” messages. Empty space in these messages is filled with buffer bytes. Although a new GSM standard was put in place several years ago to turn these buffers into random bytes, they in fact remain largely identical today, under a much older standard.
This allows the researchers to predict with a high degree of probability the plain-text content of these encrypted system messages. This, combined with a two-terabyte table of precomputed encryption keys (a so-called rainbow table), allows a cracking program to discover the secret key to the session’s encryption in about 20 seconds.
Did you notice that? A two-terabyte rainbow table. A few years ago, that kind of storage was largely theoretical. Now it’s both cheap and portable.
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