GSM crack inexpensive, says researcher

Intercepting mobile phone calls, and decrypting the data, can now be achieved using inexpensive equipment, according to security researcher Karsten Nohl.

In a presentation at the Chaos Computer Congress (27C3) in Berlin on 28 December 2010, Nohl and fellow researcher Sylvain Munaut demonstrated how calls can be intercepted and decrypted using a Universal Software Radio Peripheral (USRP) and inexpensive Motorola phones.

Nohl said last year that the encryption algorithm behind most GSM calls, a variant of A5/1, had been cracked. Nohl said on Tuesday that he had demonstrated a practical attack on GSM communications.

"The capability to record GSM data and the capability to crack A5/1 have been publicly combined into a practical attack on cheap equipment," Nohl told ZDNet UK.

The attack works by using tools to capture, decrypt, and analyse GSM calls and SMS texts, according to slides of Nohl's presentation at 27C3 (PDF). Nohl used Motorola phones running OsmocommBB, open source GSM software, that had been modified to remove uplink filters. The phones were used in conjunction with a USRP to record calls and texts, which were cracked using a code book.

Nohl told ZDNet UK that the GSM Association (GSMA), which represents operators and manufacturers, would be unlikely to push for members to use the updated A5/3 algorithm as a basis for GSM.

"Despite this readily available attack potential, A5/1 will not be replaced by A5/3 due to the cost of an upgrade," said Nohl.

GSMA was unable to comment at the time of writing.