Since Western Europe
can't export encryption products to certain countries, such as
targets of United Nations sanctions, the default version of the GSM
protocol does not use encryption. This in itself isn't necessarily a
problem, said David Wagner, a professor of computer science at the
University of California-Berkeley, but GSM also does not
authenticate its base stations, the hardware that communicates with
the handsets -- and that is potentially troublesome.
Experts said it is possible to build a phony base station that
jams the signal from the real base station and forces the cell phone
to connect to it. The base station then tells the cell phone, in
essence, "You're in Iraq, don't use encryption," and the call
proceeds unprotected with the false base station relaying
information between the real base station and the handset.
A handful of researchers have been aware of the loophole for
several years now, but it's been "a well-kept secret," Wagner said.
Security experts call this a "man-in-the-middle" attack because
the phony base station sits between the handset and the real base
station, intercepting their communications, but neither the real
base station nor the handset knows it's there.
"We know about it as a technical issue, but we haven't seen it
demonstrated," said James Moran, fraud and security director at the
GSM Association. He added that building an interception device would
require considerable technical skill. Moran said the next GSM
standard would address the problem.
Other cell phone standards probably don't authenticate base
stations either, Wagner said, perhaps because their designers were
more concerned with preventing handset cloning, which allows someone
to bill his or her calls to someone else's number. But the
phony-base-station trick is a particular problem for GSM because
different strengths of encryption are used in different places.
"Whenever you have to support both weak and strong cryptography,
one very real risk is that you end up with 'least common
denominator' security," Wagner said.
Cracking different pieces of the cryptography that protects GSM
cell phones from eavesdropping has long been a favorite pastime for
computer security researchers. Just last December, two Israeli
researchers announced that they had found a fast method of cracking
the A5/1 algorithm, the strong encryption used to protect GSM phone
calls in Europe and the United States. But the phony-base-station
strategy obviates the need for any encryption busting.
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