| Thursday April 24th 2014

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HOWTO: Kill wiretaps when making a phone call


CALEA is the terrible US federal law that requires that all switches that carry voice-traffic be built with an easy-to-access remote wiretapping capability so that cops (or bad guys who know cop secrets) can listen in on your voice conversations without cooperation from the phone company. A team of University of Pennsylvania researchers (already notorious for finding flaws in the previous version of the CALEA standard that let callers lock out wiretaps) have found a solid theoretical attack against the newer, shinier CALEA standard.

“We asked ourselves the question of whether this standard is sufficient to have reliable wiretapping,” said Micah Sherr, a post-doctoral researcher at the university and one of the paper’s co-authors. Eventually they were able to develop some proof-of-concept attacks that would disrupt devices. phone fingerprintAccording to Sherr, the standard “really didn’t consider the case of a wiretap subject who is trying to thwart or confuse the wiretap itself.”It turns out that the standard sets aside very little bandwidth — 64K bits per second — for keeping track of information about phone calls being made on the tapped line. When a wire tap is on, the switch is supposed to set up a 64Kbps Call Data Channel to send this information between the telco and the law enforcement agency doing the wiretap. Normally this channel has more than enough bandwidth for the whole system to work, but if someone tries to flood it with information by making dozens of SMS messages or VoIP (voice over Internet protocol) phone calls simultaneously, the channel could be overwhelmed and simply drop network traffic.

That means that law enforcement could lose records of who was called and when, and possibly miss entire call recordings as well, Sherr said.


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