An Improved Clock-skew Measurement Technique for Revealing Hidden Services

The Tor anonymisation network allows services, such as web servers, to be operated under a pseudonym. In previous work Murdoch described a novel attack to reveal such hidden services by correlating clock skew changes with times of increased load, and hence temperature. Clock skew measurement suffers from two main sources of noise: network jitter and timestamp quantisation error. Depending on the target's clock frequency the quantisation noise can be orders of magnitude larger than the noise caused by typical network jitter. Quantisation noise limits the previous attacks to situations where a high frequency clock is available. It has been hypothesised that by synchronising measurements to the clock ticks, quantisation noise can be reduced. We show how such synchronisation can be achieved and maintained, despite network jitter. Our experiments show that synchronised sampling significantly reduces the quantisation error and the remaining noise only depends on the network jitter (but...

• Type: Conference
• Year: 2008
• Where: USS
• Authors: Sebastian Zander, Steven J. Murdoch
• Keywords: Clock, Clock Skew, Network Jitter, Operating System, USS 2008