Next month I will be presenting my paper “Hot or Not: Revealing Hidden Services by their Clock Skew” at the 13th ACM Conference on Computer and Communications Security (CCS) held in Alexandria, Virginia.

It is well known that quartz crystals, as used for controlling system clocks of computers, change speed when their temperature is altered. The paper shows how to use this effect to attack anonymity systems. One such attack is to observe timestamps from a PC connected to the Internet and watch how the frequency of the system clock changes.

Absolute clock skew has been previously used to tell whether two apparently different machines are in fact running on the same hardware. My paper adds that because the skew depends on temperature, in principle, a PC can be located by finding out when the day starts and how long it is, or just observing that the pattern is the same as a computer in a known location.

However, the paper is centered around hidden services. This is a feature of Tor which allows servers to be run without giving away the identity of the operator. These can be attacked by repeatedly connecting to the hidden service, causing its CPU load, hence temperature, to increase and so change the clockskew. Then the attacker requests timestamps from all candidate servers and finds the one demonstrating the expected clockskew pattern. I tested this with a private Tor network and it works surprisingly well.

In the graph below, the temperature (orange circles) is modulated by either exercising the hidden service or not. This in turn alters the measured clock skew (blue triangles). The induced load pattern is clear in the clock skew and an attacker could use this to de-anonymise a hidden service. More details can be found in the paper (PDF 1.5M).

I happened upon this effect in a lucky accident, while trying to improve upon the results of the paper “Remote physical device fingerprinting“. A previous paper of mine, “Embedding Covert Channels into TCP/IP” showed how to extract high-precision timestamps from the Linux TCP initial sequence number generator. When I tested this hypothesis it did indeed improve the accuracy of clock skew measurement, to the extent that I noticed an unusual peak at about the time cron caused the hard disk on my test machine to spin-up. Eventually I realised the potential for this effect and ran the necessary further experiments to write the paper.