Monthly Archives: June 2008

Security psychology

I’m currently in the first Workshop on security and human behaviour; at MIT, which brings together security engineers, psychologists and others interested in topics ranging from deception through usability to fearmongering. Here’s the agenda and here are the workshop papers.

The first session, on deception, was fascinating. It emphasised the huge range of problems, from detecting deception in interpersonal contexts such as interrogation through the effects of context and misdirection to how we might provide better trust signals to computer users.

Over the past seven years, security economics has gone from nothing to a thriving research field with over 100 active researchers. Over the next seven I believe that security psychology should do at least as well. I hope I’ll find enough odd minutes to live blog this first workshop as it happens!

[Edited to add:] See comments for live blog posts on the sessions; Bruce Schneier is also blogging this event.

An improved clock-skew measurement technique for revealing hidden services

In 2006 I published a paper on remotely estimating a computer’s temperature, based on clock skew. I showed that by inducing load on a Tor hidden service, an attacker could cause measurable changes in clock skew and so allow the computer hosting the service to be re-identified. However, it takes a very long time (hours to days) to obtain a sufficiently accurate clock-skew estimate, even taking a sample every few seconds. If measurements are less granular than the 1 kHz TCP timestamp clock source I used, then it would take longer still.

This limits the attack since in many cases TCP timestamps may be unavailable. In particular, Tor hidden services operate at the TCP layer, stripping all TCP and IP headers. If an attacker wants to estimate clock skew over the hidden service channel, the only directly available clock source may be the 1 Hz HTTP timestamp. The quantization noise in this case is three orders of magnitude above the TCP timestamp case, making the approach I used in the paper effectively infeasible.

While visiting Cambridge in summer 2007, Sebastian Zander developed an improved clock skew measurement technique which would dramatically reduce the noise of clock-skew measurements from low-frequency clocks. The basic idea, shown below, is to only request timestamps very close to a clock transition, where the quantization noise is lowest. This requires the attacker to firstly lock-on to the phase of the clock, then keep tracking it even when measurements are distorted by network jitter.

Synchronized vs random sampling

Sebastian and I wrote a paper — An Improved Clock-skew Measurement Technique for Revealing Hidden Services — describing this technique, and showing results from testing it on a Tor hidden service installed on PlanetLab. The measurements show a large improvement over the original paper, with two orders of magnitude lower noise for low-frequency clocks (like the HTTP case). This approach will allow previous attacks to be executed faster, and make previously infeasible attacks possible.

The paper will be presented at the USENIX Security Symposium, San Jose, CA, US, 28 July – 1 August 2008.

Operational security failure

A shocking article appeared yesterday on the BMJ website. It recounts how auditors called 45 GP surgeries asking for personal information about 51 patients. In only one case were they asked to verify their identity; the attack succeeded against the other 50 patients.

This is an old problem. In 1996, when I was advising the BMA on clinical system safety and privacy, we trained the staff at one health authority to detect false-pretext phone calls, and they found 30 a week. We reported this to the Department of Health, hoping they’d introduce some operational security measures nationwide; instead the Department got furious at us for treading on their turf and ordered the HA to stop cooperating (the story’s told in my book). More recently I confronted the NHS chief executive, David Nicholson, and patient tsar Harry Cayton, with the issue at a conference early last year; they claimed there wasn’t a problem nowadays now that people have all these computers.

What will it take to get the Department of Health to care about patient privacy? Lack of confidentiality already costs lives, albeit indirectly. Will it require a really high-profile fatality?

Slow removal of child sexual abuse image websites

On Friday last week The Guardian ran a story on an upcoming research paper by Tyler Moore and myself which will be presented at the WEIS conference later this month. We had determined that child sexual abuse image websites were removed from the Internet far slower than any other category of content we looked at, excepting illegal pharmacies hosted on fast-flux networks; and we’re unsure if anyone is seriously trying to remove them at all!
Continue reading Slow removal of child sexual abuse image websites

"Covert channel vulnerabilities in anonymity systems" wins best thesis award

My PhD thesis “Covert channel vulnerabilities in anonymity systems” has been awarded this year’s best thesis prize by the ERCIM security and trust management working group. The announcement can be found on the working group homepage and I’ve been invited to give a talk at their upcoming workshop, STM 08, Trondheim, Norway, 16–17 June 2008.

Update 2007-07-07: ERCIM have also published a press release.