Card Wars: The Phantom Menace

Banking security

Just like George Lucas can’t help but return to his old projects, I have been returning to mine. After three years of stagnation, I am pleased to announce the re-launch of phantomwithdrawals.com, freshly re-vamped, updated and turned into a wiki editable by the general public.

In fact, it’s not just great artists like Mr. Lucas and I starting up old projects, our honourable colleagues wearing the black hats have got the same idea. We have new victims reporting in, rumours abound of an auth system compromise at Citi, the Ombudsman is backlogged with months of disputed withdrawal cases, and some like Alain Job are even going to court.

One original contributor to the phantom case histories has just been hit by a second phantom withdrawal five years on and is chalking up another case in the files. While her new phantom is a bread-and-butter skim incident (a magstripe clone used in the far east), amongst this mass, true phantoms — the real mystery cases — are on the rise too. Two new victims with whom I have been corresponding very kindly offered to fund the hosting for the revamped site.

Let’s consider one of these mysteries. The McGaughey case has been reported in the media in Northern Ireland: dozens of withdrawals taking place over four weeks, totaling almost five thousand pounds, all within a ten mile radius of the McGaughey’s home. Summarised that way it looks like a classic first party fraud (couple short on cash withdraw money, then deny it later). But no-one in the family is short on cash, the McGaugheys look after their card details carefully, and have solid alibis at the time of many of the withdrawals, and the interlocking pattern of real and disputed withdrawals is such that any third party would have a hard time taking and returning the card (whether covertly or in collusion with the McGaugheys). No-one appears to have either the means or the motive.

Unusually the bank has been very cooperative, providing logs from their authorisation system (BASE24), including all of the cryptograms, input data and transaction parameters covering the affected transactions. Everything turns on the Application Transaction Counter (ATC), an on-card counter which increments with every transaction initiated. If an EMV chip can be fully cloned (secret keys and all), then it will have to submit an ATC value when transacting, and if used in parallel with the real card, it won’t be long before the same number pops up twice in the auth system, or large gaps in the sequence appear. The McGaughey’s ATC sequence appears to interlock perfectly: clearly the original card was used?

Of course logs can be misinterpreted (Badger) or even faked, auth systems may not work as expected, and customers may lie and cheat following all sorts of agendas; just around the corner the missing piece of the jigsaw may lie, which reveals the truth behind the case. And there is the totally separate matter of who should suffer the loss in the interim, whilst the truth remains unclear. Liability for disputed withdrawals is the most hotly contested issue of all.

phantomwithdrawals.com can’t do much more for the McGaugheys, but it can bear witness. Documenting the incidence of phantoms and the experiences of customers disputing them adds much needed transparency to the process, and helps researchers and experts seek out the really interesting cases.

Maybe we can lift the lid and discover the truth behind the “phantom menace” — everyone is united in that goal at least — but let’s also hope that Episode 2: Attack of the Clones has not yet started shooting!

PET Award 2008

Academic papers, Awards, Privacy technology

At last year’s Privacy Enhancing Technologies Symposium (PETS), I presented the paper “Sampled Traffic Analysis by Internet-Exchange-Level Adversaries”, co-authored with Piotr Zieliński. In it, we discussed the risk of traffic-analysis at Internet exchanges (IXes). We then showed that given even a small fraction of the data passing through an IX it was still possible to track a substantial proportion of anonymous communications. Our results are summarized in a previous blog post and full details are in the paper.

Our paper has now been announced as a runner-up for the Privacy Enhancing Technologies Award. The prize is presented annually, for research which makes an outstanding contribution to the field. Microsoft, the sponsor of the award, have further details and summaries of the papers in their press release.

Congratulations to the winners, Arvind Narayanan and Vitaly Shmatikov, for “Robust De-Anonymization of Large Sparse Datasets”; and the other runner-ups, Mira Belenkiy, Melissa Chase, C. Chris Erway, John Jannotti, Alptekin Küpçü, Anna Lysyanskaya and Erich Rachlin, for “Making P2P Accountable without Losing Privacy”.

Finland privacy judgment

Legal issues, News coverage, Politics, Privacy technology, Security engineering

In a case that will have profound implications, the European Court of Human Rights has issued a judgment against Finland in a medical privacy case.

The complainant was a nurse at a Finnish hospital, and also HIV-positive. Word of her condition spread among colleagues, and her contract was not renewed. The hospital’s access controls were not sufficient to prevent colleages accessing her record, and its audit trail was not sufficient to determine who had compromised her privacy. The court’s view was that health care staff who are not involved in the care of a patient must be unable to access that patient’s electronic medical record: “What is required in this connection is practical and effective protection to exclude any possibility of unauthorised access occurring in the first place.” (Press coverage here.)

A “practical and effective” protection test in European law will bind engineering, law and policy much more tightly together. And it will have wide consequences. Privacy compaigners, for example, can now argue strongly that the NHS Care Records service is illegal. And what will be the further consequences for the Transformational Government initiative – the “Database State”?

Metrics for security and performance in low-latency anonymity systems

Academic papers, Privacy technology, Security engineering

In Tor, and in other similar anonymity systems, clients choose a random sequence of computers (nodes) to route their connections through. The intention is that, unless someone is watching the whole network at the same time, the tracks of each user’s communication will become hidden amongst that of others. Exactly how a client chooses nodes varies between system to system, and is important for security.

If someone is simultaneously watching a user’s traffic as it enters and leaves the network, it is possible to de-anonymise the communication. As anyone can contribute nodes, this could occur if the first and last node for a connection is controlled by the same person. Tor takes some steps to avoid this possibility e.g. no two computers on the same /16 network may be chosen for each connection. However, someone with access to several networks could circumvent this measure.

Not only is route selection critical for security, but it’s also a significant performance factor. Tor nodes vary dramatically in their capacity, mainly due to their network connections. If all nodes were chosen with equal likelihood, the slower ones would cripple the network. This is why Tor weights the selection probability for a node proportional to its contribution to the network bandwidth.

Because of the dual importance of route selection, there are a number of proposals which offer an alternative to Tor’s bandwidth-weighted algorithm. Later this week at PETS I’ll be presenting my paper, co-authored with Robert N.M. Watson, “Metrics for security and performance in low-latency anonymity systems”. In this paper, we examine several route selection algorithms and evaluate their security and performance.

Intuitively, a route selection algorithm which weights all nodes equally appears the most secure because an attacker can’t make their node count any more than the others. This has been formalized by two measures: Gini coefficient and entropy. In fact the reality is more complex — uniform node selection resists attackers with lots of bandwidth, whereas bandwidth-weighting is better against attackers with lots of nodes (e.g. botnets).

Our paper explores the probability of path compromise of different route selection algorithms, when under attack by a range of different adversaries. We find that none of the proposals are optimal against all adversaries, and so summarizing effective security in terms of a single figure is not feasible. We also model the performance of the schemes and show that bandwidth-weighting offers both low latency and high resistance to attack by bandwidth-constrained adversaries.

Update (2008-07-25):
The slides (PDF 2.1M) for my presentation are now online.

Personal Internet Security: follow-up report

News coverage, Politics, Security economics

The House of Lords Science and Technology Committee have just completed a follow-up inquiry into “Personal Internet Security”, and their report is published here. Once again I have acted as their specialist adviser, and once again I’m under no obligation to endorse the Committee’s conclusions — but they have once again produced a useful report with sound conclusions, so I’m very happy to promote it!

Their initial report last summer, which I blogged about at the time, was — almost entirely — rejected by the Government last autumn (blog article here).

The Committee decided that in the light of the Government’s antipathy they would hold a rapid follow-up inquiry to establish whether their conclusions were sound or whether the Government was right to turn them down, and indeed, given the speed of change on the Internet, whether their recommendations were still timely.

The written responses broadly endorsed the Committee’s recommendations, with the main areas of controversy being liability for software vendors, making the banks statutorily responsible for phishing/skimming fraud, and how such fraud should be reported.

There was one oral session where, to everyone’s surprise, two Government ministers turned up and were extremely conciliatory. Baroness Vadera (BERR) said that the report “was somewhat more interesting than our response” and Vernon Coaker (Home Office) apologised to the Committee “if they felt that our response was overdefensive” adding “the report that was produced by this Committee a few months ago now has actually helped drive the agenda forward and certainly the resubmission of evidence and the re-thinking that that has caused has also helped with respect to that. So may I apologise to all of you; it is no disrespect to the Committee or to any of the members.”

I got the impression that the ministers were more impressed with the Committee’s report than were the civil servants who had drafted the Government’s previous formal response. Just maybe, some of my comments made a difference?

Given this volte face, the Committee’s follow-up report is also conciliatory, whilst recognising that the new approach is very much in the “jam tomorrow” category — we will all have to wait to see if they deliver.

The report is still in favour of software vendor liability as a long term strategy to improving software security, and on a security breach notification law the report says “we hold to our view that data security breach notification legislation would have the twin impacts of increasing incentives on businesses to avoid data loss, and should a breach occur, giving individuals timely information so that they can reduce the risk to themselves“. The headlines have been about the data lost by the Government, but recent figures from the ICO show that private industry is doing pretty badly as well.

The report also revisits the recommendations relating to banking, reiterating the committee’s view that “the liability of banks for losses incurred by electronic fraud should be underpinned by legislation rather than by the Banking Code“. The reasoning is simple, the banks choose the security mechanisms and how much effort they put into detecting patterns of fraud, so they should stand the losses if these systems fail. Holding individuals liable for succumbing to ever more sophisticated attacks is neither fair, nor economically efficient. The Committee also remained concerned that where fraud does take place, reports are made to the banks, who then choose whether or not to forward them to the police. They describe this approach as “wholly unsatisfactory and that it risks undermining public trust in the police and the Internet“.

This is quite a short report, a mere 36 paragraphs, but comes bundled with the responses received, all of which from Ross Anderson and Nicholas Bohm, through to the Metropolitan Police and Symantec are well worth reading to understand more about a complex problem, yet one where we’re beginning to see the first glimmers of consensus as to how best to move forward.

Security psychology

Politics, Privacy technology, Security economics, Security engineering

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

Academic papers, Hardware & signals, Privacy technology

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

Legal issues, Politics, Privacy technology

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

Academic papers, Banking security, Legal issues, News coverage, Security economics

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

Academic papers, Awards, Hardware & signals, Privacy technology

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.