Privacy-friendly Aggregation for the Smart-grid
Klaus Kursawe (Radboud Universiteit Nijmegen) and George Danezis and Markulf Kohlweiss (Microsoft Research)

Privacy in for smart electricity provision seems to be a rising topic, and this year there is a whole session on it at PETS 2011. The first paper (one which I am a coauthor) looks at the problem of gathering aggregate data from groups of smart meters, without allowing any third party to get the the individual measurements. This can be applied as a PET to solve real-world problems such as fraud detection, leakage detection, load estimates, demand response, weather prediction — all of which only require aggregate data (sometimes in real time), not individual measurements.

The key challenge to providing a private aggregation protocols are the specific constraints of smart meters. They are cheap devices, with modest resources, hardly any bandwidth, no ability to communicate, etc. Two specific protocols are presented: the first one allows to compare the sum of meter readings with a reference number (maybe measured from a feeder meter). This protocol allows for fancy proofs of correctness, but it slow in terms of computation and bandwidth (it requires public key operations for each reading). The second protocol is extremely fast and has no communication overhead. In both cases a pragmatic approach to the threat model is followed: we assume that the utilities will be honestly defining groups of meters and facilitating the key management protocol — for the second protocol there is no overhead of public key operations after the initial key setup.

The key highlight from this work is not as much its technical depth (tricks with DC networks and hash function that would not surprise any PETS regular). What is interesting is that the protocols were designed for a real industrial application and now fully integrated on real smart meters and their communication protocols in collaboration with our collaborators at Elster.

Plug-in privacy for Smart Metering billing
Marek Jawurek, Martin Johns, and Florian Kerschbaum (SAP Research)

This second paper looks at the problem of billing for fine-grained time of use tariffs — their energy consumption at different times costs a different rate per unit. This is a very important topic, as correct billing and time of use tariffs are a key driver of fine-grained data collection through smart meters — if we can do billing privately then maybe less personal information may be collected.

Technically the protocols proposed are based on the homomorphic properties of Pedersen commitments: readings are commitments, and you can use multiplication by a constant and addition to compute the bill, and (most importantly) prove that it is correct. The system model is that the meter outputs signed commitments of readings, a privacy component computes the bill and proofs of correctness, and those are sent to the supplier for verification (and printing the bills!).

This is the core of a nice solution for the basic billing case (which is likely to be the common one in smart grids). We have shown in related work that the protocol can be further improved to have zero communication overhead. Since it avoids expensive zero-knowledge proofs it is fast for its proofs and verification. It also provides the basic infrastructure to support further more expressive billing policies and general computations.

An Accurate System-Wide Anonymity Metric for Probabilistic Attacks
Rajiv Bagai, Huabo Lu, Rong Li, and Bin Tang (Wichita State University)

Traditional entropy based anonymity metrics look at the security of single messages. But how can you quantify the security provided by a whole system? The first paper in this session looks at a system-wide definition of anonymity by “counting” the possible number of matchings between inputs and outputs of an anonymity system. Furthermore, the metric extends to the probabilities over perfect matchings to express subtleties of modern anonymity systems. The paper first of all provides a thorough critique of the metric by Edman et al. (there was also previous work on this metric by the Leuven crew).

In a nutshell the proposed system-wide metric associates a probability to each possible matching, and computes the entropy over this distribution as a measure of anonymity (normalized). The choice of shanon entropy to summarise quality can be changed to min-entropy or other (which is very cool!) One key issue with system-wide metrics is that  how they express the properties that any individual message receives. Paul Syverson points out that these type of metrics express more the anonymity capacity of a system — namely how much anonymity the system could provide as a whole. The question of how this capacity for protection is distributed across users may need an extension to those metrics. For anyone who would like to extend metrics to capture this aspect, this paper is a very solid foundation.

DefenestraTor: Throwing out Windows in Tor
Mashael AlSabah, Kevin Bauer and Ian Goldberg (University of Waterloo), Dirk Grunwald (University of Colorado), and Damon McCoy, Stefan Savage, and Geoffrey Voelker (University of California-San Diego)

This paper looks at performance issues within the Tor network, and in particular the effects of the congestion and flow control protocols. Tor implements simple end-to-end flow control mechanism at the granularity of circuits and streams. It turns out that the implemented window based flow control has detrimental effects on performance: it does not protect intermediate routers (who are likely to be the congested ones) from congestion.

Two approaches were followed to solve this problem. First, a smaller window could be used — but this would not solve the problem; or windows can be computed dynamically. Second, the N23 congestion control protocol (used for ATM) could be used over Tor. N23 is simple and guarantees no packets are dropped, while implementing a steady flow of data. Its a credit based system, where packets are sent when credits are available (and consume them), and credits are sent up the network when bandwidth is available.

The evaluation was done under realistic conditions on ExperimenTor. The improvement over the current Tor strategy is significant when it comes to the time to get the first byte, but the time to complete larger (bulk) downloads do suffer (which is part of the point of the protocol).

I am really happy to see research on the intersection of traditional networking and anonymous communications. I have never heard of N23 before (shame on me!), and it seems that it is a good fit for the problem of congestion in anonymity networks (where reliability is not an issue when TCP is used).

Privacy Implications of Performance-Based Peer Selection by Onion Routers: A Real-World Case Study using I2P
Michael Herrmann and Christian Grothoff (Technische Universität München)

This is an attack paper on the I2P network, and in particular the performance based peer selection. It combines a denial-of-service attack to influence the selection of peers within the network, and force a victim to choose corrupt servers.

This is a cute attack that combines denial-of-service, traffic analysis for confirmation you are on the same circuit, and interactions with an infrastructure to attack. This is a very good reminder that anonymity engineering is not simply systems’ work. Every design choice about performance can affect security in dramatic ways. The evaluation was also very sensitive to protecting users: the researchers tried their attack on the real network, but targeted their own circuits (I still want to see details to make sure no other users were affected).

Tor too implements circuit selection on the basis of performance — I am wondering to what extent similar ideas could be applied there …

Quantifying Location Privacy: The Case of Sporadic Location Exposure
Reza Shokri and George Theodorakopoulos (EPFL), George Danezis (Microsoft Research), and Jean-Pierre Hubaux and Jean-Yves Le Boudec (EPFL)

This work evaluates the privacy of using location-based services sporadically using a set of location privacy mechanisms. Sporadic services include those that require location infrequently, rather than continuously (think of restaurant suggestions rather than relaying real-time GPS streams). The key novelty of the approach is that the model of location exposure, as well as privacy protection is very general. It encompasses anonymization, generalization and obfuscation of location, use of fake traffic and suppression of location. In turn the analysis relies on advanced models of location and mobility (based on markov chains) and is based on Bayesian inference. The evaluation of different location privacy techniques is done on real-world traces from SF taxis.

I am one of the authors of this work, so of course I think it is awesome! More seriously, it is one of the fist works to combine under a common framework a multitude of location privacy mechanisms, and provide a common evaluation framework for them, to quantify the degree of protection they offer relatively to each other for different adversaries. It is also one of the first systematic applications of Bayesian inference to analyze location privacy — extending the inference paradigm beyond the analysis of network anonymity systems.

Of course this is not the last word. Only a subset of protection techniques and combination of techniques were look at, and other protection mechanisms can be integrated and evaluated in the same framework (the tracing model and threat model can be unchanged). Secondly, the analysis itself may be augmented with side-information — be it commercial transactions or traces of network traffic — that may be giving some information about location, to increase the capabilities of the adversary (or make them more realistic). The model we use, based on markov chains, has the benefit of giving analytically tractable results, but numerical techniques may be used to extend it to be more true to real-life attacks.

The Location-privacy Meter tool that can be used to evaluate custom location privacy protections is available for download!

Privacy in Mobile Computing for Location-Sharing-Based Services
Igor Bilogrevic and Murtuza Jadliwala (EPFL), Kubra Kalkan (Sabanci University), Jean-Pierre Hubaux (EPFL), and Imad Aad (Nokia)

This paper looks at applications where users need to share their location. For example, two users may want to find out if they are close to each other or where they should meet in order to share a taxi ride. Yet, those users do not want to leak any of their location information to the other users or the service provider. More specifically two users specify a set of ranked prefered location they could meet and the system needs to determine on of those fairly without revealing the current location or other preferences (except the one chosen to meet). This is called the fair rendez-vous problem.

The key contribution of this work is to show that this problem can be set with a set of concrete cryptographic protocols. It also presents an implementation of these algorithms on a real mobile phone to show that it is practical. The cryptographic computations are based on homomorphic encryption schemes as well as interactions with the service (to do multiplication that is not possible with Paillier). The implementation on a mobile phone takes a few seconds on the client and the server, and is paralelizable in the number of users. Untypically, the authors also did a user study: users were asked what their concerns were, and after using the application of the phone they were asked how usable it was, and whether they appriciated the privacy provided by the application.

This is a really nice example of a privacy application, that applies advanced crypto, but also evaluates it on a real platform for performance as well as users’ reaction to it. The obvious extensions to this work would be to generalize it to more complex rendez-vous protocols, as well as other location sharing applications. It is good to see that modern mobile devices can do plenty of crypto in a few seconds, so I am very hopeful we will see more work in this field.

On The Practicality of UHF RFID Fingerprinting: How Real is the RFID Tracking Problem?
Davide Zanetti, Pascal Sachs, and Srdjan Capkun (ETH Zurich)

This paper looks that UHF tags — they are the dumb tags that can be read at about 2m that are attached to things you buy to facilitate stock management or customer aftercare. Interestingly this study looks at how identifiable the tags are at the physical layer, not using the actual tag ID! Therefore these techniques may bypass any privacy protection that attempt to prevent access to the tag ID. It turns our that one can build a unique and reliable ID for a tag from its physical characteristics that can be used to trace people as they move around.

What is new about this work is that the focus was on practicality and cost of extracting a reliable fingerprint (previous approaches relied on expensive equipment and laboratory conditions). The solution was implemented using a cheap software radio (USRP2 device + PC).

I am not quite sure what to conclude from the evaluation on the quality of the fingerprint. It seems that an adversary can place tags within one of 83 to 100 groups. Is this really a good results or not? I guess it depends on the application and the density of tags. Of course if more than one tag is carried, then the adversary could combine fingerprints to identify individuals more easily — if you carry 5 tags you have a 20 bit IDs. Interestingly, there is extensive evaluation of the stability of the tag to temperature and mobility — it turns out that these factors do affect the quality of the fingerprint and further reduce the effective number of unique IDs that can be extracted (down to about 49 classes).

It would be interesting to combine this attack vector with the ideas from the first paper (pretending that the short physical IDs are a version of a privacy protection system) to evaluate the effectivness of tracing a set of individual throughout town.

Andreas Pfitzmann has sadly passed away last year, and a special pannel session is taking place right now at PETS 2011 commemorating his work on anonymous communications and privacy. Andreas’ technical contribution span about 30 years, and as such he can be considered a founding father of the field of anonymous communications. His work in educating policy makers, and advocating privacy in the public sphere had a profound impact on German technology policy.

The pannel includes a short excerpt from an interview with Andreas, as well as recorded contributions, by collaborators (Michael Waidner and Marit Hansen), former students (Anna Krasnova and Hannes Federrath) and people in the PET community (Paul Syverson and Caspar Bowden).

I am currently sitting at the PETS 2011 symposium in Waterloo, CA. I will be blogging about selected papers (depending on the sessions I attend) over the next couple of days — authors and other participants are welcome to comment!

The first session is about data mining and privacy.

“How Unique and Traceable are Usernames?”
Daniele Perito, Claude Castelluccia, Mohamed Ali Kaafar, and Pere Manils (INRIA)

The first paper looks at the identifiably of on-line usernames. The authors looked at user names from different sites and assess the extent to which they can be linked together, as well as link them to a real person. Interestingly they used Google Profiles as ground truth, since they allow users to provide links to other accounts. First they assess the uniqueness of pseudonyms based on a probabilistic model: a k-th order markov chain is used to compute the probability of each pseudonym, and its information content (i.e. -log_2 P(username)). The authors show that most of the usernames observed have “high entropy” and should therefore be linkable.

The second phase of the analysis looks at usernames from different services, and attempts to link them even given small modifications to the name. The key dataset used was 300K google profiles, that list (sometimes — they used 10K tuples of usernames) other accounts as well. They then show that the Levenshtein distance (i.e. edit distance) of usernames from the same person is small compared to the distance of two random user names. A classifier is built, based on a threshold of the probabilistic Levenshtein distance, to assess whether a pair of usernames belongs to the same or a different user. The results seem good: about 50% of usernames are linkable with no mistakes.

So what are the interesting avenues for future work here? First, the analysis used is a simple threshold on the edit distance metric. It would be surprising if more advanced classifiers could not be applied to the task. I would definitely try to use random forests for the same task. Second, the technique can be used for good not evil: as users try to migrate between services, they need an effective way to find their contacts — maybe the proposed techniques can help with that.

“Text Classification for Data Loss Prevention” (any public PDF?)
Michael Hart (Symantec Research Labs), Pratyusa Manadhata (HP Labs), and Rob Johnson (Stony Brook University)

The paper looks at the automatic classification of documents as sensitive or not. This is to assist “data loss prevention” systems, that raise an alarm when personal data is about to be leaked (i.e. when it is about to be emailed or stored on-line — mostly by mistake). Traditionally DLP try to describe what is confidential through a set of simple rules, that are not expressive enough to describe and find what is confidential — thus the authors present a machine learning approach to automatically classify documents using training data as sensitive or not. As with all ML techniques there is a fear of mistakes: the technique described tries to minimise errors when it comes to classifying company media (ie. public documents) and internet documents, to prevent the system getting on the way of day to day business activities.

The results were rather interesting: the first SVN classifier looked at unigrams with binary weights to classify documents. Yet, it first had a very high rate of false positives for public documents. It seems the classifiers also had a tendency to classify documents as “secret”. A first solution was to supplement the training set with public documents (from wikipedia), to best described “what is public”. Second, the classifier was tweaked to (in a rather mysterious way to me) by “pushing the decision boundary closer to the true negative”. A further step does 3-category classification as secret, public and non-enterprise, rather than just secret and not-secret.

Overall: They manage to get the false positive / false negative rate down to less than 2%-3% on the largest datasets evaluated. That is nice. The downside of the approach, is common to most work that I have seen using SVNs. It requires a lot of manual tweaking, and further it does not really make much sense — it is possible to evaluate how well the technique performs on a test corpus, but difficult to tell why it works, or what makes it good or better than other approaches. As a resut, even early positive resutls should be considered as preliminary until more extensive evaluation is done (more like medicine rather than engineering). I would personally like to see a probabilistic model based classifier on similar features that integrates the two-step classification process into one model, to really understand what is going on — but then I tend to have a Baysian bias.

P3CA: Private Anomaly Detection Across ISP Networks
Shishir Nagaraja (IIIT Delhi) and Virajith Jalaparti, Matthew Caesar, and Nikita Borisov (University of Illinois at Urbana-Champaign)

The final paper in the session looks at privacy preserving intrusion detection to enable cooperation between internet service providers. ISPs would like to pool data from their networks to detect attacks: either because the volume of communications is abnormal at certain times, or because some frequency component is odd. Cooperation between multiple ISPs is important, but this cooperation should not leak volumes of traffic at each IPS since they are a commercial secret.

Technically, privacy of computations is achieved by using two semi-trusted entities, a coordinator and key holder. All ISPs encrypt their traffic under an additive homomorphic scheme (Paillier) under the keyholder key, and send it to the coordinator. The coordinator is using the key-holder as an oracle to perform a PCA computation to output the top-n eighen vectors and values of traffic. The cryptographic techniques are quite but standard, and involve doing additions, subtraction, multiplication, comparison and normalization of matrices privately though a joint private two-party computation.

Surprisingly, the performance of the scheme is quite good! Using a small cluster, can process a few tens of time slots from hundresds of different ISPs in tens of minutes. A further incremental algorithm allows on-line computations of eighen vector/value pairs in seconds — making real-time use of the privacy preserving algorithm possible (5 minutes of updates takes about 10 seconds to process).

This is a surprising result: my intuition would be that the matrix multiplication would make the approach impractically slow. I would be quite interested to compare the implementation and algorithm used here with a general MPC compiler (under the same honest-but-curious model).

Shishir Nagaraja has pointed out that our Drac anonymity system is not the first one to consider an anonymity network overlayed on a social network. The performance versus security of routing messages over a social network was already considered in his work entitled ‘anonymity in the wild’.

Shishir Nagaraja: Anonymity in the Wild: Mixes on Unstructured Networks. Privacy Enhancing Technologies 2007: 254-271 [pdf][ppt]

This is important prior work and we should have cited it properly. It presents an analysis of an anonymity provided by different synthetic social network topologies, as well as real-world data from LiveJournal.