[tor-project] Constructing a real-world dataset for studying website fingerprinting

Mon Apr 24 14:17:21 UTC 2023

> On Apr 22, 2023, at 7:22 PM, Tobias Pulls <tobias.pulls at kau.se> wrote:
> 
> Hi Rob,
> 

Hi Tobias!

> Your earlier work on online WF and this proposal are exciting. We can learn a lot from such a proposed dataset, so please collect it, but I think that there are some challenges around framing it as a dataset for assessing WF attacks and defenses.
> 
> To be brief, I think we solve traffic diversity with BigEnough-style datasets that never overlap subpages [0] and that data staleness and network overheads are minor problems. Capable attackers (we should consider) can build closed worlds of ~100 websites [1], no need for open-world data.
> 
> The biggest strength of the proposed dataset is also its biggest weakness: capturing real-world user diversity in traces. It is more user/client diversity than browser diversity (in the sense of more broad), because the dataset captures various browsers and as well as different Tor clients implementations, other Tor configurations like VPN mode, running on routers, in strange VMs, headless browsers etc., as well as a wide range of network configurations and probably more I missed used by the very diverse Tor-network userbase. While capturing this diversity is super interesting and valuable in different ways, it also risks being too filled with junk to help assess WF attacks or defenses.
> 
> Do you have any approaches or thoughts around pruning the dataset or further refining labels beyond the first domain?

I should note that we will include both the HMAC of first domain and an HMAC of its shortest private suffix (computed using Mozilla’s public suffix list and libpsl (publicsuffix.org <http://publicsuffix.org/>)).

IMO, the first domain is by far the most important. Other domains accessed on a typical TB circuit will primarily be due to fetching the embedded objects fetched during page loads. We’ll get a new circuit if the domain in the URL bar changes, and that will have its own first domain.

I’m trying to collect the minimal thing here to balance privacy and utility; while not perfect from a ML “give me all the data” perspective, I think the dataset would allow us to make significant moves in the right direction.

> Some worries:
> 
> - I fear that we will likely have to do much guesswork on interpreting results based on the dataset. Do we want to be assessing WF defenses and attacks based on random torified curl scrapers, python scripts, and who knows what? Right now, we are creeping closer and closer to Internet~web, reflected in Tor-traffic. Without any ground truth, how can we avoid that most labels aren't mostly junk? What does it mean to reach 50% accuracy in evaluating the dataset? If a WF attack trained on TB "fails" to associate a website visit using curl that's probably a feature (not what the attacker was after anyway), not a bug. It would be fantastic if it were possible to have a sizeable subset of traces for some/most labels containing known configurations (or even just confirmed website visits with, say vanilla TB, maybe some more inline domain fingerprinting to set a bool for key website domains of Tranco top-100 requested on the circuit or something?).

First, I don’t think “junk” will be anywhere near the problem you imagine. Yes, there will be some noise from non-TB activity. But I believe that the *overwhelming* activity is people using TB going to top websites (see our IMC 2018 paper [0]). Second, you will be able to count the number of circuits that exist for each domain key. While this may not exactly match up with Tranco, it will probably be pretty close [0]. Third, I’m very happy to introduce this noise into the WF evaluation process. The adversary has to deal with it at least during testing, so you should too :)

[0] https://www.robgjansen.com/publications/torusage-imc2018.pdf

> - Filled with an unknown rate of junk, the dataset alone will be insufficient to train WF attacks in noisy environments (you want ~1000+ samples per class of something coherent, at least with current sota DL models pushed to their limits: as we move on to transformers probably even more). Suppose you subscribe to the claims of data staleness being a factor. In that case, researchers cannot even go through the very consuming process of collecting adequate labeled training data in the same way to show success on the proposed dataset. The exit collection vantage point makes this worse.
> 

We’ll have enough data that concept drift can be studied within our set exclusively. If our set isn’t big enough, or if it becomes stale after a while and we’re wanting more realistic threat estimates with current data, then yeah that’s another TRSB submission and we’ll have to think through the risks and relative benefits again. As it is now, we have a very large step forward in terms of benefit because we are starting from zero.

If we determine with this first set that we really need a continuous stream of the latest data and lots more of it, and want to establish some permanent measurement setup, that of course would be useful but also has its own risks. Right now I would argue against a permanent measurement because we haven’t even done the more basic thing yet.

> - Using the dataset as a basis for simulating defenses, one would have to simulate the corresponding client and middle traces to feed into the simulated defenses based on exit traces. Kinda messy with poor signalling for Tor-network characteristics in the dataset. When collecting at the client, getting realistic client traces (for the particular configuration) is basically for free. At the same time, middles change for every circuit, so a half-assed approach seems to get you far (speaking from experience of half-assing things here!). I worry about half-assing both client and middle traces, however.

This is solvable. You could construct a website simulator using the downstream cell streams, which we collect on the exit before Tor’s queuing plays a part in messing with the timing. Once you have that, use Shadow to fetch the simulated sites through a simulated Tor network, which should do a reasonable (and getting better) job of adding back in the performance effects on top of the cell streams. Not perfect, but maybe passes your half-ass bar?

> If we make this proposed dataset and its method the bar of doing "real-world WF", it might lead to too high of a bar. We want more real-world implementations and data collection with defenses, not less, I think.

Yes, I absolutely want us to raise the bar for doing this type of research. If that means professors can write fewer papers, and the ones we get will be more informative, then I will have succeeded!

> Sorry if the above may come across as a bit negative, it's not my intent: I *want* the dataset you describe, we can learn a lot from it for sure. Wish I had a chance to chat in person in Costa Rica! Please don't feel obliged to reply, just food for thought.
> 

How could I resist in responding? :)

I really do appreciate your thoughts, thanks for sharing!

Peace, love, and positivity,
Rob

> Best,
> Tobias
> 
> [0]: "SoK: A Critical Evaluation of Efficient Website Fingerprinting Defenses", https://www-users.cse.umn.edu/~hoppernj/sok_wf_def_sp23.pdf
> [1]: "Website Fingerprinting with Website Oracles", https://petsymposium.org/2020/files/papers/issue1/popets-2020-0013.pdf
> 
> On 20/04/2023 23:16, Jansen, Robert G CIV USN NRL (5543) Washington DC (USA) via tor-project wrote:
>> Hello Tor friends,
>> We are planning to construct a real-world dataset for studying Tor website
>> fingerprinting that researchers and developers can use to evaluate potential
>> attacks and to design informed defenses that improve Tor’s resistance to such
>> attacks. We believe the dataset will help us make Tor safer, because it will
>> allow us to design defenses that can be shown to protect *real* Tor traffic
>> instead of *synthetic* traffic. This will help ground our evaluation of proposed
>> defenses in reality and help us more confidently decide which, if any, defense
>> is worth deploying in Tor.
>> We have submitted detailed technical plans for constructing a dataset to the Tor
>> Research Safety Board and after some iteration have arrived at a plan in which
>> we believe the benefits outweigh the risks. We are now sharing an overview of
>> our plan with the broader community to provide an opportunity for comment.
>> More details are below. Please let us know if you have comments.
>> Peace, love, and positivity,
>> Rob
>> P.S. Apologies for posting near the end of the work-week, but I wanted to get
>> this out in case people want to talk to me about it in Costa Rica.
>> ===
>> BACKGROUND
>> Website fingerprinting attacks distill traffic patterns observed between a
>> client and Tor entry into a sequence of packet directions: -1 if a packet is
>> sent toward the destination, +1 if a packet is sent from toward the client. An
>> attacker can collect a list of these directions and then train machine learning
>> classifiers to associate a website domain name or url with the particular list
>> of directions observed when visiting that website. Once it does this training,
>> then when it observes a new list of directions it can use the trained model to
>> predict which website corresponds to that pattern.
>> For example, suppose [-1,-1,+1,+1] is associated with website1 and [-1,+1,-1,+1]
>> is associated with website2. There are two steps in an attack:
>> Step 1:
>> In the first step the attacker itself visits website1 and website2 many times
>> and learns:
>> [-1,-1,+1,+1] -> website1
>> [-1,+1,-1,+1] -> website2
>> It trains a machine learning model to learn this association.
>> Step 2:
>> In the second step, with the trained model in hand, the attacker monitors a Tor
>> client (maybe the attacker is the client’s ISP, or some other entity in a
>> position to observe a client’s traffic) and when it observes the pattern:
>> [-1,-1,+1,+1]
>> the model will predict that the client went to website1. This example is
>> *extremely* simplified, but I hope gives an idea how the attack works.
>> PROBLEM
>> Because researchers don’t know which websites Tor users are visiting, it’s hard
>> to do a very good job creating a representative dataset that can be used to
>> accurately evaluate attacks or defenses (i.e., to emulate steps 1 and 2). The
>> standard technique has been to just select popular websites from top website
>> lists (e.g., Alexa or Tranco) and then set up a Tor webpage crawler to visit the
>> front-pages of those websites over and over and over again. Then they use that
>> data to write papers. This approach has several problems:
>> - Low traffic diversity: Tor users don’t only visit front-pages. For example,
>> they may conduct a web search and then click a link that brings them directly to
>> an internal page of a website. The patterns produced from front-page visits may
>> be simpler and unrepresentative of the patterns that would be observed from more
>> complicated internal pages.
>> - Low browser diversity: It has been shown by research from Marc Juarez [0] and
>> others that webpage crawlers used by researchers lack diversity in important
>> aspects that cause us to overestimate the accuracy of WF attacks. For example,
>> the browser versions, configuration choices, variation in behavior (e.g., using
>> multiple tabs at once), and network location of the client can all significantly
>> affect the observable traffic patterns in ways that a crawler methodology does
>> not capture.
>> - Data staleness: Researchers collect data over a short time-frame and then
>> evaluate the attacks assuming this static dataset. In the real world, websites
>> are being updated over time, and a model trained on an old version of a website
>> may not transfer to the new version.
>> In addition to the above problems in methodology, current research also causes
>> incidental consequences for the Tor network:
>> - Network overhead: machine learning is a hot topic and several research groups
>> have crawled tens of thousands of websites over Tor many times each. While each
>> individual page load might be insignificant compared with the normal usage of
>> Tor, crawling does add additional load to the network and can contribute to
>> congestion and performance bottlenecks.
>> Researchers have been designing attacks that are shown to be extremely accurate
>> using the above synthetic crawling methodology. But because of the above
>> problems, we don’t properly understand the *true* threat of the attack against
>> the Tor network. It is possible that the simplicity of the crawling approach is
>> what makes the attacks work well, and that the attacks would not work as well if
>> evaluated with more realistic traffic and browser diversity.
>> PLAN
>> So our goal is to construct a real-world dataset for studying Tor website
>> fingerprinting that researchers and developers can use to evaluate potential
>> attacks and to design informed defenses that improve Tor’s resistance to such
>> attacks. This dataset would enable researchers to use a methodology that does
>> not have any of the above limitations. We believe that such a dataset will help
>> us make Tor safer, because it will allow us to design defenses that can be shown
>> to protect *real* Tor traffic instead of *synthetic* traffic. This would lead to
>> a better understanding of proposed defenses and enable us to more confidently
>> decide which, if any, defense is worth deploying in Tor.
>> The dataset will be constructed from a 13-week exit relay measurement that is
>> based on the measurement process established in recent work [1]. The primary
>> information being measured is the directionality of the first 5k cells sent on a
>> measurement circuit, and a keyed-HMAC of the first domain name requested on the
>> circuit. We also measure relative circuit and cell timestamps (relative to the
>> start of measurement). The measurement data is compressed, encrypted using a
>> public-key encryption scheme (the secret key is stored offline), and then
>> temporarily written to persistent storage before being securely retrieved from
>> the relay machine.
>> We hope that this dataset can become a standard tool that website fingerprinting
>> researchers and developers can use to (1) accelerate their study of attacks and
>> defenses, and (2) produce evaluation and results that are more directly
>> applicable to the Tor network. We plan to share it upon request only to other
>> researchers who appear to come from verifiable research organizations, such as
>> students from well-known universities. We will require researchers with whom we
>> share the data to (1) keep the data private, and (2) direct others who want a
>> copy of the data to us to mitigate unauthorized sharing.
>> [0] A Critical Evaluation of Website Fingerprinting Attacks. Juarez et al., CCS 2014. https://www1.icsi.berkeley.edu/~sadia/papers/ccs-webfp-final.pdf
>> [1] Online Website Fingerprinting: Evaluating Website Fingerprinting Attacks on Tor in the Real World. Cherubin et al., USENIX Security 2022. https://www.usenix.org/conference/usenixsecurity22/presentation/cherubin
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