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A framework for evaluating Website Fingerprinting attacks/defences, accompaining the paper "Bayes, not Naïve: Security Bounds on Website Fingerprinting Defenses" (Cherubin, 2017)

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README.md

Website Fingerprinting Evaluation Suite

This is a suite for evaluating Website Fingerprinting (WF) attacks and defenses, associated with the paper "Bayes, not Naïve: Security Bounds on Website Fingerprinting Defenses" (G. Cherubin, 2017).

It provides:

  • a standard interface to use the code from previous attacks/defences,
  • a method to estimate security bounds and $(\varepsilon, \Phi)$-privacy of a WF defense.

Particularly, it gives a standard interface to use the code from other WF researchers, who are acknowledged below. Code from other researchers was adapted to fit the API. With this regard, I tried making as few changes as possible, so as to keep the results close to the original ones; the changes I made are documented by diff files.

An introduction to computing security bounds is at https://giocher.com/pages/bayes.html.

Installation

This was tested on Alpine Linux 3.8, although I expect it to work for most BSD and Linux distributions. Please, open an issue should you encounter any problems.

The code works for Python 2.7.

mkvirtualenv wfes
pip install -r requirements.txt

Install weka >=3.8 in some directory $WEKA. Then install the package LibSVM:

java -classpath $WEKA/weka.jar weka.core.WekaPackageManager -install-package LibSVM

Download, patch, and build attack code.

cd code/attacks && make && cd -

Also, edit WEKA_ROOT in code/attacks/dyer/config.py with the directory containing your weka installation.

The following sections should allow you to reproduce the experiments and to replicate them on your data.

The WCN+ dataset (and data format explanation)

We consider the dataset collected by Wang et al. 2014 ("WCN+").

Download the dataset:

mkdir -p data/WCN+
cd data/WCN+
wget https://www.cse.ust.hk/~taow/wf/data/knndata.zip
unzip knndata.zip
mv batch original

This dataset is constituted of packet sequences corresponding to different page loads. Each packet sequence is contained in a file with name $W-$L, where $W is the webpage's id, and $L indicates the page load. For instance, "0-4" is the fourth page load of webpage 0.

Each of these files contains, per row:

t_i<tab>s_i

with t_i and s_i indicating respectively time and size of the i-th packet. The sign of s_i indicates the packet's direction (positive means outgoing). Note: because this dataset represents Tor traffic, where packets' sizes are fixed, s_i will effectively only indicate the direction, taking value in {-1, +1}.

Defending the dataset

You can measure security bounds for any defence. In this example, we apply the defence directly to the packet sequence files to morph them; specifically, the defence scripts that follow take as input a packet sequence file and output a new (morphed) packed sequence file. If you wish to evaluate other defences, you can simply collect live network data for them, and estimate security on the generated packet sequence files -- which should have the format specified above.

Some of the defenses' scripts are downloaded and patched using:

cd code/defenses && make && cd -

Scripts to defend traces can be called as:

python defend.py $DATASET

and they will put the defended traces into ./defended. This should change in the future.

For example:

python defenses/CS-BuFLO/cs_buflo.py data/WCN+/original

NOTE: most of these scripts assume traces' files are in the format $W-$L, with $W in {0..99}, $L in {0..89} as in the WCN+ dataset. For decoy-pages, the dataset will need to contain "open world" traces $W, i=0..8999. It's fairly simple to make this more general, but I didn't have the time to change this in Tao Wang's code.

Extracting features

In order to perform an attack or to measure security bounds you need to first extract feature vectors ("objects") from traces.

Each page load $W-$L corresponds to a feature vector, and, for the purpose of this document, each feature vector is contained in a file "$W-$L.features". We create a directory, $FEAT_DIR, that will contain the feature vectors.

cd into code/.

In general, you can extract features for attack $attack as follows:

python extract_features.py --traces $DATASET --out $FEAT_DIR --attack $attack

For instance:

python extract_features.py --traces ../data/WCN+/original --attack knn --out ../data/features/knn/

where ../data/WCN+/ is the directory containing the (possibly defended) packet sequence files, and ../data/features/knn/ is the output folder that will contain the resulting feature files.

For a list of attacks run:

python extract_features.py -h

The --type option can be used to trim the features for specific attacks, namely k-NN and k-FP; it takes parameter either "knn" or "kfp".

NOTES

k-NN features. If the argument "--type knn" is added, weights are applied to features. This needs to be done for evaluating the attack. As for computing bounds, this option clearly gives a small advantage (i.e., bounds are smaller); in the paper, however, I computed bounds without this option in order to show that the method is robust w.r.t. small modifications of the feature set.

k-FP features. If the argument "--type kfp" is added, features are extracted using Random Forest as in the paper by Hayes and Danezis. To my understanding, this is an advantage only in the Open World scenario. In experments, I did not use this option for attacks nor bounds, as I observed it produced worse results.

Classification (attack)

To evaluate an attack, launch:

python classify.py --features $FEAT_DIR --train 0.8 --test 0.2 --attack $ATTACK --out $OUT_FNAME

where --train and --test specify the percentage of training and test examples -- whose value needs not to sum up to 1.

The output is a json file.

For more options, run:

python classify.py -h

Measuring security

Computing bounds is done in two phases.

Computing distances

First, you need to compute the pairwise distances between feature vectors:

python compute_distances.py --features $FEAT_DIR --out $OUT

The $OUT file can be opened using dill, should you want to inspect it.

An alternative to computing distances (and bounds) on feature vectors is to compute them directly on packet sequences (see experiment in Section 7.4 of the paper):

python compute_distances --features $TRACES_DIR --sequences --out $OUT

Note that this did not produce good results (Fig.5); indeed, for most defences one should compute security bounds after a feature transformation (e.g., see the blog post).

Computing bounds

Then, you can compute the bounds using:

python bounds.py --distances $DISTANCES --train 0.8 --test 0.2 --out $OUT

The output is a json file.

Hacking

TODO How to add new attacks/defences.

How to add new distance metrics.

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A framework for evaluating Website Fingerprinting attacks/defences, accompaining the paper "Bayes, not Naïve: Security Bounds on Website Fingerprinting Defenses" (Cherubin, 2017)

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