commit fb31be566ba61aa076bed0f22c6f5228f8a67513 Author: Karsten Loesing karsten.loesing@gmx.net Date: Wed Aug 8 20:13:17 2012 +0200
Add five-ways-test-bridge-reachability blog post. --- 2011/five-ways-test-bridge-reachability/.gitignore | 3 + .../five-ways-test-bridge-reachability.bib | 70 ++++ .../five-ways-test-bridge-reachability.tex | 349 ++++++++++++++++++++ .../tortechrep.cls | 1 + 4 files changed, 423 insertions(+), 0 deletions(-)
diff --git a/2011/five-ways-test-bridge-reachability/.gitignore b/2011/five-ways-test-bridge-reachability/.gitignore new file mode 100644 index 0000000..05419b2 --- /dev/null +++ b/2011/five-ways-test-bridge-reachability/.gitignore @@ -0,0 +1,3 @@ +five-ways-test-bridge-reachability.pdf +five-ways-test-bridge-reachability-2011-12-01.pdf + diff --git a/2011/five-ways-test-bridge-reachability/five-ways-test-bridge-reachability.bib b/2011/five-ways-test-bridge-reachability/five-ways-test-bridge-reachability.bib new file mode 100644 index 0000000..85d135f --- /dev/null +++ b/2011/five-ways-test-bridge-reachability/five-ways-test-bridge-reachability.bib @@ -0,0 +1,70 @@ +@techreport{dingledine2011strategies, + title = {Strategies for getting more bridge addresses}, + author = {Roger Dingledine}, + institution = {The Tor Project}, + year = {2011}, + month = {May}, + number = {2011-05-001}, + note = {\url{https://research.torproject.org/techreports/strategies-getting-more-bridge-a..., +} + +@inproceedings{loesing2010case, + title = {A Case Study on Measuring Statistical Data in the {Tor} Anonymity Network}, + author = {Karsten Loesing and Steven J. Murdoch and Roger Dingledine}, + booktitle = {Proc.\ Workshop on Ethics in Computer Security Research}, + year = {2010}, + month = Jan, + address = {Tenerife, Canary Islands, Spain}, + note = {\url{https://metrics.torproject.org/papers/wecsr10.pdf%7D%7D, +} + +@techreport{danezis2011anomaly, + title = {An anomaly-based censorship-detection system for {Tor}}, + author = {George Danezis}, + institution = {The Tor Project}, + year = {2011}, + month = {September}, + number = {2011-09-001}, + note = {\url{https://research.torproject.org/techreports/detector-2011-09-09.pdf%7D%7D, +} + +@techreport{loesing2011case, + title = {Case study: Learning whether a {Tor} bridge is blocked by + looking at its aggregate usage statistics, Part One}, + author = {Karsten Loesing}, + institution = {The Tor Project}, + year = {2011}, + month = {September}, + number = {2011-09-002}, + note = {\url{https://research.torproject.org/techreports/blocking-2011-09-15.pdf%7D%7D, +} + +@techreport{dingledine2011ten, + title = {Ten ways to discover {Tor} bridges}, + author = {Roger Dingledine}, + institution = {The Tor Project}, + year = {2011}, + month = {October}, + number = {2011-10-002}, + note = {\url{https://research.torproject.org/techreports/ten-ways-discover-tor-bridges-20..., +} + +@inproceedings{proximax11, + title = {Proximax: Fighting Censorship With an Adaptive System for Distribution of Open + Proxies}, + author = {Kirill Levchenko and Damon McCoy}, + booktitle = {Proceedings of Financial Cryptography and Data Security (FC'11)}, + year = {2011}, + month = {February}, + note = {\url{http://freehaven.net/anonbib/#proximax11%7D%7D, +} + +@inproceedings{oakland11-formalizing, + title = {Formalizing Anonymous Blacklisting Systems}, + author = {Ryan Henry and Ian Goldberg}, + booktitle = {Proceedings of the 2011 IEEE Symposium on Security and Privacy}, + year = {2011}, + month = {May}, + note = {\url{http://freehaven.net/anonbib/#oakland11-formalizing%7D%7D, +} + diff --git a/2011/five-ways-test-bridge-reachability/five-ways-test-bridge-reachability.tex b/2011/five-ways-test-bridge-reachability/five-ways-test-bridge-reachability.tex new file mode 100644 index 0000000..84c95ee --- /dev/null +++ b/2011/five-ways-test-bridge-reachability/five-ways-test-bridge-reachability.tex @@ -0,0 +1,349 @@ +\documentclass{tortechrep} +\begin{document} + +\author{Roger Dingledine} +\contact{arma@torproject.org} +\reportid{2011-12-001} +\date{December 1, 2011} +\title{Five ways to test bridge reachability} +\maketitle + +\section{Introduction} + +Once we get more (and more diverse) bridge addresses +\cite{dingledine2011strategies}, +the next research step is that we'll need to get better at telling which +bridges are blocked in which jurisdictions. +For example, most of the bridges we give out via https and gmail are +blocked in China. +But which ones exactly? +How quickly do they get blocked? +Do some last longer than others? +Do they ever get unblocked? +Is there some pattern to the blocking, either by time, by IP address or +network, by user load on the bridge, or by distribution strategy? +We can't evaluate new bridge distribution strategies% +\footnote{\url{https://blog.torproject.org/blog/bridge-distribution-strategies%7D%7D +if we can't track whether the bridges in each strategy are being blocked. + +Generally speaking, bridge reachability tests break down into two +approaches: passive and active. +Passive tests don't involve any new connections on the part of Tor clients +or bridges, whereas active tests follow the more traditional ``scanning'' +idea. +None of the reachability tests we've thought of are perfect. +Instead, here we discuss how to combine imperfect tests and use feedback +from the tests to balance their strengths and weaknesses. + +\section{Passive approaches} + +We should explore two types of passive testing approaches: reporting from +bridges and reporting from clients. + +\subsection{Reporting from bridges} + +Right now Tor relays and bridges publish aggregate user counts +\cite{loesing2010case}---rough number of users per country per day. +In theory we can look at the user counts over time to detect statistical +drops in usage for a given country. +That approach has produced useful results in practice for overall +connections to the public Tor relays from each country: +see George Danezis's initial work \cite{danezis2011anomaly} on a Tor +censorship detector. + +But there are two stumbling blocks when trying to apply the censorship +detector model to individual bridges. +First, we don't have ground truth about which bridges were \emph{actually} +blocked or not at a given time, so we have no way to validate our models. +Second, while overall usage of bridges in a given country might be high, +the load \emph{on a given bridge} tends to be quite low, which in turns +makes it difficult to achieve statistical significance +\cite{loesing2011case} when looking at usage drops. + +Ground truth needs to be learned through active tests: we train the models +with usage patterns for bridges that get blocked and bridges that don't +get blocked, and the model predictions should improve. +The question of statistical significance can be overcome by treating the +prediction as a hint: even if our models don't give us enough confidence +to answer ``blocked for sure'' or ``not blocked for sure'' about a given +bridge, they should be able to give us a number reflecting likelihood that +the bridge is now blocked. +That number should feed back into the active tests, for example so we pay +more attention to bridges that are more likely to be newly blocked. + +\subsection{Reporting from clients} + +In addition to the usage reporting by bridges, we should also consider +reachability reporting by clients. +Imagine a Tor client that has ten bridges configured. +It tries to connect to each of them, and finds that two work and eight +don't. +This client is doing our scanning for us, if only we could safely learn +about its results. +The first issue that comes up is that it could mistakenly report that a +bridge is blocked if that bridge is instead simply down. +So we would want to compare the reports to concurrent active scans from a +``more free'' jurisdiction, to pick out the bridges that are up in one +place yet down in another. + +From there, the questions get trickier: +1) does the set of bridges that a +given user reports about create a fingerprint that lets us recognize that +user later? +Even if the user reports about each bridge through a separate Tor circuit, +we'd like to know the time of the scan, and nearby times can be used to +build a statistical profile. +2) What if users submit intentionally misleading reports? +It seems there's a tension between wanting to build a profile for the user +(to increase our confidence in the validity of her reports) versus wanting +to make sure the user doesn't develop any recognizable profile. +Perhaps the Nymble \cite{oakland11-formalizing} design family can +contribute an ``unlinkable reputation'' trick to resolve the conflict, but +as we find ourselves saying so often at Tor, more research remains. + +\section{Active approaches} + +The goal of active scanning is to get ground truth on whether each bridge +is really blocked. +There's a tradeoff here: frequent scans give us better resolution and +increased confidence, but too many scan attempts draw attention to the +scanners and thus to the addresses being scanned. + +We should use the results of the passive and indirect scans to give hints +about what addresses to do active scans on. +In the steady-state, we should aim to limit our active scans to bridges +that we think just went from unblocked to blocked or vice versa, and to a +sample of others for spot checks to keep our models trained. + +There are three pieces to active scanning: direct scans, reverse scans, +and indirect scans. + +\subsection{Direct scans} + +Direct scans are what we traditionally think of when we think of scanning: +get access to a computer in the target country, give it a list of bridges, +and have it connect directly to each bridge on the list. + +Before I continue though, I should take an aside to discuss types of +blocking. +In September 2009 when China first blocked some bridges, I spent a while +probing the blocked bridges from a computer in Beijing. +From what I could tell, China blocked the bridges in two ways. +If the bridge had no other interesting services running (like a +webserver), they just blackholed the IP address, meaning no packets to or +from the IP address made it through the firewall. +But if there \emph{was} an interesting service, they blocked the bridge by +IP and port. +(I could imagine this more fine-grained blocking was done by dropping SYN +packets, or by sending TCP RST packets; but I didn't get that far in my +investigation.) + +So there are two lessons to be learned here. +First, the degree to which our active scans match real Tor client behavior +could influence the accuracy of the scans. +Second, some real-world adversaries are putting considerable +effort---probably manual effort---into examining the bridges they find and +choosing how best to filter them. +After all, if they just blindly filtered IP addresses we list as bridges, +we could add Baidu's address as a bridge and make them look foolish. +(We tried that; it didn't work.) + +These lessons leave us with two design choices to consider. + +First, how much of the Tor protocol should we use when doing the scans? +The spectrum ranges from a simple TCP scan (or even just a SYN scan), to a +vanilla SSL handshake, to driving a real Tor client that does a genuine +Tor handshake. +The less realistic the handshake, the more risk that we conclude the +bridge is reachable when in fact it isn't; but the more realistic the +handshake, the more we stand out to an adversary watching for Tor-like +traffic. + +The mechanism by which the adversary is discovering bridges +\cite{dingledine2011ten} also impacts which reachability tests are a smart +idea. +For example, it appears that China may have recently started doing deep +packet inspection (DPI) for Tor-like connections over the Great Firewall +and then doing active-followup SSL handshakes to confirm which addresses +are Tor bridges. +If that report turns out to be true, testing bridge reachability via +active scans that include handshaking would be counterproductive: the act +of doing the test would influence the answer. +We can solve their attack by changing our handshake% +\footnote{\url{https://blog.torproject.org/blog/iran-blocks-tor-tor-releases-same-day-fix%7... +so they don't recognize it anymore, or by introducing scanning-resistance% +\footnote{\url{https://svn.torproject.org/svn/projects/design-paper/blocking.html%5C#tth%5C... +measures like bridge passwords. +In the shorter-term, we should confirm that simple connection scanning +(without a handshake) doesn't trigger any blocks, and then restrict +ourselves to that type of scanning in China until we've deployed a better +answer. + +Second, should we scan ``decoy'' addresses as well, to fool an observer +into thinking that we're not scanning for Tor bridges in particular, +and/or to drive up the work the observer needs to do to distinguish the +``real'' bridges? +Whether this trick is useful depends on the level of sophistication and +dedication of the adversary. +For example, China has already demonstrated that they check IP addresses +before blocking them, and in general I worry that the more connections you +make to \emph{anything}, the more likely you are to attract attention for +further scrutiny. +How would we generate the list of decoy addresses? +If we choose it randomly from the space of IP addresses, a) most of them +will not respond, and b) we'll invite abuse complaints from security +people looking for worms. +Driving up the work factor sounds like a great feature, but it could have +the side effect that it encourages the adversary to invest in an automated +``is this a Tor bridge'' checker, which would be an unfortunate step for +them to take if they otherwise wouldn't. + +Active direct scans come with a fundamental dilemma: the more we think a +bridge has been blocked, the more we want to scan it; but the more likely +it is to be blocked, the more the adversary might already be watching for +connections to it, for example to do a ``zig-zag'' bridge enumeration +attack \cite{dingledine2011ten}. +So we need to avoid scanning bridges that we think are not blocked. +But we also need to explore more subtle scanning techniques such as the +ones below. + +\subsection{Reverse scans} + +A bridge that gets duplex% +\footnote{\url{http://en.wikipedia.org/wiki/Duplex%5C_%5C%28telecommunications%5C%29%7D%7D +blackholed by a government firewall can learn that it has been filtered by +trying to make a connection \emph{into} the filtered country. + +For example, each bridge might automatically connect to baidu.com +periodically, and publish the results of its reachability test in its +extrainfo descriptor. +We could either feed this information into the models and follow up with +other active probes if the bridge thinks it's been blocked; or we could +use it the other way by instructing bridges that we think have been +blocked to launch a reverse scan. + +We can actually take advantage of the flexibility of the Tor protocol to +do scanning from each bridge to Baidu without changing the bridge code at +all: we simply try to extend an ordinary circuit from the bridge to the +target destination, and learn at what stage the 'extend' request failed. +(We should extend the Tor control protocol% +\footnote{\url{https://trac.torproject.org/projects/tor/ticket/2576%7D%7D to +expose the type of failure to the Tor controller, but that's a simple +matter of programming.) + +Note that these reverse scans can tell us that a bridge has been blocked, +but it can't tell us that a bridge \emph{hasn't} been blocked, since it +could just be blocked in a more fine-grained way. + +Finally, how noticeable would these reverse scans be? +That is, could the government firewall enumerate bridges by keeping an eye +out for Tor-like connection attempts to Baidu? +While teaching the bridges to do the scanning themselves would require +more work, it would give us more control over how much the scans stick +out. + +\subsection{Indirect scans} + +Indirect scans use other services as reflectors. +For example, you can connect to an FTP server inside the target country, +tell it that your address is the address of the bridge you want to scan, +and then try to fetch a file. +How it fails should tell you whether that FTP server could reach the +bridge or not. + +There are many other potential reflector protocols out there, each with +their own tradeoffs. +For example, can we instruct a DNS request in-country to recurse to the +target bridge address, and distinguish between ``I couldn't reach that DNS +server'' and ``that wasn't a DNS server''? +(DNS is probably not the right protocol to use inside China, given the +amount of DNS mucking they are already known to do.) + +Another avenue is a variant on idle scanning,% +\footnote{\url{http://nmap.org/book/idlescan.html%7D%7D which takes advantage +of predictable TCP IPID patterns: send a packet directly to the bridge to +learn its current IPID, then instruct some computer in-country to send a +packet, and then send another packet directly to find the new IPID and +learn whether or not the in-country packet arrived. + +What other services can be bent to our will? +Can we advertise the bridge address on a bittorrent tracker that's popular +in China and see whether anybody connects? +Much creative research remains here. + +\section{Putting it all together} + +One of the puzzle pieces holding us back from rolling out the ``tens of +thousands of bridge addresses offered by volunteers with spare net +blocks'' plan is that we need better ways to get feedback on when +addresses get blocked. +The ideas in this blog post hopefully provide a good framework for +thinking about the problem. + +For the short term, we should deploy a basic TCP connection scanner from +inside several censoring countries (China, Iran, and Syria come to mind). +Since the ``clients report'' passive strategy still has some open research +questions, we should get all our hints from the ``bridges report'' passive +strategy. +As we're ramping up, and especially since our current bridges are either +not blocked at all (outside China), or mostly blocked (inside China), we +should feel free to do more thorough active scans to get a better +intuition about what scanning can teach us. + +In the long term, I want to use these various building blocks in a +feedback loop to identify and reward successful bridge distribution +strategies, as outlined in Levchenko and McCoy's FC 2011 paper +\cite{proximax11}. + +Specifically, we need these four building blocks: + +\begin{enumerate} + +\item A way to discover how much use a bridge is seeing from a given +country. Done: see the WECSR10 paper \cite{loesing2010case} +and usage graphs% +\footnote{\url{https://metrics.torproject.org/users.html%5C#bridge-users%7D%7D. + +\item A way to get fresh bridge addresses over time. +The more addresses we can churn through, the more aggressive we can be in +experimenting with novel distribution approaches. +See the ``more bridge addresses'' report \cite{dingledine2011strategies} +for directions here. + +\item A way to discover when a bridge is blocked in a given country. +That's what this report is about. + +\item Distribution strategies that rely on different mechanisms to make +enumeration difficult. +Beyond our ``https'' and ``gmail'' distribution strategies, we know a +variety of people in censored countries, and we can think of each of these +people as a distribution channel. + +\end{enumerate} + +We can define the \emph{efficiency} of a bridge address in terms of how +many people use it and how long before it gets blocked. +So a bridge that gets blocked very quickly scores a low efficiency, a +bridge that doesn't get blocked but doesn't see much use scores a medium +efficiency, and a popular bridge that doesn't get blocked scores high. +We can characterize the efficiency of a distribution channel as a function +of the efficiency of the bridges it distributes. +The key insight is that we then adapt how many new bridges we give to each +distribution channel based on its efficiency. +So channels that are working well automatically get more addresses to give +out, and channels that aren't working well automatically end up with fewer +addresses. + +Of course, there's more to it than that. +For example, we need to consider how to handle the presence of bridge +enumeration attacks that work independently of which distribution channel +a given bridge address was given to. +We also need to consider attacks that artificially inflate the efficiency +of bridges (and thus make us overreward distribution channels), or that +learn about a bridge but choose not to block it. +But that, as they say, is a story for another time. + +\bibliography{five-ways-test-bridge-reachability} + +\end{document} + diff --git a/2011/five-ways-test-bridge-reachability/tortechrep.cls b/2011/five-ways-test-bridge-reachability/tortechrep.cls new file mode 120000 index 0000000..4c24db2 --- /dev/null +++ b/2011/five-ways-test-bridge-reachability/tortechrep.cls @@ -0,0 +1 @@ +../../tortechrep.cls \ No newline at end of file