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You need to change some of your habits, as some things won't work exactly as you are used to. Please read the full list of warnings for details.

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Want Tor to really work?

You need to change some of your habits, as some things won't work exactly as you are used to.

1. Use the Tor Browser Tor does not protect all of your computer's Internet traffic when you run it. Tor only protects your applications that are properly configured to send their Internet traffic through Tor. To avoid problems with Tor configuration, we strongly recommend you use the Tor Browser Bundle. It is pre-configured to protect your privacy and anonymity on the web as long as you're browsing with the Tor Browser itself. Almost any other web browser configuration is likely to be unsafe to use with Tor.
   
2. Don't enable or install browser plugins The Tor Browser will block browser plugins such as Flash, RealPlayer, Quicktime, and others: they can be manipulated into revealing your IP address. Similarly, we do not recommend installing additional addons or plugins into the Tor Browser, as these may bypass Tor or otherwise harm your anonymity and privacy. The lack of plugins means that Youtube videos are blocked by default, but Youtube does provide an experimental opt-in feature (enable it here) that works for some videos.
   
3. Use HTTPS versions of websites Tor will encrypt your traffic to and within the Tor network, but the encryption of your traffic to the final destination website depends upon on that website. To help ensure private encryption to websites, the Tor Browser Bundle includes HTTPS Everywhere to force the use of HTTPS encryption with major websites that support it. However, you should still watch the browser URL bar to ensure that websites you provide sensitive information to display a blue or green URL bar button, include https:// in the URL, and display the proper expected name for the website.
   
4. Don't open documents downloaded through Tor while online The Tor Browser will warn you before automatically opening documents that are handled by external applications. DO NOT IGNORE THIS WARNING. You should be very careful when downloading documents via Tor (especially DOC and PDF files) as these documents can contain Internet resources that will be downloaded outside of Tor by the application that opens them. This will reveal your non-Tor IP address. If you must work with DOC and/or PDF files, we strongly recommend either using a disconnected computer, downloading the free VirtualBox and using it with a virtual machine image with networking disabled, or using Tails. Under no circumstances is it safe to use BitTorrent and Tor together, however.
   
5. Use bridges and/or find company Tor tries to prevent attackers from learning what destination websites you connect to. However, by default, it does not prevent somebody watching your Internet traffic from learning that you're using Tor. If this matters to you, you can reduce this risk by configuring Tor to use a Tor bridge relay rather than connecting directly to the public Tor network. Ultimately the best protection is a social approach: the more Tor users there are near you and the more diverse their interests, the less dangerous it will be that you are one of them. Convince other people to use Tor, too!
   

Be smart and learn more. Understand what Tor does and does not offer. This list of pitfalls isn't complete, and we need your help identifying and documenting all the issues.

A few things everyone can do now:

1. Please consider running a relay to help the Tor network grow.
2. Do you have an Amazon account? Are you willing to spend up to $3 a month? Then spin up your own Tor bridge in less than 10 minutes with tor cloud!
3. Tell your friends! Get them to run relays. Get them to run hidden services. Get them to tell their friends.
4. If you like Tor's goals, please take a moment to donate to support further Tor development. We're also looking for more sponsors — if you know any companies, NGOs, agencies, or other organizations that want anonymity / privacy / communications security, let them know about us.
5. We're looking for more good examples of Tor users and Tor use cases. If you use Tor for a scenario or purpose not yet described on that page, and you're comfortable sharing it with us, we'd love to hear from you.

Documentation

1. Help translate the documentation into other languages. See the translation guidelines if you want to help out. We especially need Arabic or Farsi translations, for the many Tor users in censored areas.
2. Evaluate and document our list of programs that can be configured to use Tor.
3. We have a huge list of potentially useful programs that interface to Tor. Which ones are useful in which situations? Please help us test them out and document your results.

Advocacy

1. Create a presentation that can be used for various user group meetings around the world.
2. Create a video about the positive uses of Tor, what Tor is, or how to use it. Some have already started on Tor's Media server, Howcast, and YouTube.
3. Create a poster, or a set of posters, around a theme, such as "Tor for Freedom!"
4. Create a t-shirt design that incorporates "Congratulations! You are using Tor!" in any language.

Projects

Below are a list of Tor related projects we're developing and/or maintaining. Most discussions happen on IRC so if you're interested in any of these (or you have a project idea of your own), then please join us in #tor-dev. Don't be shy to ask questions, and don't hesitate to ask even if the main contributors aren't active at that moment.

Name	Category	Language	Activity	Contributors
Tor	Core	C	Heavy	nickm, athena, arma
*JTor	Core	Java	None	bleidl
TBB	Bundle	Sys Admin	Moderate	Erinn
Tor Browser	Browser	C	Moderate	mikeperry, pearl crescent
Torbutton	Browser Add-on	Javascript	Moderate	mikeperry
HTTPS Everywhere	Browser Add-on	Javascript	Moderate	pde, mikeperry
Vidalia	User Interface	C++, Qt	None	chiiph
Arm	User Interface	Python, Curses	Light	atagar
Orbot	User Interface	Java	Light	n8fr8, ioerror
Tails	OS image	Sys Admin	Heavy	#tails
tor-ramdisk	OS image	Sys Admin	Light	blueness
*Torouter	OS image	Sys Admin	None	ioerror
Torsocks	Usability	C	Moderate	ioerror, nickm
TorBirdy	Browser Add-on	JavaScript	Heavy	Sukhbir (sukhe), ioerror
Obfsproxy	Client Add-on	C, Python	Moderate	asn, nickm
Flash Proxy	Client Add-on	Python, JavaScript, Go	Heavy	dcf, aallai, jct
*Thandy	Updater	Python	None	chiiph, Erinn, nickm
Shadow	Simulator	C, Python	Heavy	robgjansen
Stem	Library	Python	Heavy	atagar, neena
Txtorcon	Library	Python, Twisted	Heavy	meejah
Tlsdate	Utility	C	Light	ioerror
Metrics	Client Service	Java	Heavy	karsten
Atlas	Client Service	JavaScript	Light	hellais
TorStatus	Client Service	Python, Django	None
Compass	Client Service	Python	Light	gsathya, karsten
Onionoo	Backend Service	Java, Python	Moderate	karsten, gsathya
Weather	Client Service	Python	None	kaner
GetTor	Client Service	Python	None	kaner
TorCheck	Client Service	Python, Perl	None	ioerror
BridgeDB	Backend Service	Python	None	kaner, nickm
Ooni Probe	Scanner	Python	Heavy	hellais, isis, ioerror
TorFlow	Backend Service	Python	None	aagbsn, mikeperry
*TorBEL	Backend Service	Python	None	Sebastian
Tor2web	Client Service	Python	Heavy	hellais
Anonbib	Website	Python	Light	arma, nickm

_{* Project is still in an alpha state.}

Tor (code, bug tracker)

Central project, providing the core software for using and participating in the Tor network. Numerous people contribute to the project to varying extents, but the chief architects are Nick Mathewson and Roger Dingledine.
Project Ideas:
Run With Limited Capabilities
Tor Codebase Cleanup
HTTPS Server Impersonation
Make Chutney Do More, More Reliably

JTor (code, bug tracker)

Java implementation of Tor and successor to OnionCoffee. This project isn't yet complete, and has been inactive since Fall 2010.

Tor Browser Bundle (code, bug tracker, design doc)

The Tor Browser Bundle is an easy-to-use portable package of Tor, Vidalia, Torbutton, and a Firefox fork preconfigured to work together out of the box. This is actively being worked on by Erinn Clark.

Tor Browser (code, bug tracker)

The Tor Browser is a modified Firefox that aims to resolve the privacy and security issues in mainline Firefox.

Torbutton (code, bug tracker)

Firefox addon that addresses many of the client-side threats to browsing the Internet anonymously. Mike has since continued to adapt it to new threats, updated versions of Firefox, and possibly Chrome as well.

HTTPS Everywhere (code, bug tracker)

HTTPS Everywhere is a Firefox and Chrome extension that encrypts your communications with many major websites, making your browsing more secure.

Vidalia (code, bug tracker)

The most commonly used user interface for Tor. Matt Edman started the project in 2006 and brought it to its current stable state. Development slowed for several years, though Tomás Touceda has since taken the lead with pushing the project forward.

Arm (code, bug tracker)

The anonymizing relay monitor (arm) is a terminal status monitor for Tor, intended for command-line aficionados, ssh connections, and anyone with a tty terminal. This works much like top does for system usage, providing real time statistics for bandwidth, resource usage, connections, and quite a bit more.

Orbot (code, bug tracker)

Provides Tor on the Android platform. This was under very active development up through Fall 2010, after which things have been quiet.

The Amnesic Incognito Live System (code, bug tracker)

The Amnesic Incognito Live System is a live CD/USB distribution preconfigured so that everything is safely routed through Tor and leaves no trace on the local system. This is a merger of the Amnesia and Incognito projects, and still under very active development.

Tor-ramdisk (code, documentation)

Tor-ramdisk is a uClibc-based micro Linux distribution whose sole purpose is to securely host a Tor server purely in RAM.

Torouter (code, bug tracker)

Project to provide an easy-to-use, embedded Tor instance for routers. This had high activity in late 2010, but has since been rather quiet.

Torsocks (code, bug tracker)

Utility for adapting other applications to work with Tor. Development has slowed and compatibility issues remain with some platforms, but it's otherwise feature complete.

TorBirdy (code, bug tracker)

TorBirdy is Torbutton for Thunderbird and related Mozilla mail clients.
Project Ideas:
Improving TorBirdy

Obfsproxy (C codebase, python codebase, bug tracker)

A proxy that shapes Tor traffic, making it harder for censors to detect and block Tor. This has both a C and python implementation.

Flash Proxy (code, bug tracker)

Pluggable transport using proxies running in web browsers to defeat address-based blocking.

Thandy (code)

Updater for Tor. The project began in the Summer of 2008 and produced a functional secure updater. The remaining steps are to deploy a Thandy repository, and integrate Thandy into one of our bundles — which requires UI changes in the bundles, Python packaging on various platforms, etc. Later interest in it was rekindled and many aspects of its design (including the language it'll be in) are currently in flux. This is related to TUF.

Shadow (code, bug tracker)

Shadow is a discrete-event network simulator that runs the real Tor software as a plug-in. Shadow is open-source software that enables accurate, efficient, controlled, and repeatable Tor experimentation. For another simulator, see ExperimenTor.

Stem (code, bug tracker)

Python controller library for scripts and controller applications using Tor.
Project Ideas:
Stem Usability Improvements

Txtorcon (code, docs)

Twisted-based asynchronous Tor control protocol implementation. Includes unit-tests, examples, state-tracking code and configuration abstraction. Used by OONI and APAF.

Tlsdate (code)

tlsdate: secure parasitic rdate replacement
tlsdate sets the local clock by securely connecting with TLS to remote servers and extracting the remote time out of the secure handshake. Unlike ntpdate, tlsdate uses TCP, for instance connecting to a remote HTTPS or TLS enabled service, and provides some protection against adversaries that try to feed you malicious time information.

Metrics (code: db, utils, web, bug tracker)

Processing and analytics of consensus data, provided to users via the metrics portal. This has been under active development for several years by Karsten Loesing. See also TorPerf.
Project Ideas:
Searchable Tor descriptor and Metrics data archive (Python/Django?)

Atlas (code)

Atlas is a web application to discover Tor relays and bridges. It provides useful information on how relays are configured along with graphics about their past usage. This is the third evolution of the TorStatus application.

TorStatus (code)

Portal providing an overview of the Tor network, and details on any of its current relays. Though very actively used, this project has been unmaintained for a long while. The original codebase was written in PHP, and students from Wesleyan wrote the new Django counterpart.

Compass (code, bug tracker)

Compass is a web and command line application that filters and aggregates the Tor relays based on various attributes.
Project Ideas:
Compass Refactoring

Onionoo (java codebase, python codebase, bug tracker)

Onionoo is a JSON based protocol to learn information about currently running Tor relays and bridges.

Weather (code, bug tracker)

Provides automatic notification to subscribed relay operators when their relay's unreachable. This underwent a rewrite by the Wesleyan HFOSS team, which went live in early 2011.

GetTor (code, bug tracker)

E-mail autoresponder providing Tor's packages over SMTP. This has been relatively unchanged for quite a while.

TorCheck (code, bug tracker)

Provides a simple site for determining if the visitor is using Tor or not. This has been relatively unchanged for quite a while.

BridgeDB (code, bug tracker)

Backend bridge distributor, handling the various pools they're distributed in. This was actively developed until Fall of 2010.

Ooni Probe (code, bug tracker)

Censorship scanner, checking your local connection for blocked or modified content.

TorFlow (code, bug tracker)

Library and collection of services for actively monitoring the Tor network. These include the Bandwidth Scanners (measuring throughput of relays) and SoaT (scans for malicious or misconfigured exit nodes). SoaT was last actively developed in the Summer of 2010, and the Bandwidth Scanners a few months later. Both have been under active use since then, but development has stopped.

TorBEL (code, bug tracker)

The Tor Bulk Exitlist provides a method of identifying if IPs belong to exit nodes or not. This is a replacement for TorDNSEL which is a stable (though unmaintained) Haskell application for this purpose. The initial version of TorBEL was started in GSOC 2010 but since then the project has been inactive.

Tor2web (code)

Tor2web allows Internet users to browse websites running in Tor hidden services. It trades user anonymity for usability by allowing anonymous content to be distributed to non-anonymous users.

Anonymity Bibliography (code)

Anonbib is a list of important papers in the field of anonymity. It's also a set of scripts to generate the website from Latex (bibtex). If we're missing any important papers, please let us know!

Project Ideas

You may find some of these projects to be good ideas for Google Summer of Code and the Outreach Program for Women. We have labelled each idea with how useful it would be to the overall Tor project (priority), how much work we expect it would be (effort level), how much clue you should start with (skill level), and which of our core developers would be good mentors. If one or more of these ideas looks promising to you, please contact us to discuss your plans rather than sending blind applications. You may also want to propose your own project idea — which often results in the best applications.

1. Compass Refactoring
   Effort Level: Medium
   Skill Level: Medium
   Likely Mentors: gsathya, karsten Compass was first designed to be a cli app and then hacked into a web app. The codebase needs to be refactored such that the main processing code is separated from the display functions(probably into separate files) and made modular so that adding more features (#6612) is easy. For example, the main processing logic could be in compass.py, whereas print_groups() and other display related functions could be part of a separate cli.py; web.py would also have to modified to make use of this new modular code. Bonus points for adding features to compass(#6619, #6612, #6728).
   
2. HTTPS Server Impersonation
   Effort Level: Medium to High
   Skill Level: Medium to High
   Likely Mentors: Nick (nickm) We have an open proposal for a way to make Tor bridges avoid censorship by impersonating an HTTPS server. Specifically, we need to hack some popular SSL "reverse proxy" (your choice) so that it relays regular web connections to an HTTP server, but certain connections to a local Tor process. Proposal 203 has a general design sketch.
   
3. Improving TorBirdy
   Effort Level: High
   Skill Level: Medium to High
   Likely Mentors: Sukhbir, Jacob TorBirdy is Torbutton for Thunderbird, Icedove and related Mozilla mail clients.
   TorBirdy is under active development and is available from our wiki and mozilla's addons site.
   The goal of this project is to improve TorBirdy by:
   
   • Writing a Thunderbird patch to plug known leaks. We have already submitted a patch to Thunderbird but we anticipate there will be much more work required before it is accepted, possibly involving rewriting the entire patch; this appears trivial, but it is not, as we are proposing a completely new privacy-safe message-ID header generation algorithm for Thunderbird.
   • Implementing a JavaScript HTTP proxy (see the ticket)
   • Auditing TorBirdy for leaks and for use with other add-ons (as an example see its ticket)
   A student undertaking this project should have some C++ and JavaScript development experience. Previous experience with Firefox/Thunderbird extension development is a plus, but not required.
   
4. Make Chutney Do More, More Reliably
   Effort Level: Medium to High, depending on scope of project
   Skill Level: Medium
   Likely Mentors: Nick (nickm) We have a little Python tool called Chutney for making small local test networks. It's small, not widely used, and not as automated as it could be.
   It would be great to see chutney extended and a set of supporting tests built to the point where we could use Chutney to exercise various Tor features as an automated integration test.
   
5. Run With Limited Capabilities
   Effort Level: Medium to High
   Skill Level: High
   Likely Mentors: Nick (nickm) Many modern operating systems give a running program the ability to drop capabilities that it no longer needs, and other ways for a program to run pieces of itself in a sandbox with diminished privileges.
   We'd like to do this with Tor, to improve its resistance to attacks. The easiest areas to address would be on systems like recent Linux kernels that make it easy to drop or restrict the set of syscalls that a program can invoke. That's a great project, but probably not big enough for an internship just on its own. For that, we'd want to make progress on at least multiple platforms, or look into refactoring Tor into pieces that need more privileges and pieces that don't with an eye towards sandboxing them differently.
   See tickets #7005 and #5219, and their descendants, for more information.
   
6. Searchable Tor descriptor and Metrics data archive
   Effort Level: Medium
   Skill Level: Medium
   Likely Mentors: Karsten The Metrics data archive of Tor relay descriptors and other Tor-related network data has grown to over 100G in size, bz2-compressed. We have developed two search interfaces: the relay search finds relays by nickname, fingerprint, or IP address in a given month; ExoneraTor finds whether a given IP address was a relay on a given day.
   We'd like to have a more general search application for Tor descriptors and metrics data. There are more descriptor types that we'd like to include in the search. The search application should handle most of them and understand some semantics like what's a timestamp, what's an IP address, and what's a link to another descriptor. Users should then be able to search for arbitrary strings or limit their search to given time periods or IP address ranges. Descriptors that reference other descriptors should contain links, and descriptors should be able to say from where they are linked. The goal is to make the archive easily browsable.
   The search application shall be separate from the metrics website and shouldn't rely on the metrics website codebase. The search application will contain hourly updated descriptor data from the metrics website via rsync. Programming language and database system are not specified yet, though there's a slight preference for Python/Django and Postgres for maintenance reasons. If there are good reasons to pick something else, e.g, some NoSQL variant or some search application framework, that's fine, too. Further requirements are that lookups should be really fast and that changes to the search application can be implemented in reasonable time.
   Applications for this project should come with a design of the proposed search application, ideally with a proof-of-concept based on a subset of the available data to show that it will be able to handle the 100G+ of data.
   
7. Stem Usability Improvements
   Effort Level: Medium
   Skill Level: Medium
   Likely Mentors: Damian (atagar) Stem is a python controller library for tor. Like it's predecessor, TorCtl, it uses tor's control protocol to help developers program against the tor process, enabling them to build things similar to Vidalia and arm.
   While TorCtl provided a fine first draft for this sort of functionality, it has not proved to be extensible nor maintainable. Stem is a rewrite of TorCtl with a heavy focus on testing, documentation, and providing a developer friendly API.
   Stem is very nearly feature complete but presently has no users. We want to change that prior to making our first release for a couple reasons...
   
   • Make sure that we have a reasonably good API, and improve the rough edges that hurt its usability.
   • Provide examples for how stem can be used.
   This project involves several tasks...
   
   1. Move stem's site to Tor's website (ticket)
   2. Set up Piwik for our site (ticket)
   3. Come up with a better, more developer friendly "Module Overview" for our documentation (example page). For instance, it would be nice to provide interlinking between the overview and the classes/methods that it lists. This will probably involve asking for help from the Sphinx user list.
   4. Finally get your hands dirty using stem. We want to expand stem's tutorial page with more examples. To do this you'll want to both brainstorm some of your own and contact the tor-dev@ email list to solicit ideas. This last step is pretty open ended, so go nuts with whatever you think will improve stem's usability!
8. Tor Codebase Cleanup
   Effort Level: Low to High, depending on subproject chosen
   Skill Level: Medium to High
   Likely Mentors: Nick (nickm) The Tor code is almost 10 years old in places, and we haven't always had enough time or wisdom to write things as well as we could have. Our unit test coverage is shamefully low, and the dependency graph of our modules is shamefully convoluted . We could use refactoring and unit tests! Please look through the Tor source code and look for ugly or tricky code or dependencies -- the uglier and trickier the better -- and think about how you could make the code look better, read better, and (subject to testing) work better.
   If this is for a fun side-project, it would be great for you to work on anything that can be made better and more tested. For an internship-level position, we'd hope that you could find a number of particularly tricky or knotty piece of the code to clean up, and aim for resolving the ugliest problems, not necessarily the easiest.
   For a big project here, it would be great to pick one of the major "submodules" of Tor -- path selection, node discovery, directory authority operations, directory service -- and refactor its interface completely, to minify and codify its points of contact with the rest of Tor.
   
9. Website and video documentation update
   Effort Level: Medium
   Skill Level: Medium
   Likely Mentors: Runa, Karen Identify outdated and/or weak pages on torproject.org and re-write the documentation to appeal to a less technical, more goal-oriented audience. Create a number of 3-5 minute videos with graphically portray the written documentation. See the website tickets for more information and a starting point.
   
10. Bring up new ideas!
    Don't like any of these? Look at the Tor development roadmap for more ideas, or just try out Tor, Vidalia, and Torbutton, and find out what you think needs fixing. Some of the current proposals might also be short on developers.

Other Coding and Design related ideas

1. Tor relays don't work well on Windows XP. On Windows, Tor uses the standard select() system call, which uses space in the non-page pool. This means that a medium sized Tor relay will empty the non-page pool, causing havoc and system crashes. We should probably be using overlapped IO instead. One solution would be to teach libevent how to use overlapped IO rather than select() on Windows, and then adapt Tor to the new libevent interface. Christian King made a good start on this in the summer of 2007.
2. We need to actually start building our blocking-resistance design. This involves fleshing out the design, modifying many different pieces of Tor, adapting Vidalia so it supports the new features, and planning for deployment.
3. We need a flexible simulator framework for studying end-to-end traffic confirmation attacks. Many researchers have whipped up ad hoc simulators to support their intuition either that the attacks work really well or that some defense works great. Can we build a simulator that's clearly documented and open enough that everybody knows it's giving a reasonable answer? This will spur a lot of new research. See the entry below on confirmation attacks for details on the research side of this task — who knows, when it's done maybe you can help write a paper or three also.
4. Tor 0.1.1.x and later include support for hardware crypto accelerators via OpenSSL. It has been lightly tested and is possibly very buggy. We're looking for more rigorous testing, performance analysis, and optimally, code fixes to OpenSSL and Tor if needed.
5. Perform a security analysis of Tor with "fuzz". Determine if there are good fuzzing libraries out there for what we want. Win fame by getting credit when we put out a new release because of you!
6. Tor uses TCP for transport and TLS for link encryption. This is nice and simple, but it means all cells on a link are delayed when a single packet gets dropped, and it means we can only reasonably support TCP streams. We have a list of reasons why we haven't shifted to UDP transport, but it would be great to see that list get shorter. We also have a proposed specification for Tor and UDP — please let us know what's wrong with it.
7. We're not that far from having IPv6 support for destination addresses (at exit nodes). If you care strongly about IPv6, that's probably the first place to start.
8. We need a way to generate the website diagrams (for example, the "How Tor Works" pictures on the overview page from source, so we can translate them as UTF-8 text rather than edit them by hand with Gimp. We might want to integrate this as an wml file so translations are easy and images are generated in multiple languages whenever we build the website.
9. How can we make the various LiveCD/USB systems easier to maintain, improve, and document? One example is The Amnesic Incognito Live System.
10. Another anti-censorship project is to try to make Tor more scanning-resistant. Right now, an adversary can identify Tor bridges just by trying to connect to them, following the Tor protocol, and seeing if they respond. To solve this, bridges could act like webservers (HTTP or HTTPS) when contacted by port-scanning tools, and not act like bridges until the user provides a bridge-specific key. To start, check out Shane Pope's thesis and prototype.

Research

1. The "end-to-end traffic confirmation attack": by watching traffic at Alice and at Bob, we can compare traffic signatures and become convinced that we're watching the same stream. So far Tor accepts this as a fact of life and assumes this attack is trivial in all cases. First of all, is that actually true? How much traffic of what sort of distribution is needed before the adversary is confident he has won? Are there scenarios (e.g. not transmitting much) that slow down the attack? Do some traffic padding or traffic shaping schemes work better than others?
2. A related question is: Does running a relay/bridge provide additional protection against these timing attacks? Can an external adversary that can't see inside TLS links still recognize individual streams reliably? Does the amount of traffic carried degrade this ability any? What if the client-relay deliberately delayed upstream relayed traffic to create a queue that could be used to mimic timings of client downstream traffic to make it look like it was also relayed? This same queue could also be used for masking timings in client upstream traffic with the techniques from adaptive padding, but without the need for additional traffic. Would such an interleaving of client upstream traffic obscure timings for external adversaries? Would the strategies need to be adjusted for asymmetric links? For example, on asymmetric links, is it actually possible to differentiate client traffic from natural bursts due to their asymmetric capacity? Or is it easier than symmetric links for some other reason?
3. Repeat Murdoch and Danezis's attack from Oakland 05 on the current Tor network. See if you can learn why it works well on some nodes and not well on others. (My theory is that the fast nodes with spare capacity resist the attack better.) If that's true, then experiment with the RelayBandwidthRate and RelayBandwidthBurst options to run a relay that is used as a client while relaying the attacker's traffic: as we crank down the RelayBandwidthRate, does the attack get harder? What's the right ratio of RelayBandwidthRate to actually capacity? Or is it a ratio at all? While we're at it, does a much larger set of candidate relays increase the false positive rate or other complexity for the attack? (The Tor network is now almost two orders of magnitude larger than it was when they wrote their paper.) Be sure to read Don't Clog the Queue too.
4. The "routing zones attack": most of the literature thinks of the network path between Alice and her entry node (and between the exit node and Bob) as a single link on some graph. In practice, though, the path traverses many autonomous systems (ASes), and it's not uncommon that the same AS appears on both the entry path and the exit path. Unfortunately, to accurately predict whether a given Alice, entry, exit, Bob quad will be dangerous, we need to download an entire Internet routing zone and perform expensive operations on it. Are there practical approximations, such as avoiding IP addresses in the same /8 network?
5. Other research questions regarding geographic diversity consider the tradeoff between choosing an efficient circuit and choosing a random circuit. Look at Stephen Rollyson's position paper on how to discard particularly slow choices without hurting anonymity "too much". This line of reasoning needs more work and more thinking, but it looks very promising.
6. Tor doesn't work very well when relays have asymmetric bandwidth (e.g. cable or DSL). Because Tor has separate TCP connections between each hop, if the incoming bytes are arriving just fine and the outgoing bytes are all getting dropped on the floor, the TCP push-back mechanisms don't really transmit this information back to the incoming streams. Perhaps Tor should detect when it's dropping a lot of outgoing packets, and rate-limit incoming streams to regulate this itself? I can imagine a build-up and drop-off scheme where we pick a conservative rate-limit, slowly increase it until we get lost packets, back off, repeat. We need somebody who's good with networks to simulate this and help design solutions; and/or we need to understand the extent of the performance degradation, and use this as motivation to reconsider UDP transport.
7. A related topic is congestion control. Is our current design sufficient once we have heavy use? Maybe we should experiment with variable-sized windows rather than fixed-size windows? That seemed to go well in an ssh throughput experiment. We'll need to measure and tweak, and maybe overhaul if the results are good.
8. Our censorship-resistance goals include preventing an attacker who's looking at Tor traffic on the wire from distinguishing it from normal SSL traffic. Obviously we can't achieve perfect steganography and still remain usable, but for a first step we'd like to block any attacks that can win by observing only a few packets. One of the remaining attacks we haven't examined much is that Tor cells are 512 bytes, so the traffic on the wire may well be a multiple of 512 bytes. How much does the batching and overhead in TLS records blur this on the wire? Do different buffer flushing strategies in Tor affect this? Could a bit of padding help a lot, or is this an attack we must accept?
9. Tor circuits are built one hop at a time, so in theory we have the ability to make some streams exit from the second hop, some from the third, and so on. This seems nice because it breaks up the set of exiting streams that a given relay can see. But if we want each stream to be safe, the "shortest" path should be at least 3 hops long by our current logic, so the rest will be even longer. We need to examine this performance / security tradeoff.
10. It's not that hard to DoS Tor relays or directory authorities. Are client puzzles the right answer? What other practical approaches are there? Bonus if they're backward-compatible with the current Tor protocol.
11. Programs like Torbutton aim to hide your browser's UserAgent string by replacing it with a uniform answer for every Tor user. That way the attacker can't splinter Tor's anonymity set by looking at that header. It tries to pick a string that is commonly used by non-Tor users too, so it doesn't stand out. Question one: how badly do we hurt ourselves by periodically updating the version of Firefox that Torbutton claims to be? If we update it too often, we splinter the anonymity sets ourselves. If we don't update it often enough, then all the Tor users stand out because they claim to be running a quite old version of Firefox. The answer here probably depends on the Firefox versions seen in the wild. Question two: periodically people ask us to cycle through N UserAgent strings rather than stick with one. Does this approach help, hurt, or not matter? Consider: cookies and recognizing Torbutton users by their rotating UserAgents; malicious websites who only attack certain browsers; and whether the answers to question one impact this answer.
12. How many bridge relays do you need to know to maintain reachability? We should measure the churn in our bridges. If there is lots of churn, are there ways to keep bridge users more likely to stay connected?

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