Seven things everyone can do now:

1. We need users like you to try Tor out, and let the Tor developers know about bugs you find or features you don't find.
2. Please consider running a server to help the Tor network grow.
3. We especially need people with Windows programming skills to run an exit server on Windows, to help us debug.
4. Run a Tor hidden service and put interesting content on it.
5. Take a look at the Tor GUI Competition, and come up with ideas or designs to contribute to making the Tor interface better. Free T-shirt for each submission!
6. Tell your friends! Get them to run servers. Get them to run hidden services. Get them to tell their friends.
7. Consider joining the Electronic Frontier Foundation. More EFF donations means more freedom in the world, including more Tor development.

Usability and Installers

1. Make an entry to the GUI competition. This requires looking at how Tor interacts with the operating system and other applications, deciding where the critical usability issues are, and trying to make an application (or applet or plugin) that helps the user experience.
2. Extend our NSIS-based windows installer to include FreeCap and/or Privoxy. When we ship Privoxy, include a config file that's preconfigured to work well with Tor.
3. Develop a way to handle OS X installation and uninstallation.
4. Choosing exit node by meta-data, e.g. country.
5. Tor provides anonymous connections, but if you want to keep multiple pseudonyms in practice (say, in case you frequently go to two websites and if anybody knew about both of them they would conclude it's you), we don't support that well yet. We should find a good approach and interface for handling pseudonymous profiles in Tor. See this post and followup for details.

Coding, Design, and Software Development

1. We need a better option for a scrubbing web proxy than just Privoxy, since it's unmaintained and still has bugs, especially on Windows. While we're at it, what sensitive information is not kept safe by Privoxy? Are there other scrubbing web proxies that are more secure?
2. We need better applications for dynamically intercepting connections and sending them through Tor. tsocks (Linux) and freecap (Windows) seem to be good candidates, and there are a variety of other possibilities listed at the bottom of our support page.
3. Speaking of tsocks, it appears to be unmaintained: we have submitted several patches with no response. Can somebody volunteer to start maintaining a new tsocks branch? We'll help.
4. We need a way to intercept DNS requests so they don't "leak" while we're trying to be anonymous. One option is to use Tor's built-in support for doing DNS resolves; but you need to ask via our new socks extension for that, and so no applications do this yet. Another option is to use Tor's controller interface: the application connects to Tor, hands it the DNS query, and Tor replies with a dummy IP address. Then the application makes a connection through Tor to that dummy IP address, and Tor automatically maps it back to the original query.
5. Right now the hidden service descriptors are being stored on just a few directory servers. This is bad for privacy and bad for robustness. To get more robustness, we're going to need to make hidden service descriptors even less private because we're going to have to mirror them onto many places. Ideally we'd like to separate the storage/lookup system from the Tor directory servers entirely. Any reliable distributed storage system will do, as long as it allows authenticated updates. As far as we know, no implemented DHT code supports authenticated updates. What's the right next step?
6. Tor exit servers need to do many DNS resolves in parallel. But gethostbyname() is poorly designed --- it blocks until it has finished resolving a query --- so it requires its own thread or process. So Tor is forced to spawn many separate DNS "worker" threads. There are some asynchronous DNS libraries out there, but traditionally they are buggy and abandoned. There are rumors that some of them are becoming better. Are any of them stable, fast, clean, and free software? If so (or if we can make that so), we should integrate them into Tor so we don't need to spawn dozens of dns resolver threads. See Agl's post for a potential start.
7. Currently Tor ships with its own AES, since when we started OpenSSL had missing/broken AES support. But now that it's gotten more mainstream, we should change things so we only use our bundled AES if OpenSSL doesn't support it natively.
8. Because Tor servers need to store-and-forward each cell they handle, high-bandwidth Tor servers end up using dozens of megabytes of memory just for buffers. We need better heuristics for when to shrink/expand buffers. Maybe this should be modelled after the Linux kernel buffer design, where you have many smaller buffers that link to each other, rather than monolithic buffers?
9. How do ulimits work on Win32, anyway? We're having problems especially on older Windowses with people running out of file descriptors, connection buffer space, etc. (We should handle WSAENOBUFS as needed, look at the MaxConnections registry entry, look at the MaxUserPort entry, and look at the TcpTimedWaitDelay entry. We may also want to provide a way to set them as needed. See bug 98.)
10. Encrypt identity keys on disk, and implement passphrase protection for them. Right now they're just stored in plaintext.
11. Periodically people running servers tell us they want to have one BandwidthRate during some part of the day, and a different BandwidthRate at other parts of the day. Rather than coding this inside Tor, we should have a little script that speaks via the Tor Controller Interface, and does a setconf to change the bandwidth rate. Perhaps it would run out of cron, or perhaps it would sleep until appropriate times and then do its tweak (that's probably more portable). Can somebody write one for us and we'll put it into tor/contrib/?
12. Patches to Tor's autoconf scripts. First, we'd like our configure.in to handle cross-compilation, e.g. so we can build Tor for obscure platforms like the Linksys WRTG54. Second, we'd like to make with-ssl-dir disable the search for ssl's libraries.
13. Implement reverse DNS requests inside Tor (already specified in Section 5.4 of tor-spec.txt).
14. Perform a security analysis of Tor with "fuzz". Determine if there good fuzzing libraries out there for what we want. Win fame by getting credit when we put out a new release because of you!
15. How hard is it to patch bind or a DNS proxy to redirect requests to Tor via our tor-resolve socks extension? What about to convert UDP DNS requests to TCP requests and send them through Tor?

Documentation

1. Volunteer to help maintain this website: code, content, css, layout.
2. We have too much documentation --- it's spread out too much and duplicates itself in places.
3. Help translate the web page and documentation into other languages. See the translation guidelines if you want to help out.
4. Investigate privoxy vs. freecap vs. sockscap for win32 clients. Are there usability or stability issues that we can track down and resolve?
5. Evaluate, create, and document a list of programs that can be routed through Tor.
6. We have a huge list of potentially useful programs to interface to Tor. Which ones are useful in which situations? Please help us test them out and document your results.

Research

1. To let dissidents in remote countries use Tor without being blocked at their country's firewall, we need a way to get tens of thousands of relays, not just a few hundred. We can imagine a Tor client GUI that has a "help China" button at the top that opens a port and relays a few KB/s of traffic into the Tor network. (A few KB/s shouldn't be too much hassle, and there are few abuse issues since they're not being exit nodes.) But how do we distribute a list of these volunteer clients to the good dissidents in an automated way that doesn't let the country-level firewalls intercept and enumerate them? Probably needs to work on a human-trust level.
2. Tor doesn't work very well when servers 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.
3. 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.
4. The "website fingerprinting attack": make a list of a few hundred popular websites, download their pages, and make a set of "signatures" for each site. Then observe a Tor client's traffic. As you watch him receive data, you quickly approach a guess about which (if any) of those sites he is visiting. Do this attack on the deployed Tor network, and find out how effective it is. Then start exploring defenses: for example, we could change Tor's cell size from 512 bytes to 1024 bytes, we could employ padding techniques like defensive dropping, or we could add traffic delays. How much of an impact do these have, and how much usability impact (using some suitable metric) is there from a successful defense in each case?
5. 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?
6. Come up with practical approximations to picking entry and exit in different routing zones.
7. Find ideal churn rate for helper nodes; how safe is it?
8. Is exiting from the middle of the circuit always a bad idea?
9. IPv6 support (For exit addresses)
10. It's not that hard to DoS Tor servers or dirservers. Are puzzles the right answer? What other practical approaches are there?

Drop by the #tor IRC channel at irc.oftc.net or email tor-volunteer@freehaven.net if you want to help out!