July 2021 - tor-dev - lists.torproject.org

GSoC 2021 - Alexa Top Sites Captcha and Tor Block Monitoring #Update
by Apratim Ranjan Chakrabarty 20 Jul '21

20 Jul '21

Hi everyone! Been quite sometime since the last update but if one wants to see the details in between one could go to the DIAL blogs for the project[1]. As of now, we do have a working project with the following details implemented [2] and further the dotted[consensus module], idea taken from Senser paper[3] haven't been implemented yet, but hopefully I'll implement it within a week or two at most. I personally was tilted towards the similarity of the structure but after some discussions with woswos and Micah Sherr[4], I've thought of implementing the content based approach too. I'll briefly describe both the methods below: + Structure of the website: This was thought of because we don't really know what various changes would be there for a website. More specifically would be useful for dynamic websites, websites with language based on geolocation (Geotargeting). But I have to use a filter list and statistical method to approach the problem. + Content based Approach: Compares the content of the HTML data using tree like structure and hashes to know how the structure is different or similar. Usage of proxies of the same locations as vantage points to get better results. That said, the above mentioned methods are used for the case where websites partially block tor. One good example for this case would be https://dan.me.uk/ which doesn't block tor exit relay nodes completely, but gives an error page (partial block) and no error HTTP response code. The checking of the HTTP response codes being a low-hanging-fruitish algorithm is our first step which is seen performing good and might sometimes result in false positives (Says a website like https://cloudflare.com to be blocked completely, when it returns captcha or is partially blocked). Further for the demo purpose, one can refer to the Experimental code[5] and it's log[6] (Isn't much of a good code and is a bit old but wrote to serve the purpose of backing up the first method (Structure of the website)). Also one could look into the `Analyzer.py`[7,8] which would contain the most recent and improved logic to the analysis. Hope to improve it with every passing day. I also plan to create a FAQ[9] page which would have excerpts of discussions or answers to as why a following approach was taken. Thanks, Apratim (irc: _ranchak_) ** Looking forward for suggestions and comments as to how to improve on it. Also materials like research paper in this domain would be helpful ** References: [1] https://hub.osc.dial.community/t/tor-project-alexa-top-sites-captcha-and-bl… [2] https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/wikis/GSoC-2021#upda… [3] http://people.cs.georgetown.edu/~wzhou/publication/senser-acsac13.pdf [4] https://seclab.cs.georgetown.edu/msherr/ [5] https://github.com/Hackhard/Fetcher/blob/b9f2fa8d09061862cf954537cbaad7921d… [6] https://raw.githubusercontent.com/Hackhard/Fetcher/main/status%20code/test_… [7] Consensus_lite branch: https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/blob/consensus_lite/… [8] Master branch: https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/blob/master/src/capt… [9] https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/wikis/GSoC-2021/Faqs

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Mostly Automatic Censorship Circumvention in Tor Browser
by Richard Pospesel 08 Jul '21

08 Jul '21

Hi Everyone, As part of our Sponsor 30 work, we are looking to improve the new about:torconnect experience by adding automatic tor settings configuration for censorship circumvention. This document outlines and discusses the *technical* challenges associated with this work, and does not go into any great detail on the right UX would be (in terms of easy of use, user trust, etc). Anyway, if you see any pitfalls or problems with anything here, do let us know. ------------------------------8<-------------------------------------- # Mostly Automatic Bridge Configuration to Bypass Internet Censorship Our goal for this work is to enable Tor Browser users to access tor without having to navigate to about:preferences#tor to configure bridges. Technically speaking, this is a trivial problem assuming you know: - which bridge settings work at the user's location - the location of the user ## Circumvention Settings Map For now, it seems sufficient to maintain a map of countries to some data-structure containing information about which censorship circumvention techniques work and which ones do not. A proposed example format can be found here: - https://gitlab.torproject.org/tpo/anti-censorship/state-of-censorship/-/blo… This map would at be distributed and updated through tor-browser releases. ### Problems #### Censorship Changes Invalidate the Map The obvious problem with distributing the censorship-circumvention settings map with Tor Browser is that if the techniques used in a location change such that old settings no longer work, you will be left with a non-functional Tor Browser with no way to update it apart from acquiring a fresh install with the updated settings or by manually configuring Tor Browser's bridge settings (so what users have to do now) A fix for this would be to provide a rules update mechanism whereby updated rules could be fetched outside of tor (via the clearnet, or over moat). Special care would need to be taken to ensure the rule updates from this automatic mechanism actually came from the Tor Project (via some sort of signature verification scheme, for example). Another wrinkle here is that rules would also need to be distributed somewhere that is difficult to censor. It seems likely that we may need different locations and mechanisms for acquiring the rule-set based on the user's location. Whatever the mechanism, updates should happen at least before the user attempts to auto-configure. Otherwise, perhaps we should periodically auto-update the the settings at a reasonable cadence. #### Time Investment to Update Map Another problem with solely distributing the rules through Tor Browser, is that censorship events would now require a Tor Browser release just to push new rules out to people. Publishing new Tor Browser releases is not a simple task, and enabling adversaries to force Tor Browser releases by tweaking their censorship systems seems like a cute way to DDOS the Applications team. An alternate update channel is definitely necessary outside of periodic Tor Browser releases. #### Are Per-Country Entries Granular Enough? One could imagine highly localized censorship events occurring which require special settings that are not needed in the rest of the country. For instance, if there is a clearnet blackout in Minneapolis, would we want to pipe *all* of our US users through the same bridges? Seems like a potential scalability problem for countries with large populations. ## Determining User Location A user's location can be determined by accessing location services through the clearnet. Mozilla offers a such a service ( https://location.services.mozilla.com/ ) with a very simple HTTP interface. Prior to bootstrapping, Tor Browser can access the location service by temporarily enabling network DNS: - network.dns.disabled=false and making an exception for the location service URL to bypass the proxy by: - network.proxy.no_proxies_on="location.services.mozilla.com" The location service would send back a country code in a JSON object which we can use to look up appropriate bridge settings in our map described above. ### Problems So the functionality of this approach is pretty easy to implement: tweak some prefs, make an XMLHttpRequest, change the prefs back. One possible problem we may face is if censors start blocking Mozilla's location services. Maybe we should have a pool of location service providers to make this more difficult (though we would need to do the research and figure out how feasible this is from a cost perspective). It is also possible to add location service functionality to moat, though this would also be a bit of an engineering endeavor. If we move forward with Mozilla's location services, we will need to acquire an API key, but I would not expect this to be an issue. We will also need to make arrangements with them to surpass the current limit of 100,000 daily API requests ( see: https://location.services.mozilla.com/terms ) The big challenge here is engineering the right UX which maintains our users trust. I think we need to be very explicit with this convenience feature, and definitely not just have it silently happen in the background. Users should also be able to opt-out, and manually select their country for the purposes of getting the right settings out of the above mentioned map. It should be very difficult to accidentally enable this automatic lookup. This will likely require a fair bit of iteration on the about:torconnect page design and flow.

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Arti development report: 23 June through July 7
by Nick Mathewson 08 Jul '21

08 Jul '21

# Arti Report 1: 23 June through July 7 We started with a project kickoff meeting. We're planning to continue meeting every 2 weeks, making our meetings increasingly public, and posting them on youtube. We wrote up an announcement to explain what Arti is and why we're doing it. The [announcement](https://blog.torproject.org/announcing-arti) goes live on July 8; it's a good place to start if you're not already familiar with the project, or if you're looking for more helpful links. To get ready for future development, and check the state of our packages we put out an [0.0.0 release](https://crates.io/crates/arti) and uploaded it to crates.io. We found a couple of issues while doing so ([#129][ticket-129], [#130][ticket-130]). We want to switch as much new-feature development to Arti as we can. With that in mind, we're developing a set of guidelines for discouraging features in C -- especially features with the potential to destabilize our code. (See [#128][ticket-128].) We're trying a bottom-up approach here, starting by identifying what kinds of features we've done recently that maybe we should skip in the future. We had an excellent meeting with developers on various Zcash projects, to gather requirements, set expectations, and open lines of communication. Based on their suggestions, we opened a set of new tickets to work on. ([#132][ticket-132] [#133][ticket-133] [#134][ticket-134] [#135][ticket-135] [#136][ticket-136] [#137][ticket-137] [#138][ticket-138] [#139][ticket-139] [#140][ticket-140]) In terms of new code written, things have been a bit slow: we're still ramping up, and a lot of folks take vacations in the summer. We [revised our asynchronous runtime backend][ticket-129] based on issues we found while uploading our packages. More recently, I've started working on an [implementation][ticket-57] of Tor's [circuit timeout inference][cti] code, which is necessary for good performance and security. ## Next steps In our next couple of weeks, I hope we get a first cut at our circuit timeout code finished and working, and figure out a reasonable initial solution for our [persistence needs][ticket-59]. We'll also be ramping up on implementations for [guard nodes][ticket-58] and [stream isolation][ticket-73], both of which are necessary for security. [ticket-57]: https://gitlab.torproject.org/tpo/core/arti/-/issues/57 [ticket-58]: https://gitlab.torproject.org/tpo/core/arti/-/issues/58 [ticket-59]: https://gitlab.torproject.org/tpo/core/arti/-/issues/59 [ticket-73]: https://gitlab.torproject.org/tpo/core/arti/-/issues/73 [ticket-128]: https://gitlab.torproject.org/tpo/core/arti/-/issues/128 [ticket-129]: https://gitlab.torproject.org/tpo/core/arti/-/issues/129 [ticket-130]: https://gitlab.torproject.org/tpo/core/arti/-/issues/130 [ticket-132]: https://gitlab.torproject.org/tpo/core/arti/-/issues/132 [ticket-133]: https://gitlab.torproject.org/tpo/core/arti/-/issues/133 [ticket-134]: https://gitlab.torproject.org/tpo/core/arti/-/issues/134 [ticket-135]: https://gitlab.torproject.org/tpo/core/arti/-/issues/135 [ticket-136]: https://gitlab.torproject.org/tpo/core/arti/-/issues/136 [ticket-137]: https://gitlab.torproject.org/tpo/core/arti/-/issues/137 [ticket-138]: https://gitlab.torproject.org/tpo/core/arti/-/issues/138 [ticket-139]: https://gitlab.torproject.org/tpo/core/arti/-/issues/139 [ticket-140]: https://gitlab.torproject.org/tpo/core/arti/-/issues/140 [cti]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/path-spec.txt#…

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