Hi, I just had a really great conversation with some of the developers at Briar about the recent work they've done in integrating some pluggable transports into their messaging application. I thought I would summarize some key points from the conversation here. In particular, this information might prove useful for both the metrics and anti-censorship teams to know how other projects are using our tools and what they would like help with. This summary is organized as follows: I. How they used metrics data II. Independent reachability tests they performed III. What they did to integrate pluggable transports into Briar IV. What they want help with I. How they used metrics data One goal the Briar project had was to automatically detect where bridges are needed so they can be enabled automatically and for Briar to "work out of the box" with minimal developer overhead as they do not currently have dedicated anti-censorship funding. Here is the related ticket with a lot of useful discussion: https://code.briarproject.org/briar/briar/issues/1266 They wrote a collection of analytics scripts that use both OONI and Tor Project metrics data to determine where bridges are being blocked. Source code for the scripts: https://code.briarproject.org/briar/tor-circumvention-analytics/ Results (outdated): https://grobox.de/tor/bridges.html They rely mostly on OONI data but also use some Tor metrics to look at the ratio of bridge users in a country. Their reasoning being that while OONI shows what doesn't work, Tor metrics data shows what is currently working. Source code for Tor metrics script: https://code.briarproject.org/briar/tor-circumvention-analytics/blob/master/... The ticket linked above has more discussion that might be useful to the metrics team as well. II. Independent reachability tests they performed To validate some of the results from OONI and Tor metrics, they ran a private bridge and did some reachability tests to it from China. They found that the bridge was not blocked, however they did not perform bandwidth tests to determine whether or not obfs4 bridges are being throttled. This makes sense as briar produces probably fairly low bandwidth traffic. When I asked them about it they suggested performing the following test: Set up obfs4 to forward connections to a local http server that servers a large, static file. Have a contact in China set up a cron job that periodically downloads the file from the bridge via obfs4. Their original reachability test was not running for very long, so it's still possible that prolonged use would result in its discovery but the fact that it wasn't blocked suggests that China might be enumerating bridges through the distribution mechanism rather than by something identifiable from obfs4 itself. We should perform our own tests to verify this, and to check for throttling. III. What they did to integrate pluggable transports into Briar My understanding from the issue text and commit messages about how briar decides whether to use pluggable transports is to use the output of their reachability analysis to determine user needs by country. If Tor is not blocked, users just use vanilla Tor If it is they use obfs4. If obfs4 is blocked, then they use meek (meek lite). Right now the countries that use meek in briar are: China, Iran, Egypt, Belarus, Turkey, Syria, and Venezuela. As far as the integration, they wrote some code that makes reproducible builds of obfs4 and meek and spits out a java/android library: https://code.briarproject.org/briar/go-reproducer Briar already uses Tor, so they configure these bridges in the usual way using a torrc file and a hardcoded, shipped file of bridge information (which I believe are also the default bridges used by Tor Browser). They decided they didn't want to maintain any private bridges like the one they used for their reachability tests. One of the concerns there was the ability to fingerprint Briar traffic by bridge connection and differentiate it from other Tor traffic. IV. What they want help with Because they do not have dedicated anti-censorship funding, they mentioned a few things that would help them maintain their pluggable transport use going forward and ease the integration of pluggable transports. The main thing they would like on the metrics side is up-to-date information about which PT works in which country and where PTs are needed at all in order to make quick and easy decisions based on location about which transports to use. They started to work with OONI to expand their tests but it turned out to be too much work for their time/funding: https://code.briarproject.org/briar/briar/issues/1414 It's on our roadmap to work with OONI and other censorship measurement tools (like Censored Planet) to expand our tests so we should get into contact with them again once we have gotten farther with this. On the PT development side, they expressed a desire to transfer maintenance of their reproducible builds of obfs4 and meek to someone else (that's the go-reproducer code linked above).