- anti-censorship-team - lists.torproject.org

Snowflake rendezvous design meeting 2021-05-18 14:00
by David Fifield 18 May '21

18 May '21

We'll have a meeting tomorrow to talk about redesigning changing the format of client–broker rendezvous messages, in order to support client registration methods other than domain-fronted HTTPS. To save time, this message has a proposed agenda and background information to read before the meeting. - Answer questions about background, the status quo, and future broker development goals. - Message versioning - Inside or outside JSON? - Backward compatibility with legacy-format messages. - Future need for encrypted/signed messages. - How do alternative rendezvous methods interface with the broker? - Same process or separate processes? - If separate, then how does IPC with the broker work? Convert registrations to localhost HTTP, or create some other local communication protocol? Recent work on this topic is: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… Background A client sends a message to the Snowflake broker to request proxy service. See https://www.bamsoftware.com/papers/thesis/#p279 for a high-level description. Concretely, the client sends a domain-fronted HTTP POST request with a body that is a JSON serialization of an SDP offer (https://pkg.go.dev/github.com/pion/webrtc/v3#SessionDescription): https://gitweb.torproject.org/pluggable-transports/snowflake.git/tree/clien… A request looks like this: POST /client HTTP/1.1 Snowflake-NAT-Type: restricted {"type":"offer","sdp":"v=0\r\n..."} If the broker can match the client with a proxy, the response has status 200 and the body is the JSON serialization of an SDP answer: HTTP/1.1 200 OK {"type":"answer","sdp":"v=0\r\n..."} If the broker cannot make a match, the response has a status of 503 or 504, and an empty body: HTTP/1.1 503 Service Unavailable If there is a syntax problem with the client's offer, the response has status 400: HTTP/1.1 400 Bad Request The problem with the current message formats is that they rely of features of HTTP not present in other protocols, and they are not easily extensible. The HTTP bodies are assumed to contain an SDP offer/answer and that only, so any side information has to go somewhere else, such as Snowflake-NAT-Type which is sent in an HTTP header (#34129). The 200/503/400 status codes also are not present in other protocols. Parts of what is conceptually a single message are scattered across the HTTP body, header, and status code. The goal is to move all the information into a common message format that can also be used by other rendezvous channels. That means *all* the necessary information will be present in the HTTP body, and status codes will always be 200. Some past design sketches: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… When the client requests /client, the broker returns either a 200 with a response body, an empty 503, or an empty 400. This is awkward when doing rendezvous over non-HTTP channels, or even over AMP cache, which doesn't reliably pass through the server's original status code... It would be easier if all the necessary information in the broker's response were in the HTTP body, because that's easier to port to other channels. https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… I propose to remove this ambiguity by including the data payload inside the JSON object that represents an error. Just like in an HTTP response, we have two things—a status code and a message—inside one package. We have already made a similar change to /proxy messages, for example changing status codes 200 and 504 to the strings "client match" and "no match": https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… https://github.com/cohosh/snowflake/pull/8/files We don't have plans to use anything other than HTTP for proxy–broker messages, so it was less important in that case to remove all reliance on HTTP. We are also generally less concerned about backward compatibility for proxies than we are for clients. cohosh has drafted a merge request that moves almost all message information into JSON in HTTP bodies: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… The above client request would become: POST /client HTTP/1.1 {"Version":"1.0","Offer":{"type":"offer","sdp":"v=0\r\n..."},"NAT":"restricted"} In the event of a match, the responses would be: HTTP/1.1 200 OK {"Answer":{"type":"answer","sdp":"v=0\r\n..."}} If no match: HTTP/1.1 200 OK {"Error":"timed out waiting for answer!"} Parsing errors currently still use an HTTP status code: HTTP/1.1 400 Bad Request This work is primarily towards the goal of supporting other rendezvous methods: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… But we also want to keep in mind other development goals for the Snowflake broker. We want to encrypt and authenticate broker messages in the future, and the message format should facilitate that, without our having to think of another backcompat scheme in the future: https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… And we want to be able to move the broker away from being one big process that manages everything. Ideally, different rendezvous methods run in different processes, with reduced privileges, and can crash and be restarted independently of the main broker. https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow…

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Branch name changes on anti-censorship team repositories
by Cecylia Bocovich 11 May '21

11 May '21

Hey everyone, We're in the process of switching to using `main` as the default branch in our repositories. I've created a ticket to document and manage this switch: https://gitlab.torproject.org/tpo/anti-censorship/team/-/issues/6 The impact to contributors will be small. Those who have already cloned the repository just need to remember to develop new patches off of the `main` branch. Merge requests in Gitlab will automatically open against the default branch, so if your MR has more commits than you expect you'll just need to rebase it onto the main branch. The hardest part is on the maintainer side since we manage so many repositories. I've tried to document all the repositories in the ticket above, but left out some very old unmaintained repos (such as pyptlib) that I don't have permission to maintain. If in the unlikely event these projects are brought back to life we can deal with a branch change then. If you think anything on or off this list is a mistake, feel free to comment on the ticket! For the procedure, I'll be running this script written by anarcat (our sysadmin): https://gitlab.com/anarcat/scripts/-/blob/main/git-branch-rename-remote The script will generally: - run `git branch --move master main` - change symbolic refs for remote: `git symbolic-ref refs/remotes/remote/HEAD \ refs/remotes/remote/main` - set local branch to track new upstream `git branch -u remote/main` - push new main branch to remote `git push -u remote main` - remove old master branch from remote: `git push -d remote master` - For GitLab, update the default branch to main My plan will be to make this change on Friday unless we realize there is more prerequisite work to do. Let me know if you have questions. Cecylia

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Results of Turbo Tunnel security audit that affect Snowflake
by David Fifield 24 Apr '21

24 Apr '21

There was recently a security audit of Turbo Tunnel software artifacts, including dnstt and Snowflake. I posted the report on the dnstt page: https://www.bamsoftware.com/software/dnstt/cure53-turbotunnel-2021.pdf The report finds three issues that have to do with Snowflake, rated from Informational to Low. UCB-02-001: Memory leak in Handler() routine of Snowflake client library (Low) UCB-02-008: Lack of rate limiting in Snowflake and dnstt (Info) UCB-02-009: Brokers and proxies are not authenticated (Low) For UCB-02-001, I have already opened https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow…. UCB-02-008 is not a vulnerability, but only a suggestion that rate-limiting interactions may help mitigate certain kinds of resource-exhaustion attacks. Some related tickets are: "Broker needs better resilience against DoS" https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… "Make it more expensive (CPU wise, or other thing) to make the initial connection to a snowflake" https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… UCB-02-009 is something we have already discussed in the team across various issues. I don't know if we can meaningfully authenticate proxies, but the broker's messages ought to be signed and encrypted. "End-to-end confidentiality for Snowflake client registrations" https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow… "Authentication for proxy--bridge connections" https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snow…

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Next anti-censorship reading group: 29 April 2021
by Cecylia Bocovich 22 Apr '21

22 Apr '21

Hey everyone! We will discuss "Domain Shadowing: Leveraging Content Delivery Networks for Robust Blocking-Resistant Communications" next Thursday at our regular team meeting on 2021-04-29, at 16 UTC. Relevant links: - https://www.usenix.org/system/files/sec21fall-wei.pdf Questions to ask and goals to have: - What can we learn from this work? - Are there immediate actions we can take based on this work? - Are there long-term actions we can take based on this work? - Is there future work that we want to call out, in hopes that others will pick it up? Hope to see you all there! Cecylia

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Crash in Snowflake users since 2021-01-17
by David Fifield 17 Apr '21

17 Apr '21

I happened to look at the Snowflake bridge's bandwidth and user history page, and the counts are close to zero since yesterday. I haven't investigated it at all. https://metrics.torproject.org/rs.html#details/5481936581E23D2D178105D44DB6…

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Throttling: What's old is new again
by Matthew Finkel 07 Apr '21

07 Apr '21

I found the premise of this article [0], that throttling connections (based on SNI) is new, a little amusing. It reminds me of how Iran implemented a very similar technique almost a decade ago [1]. I don't know of earlier uses of throttling as a censorship mechanism, at this scale, before the early-2010's (but I'd be interested in learning about them, if anyone knows). An earlier article about Roskomnadzor's implementation is interesting [2], for those who missed it. [0] https://arstechnica.com/gadgets/2021/04/russias-twitter-throttling-may-give… [1] https://ooni.org/post/iran-internet-censorship/#reported-cases-of-internet-… [2] https://arstechnica.com/gadgets/2021/03/a-russian-isp-confirms-roskomnadzor…

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Microsoft announces shutting down domain fronting attempts in Azure
by Cecylia Bocovich 06 Apr '21

06 Apr '21

It looks like Azure is going to shutdown domain fronting: https://www.microsoft.com/security/blog/2021/03/26/securing-our-approach-to… There isn't a time frame listed in the article, and I haven't gotten any notifications through my Azure account yet. Cecylia

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Plans for Orbot and OnionBrowser domain fronting update
by Nathan of Guardian 02 Apr '21

02 Apr '21

It seems like Azure Domain Fronting may already be going offline, according to some reports. Our own testing from US and EU show that it is still working for now. That said, here is our plan for updating Orbot and Onion Browser in response to what may come at any moment: 1) Move to Fastly for Snowflake and Moat as soon as they are ready. Please keep us posted on this. 2) Remove Meek as a built-in option. 3) Promote "social distribution" of bridge URLs via links and QR codes through communities that need them 4) Work on setting up our own additional pool of CDN front addresses for Moat and the Snowflake broker(s) that we can round-robin/cat-and-mouse through for both Snowflake and Moat. These would be compiled into our apps, or provided through some kind of S3/hard to block bootstrap URL. 5) Continue our own work in mobile-specific bridge distribution (push messages, SMS, chat bots, social etc) options we can employ in the future. .... any other things to know, that we missed, that we are being naive about? Thanks! +n

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TurboTunnel Packet Scheduler Tips and Traffic Analysis Benefits
by Mike Perry 31 Mar '21

31 Mar '21

Hi all, I recently posted a draft proposal for traffic splitting for Tor: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/proposals/329-… It has similar properties as TurboTunnel, but I put quite a bit of work and consulted with several TCP and multipath TCP researchers to develop the packet scheduling algorithms defined in Section 3: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/proposals/329-… You may be interested in adopting those algorithms, if possible. There are variants that optimize for latency, throughput, or both in combination. The proposal also supports resumption of closed circuits. I am writing this mail because while these things are nice, resumption in particular does not defend against all classes of issues I am worried about with respect to confirmation attacks from closing or blocking connections, particularly onion service connections: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/proposals/329-… Briefly, if an adversary suspects an onion service is in use at a particular guard, it can make a circuit to that onion service, hold it open, and then close TCP connections of that guard to see if the circuit closes. This can even be done on the side with TCP RST injection, and possibly even blindly with brute force RST spoofing and spamming, if a good enough guess can be made about the IP address range of the service. Worse, in this case, the resumption I specified for Conflux does not help here, because this adversary will see the service's resumption attempt, and use that for confirmation instead of circuit close. TurboTunnel's resumption and automatic bridge discovery and replacement should greatly mitigate this attack, because it happens at a layer that is not necessarily controlled by the adversary, and thus resumption is mostly invisible to them (save for delay, which Cecylia is working on minimizing). Additionally, since traffic splitting has been shown to help against website fingerprinting, it stands to reason that doing traffic splitting at multiple layers and multiple endpoints may actually be better than doing it at just one layer, and costs us little-to-nothing in terms of overhead: https://www.comsys.rwth-aachen.de/fileadmin/papers/2020/2020-delacadena-tra… So please, keep on improving TurboTunnel! And let's compare notes about what we learn! Looking forward to obfs4 TurboTunnel, too! -- Mike Perry

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Ansible role that installs snowflake proxy on Debian
by Jacobo Nájera 17 Mar '21

17 Mar '21

Hi, I wrote an ansible role to install snowflake proxy on Debian Buster. I will extend its support to Fedora and Ubuntu and improvements soon. https://galaxy.ansible.com/nvjacobo/snowflake Repo: https://github.com/nvjacobo/snowflake Feedback is welcome Thanks, Jacobo

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