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

Nov 27 (Thursday) meeting cancelled
by Philipp Winter 27 Nov '19

27 Nov '19

This week's meeting falls on Thanksgiving, a U.S. holiday, so several of us won't be around. Let's cancel our Thursday meeting and meet again next week, on December 4. Cheers, Philipp

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day for roadmaping January & February?
by Gaba 26 Nov '19

26 Nov '19

Hi! We need to work on roadmapping January and February, mostly for the sponsor 28. Can you all meet on December 16th or 17th? cheers, gaba -- Project Manager: Network, Anti-Censorship and Metrics teams gaba at torproject.org she/her are my pronouns GPG Fingerprint EE3F DF5C AD91 643C 21BE 8370 180D B06C 59CA BD19

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raw.githubusercontent.com blocked in China (affects GetTor)
by David Fifield 21 Nov '19

21 Nov '19

It looks like the domain raw.githubusercontent.com was blocked in China a few days ago. https://ntc.party/t/raw-githubusercontent-com-blocked-in-china-since-2019-1… This affects GetTor, because it uses links like https://github.com/TheTorProject/gettorbrowser/raw/torbrowser-releases/torb… which redirect to https://raw.githubusercontent.com/TheTorProject/gettorbrowser/torbrowser-re…

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IRC keywords to reach anti-censorship team
by Philipp Winter 14 Nov '19

14 Nov '19

Other teams have started to use IRC keywords to reach team members, e.g.: "network-team: somebody review #foo". We discussed this in today's meeting and agreed to use both "ac-team" and "anti-censorship-team" for our team: <http://meetbot.debian.net/tor-meeting/2019/tor-meeting.2019-11-14-18.00.html> Cheers, Philipp

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Cancelling IRC meeting on November 7
by Philipp Winter 30 Oct '19

30 Oct '19

Both Cecylia and I will be travelling and won't be able to host the anti-censorship meeting on November 7. Let's cancel this one. Cheers, Philipp

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Turbo Tunnel in obfs4proxy (survives TCP connection termination)
by David Fifield 21 Oct '19

21 Oct '19

I posted this at https://github.com/net4people/bbs/issues/14#issuecomment-544747519 but it's relevant to anti-censorship in Tor. Here are proof-of-concept branches implementing the turbo tunnel idea in obfs4proxy, one using kcp-go/smux and one using quic-go: * https://dip.torproject.org/dcf/obfs4/tree/reconnecting-kcp * https://dip.torproject.org/dcf/obfs4/tree/reconnecting-quic As diffs: * [Changes needed to add kcp-go/smux to plain obfs4proxy](https://dip.torproject.org/dcf/obfs4/compare/aa7313b7eb78b7546a… * [Changes needed to adapt kcp-go/smux to quic-go](https://dip.torproject.org/dcf/obfs4/compare/b5d6895542b8fce929739… Using either of these branches, your circumvention session is decoupled from any single TCP connection. If a TCP connection is terminated, the obfs4proxy client will establish a new connection and pick up where it left off. An error condition is signaled to the higher-level application only when there's a problem establishing a new connection. Otherwise, transient connection termination is invisible (except as a brief increase in RTT) to Tor and whatever other application layers are being tunnelled. I did a small experiment showing how a Tor session can persist, despite the obfs4 layer being interrupted every 20 seconds. I configured the "little bastard" connection terminator to forward from a local port to a remote bridge, and terminate connections after 20 seconds. ``` lilbastard$ cargo run -- -w 20 127.0.0.1:3000 192.81.135.242:4000 ``` On the bridge, I ran tor using either plain obfs4proxy, or one of the two turbo tunnel branches. (I did the experiment once for each of the three configurations.) ``` DataDirectory datadir.server SOCKSPort 0 ORPort auto BridgeRelay 1 AssumeReachable 1 PublishServerDescriptor 0 ExtORPort auto ServerTransportListenAddr obfs4 0.0.0.0:4000 ServerTransportPlugin obfs4 exec ./obfs4proxy -enableLogging -unsafeLogging -logLevel DEBUG # ServerTransportPlugin obfs4 exec ./obfs4proxy.kcp -enableLogging -unsafeLogging -logLevel DEBUG # ServerTransportPlugin obfs4 exec ./obfs4proxy.quic -enableLogging -unsafeLogging -logLevel DEBUG ``` On the client, I configured tor to use the corresponding obfs4proxy executable, and connect to the bridge through the "little bastard" proxy. (If you do this, your bridge fingerprint and cert will be different.) ``` DataDirectory datadir.client SOCKSPort 9250 UseBridges 1 Bridge obfs4 127.0.0.1:3000 94E4D617537C3E3CEA0D1D6D0BC852B5A7613B77 cert=6rB8kVd981U0G2b9nXioB5o0Zu7tDpDkoZyPe2aCmqFzGmfaSiNIfQvkJABakH+DfYwWRw iat-mode=0 ClientTransportPlugin obfs4 exec ./obfs4proxy -enableLogging -unsafeLogging -logLevel DEBUG # ClientTransportPlugin obfs4 exec ./obfs4proxy.kcp -enableLogging -unsafeLogging -logLevel DEBUG # ClientTransportPlugin obfs4 exec ./obfs4proxy.quic -enableLogging -unsafeLogging -logLevel DEBUG ``` Then, I captured traffic for 90 seconds while downloading a video file through the tor proxy. In the "plain obfs4proxy" case, the download stopped after the first connection termination at 20 s. Every 20 s after that, there was a small amount of activity, which was tor reconnecting to the bridge (and the resulting obfs4 handshake). But it didn't matter, because tor has already signaled the first connection termination to the application layer, which gave up: ``` curl: (18) transfer closed with 111535615 bytes remaining to read ``` In comparison, the "kcp" and "quic" cases kept on downloading, being only momentarily delayed by a connection termination. Notes: * How this works architecturally, on the client side, we replace the original TCP Dial call with either kcp.NewConn2 or quic.Dial, over an abstract packet-sending interface (clientPacketConn). clientPacketConn runs a loop that repeatedly connects to the same destination and exchanges packets (represented as length-prefixed blobs in a TCP stream) as long as the connection is good, reporting an error only when a connection attempt fails. On the server side, we replace the TCP Listen call with either kcp.ServeConn or quic.Listen, over an abstract serverPacketConn. serverPacketConn opens a single TCP listener, takes length-prefixed packets from *all* the TCP streams that arrive at the listener, and feeds them into a *single* KCP or QUIC engine. Whenever we need to send a packet for a particular connection ID, we send it on the TCP stream that most recently sent us a packet for that connection ID. * There's no need for this functionality to be built into obfs4proxy itself. It could be done as a separate program: ``` ------------ client ------------ ------------ bridge ------------ tor -> turbotunnel -> obfs4proxy -> internet -> obfs4proxy -> turbotunnel -> tor ``` But this kind of process layering is cumbersome with pluggable transports. * I'm passing a blank client IP address to the pt.DialOr call—this information is used for geolocation in Metrics graphs. That's because an OR connection no longer corresponds to a single incoming IP address with its single IP address—instead it corresponds to an abstract "connection ID" that remains constant across potentially many TCP connections. In order to make this work, you would have to define some heuristic such as "the client IP address associated with the OR connection is that of the first TCP connection that carried that connection ID."

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IPv6 Snowflake broker (#29258) IP addresses and SSH key fingerprints
by David Fifield 17 Oct '19

17 Oct '19

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 2019-10-17 This is a signed statement of the IP addresses and SSH host key fingerprints of the Snowflake broker set up by David Fifield at https://bugs.torproject.org/#comment:11. 37.218.245.111 2a00:c6c0:0:154:4:d8aa:b4e6:c89f RSA: 2048 SHA256:dp0Xo/oN1qZfMuZnqgKEbeOsbU2qpDR60B5MLIRaAgg DSA: 1024 SHA256:DF5ofogjGur02gv8/ciU3wFA+YHNuAhUlel9Uv2KBlo ECDSA: 256 SHA256:6cskO6ch/kv2RbIMhTdwqpsd9vB8npzlZTlkWZJLoek ED25519: 256 SHA256:fEkLvmu5woTvU6162I8Jxd2eHzsTnBshumtWICclWA4 -----BEGIN PGP SIGNATURE----- iQIzBAEBCgAdFiEEeXoyauxKR4rwUMw64rk9gVzTiOUFAl2ormoACgkQ4rk9gVzT iOX7xQ/+KyWHU7f5Oj2zFFVFu/vkD2xNIDEuPgnAUmVp879QnfzqmHxBifNzgpXQ t+357gSWlMAHP6IO0spnKxQqFsugvH3QqH5fyEXmEKQcJw7UI5VYNR5aGd7soSOx meHtgVkmeyWJDNZZSjVrk9C0nnEnteOXMx02NVD4kC1hJkSBvOJT4+M3Ekxx0nnQ AOfc7ZEBhchEMYsf2wxY04O0dR4HCgwcRFUGV+gT5bKw4eoqT+0sMetMzi5CHAqd EjsRQhDCHOp1v7xkkxx0lMOKPL4IYQI4l5qOIMWfpFS3ZwWX18bm3qK4UsDVAxIJ FouPEuVz7CAsLrlc5hnaDrmM2VsH8hqRFW4PHhmycaHVrW1AKkt0h/qHxDfpIkgK xGNbROcpzfV3FmqGWe09sx1aSLI34nMzIPX3aB6EmZ3Qn3Dx3VZExrBQSlcJ5NLy 3fFvqEpOhRheuE4ohD48/jfj0hsX/Sqc5cKNcmQFPbjk3T2LzUx8TYsNMq/Us0NC XrNk3ph5ydssMCLSEZqFV+xTY/08u6bKMYyxrC7bSsRJo1WIBLOVlWbHVYc6WGgD i/XCDMOXSFawZVIQm5PJiqR87y5q9RBnitd31uNnYZLlty3SFvj2H6PqSu7iIw46 I5Gc/rPxRcWggjOOXxCm0IiTqnM94WgoWWMRFNpQqHzTio+J6fc= =sJTV -----END PGP SIGNATURE-----

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obfs4 bridge option on eclips.is
by David Fifield 14 Oct '19

14 Oct '19

While setting up a new VPS on eclips.is, which is the service we use to host the Snowflake broker and bridge, I noticed that they have a preconfigured option to set up an obfs4 bridge. (And an Outline server, which is the Shadowsocks-based VPN from Jigsaw.)

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Summary of Point Break (Jansen et al. USENIX Security '19) as it pertains to bridges
by David Fifield 26 Aug '19

26 Aug '19

Rob Jansen, Tavish Vidya, and Micah Sherr have a paper about bandwidth-based DoS against Tor. Section 5 is about default bridges. https://www.usenix.org/conference/usenixsecurity19/presentation/jansen https://www.usenix.org/system/files/sec19-jansen.pdf#page=6 While elsewhere in the paper they discuss in-protocol attacks, in the context of bridges they limit themselves to attacks using third-party paid "stresser" DoS services, which you can rent for about $1/Gbps/hour (Section 3.1). They looked at the default bridges in Tor Browser 8.0.3 (October 2018). Only 12 of 25 default obfs4 bridges were working. (I think most of the non-working bridges have since been pruned, e.g. in #29378, #30264.) The median bandwidth of the 12 working bridges was 368 KB/s, with a large variance: minimum of 67 KB/s and maximum of 1190 KB/s. Besides the default bridges, they requested 135 bridges from BridgeDB, and found that only 70% (95/135) of them worked. (This has also been at least partially addressed by #30441.) BridgeDB bridges are faster than default bridges, with a median bandwidth of closer to 600 KB/s (Figure 1). They estimate that disabling the 12 default obfs4 bridges would require 30 stresser jobs, at a rate of $22/hour or $17K/month. If all users of default obfs4 bridges switched to BridgeDB bridges, the median bandwidth of BridgeDB bridges would slow down to under 100 KB/s (Figure 2). If even half of default obfs4 users switch to using meek, the cost to operate meek will at least double (Figure 3).

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Turbo Tunnel: let's build a sequencing/reliability layer into our circumvention protocols
by David Fifield 16 Aug '19

16 Aug '19

I've just posted a manifesto on circumvention protocol design at the net4people BBS: https://github.com/net4people/bbs/issues/9 I'll include the text here as well. Code name: Turbo Tunnel Designing circumvention protocols for speed, flexibility, and robustness In working on circumvention protocols, I have repeatedly felt the need for a piece that is missing from our current designs. This document summarizes the problems I perceive, and how I propose to solve them. In short, I think that every circumvention transport should incorporate some kind of session/reliability protocol—even the ones built on reliable channels that seemingly don't need it. It solves all kinds of problems related to performance and robustness. By session/reliability protocol, I mean something that offers a reliable stream abstraction, with sequence numbers and retransmissions, like [QUIC](https://quicwg.org/) or [SCTP](https://tools.ietf.org/html/rfc4960#section-1.5.2). Instead of a raw unstructured data stream, the obfuscation layer will carry encoded datagrams that are provided by the session/reliability layer. When I say that circumvention transports should incorporate something like QUIC, for example, I don't mean that QUIC UDP packets are what we should send on the wire. No—I am not proposing a new *outer* layer, but an additional *inner* layer. We take the datagrams provided by the session/reliability layer, and encode them as appropriate for whatever obfuscation layer we happen to be using. So with meek, for example, instead of sending an unstructured blob of data in each HTTP request/response, we would send a handful of QUIC packets, encoded into the HTTP body. The receiving side would decode the packets and feed them into a local QUIC engine, which would reassemble them and output the original stream. A way to think about it is that the the sequencing/reliability layer is the "TCP" to the obfuscation layer's "IP". The obfuscation layer just needs to deliver chunks of data, on a best-effort basis, without getting blocked by a censor. The sequencing/reliability layer builds a reliable data stream atop that foundation. I believe this design can improve existing transports, as well as enable new transports that are now possible now, such as those built on unreliable channels. Here is a list of selected problems with existing or potential transports, and how a sequencing/reliability layer helps solve them: Problem: Censors can disrupt obfs4 by terminating long-lived TCP connections, as Iran did in 2013, killing connections after 60 seconds. This problem exists because the obfs4 session is coupled with the TCP connection. The obfs4 session begins and ends exactly when the TCP connection does. We need an additional layer of abstraction, a virtual session that exists independently of any particular TCP connection. That way, if a TCP connection is terminated, it doesn't destroy all the state of the obfs4 session—you can open another TCP connection and resume where you left off, without needing to re-bootstrap Tor or your VPN or whatever was using the channel. Problem: The performance of meek is limited because it is half-duplex: it never sends and receives at the same time. This is because, while the bytes in a single HTTP request arrive in order, the ordering of multiple simultaneous requests is not guaranteed. Therefore, the client sends a request, then waits for the server's response before sending another, resulting in a delay of an RTT between successive sends. The session/reliability layer provides sequence numbers and reordering. Both sides can send data whenever is convenient, or as needed for traffic shaping, and any unordered data will be put back in order at the other end. A client could even split its traffic over two or more CDNs, with different latency characteristics, and know that the server will buffer and reorder the encoded packets to correctly recover the data stream. Problem: A Snowflake client can only use one proxy at a time, and that proxy may be slow or unreliable. Finding a working proxy is slow because each non-working one must time out in succession before trying another one. The problem exists because even though each WebRTC DataChannel is reliable (DataChannel uses SCTP internally), there's no ordering between multiple simultaneous DataChannels on separate Snowflake proxies. Furthermore, if and when a proxy goes offline, we cannot tell whether the last piece of data we sent was received by the bridge or not—the SCTP ACK information is not accessible to us higher in the stack—so even if we reconnect to the bridge through another proxy, we don't know whether we need to retransmit the last piece of data or not. All we can do is tear down the entire session and start it up again from scratch. As in the obfs4 case, this problem is solved by having an independent virtual session that persists across transient WebRTC sessions. An added bonus is the opportunity to use more than one proxy at once, to increase bandwidth or as a hedge against one of them disappearing. Problem: [DNS over HTTPS](https://groups.google.com/d/msg/traffic-obf/ZQohlnIEWM4/09N7zsxjBgAJ) is an unreliable channel: it is reliable TCP up to the DoH server, but after that, recursive resolutions are plain old unreliable UDP. And as with meek, the ordering of simultaneous DNS-over-HTTPS requests is not guaranteed. Solved by retransmission in the session layer. There's no [DNS pluggable transport](https://trac.torproject.org/projects/tor/wiki/doc/DnsPluggableTr… yet, but I think some kind of retransmission layer will be a requirement for it. Existing DNS tunnel software uses various ad-hoc sequencing/retransmission protocols. I think that a proper user-space reliability layer is the "right" way to do it. Problem: Shadowsocks opens a separate encrypted TCP connection for every connection made to the proxy. If a web page loads resources from 5 third parties, then the Shadowsocks client makes 5 parallel connections to the proxy. This problem is really about multiplexing, not session/reliability, but several candidate session/reliability protocols additionally offer multiplexing, for example streams in QUIC, streams in SCTP, or smux for KCP. Tor does not have this problem, because Tor already is a multiplexing protocol, with multiple virtual circuits and streams in one TCP/TLS connection. But every system could benefit from adding multiplexing at some level. Shadowsocks, for example, could open up one long-lived connection, and each new connection to the proxy would only open up a new stream inside the long-lived connection. And if the long-lived connection were killed, all the stream state would still exist at both endpoints and could be resumed on a new connection. As an illustration of what I'm proposing, here's the protocol layering of meek (which [sends chunks of the Tor TLS stream](https://trac.torproject.org/projects/tor/wiki/doc/AChildsGardenOfPl… inside HTTP bodies), and where the new session/reliability layer would be inserted. Tor can remain oblivious to what's happening: just as before it didn't "know" that it was being carried over HTTP, it doesn't now need to know that it is being carried over QUIC-in-HTTP (for example). ``` [TLS] [HTTP] [session/reliability layer] ⇐ 🆕 [Tor] [application data] ``` I've done a little survey and identified some suitable candidate protocols that also seem to have good Go packages: * [QUIC](https://quicwg.org/) with [quic-go](https://github.com/lucas-clemente/quic-go) * KCP with [kcp-go](https://github.com/xtaci/kcp-go) * [SCTP](https://tools.ietf.org/html/rfc4960) with [pion/sctp](https://github.com/pion/sctp) I plan to evaluate at least these three candidates and develop some small proofs of concept. The overall goal of my proposal is to liberate the circumvention context from particular network connections and IP addresses. ### Related work The need for a session and sequencing layer has been felt—and dealt with—repeatedly in many different projects. It has not yet, I think, been treated systematically or recognized as a common need. Systems typically implement some form of TCP-like SEQ and ACK numbers. The ones that don't, are usually built on the assumption of one long-lived TCP connection, and therefore are really using the operating system's sequencing and reliability functions behind the scenes. Here are are few examples: * Code Talker Tunnel (a.k.a. SkypeMorph) uses [SEQ and ACK numbers](https://www.cypherpunks.ca/~iang/pubs/skypemorph-ccs.pdf#page=7) and mentions selective ACK as a possible extension. I think it uses the UDP 4-tuple to distinguish sessions, but I'm not sure. * OSS used [SEQ and ACK numbers](https://www.freehaven.net/anonbib/papers/pets2013/paper_29.pdf#pag… and a random ID to distinguish sessions. * I [wasted time](https://www.bamsoftware.com/papers/thesis/#p227) in the early development of meek grappling with sequencing, before [punting by strictly serializing requests](https://www.bamsoftware.com/papers/fronting/#sec:deploy-tor), sacrificing performance for simplicity. meek uses an [X-Session-Id](https://trac.torproject.org/projects/tor/wiki/doc/AChildsGard… HTTP header to distinguish sessions. * [DNS tunnels](https://trac.torproject.org/projects/tor/wiki/doc/DnsPluggableTran… all tend to do their own idiosyncratic thing. dnscat2, one of the better-thought-out ones, uses [explicit SEQ and ACK numbers](https://github.com/iagox86/dnscat2/blob/master/doc/protocol.md#seq…. My position is that SEQ/ACK schemes are subtle enough and independent enough that they should be treated as a separate layer, not as an underspecified and undertested component of some specific system. Psiphon can use [obfuscated QUIC](https://github.com/Psiphon-Labs/psiphon-tunnel-core/tree/52de11dbabae… as a transport. It's directly using QUIC UDP on the wire, except that each UDP datagram is [additionally obfuscated](https://github.com/Psiphon-Labs/psiphon-tunnel-core/blob/52de11… before being sent. You can view my proposal as an extension of this design: instead of always sending QUIC packets as single UDP datagrams, we allow them to be encoded/encapsulated into a variety of carriers. [MASQUE](https://davidschinazi.github.io/masque-drafts/draft-schinazi-masque.html#RFC8441) tunnels over HTTPS and can use QUIC, but is not really an example of the kind of design I'm talking about. It leverages the multiplexing provided by HTTP/2 (over TLS/TCP) or HTTP/3 (over QUIC/UDP). In HTTP/2 mode it does not introduce its own session or reliability layer (instead using that of the underlying TCP connection); and in HTTP/3 mode it directly exposes the QUIC packets on the network as UDP datagrams, instead of encapsulating them as an inner layer. That is, it's using QUIC as a carrier for HTTP, rather than HTTP as a carrier for QUIC. The main similarity I spot in the MASQUE draft is the envisioned [connection migration](https://davidschinazi.github.io/masque-drafts/draft-schinazi-masque.html#rfc.section.7.1) which frees the circumvention session from specific endpoint IP addresses. Mike Perry wrote a [detailed summary](https://lists.torproject.org/pipermail/tor-dev/2018-March/013026.h… of considerations for migrating Tor to use end-to-end QUIC between the client and the exit. What Mike describes is similar to what is proposed here—especially the subtlety regarding protocol layering. The idea is not to use QUIC hop-by-hop, replacing the TLS/TCP that is used today, but to *encapsulate* QUIC packets end-to-end, and use some other *unreliable* protocol to carry them hop-by-hop between relays. Tor would not be using QUIC as the network transport, but would use features of the QUIC protocol. ### Anticipated questions Q: Why don't VPNs like Wireguard have to worry about all this? A: Because they are implemented in kernel space, not user space, they are, in effect, using the operating system's own sequencing and reliability features. Wireguard just sees IP packets; it's the kernel's responsibility to notice, for example, when a TCP segment need to be retransmitted, and retransmit it. We should do in user space what kernel-space VPNs have been doing all along! Q: You're proposing, in some cases, to run a reliable protocol inside another reliable protocol (e.g. QUIC-in-obfs4-in-TCP). What about the reputed inefficiency TCP-in-TCP? A: Short answer: don't worry about it. I think it would be premature optimization to consider at this point. The fact that the need for a session/reliability layer has been felt for so long by so many systems indicates that we should start experimenting, at least. There's contradictory information online as to whether TCP-in-TCP is as bad as they say, and anyway there are all kinds of performance settings we may tweak if it turns out to be a problem. But again: let's avoid premature optimization and not allow imagined obstacles to prevent us from trying. Q: QUIC is not just a reliability protocol; it also has its own authentication and encryption based on TLS. Do we need that extra complexity if the underlying protocol (e.g. Tor) is already encrypted and authenticated independently? A: The transport and TLS parts of QUIC are specified separately (https://tools.ietf.org/html/draft-ietf-quic-transport and https://tools.ietf.org/html/draft-ietf-quic-tls) so in principle they are separable and we could just use the transport part without encryption or authentication, as if it were SCTP or some other plaintext protocol. In practice, quic-go [assumes](https://godoc.org/github.com/lucas-clemente/quic-go#Dial) right in the API that you'll be using TLS, so separating them may be more trouble than it's worth. Let's start with simple layering that is clearly correct, and only later start breaking abstraction for better performance if we need to. Q: What about traffic fingerprinting? If you simply obfuscate and send each QUIC packet as it is produced, you will leak traffic features through their size and timing, especially when you [consider retransmissions and ACKs](https://www-users.cs.umn.edu/~hoppernj/ccs13-cya.pdf#page=5). A: Don't do that, then. There's no essential reason why the traffic pattern of the obfuscation layer needs to match that of the sequencing/reliability layer. Part of the job of the obfuscation layer is to erase such traffic features, if doing so is a security requirement. That implies, at least, *not* simply sending each entire QUIC packet as soon as it is produced, but padding, splitting, and delaying as appropriate. An ideal traffic scheduler would be *independent* of the underlying stream—its implementation would [not even *know*](https://lists.torproject.org/pipermail/tor-dev/2017-June/012310.html) how much actual data is queued to send at any time. But it's likely we don't have to be quite that rigorous in implementation, at least at this point.

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