path: root/proposals/ideas/xxx-pluggable-transport.txt

Filename: xxx-pluggable-transport.txt
Title: Pluggable transports for circumvention
Author: Jacob Appelbaum, Nick Mathewson
Created: 15-Oct-2010
Status: Draft

Overview

  This proposal describes a way to decouple protocol-level obfuscation
  from the core Tor protocol in order to better resist client-bridge
  censorship.  Our approach is to specify a means to add pluggable
  transport implementations to Tor clients and bridges so that they can
  negotiate a superencipherment for the Tor protocol.

Scope

  This is a document about transport plugins; it does not cover
  discovery improvements, or bridgedb improvements.  While these
  requirements might be solved by a program that also functions as a
  transport plugin, this proposal only covers the requirements and
  operation of transport plugins.

Motivation

  Frequently, people want to try a novel circumvention method to help
  users connect to Tor bridges.  Some of these methods are already
  pretty easy to deploy: if the user knows an unblocked VPN or open
  SOCKS proxy, they can just use that with the Tor client today.

  Less easy to deploy are methods that require participation by both the
  client and the bridge.  In order of increasing sophistication, we
  might want to support:

  1. A protocol obfuscation tool that transforms the output of a TLS
     connection into something that looks like HTTP as it leaves the
     client, and back to TLS as it arrives at the bridge.
  2. An additional authentication step that a client would need to
     perform for a given bridge before being allowed to connect.
  3. An information passing system that uses a side-channel in some
     existing protocol to convey traffic between a client and a bridge
     without the two of them ever communicating directly.
  4. A set of clients to tunnel client->bridge traffic over an existing
     large p2p network, such that the bridge is known by an identifier
     in that network rather than by an IP address.

  We could in theory support these almost fine with Tor as it stands
  today: every Tor client can take a SOCKS proxy to use for its outgoing
  traffic, so a suitable client proxy could handle the client's traffic
  and connections on its behalf, while a corresponding program on the
  bridge side could handle the bridge's side of the protocol
  transformation.  Nevertheless, there are some reasons to add support
  for transportation plugins to Tor itself:

  1. It would be good for bridges to have a standard way to advertise
     which transports they support, so that clients can have multiple
     local transport proxies, and automatically use the right one for
     the right bridge.

  2. There are some changes to our architecture that we'll need for a
     system like this to work.  For testing purposes, if a bridge blocks
     off its regular ORPort and instead has an obfuscated ORPort, the
     bridge authority has no way to test it.  Also, unless the bridge
     has some way to tell that the bridge-side proxy at 127.0.0.1 is not
     the origin of all the connections it is relaying, it might decide
     that there are too many connections from 127.0.0.1, and start
     paring them down to avoid a DoS.

  3. Censorship and anticensorship techniques often evolve faster than
     the typical Tor release cycle.  As such, it's a good idea to
     provide ways to test out new anticensorship mechanisms on a more
     rapid basis.

  4. Transport obfuscation is a relatively distinct problem
     from the other privacy problems that Tor tries to solve, and it
     requires a fairly distinct skill-set from hacking the rest of Tor.
     By decoupling transport obfuscation from the Tor core, we hope to
     encourage people working on transport obfuscation who would
     otherwise not be interested in hacking Tor.

  5. Finally, we hope that defining a generic transport obfuscation plugin
     mechanism will be useful to other anticensorship projects.

Non-Goals

  We're not going to talk about automatic verification of plugin
  correctness and safety via sandboxing, proof-carrying code, or
  whatever.

  We need to do more with discovery and distribution, but that's not
  what this proposal is about.  We're pretty convinced that the problems
  are sufficiently orthogonal that we should be fine so long as we don't
  preclude a single program from implementing both transport and
  discovery extensions.

  This proposal is not about what transport plugins are the best ones
  for people to write.  We do, however, make some general
  recommendations for plugin authors in an appendix.

  We've considered issues involved with completely replacing Tor's TLS
  with another encryption layer, rather than layering it inside the
  obfuscation layer.  We describe how to do this in an appendix to the
  current proposal, though we are not currently sure whether it's a good
  idea to implement.

  We deliberately reject any design that would involve linking more code
  into Tor's process space.

Design overview

  To write a new transport protocol, an implementer must provide two
  pieces: a "Client Proxy" to run at the initiator side, and a "Server
  Proxy" to run a the server side.  These two pieces may or may not be
  implemented by the same program.

  Each client may run any number of Client Proxies.  Each one acts like
  a SOCKS proxy that accepts accept connections on localhost.  Each one
  runs on a different port, and implements one or more transport
  methods.  If the protocol has any parameters, they passed from Tor
  inside the regular username/password parts of the SOCKS protocol.

  Bridges (and maybe relays) may run any number of Server Proxies: these
  programs provide an interface like stunnel-server (or whatever the
  option is): they get connections from the network (typically by
  listening for connections on the network) and relay them to the
  Bridge's real ORPort.

  To configure one of these programs, it should be sufficient simply to
  list it in your torrc.  The program tells Tor which transports it
  provides.  The Tor consensus should carry a new approved version number that
  is specific for pluggable transport; this will allow Tor to know when a
  particular transport is known to be unsafe safe or non-functional.

  Bridges (and maybe relays) report in their descriptors which transport
  protocols they support.  This information can be copied into bridge
  lines.  Bridges using a transport protocol may have multiple bridge
  lines.

  Any methods that are wildly successful, we can bake into Tor.

Specifications: Client behavior

  Bridge lines can now follow the extended format "bridge method
  address:port [[keyid=]id-fingerprint] [k=v] [k=v] [k=v]". To connect
  to such a bridge, a client must open a local connection to the SOCKS
  proxy for "method", and ask it to connect to address:port.  If
  [id-fingerprint] is provided, it should expect the public identity key
  on the TLS connection to match the digest provided in
  [id-fingerprint].  If any [k=v] items are provided, they are
  configuration parameters for the proxy: Tor should separate them with
  semicolons and put them user and password fields of the request,
  splitting them across the fields as necessary.  If a key or value
  value must contain a semicolon or a backslash, it is escaped with a
  backslash.

  The "id-fingerprint" field is always provided in a field named
  "keyid", if it was given.  Method names must be C identifiers.

  Example: if the bridge line is "bridge trebuchet www.example.com:3333
     rocks=20 height=5.6m" AND if the Tor client knows that the
     'trebuchet' method is provided by a SOCKS5 proxy on
     127.0.0.1:19999, the client should connect to that proxy, ask it to
     connect to www.example.com, and provide the string
     "rocks=20;height=5.6m" as the username, the password, or split
     across the username and password.

  There are two ways to tell Tor clients about protocol proxies:
  external proxies and managed proxies.  An external proxy is configured
  with "ClientTransportPlugin trebuchet socks5 127.0.0.1:9999".  This
  tells Tor that another program is already running to handle
  'trubuchet' connections, and Tor doesn't need to worry about it.  A
  managed proxy is configured with "ClientTransportPlugin trebuchet
  exec /usr/libexec/tor-proxies/trebuchet [options]", and tells Tor to launch
  an external program on-demand to provide a socks proxy for 'trebuchet'
  connections. The Tor client only launches one instance of each
  external program, even if the same executable is listed for more than
  one method.

  The same program can implement a managed or an external proxy: it just
  needs to take an argument saying which one to be.

Client proxy behavior

   When launched from the command-line by a Tor client, a transport
   proxy needs to tell Tor which methods and ports it supports.  It does
   this by printing one or more CMETHOD: lines to its stdout.  These look
   like

   CMETHOD: trebuchet SOCKS5 127.0.0.1:19999 ARGS:rocks,height \
              OPT-ARGS:tensile-strength

   The ARGS field lists mandatory parameters that must appear in every
   bridge line for this method. The OPT-ARGS field lists optional
   parameters.  If no ARGS or OPT-ARGS field is provided, Tor should not
   check the parameters in bridge lines for this method.

   The proxy should print a single "METHODS: DONE" line after it is
   finished telling Tor about the methods it provides.

   The transport proxy MUST exit cleanly when it receives a SIGTERM from
   Tor.

   The Tor client MUST ignore lines beginning with a keyword and a colon
   if it does not recognize the keyword.

   In the future, if we need a control mechanism, we can use the
   stdin/stdout from Tor to the transport proxy.

   A transport proxy MUST handle SOCKS connect requests using the SOCKS
   version it advertises.

   Tor clients SHOULD NOT use any method from a client proxy unless it
   is both listed as a possible method for that proxy in torrc, and it
   is listed by the proxy as a method it supports.

   [XXXX say something about versioning.]

Server behavior

   Server proxies are configured similarly to client proxies.

   

Server proxy behavior



   [so, we can have this work like client proxies, where the bridge
   launches some programs, and they tell the bridge, "I am giving you
   method X with parameters Y"?  Do you have to take all the methods? If
   not, which do you specify?]

   [Do we allow programs that get started independently?]

   [We'll need to figure out how this works with port forwarding.  Is
   port forwarding the bridge's problem, the proxy's problem, or some
   combination of the two?]

   [If we're using the bridge authority/bridgedb system for distributing
   bridge info, the right place to advertise bridge lines is probably
   the extrainfo document.  We also need a way to tell the bridge
   authority "don't give out a default bridge line for me"]

Server behavior

Bridge authority behavior

Implementation plan

   Turn this into a draft proposal

   Circulate and discuss on or-dev.

   We should ship a couple of null plugin implementations in one or two
   popular, portable languages so that people get an idea of how to
   write the stuff.

   1. We should have one that's just a proof of concept that does
      nothing but transfer bytes back and forth.

   1. We should not do a rot13 one.

   2. We should implement a basic proxy that does not transform the bytes at all

   1. We should implement DNS or HTTP using other software (as goodell
      did years ago with DNS) as an example of wrapping existing code into
      our plugin model.

   2. The obfuscated-ssh superencipherment is pretty trivial and pretty
   useful.  It makes the protocol stringwise unfingerprintable.

      1. Nick needs to be told firmly not to bikeshed the obfuscated-ssh
        superencipherment too badly

         1. Go ahead, bikeshed my day

   1. If we do a raw-traffic proxy, openssh tunnels would be the logical choice.

Appendix: recommendations for transports

  Be free/open-source software.  Also, if you think your code might
  someday do so well at circumvention that it should be implemented
  inside Tor, it should use the same license as Tor.

  Use libraries that Tor already requires. (You can rely on openssl and
  libevent being present if current Tor is present.)

  Be portable: most Tor users are on Windows, and most Tor developers
  are not, so designing your code for just one of these platforms will
  make it either get a small userbase, or poor auditing.

  Think secure: if your code is in a C-like language, and it's hard to
  read it and become convinced it's safe then, it's probably not safe.

  Think small: we want to minimize the bytes that a Windows user needs
  to download for a transport client.

  Specify: if you can't come up with a good explanation

  Avoid security-through-obscurity if possible.  Specify.

  Resist trivial fingerprinting: There should be no good string or regex
  to search for to distinguish your protocol from protocols permitted by
  censors.

  Imitate a real profile: There are many ways to implement most
  protocols -- and in many cases, most possible variants of a given
  protocol won't actually exist in the wild.

Appendix: Raw-traffic transports

  This section describes an optional extension to the proposal above.
  We  are not sure whether it is a good idea.