Re: [tor-dev] Bridge Guards (prop#188) & Bridge ORPort Reachability Tests

On 10 Sep 2015, at 17:01, isis <isis@xxxxxxxxxxxxxx> wrote:

Hey all,

I have a working implementation of proposal #188, [0] which specifies a
mechanism by which tor's (rather accidental) loose-source routing feature is
utilised by Bridges in order to transparently inject a Bridge Guard into all
client circuits. (See #7144. [1])

Currently, I am updating the text in proposal #188 to match my implementation,
and I am trying to decide what to do about the two types of Bridge ORPort
reachability testing. Here's the text I've written so far to explain the
issues:

(In the following, "Alice" is an OP, "Bob" is a Bridge, "Guillaume" is
Bob's chosen Entry Guard, and "Charlie" is a random Middle node.)

4.4. Bridge Reachability Testing

  Currently, a Bridge's reachability is tested both by the Bridge
  itself (called "self-testing") and by the BridgeAuthority.

4.4.1. Bridge Reachability Self-Testing

  Before a Bridge uploads its descriptors to the BridgeAuthority, it
  creates a special type of testing circuit which ends at itself:

      Bob -> Guillaume -> Charlie -> Bob

  Thus, going to all this trouble to later use loose-source routing in
  order to relay Alice's traffic through Guillaume (rather than
  connecting directly to Charlie, as Alice intended) is diminished by
  the fact that Charlie can still passively enumerate Bridges by
  waiting to be asked to connect to a node which is not
  contained within the consensus.

Extending to an ORPort not in the consensus can also be used to enumerate single onion services (prop252).

Any defences could apply to both, and if they are indistinguishable, single onion services could provide cover for bridges.

(Except, of course, for the defence of a single onion service being a relay. That doesnât help bridges.)

  We could get around this option by disabling self-testing for bridges
  entirely, by automatically setting "AssumeReachable 1" for all Bridge
  relaysâ although I am not sure if this is wise.

4.4.2. Bridge Reachability Testing by the BridgeAuthority

  After receiving Bob's descriptors, the BridgeAuthority attempts to
  connect to Bob's ORPort by making a direct TLS connection to the
  Bridge's advertised ORPort.

  Should we change this behaviour? One the one hand, at least this
  does not enable any random OR in the entire network to enumerate
  Bridges. On the other hand, any adversary who can observe packets
  from the BridgeAuthority is capable of enumeration.

So, my current questions are:

1. Bridges only do reachability self-testing in order to warn their operators
   that the Bridge (or firewall or whatever) might be misconfigured, correct?
   Is there any other reason (or future reason) why self-testing is
   important/useful?

2. How should the self-testing be changed?

   2.a. First, if there aren't any other reasons for self-testing: Is Bridge
        reachability self-testing actually helpful to Bridge operators in
        practice? Don't most Bridge operators just try to connect, as a
        client, to their own Bridge to see if it's working correctly? (This
        is what I usually do, at leastâ)

        If it's not useful to most Bridge operators, can we just disable
        self-testing for all Bridges? (And, should we disable it only for
        Bridges who are using Bridge Guards, or all of Bridges regardless?)

   2.b. If it is useful to people, then the best way I can think of so far to
        keep it, but refactor it to be safer, would be to create a circuit
        like:

            Bridge â Guard â Middle â OtherMiddle â Guard â Bridge

        Clearly, that circuit is just a little bit insaneâ but we can't do:

            Bridge â Guard â Middle â Guard â Bridge

        because the Middle would refuse to extend back to the previous node
        (all ORs follow this rule). Similarly, it would be stupid to do:

            Bridge â Guard â Middle â OtherMiddle â Bridge

        because, obviously, that merely shifts the problem and accomplishes
        nothing. But is there something smarter I could do?

I quite like this idea, and a 5-hop circuit is below the 8-hop limit.

3. Should we change how the BridgeAuthority tests Bridge ORPort reachability?
If so, how?

The bridge authority could connect via a 3-hop path, but that would suffer from the same issues as bridge reachability self-testing - the bridge authority extending to an ORPort not in the consensus would reveal the bridge to the last hop.

But the bridge authority could choose a set of guards (vanguards?, last-hop guards?) for this purpose, reducing the chances that one is an adversary.

4. If I'm going to refactor all of this, are there other (future) things I
   should take into account?

   For example, if self-testing is disabled for Bridges, and, due to #7349,
   [2] the BridgeAuthority isn't testing reachability either, then how will
   the BridgeAuthority know if (any of) the Bridge's PTs are reachable?
   (One solution might be to, in that case, have the BridgeAuthority lie and
   tell BridgeDB that such Bridges were reachable, letting BridgeDB do the PT
   reachability testing.)

Please help me brainstorm a way out of this mess!

[0]: https://gitweb.torproject.org/torspec.git/tree/proposals/188-bridge-guards.txt
[1]: https://bugs.torproject.org/7144
[2]: https://bugs.torproject.org/7349

Best Regards,
--
ââ isis agora lovecruft
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