[tor-bugs] #9777 [Tor]: Retry your path selection if you don't get an NTor-supporting relay in it?

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#9777: Retry your path selection if you don't get an NTor-supporting relay in it?
------------------------+------------------------------------
 Reporter:  arma        |          Owner:
     Type:  defect      |         Status:  new
 Priority:  normal      |      Milestone:  Tor: 0.2.4.x-final
Component:  Tor         |        Version:
 Keywords:  tor-client  |  Actual Points:
Parent ID:              |         Points:
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 Currently Tor chooses its path as normal, and then it decides for each
 relay whether it will send a TAP or NTor.

 Coupled with the use of CREATE_FAST for the first hop, that means if you
 flip two (weighted) coins wrong, you're building a circuit that's worth
 attacking by an adversary who finds breaking 1024-bit crypto doable.

 By my count, the current consensus aggregate weight for various relay
 versions is:
 {{{
 0.2.2.x: 351110
 0.2.3.x: 4036995
 0.2.4.x: 6361554
 0.2.5.x: 285012
 }}}

 So oversimplifying grossly, that's a 40% squared = 16% chance of flipping
 those coins wrong given the current network.

 But even in a world where 90% of the relay capacity has upgraded, that's
 still a 1% chance of an all-TAP circuit. Not so good.

 (Implementing proposal 221 would help here, but not make the problem go
 away entirely.)

 I think we should have a config option, probably set to auto and
 controlled by the consensus, to flip our coins again if we're not going to
 send an NTor cell to any of the relays in our path.

 Unless we want to decide that users should always want it, and not even
 make it a config option or consensus param? I don't think we should do
 that at least until we've calculated the actual odds of all all-TAP path.
 They could actually be quite a bit higher than 16% now, since your Guards
 are discounted as a function of the scarcity of guard bandwidth, and a lot
 of middle relays are small and a lot of small relays haven't upgraded yet.
 In that case turning on this 'pick another path' feature could
 significantly impact anonymity too.

 I wonder how sad 0.2.4 users would be if they don't get this feature.
 Probably pretty sad. So, setting to 0.2.4 but that's up for debate.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/9777>
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