[tor-bugs] #10969 [Core Tor/Tor]: Set of guard nodes can act as a linkability fingerprint

[Tor Bug Tracker & Wiki]

#10969: Set of guard nodes can act as a linkability fingerprint
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 Reporter:  asn                                  |          Owner:
                                                 |  mikeperry
     Type:  defect                               |         Status:
                                                 |  assigned
 Priority:  High                                 |      Milestone:  Tor:
                                                 |  unspecified
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-client, tor-guard,               |  Actual Points:
  026-triaged-1, XKEYSCORE, prop259, SponsorU-   |
  deferred, tor-03-unspecified-201612            |
Parent ID:  #11480                               |         Points:  large
 Reviewer:                                       |        Sponsor:
-------------------------------------------------+-------------------------

Comment (by asn):

 Replying to [comment:30 teor]:
 > Has the new guard design in 0.3.0 fixed this issue?
 > Has switching to one entry guard fixed this issue?

 Hey teor,

 switching to one entry guard slightly improved the situation, but did not
 fix the issue. The new guard design did not fix the issue either.

 An adversary who monitors your connection enough to be able to derive the
 first few elements of your guard list can use that info to track you down.
 This is even easier since currently we use multiple directory guards,
 which means that we easily leak the first 3 positions in our guard list
 (see `DFLT_N_PRIMARY_DIR_GUARDS_TO_USE`) (see #21006).

 Even with 1 directory guard, an adversary could get glimpses into your
 guard list when your guards are down, or when he kills your connections to
 them. To completely solve this issue we would need to use guard sets or
 some other wacky solution
 (http://www.homepages.ucl.ac.uk/~ucabaye/papers/guard_sets_for_onion_routing.pdf).
 We are pretty far away from this happening.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/10969#comment:31>
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