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Re: [tor-bugs] #9273 [Tor]: Brainstorm tradeoffs from moving to 2 (or even 1) guards
#9273: Brainstorm tradeoffs from moving to 2 (or even 1) guards
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Reporter: arma | Owner:
Type: project | Status: new
Priority: major | Milestone: Tor: 0.2.5.x-final
Component: Tor | Version:
Keywords: tor-client design analysis | Parent:
Points: | Actualpoints:
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Changes (by mikeperry):
* cc: mikeperry (added)
Comment:
FTR: I am in favor of both Conflux and reduced guards. Also note that
Conflux will naturally compensate for the performance downsides of reduced
guards. 2 seems a good choice for this reason.
I also did some back-of-the-envelope math for guard fingerprinting when we
were discussing directory guards being separate or shared in
https://trac.torproject.org/projects/tor/ticket/6526#comment:8. Basically,
if we have 9 bits of guard entropy, that is equivalent to 512 uniform
guard choices, and then you can use combinatorics from there. For two
guards, https://www.wolframalpha.com/input/?i=512+choose+2 = 130,816
combinations, or about 17 bits of identifying entropy, versus 3 guards at
512 choose 3 = 22 million/25 bits. You could also compute the Shannon
entropy directly the long way, but I think it should be the same.
Of course, my mental model of Paul Syverson just said "But that entropy
metric doesn't reflect the fact that users will be more likely to have
popular fast guards than unpopular slow guards, and this becomes even more
pronounced with two guards rather than three. Tracking users with popular
fast guards is harder than tracking users who get unlucky and pick one or
more unpopular slow guards because the anonymity sets are larger for the
fast guard users."
Usually when Paul gets inside my head like this, I just start thinking
about something that would jeopardize his security clearance and he goes
away. But in this case, I think he's right. It would be an even larger
improvement than pure entropy metrics indicate.
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/9273#comment:3>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online
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