[tor-bugs] #9273 [Tor]: Brainstorm tradeoffs from moving to 2 (or even 1) guards

Tor Bug Tracker & Wiki blackhole at torproject.org
Tue Jul 16 18:39:32 UTC 2013

#9273: Brainstorm tradeoffs from moving to 2 (or even 1) guards
 Reporter:  arma     |          Owner:                    
     Type:  project  |         Status:  new               
 Priority:  normal   |      Milestone:  Tor: 0.2.5.x-final
Component:  Tor      |        Version:                    
 Keywords:           |         Parent:                    
   Points:           |   Actualpoints:                    
 There are now many conflicting issues to consider when changing the
 default number of guards. I'd like to write a proposal suggesting we move
 to 2 (or even 1), but I don't think I'm ready to write the analysis
 section yet.

 Here's a start:

 Pro 1: Reduces chance of using an adversary's guard. This argues for 1,
 but 2 would still be a lot better. See Tariq's WPES 2012 paper for

 Pro 2: Reduces impact from guard fingerprinting: if the adversary learns
 that you have the following n guards, and later sees an anonymous user
 with the same guards, how likely is it to be you? Said another way, a trio
 of guards produces a cubic, whereas a duo of guards produces a quadratic.
 Somebody should do the math to sort out the chance of having all possible
 trios of guards, followed by the expected uniqueness of a trio. I expect
 moving to 2 gives the majority of the benefit here.

 Con 1: Increases the variance of performance. The more guards you have,
 the closer to average performance you'll be. Whereas if you have just one
 guard, your performance will be impacted a lot by that choice. It would
 seem that we need to raise the bar on getting the Guard flag if we move
 people to having just one guard.

 Con 2: Moving to 1 guard will rule out a Conflux-style design. But 2
 guards would still work fine.

 What did I miss?

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/9273>
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