[tor-bugs] #8240 [Tor]: Raise our guard rotation period, if appropriate

[Tor Bug Tracker & Wiki]

#8240: Raise our guard rotation period, if appropriate
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     Reporter:  arma     |      Owner:
         Type:  defect   |     Status:  assigned
     Priority:  major    |  Milestone:  Tor: 0.2.7.x-final
    Component:  Tor      |    Version:  Tor: 0.2.7
   Resolution:           |   Keywords:  tor-client, needs-proposal,
Actual Points:           |  023-backport, unfrozen, 027-triaged-1-in,
       Points:  medium   |  SponsorU
                         |  Parent ID:
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Comment (by asn):

 Some thoughts here about raising the rotation period to 9 months.

 Let's assume a 5% network adversary and a single guard per client. This
 means that every time we pick a guard we toss a coin with probability 0.95
 of coming out safe. Hence after `n` guard rotations, we have (0.95^n^)
 probability of coming out safe.

 This means that after 14 rotations we have more than 50% probability of
 being unsafe and after 6 rotations we have 25% probability of being
 unsafe.

 Currently, with 2 to 3 months rotation period, this means that after about
 3 years we have 50% probability of being unsafe. Or after 1.5 year we have
 25% probability of being unsafe. This does not sound terrible.

 OTOH, a 9 month rotation period gives us about 10 years before we have 50%
 prob of being unsafe, and 4.5 years before we have 25% probability of
 being unsafe. This seems like a pretty big safety guarantee compared to
 the rest of the security of Tor.

 Alternatively, with a 5 month rotation period, we have about 5 and a half
 years before we are 50% unsafe, and about 2.5 years before we are 25%
 unsafe. This might be more appropriate, considering the load balancing
 improvements that 5 months give us compared to 9 months, plus the concerns
 that people have raised against keeping a guard for too long.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/8240#comment:50>
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