[tor-bugs] #13705 [Tor]: Allow relays to promise in their descriptor that their IP address won't change

[Tor Bug Tracker & Wiki]

#13705: Allow relays to promise in their descriptor that their IP address won't
change
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 Reporter:  arma         |          Owner:
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:  Tor: 0.2.6.x-final
Component:  Tor          |        Version:
 Keywords:  tor-relay    |  Actual Points:
Parent ID:               |         Points:
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 Imagine the following scenario: Oscar runs a fast relay that gets the
 Guard flag and accumulates some users, including a user Alice. Then some
 attacker does a guard enumeration attack to identify that his victim is
 using Oscar's relay as her guard. He can get a warrant to collect Oscar's
 computer, but for whatever reason he's not allowed to tap the relay in-
 place. So he steals the computer, takes it to his location, turns it back
 on, and the relay starts up again. Alice then says "oh good, my guard is
 back online" and moves back to using it.

 One straightforward option to reduce the risk of this scenario happening
 in practice is for relays that intend to have a static IP address to set a
 line in their descriptor that tells the directory authorities to refuse
 them if they show up from a different IP address. The implementation on
 the directory authority side would be to add the IP address to fingerprint
 mapping to the router-stability file or equivalent, and then check whether
 there's a mapping when considering newly published descriptors.

 This idea wouldn't handle the attack when done on relays with dynamic or
 varying IP addresses.

 Another avenue for addressing the attack is the encrypted identity key
 proposal and friends. I'm not sure if they handle this issue, or are
 orthogonal, or would supersede this idea.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13705>
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