[tor-bugs] #17690 [Tor]: Control the number of HSDirs using a consensus parameter

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#17690: Control the number of HSDirs using a consensus parameter
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     Reporter:  asn     |      Owner:
         Type:  defect  |     Status:  new
     Priority:  Medium  |  Milestone:
    Component:  Tor     |    Version:
     Severity:  Normal  |   Keywords:  tor-hs
Actual Points:          |  Parent ID:
       Points:          |    Sponsor:
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 People are worrying that HSDirs can launch traffic confirmation attacks
 against hidden service clients. This will be harder to do after the shared
 randomness proposal gets deployed but still not impossible (without some
 sort of PIR scheme).

 Till we get there, Roger suggested we make the number of HSDirs
 configurable, and control it using a consensus parameter (similar to how
 we use `NumEntryGuards`).

 This will also be useful after prop#246 gets implemented and we merge
 HSDirs with IPs.

 On the technical side, we '''might''' need two consensus parameters. One
 for the number of replicas, and one for the number of descriptors per
 replica.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17690>
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