[tor-bugs] #19923 [Core Tor/Tor]: Single Onion Services should upgrade to ntor (if they can)

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#19923: Single Onion Services should upgrade to ntor (if they can)
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     Reporter:  teor     |      Owner:
         Type:           |     Status:  new
  enhancement            |
     Priority:  Medium   |  Milestone:
    Component:  Core     |    Version:
  Tor/Tor                |   Keywords:  029-proposed, sos, rsos, tor-hs,
     Severity:  Normal   |  TorCoreTeam201608
Actual Points:           |  Parent ID:
       Points:  1.0      |   Reviewer:
      Sponsor:           |
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 Split off #17178 and #19163, depends on both.

 Single Onion Services build a one-hop path to the client-provided
 rendezvous point. This circuit is only secured using SSL and TAP, as the
 INTRODUCE cell only contains TAP onion keys.

 But in most cases, the Single Onion Service can look up the ntor onion key
 for the rendezvous point in the consensus, and therefore it can upgrade to
 ntor. (If it doesn't find the rendezvous point in the consensus, it simply
 continues with TAP.)

 My suggested solution is to replace the entire rendezvous point
 extend_info with the extend_info from the consensus (if found). We should
 do this for both clients and services, whether using Single Onion Services
 or Tor2web or not (to avoid introducing new fingerprinting mechanisms).

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19923>
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