[tor-bugs] #30382 [Core Tor/Tor]: Provide control port event for when we are missing v3 client auth for an onion

[Tor Bug Tracker & Wiki]

#30382: Provide control port event for when we are missing v3 client auth for an
onion
------------------------------+--------------------------------------------
     Reporter:  asn           |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:  Tor: 0.4.2.x-final
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-hs, tbb-usability, hs-auth
Actual Points:                |  Parent ID:  #14389
       Points:                |   Reviewer:  6
      Sponsor:                |
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 For TB to be able to alert the user that they need to input their client
 auth credentials we need an appropriate control port event.

 In particular:

 1) When Tor fails to decrypt the second layer of desc encryption, we issue
 the `CLIENT_AUTH_NEEDED <onion> <reason>` event. Tor does not go to fetch
 more descs from the hsdir for this onion.

 2) At the same time, we store the broken descriptor into the hs cache,
 with a special flag that says "missing client auth" and hence `desc` is
 `NULL`.

 3) When TB intercepts the event it presents the user with a dialogue
 (#30237) and adds any client auth creds with the commands from #30381.

 4) As part of the #30381 commands the descriptor is decrypted.

 5) TB issues another SOCKS request which uses the right descriptor and
 goes forward.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/30382>
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