[tor-bugs] #28634 [Core Tor/Tor]: Design a useful padding machine that we can enable

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#28634: Design a useful padding machine that we can enable
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 Reporter:  asn                                  |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.4.0.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  wtf-pad, tor-relay, tor-cell,        |  Actual Points:
  padding                                        |
Parent ID:  #28632                               |         Points:
 Reviewer:  mikeperry                            |        Sponsor:
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Comment (by mikeperry):

 #28633 is done, I believe. #28780 could technically be called nice-to-have
 gravy. The machines won't be effective against any real adversary without
 it since lifetimes give away too much info, but we could at least test the
 machines.

 The harmless piece is the tricky bit, though. The Padding TODO file has
 items for sending an ordered preference list of machine choices in the
 negotiation, and a way to stop re-trying negotiation if it keeps failing.
 One (or both) of these must be done to meet the harmless property. I
 *think* that having an ordered preference will be sufficient for safety,
 if we specify a "null" machine that is the last preference/fallback in
 case of error, and if you negotiate a "null" machine, you don't try to
 negotiate anything more on that circuit.

 So if we really want something that can work in 0.4.0, we definitely need
 to implement this preference ordering idea with explicit "null" fallback.
 And then we can try to get #28780 done after that.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/28634#comment:3>
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