[tor-bugs] #25573 [Core Tor/Tor]: Create a RELAY_COMMAND_END_ACK cell type

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#25573: Create a RELAY_COMMAND_END_ACK cell type
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     Reporter:  mikeperry     |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  guard-discovery-stats
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:  SponsorV-can  |
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 In order to eliminate a side channel attack described in
 https://petsymposium.org/2018/files/papers/issue2/popets-2018-0011.pdf we
 need a way to determine if a stream id is invalid.

 Many clients (particularly Firefox) will hang up on streams that still
 have data in flight. In this case, Tor clients send RELAY_COMMAND_END when
 they are done with a stream, and immediately remove that stream ID from
 their valid stream mapping. The remaining application data continues to
 arrive, but is silently dropped by the Tor client. The result is that this
 ignored stream data currently can't be distinguished from injected dummy
 traffic with completely random stream IDs, and this fact can be used to
 mount side channel attacks.

 A similar situation exists for spurious RELAY_ENDs.

 This isn't a full fix, because malicious relays can withhold the ack, but
 having it in place would simplify a lot of the code for dealing with
 unexpected packets.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/25573>
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