[tor-bugs] #26846 [Core Tor/Tor]: prop289: Leave unused random bytes in relay cell payload

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#26846: prop289: Leave unused random bytes in relay cell payload
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 Reporter:  dgoulet                              |          Owner:  dgoulet
     Type:  enhancement                          |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.4.1.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  prop289, 035-roadmap-subtask,        |  Actual Points:
  prop289-assigned-sponsor-v, 041-proposed-on-   |
  roadmap, 0411-alpha, security 041-should       |
Parent ID:  #26288                               |         Points:
 Reviewer:                                       |        Sponsor:
                                                 |  SponsorV
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Comment (by arma):

 Replying to [comment:18 nickm]:
 > dgoulet, I'm happy to do this if you'd like.

 If you base it on my branch above, which I think is still plausible since
 it's so simple, be sure to note that dgoulet stuck a "don't mess with the
 first 4 unused bytes" rule in there, so we'd need to open up at least 5.

 Another option, which is less hacky (less probabilistic) but would require
 more code and more state, would be to have a counter (on each circuit) of
 how many full cells we've sent, and if we ever send a non-full cell then
 reset it, and if it ever reaches 1000, make some space in that cell.

 And as a final note, it's not actually required (from a technical
 coordination perspective) that we get this feature in to this release,
 since we could in theory add this "make some space" logic in a future Tor,
 and that would be the Tor that finishes doing prop289 properly. But "why
 not now, it's not that big" is a solid reason for doing it now too.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26846#comment:21>
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