[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
-------------------------------------------------+-------------------------
 Reporter:  dgoulet                              |          Owner:  dgoulet
     Type:  enhancement                          |         Status:
                                                 |  reopened
 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                  |
Parent ID:  #26288                               |         Points:
 Reviewer:                                       |        Sponsor:
                                                 |  SponsorV
-------------------------------------------------+-------------------------

Comment (by arma):

 We reopened this ticket because it was never done.

 The current code in master will fill unused bytes with randomness, but it
 will never introduce any extra unused bytes in cells. So if cells are
 always full, there will be no randomness, which will make the entire flow
 predictable to somebody who already knows what bytes they'll be receiving
 from the other side.

 We need some feature, like the one in my branch from comment:3, that
 sometimes leaves a bit of space in cells.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26846#comment:15>
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