[tor-bugs] #24182 [Core Tor/Tor]: Pad INTRODUCE cells to hide the number of link specifiers

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#24182: Pad INTRODUCE cells to hide the number of link specifiers
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     Reporter:  teor       |      Owner:  (none)
         Type:  defect     |     Status:  new
     Priority:  Medium     |  Milestone:  Tor: 0.3.3.x-final
    Component:  Core       |    Version:
  Tor/Tor                  |
     Severity:  Normal     |   Keywords:  prop224, tor-hs, defence-in-depth
Actual Points:             |  Parent ID:  #23493
       Points:  0.5        |   Reviewer:
      Sponsor:             |
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 Before #23577, there are two sizes of introduce cell data:
 * legacy id and IPv4 (24 bytes)
 * legacy id and IPv4 and ed25519 id (56 bytes)

 After #23577: there are another two sizes:
 * legacy id and IPv4 and IPv6 (40 bytes)
 * legacy id and IPv4 and ed25519 id and IPv6 (72 bytes)

 And if we ever remove IPv4 addresses, or add another link specifier, we
 create even more sizes.

 This is ok at the moment, because we pad all cells to
 HS_CELL_INTRODUCE1_MIN_SIZE (246 bytes), and no cells are larger than this
 size. But we should make sure we are padding larger cells to the maximum
 cell length, to avoid splitting the rendezvous point anonymity set with
 future link specifiers.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/24182>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
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