[tor-bugs] #23420 [Core Tor/Tor]: prop224: Pad RENDEZVOUS1 cell to match legacy cell length

Tor Bug Tracker & Wiki blackhole at torproject.org
Wed Sep 20 10:40:57 UTC 2017

#23420: prop224: Pad RENDEZVOUS1 cell to match legacy cell length
 Reporter:  asn                             |          Owner:  dgoulet
     Type:  defect                          |         Status:  needs_review
 Priority:  Medium                          |      Milestone:  Tor:
                                            |  0.3.2.x-final
Component:  Core Tor/Tor                    |        Version:
 Severity:  Normal                          |     Resolution:
 Keywords:  prop224, prop224-extra, tor-hs  |  Actual Points:
Parent ID:                                  |         Points:  1
 Reviewer:                                  |        Sponsor:

Comment (by asn):

 OK. Based on the thread from comment:4 :

 - Ian pointed out that just padding with random bytes is not the right
 approach since the `g^y` field of the legacy cell has certain algebraic
 structure that will be missing from the random pad. We could in theory add
 intelligent padding that matches the algebraic structure, but kinda
 overkill perhaps.

 - teor pointed out that there are other distinguishers between v2 and v3,
 like whether the service extends with tap vs ntor.

 We ''could'' do naive padding in v3, but only with the hope that it would
 obfuscate future cells (v4, etc.).

Ticket URL: <https://trac.torproject.org/projects/tor/ticket/23420#comment:5>
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