[tor-bugs] #22006 [Core Tor/Tor]: prop224: Validate ed25519 pubkeys to remove torsion component

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#22006: prop224: Validate ed25519 pubkeys to remove torsion component
-------------------------------------------------+-------------------------
 Reporter:  asn                                  |          Owner:  asn
     Type:  defect                               |         Status:
                                                 |  needs_review
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.2.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-hs, prop224, ed25519, review-    |  Actual Points:
  group-18                                       |
Parent ID:  #21888                               |         Points:
 Reviewer:  nickm, isis                          |        Sponsor:
                                                 |  SponsorR-can
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Comment (by catalyst):

 It would be nice to have a summary of what security goals this validation
 accomplishes, given that ed25519 signature verification already nulls out
 the cofactor.  (e.g., what can an adversary do with a non-canonical onion
 address and no private key for it?)  Also, I suspect that the HS and non-
 HS cases have slightly different security goals, and these would be good
 to distinguish.

 Terminology nit: all the sources I can easily find define a torsion point
 as one that has finite order.  So as I understand it, all ed25519 points
 are "torsion points".  Maybe use "small-torsion component or "small-order
 component" instead of "torsion component"?

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/22006#comment:22>
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