[tor-bugs] #16562 [Tor]: Harmonize curve25519-signature format with what others are doing

[Tor Bug Tracker & Wiki]

#16562: Harmonize curve25519-signature format with what others are doing
-------------------------+--------------------------------
     Reporter:  nickm    |      Owner:
         Type:  defect   |     Status:  new
     Priority:  blocker  |  Milestone:  Tor: 0.2.7.x-final
    Component:  Tor      |    Version:
   Resolution:           |   Keywords:
Actual Points:           |  Parent ID:
       Points:           |
-------------------------+--------------------------------

Comment (by nickm):

 Right now, our ref10 code's verification logic does:
 {{{
   if (signature[63] & 224) goto badsig;
 }}}

 And donna's verification logic does:
 {{{
         if ((RS[63] & 224) || !ge25519_unpack_negative_vartime(&A, pk))
                 return -1;
 }}}

 So if we're going to add support for this, we're going to need to do it in
 these steps:

 1. Alter the verification logic to accept the new sign bit location.
 2. Some time later, generate descriptors with their crosscertification
 signatures in the new format.
 3. Some time later, remove support for accepting the old signature format
 format.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/16562#comment:1>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online