[tor-bugs] #19872 [Core Tor/Tor]: Introduce prefixed sign/verify functions

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#19872: Introduce prefixed sign/verify functions
-------------------------+-------------------------------------------------
     Reporter:  asn      |      Owner:
         Type:  task     |     Status:  new
     Priority:  Medium   |  Milestone:  Tor: 0.2.???
    Component:  Core     |    Version:
  Tor/Tor                |   Keywords:  029-proposed, tor-hs,
     Severity:  Normal   |  TorCoreTeam201608
Actual Points:           |  Parent ID:
       Points:  0.3      |   Reviewer:
      Sponsor:           |
  SponsorR-can           |
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 prop224 wants us to sign messages prefixed with a constant string. For
 example:

 {{{
    SIG is a signature, using AUTH_KEY, of all contents of the cell, up
    to but not including SIG. These contents are prefixed with the string
    "Tor establish-intro cell v1".
 }}}

 dgoulet also has plans for adding a prefix to the signature of the HS
 descriptor.

 So, it would be ideal if we had some crypto util functions that will do
 that for us. We are talking about two functions called
 `ed25519_sign_prefixed()` and `ed25519_checksig_prefixed()` that are
 basically wrappers over `ed25519_sign()` and `ed25519_checksig()` that
 also accept a `const char *prefix_str`.

 If we have these functions in upstream tor, it will be easier for us to
 write code for the various prop224 subsystems that need to be implemented.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/19872>
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