[tor-bugs] #17315 [Pluggable transport]: explain replay prevention in obfs4 spec

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#17315: explain replay prevention in obfs4 spec
-------------------------------------+-----------------
     Reporter:  arma                 |      Owner:  asn
         Type:  defect               |     Status:  new
     Priority:  normal               |  Milestone:
    Component:  Pluggable transport  |    Version:
   Resolution:                       |   Keywords:
Actual Points:                       |  Parent ID:
       Points:                       |    Sponsor:
-------------------------------------+-----------------

Comment (by yawning):

 I wasn't going to specify how it was implemented...

 {{{
                 if hmac.Equal(macCmp, macRx) {
                         // Ensure that this handshake has not been seen
 previously.
                         if filter.TestAndSet(time.Now(), macRx) {
                                 // The client either happened to generate
 exactly the same
                                 // session key and padding, or someone is
 replaying a previous
                                 // handshake.  In either case, fuck them.
                                 return nil, ErrReplayedHandshake
                         }

                         macFound = true
                         hs.epochHour = epochHour

                         // We could break out here, but in the name of
 reducing timing
                         // variation, evaluate all 3 MACs.
                 }
 }}}

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/17315#comment:2>
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