[tor-bugs] #8223 [Obfsproxy]: obfsproxy: obfs2: Key derivation with shared-secret enabled does an extra hash iteration

[Tor Bug Tracker & Wiki]

#8223: obfsproxy: obfs2: Key derivation with shared-secret enabled does an extra
hash iteration
-----------------------+----------------------------------------------------
 Reporter:  asn        |          Owner:  asn             
     Type:  defect     |         Status:  new             
 Priority:  normal     |      Milestone:                  
Component:  Obfsproxy  |        Version:  Obfsproxy: 0.1.4
 Keywords:             |         Parent:                  
   Points:             |   Actualpoints:                  
-----------------------+----------------------------------------------------
 {{{
   digest_update(c, (uchar*)keytype, strlen(keytype));
   if (seed_nonzero(state->initiator_seed))
     digest_update(c, state->initiator_seed, OBFUSCATE_SEED_LENGTH);
   if (seed_nonzero(state->responder_seed))
     digest_update(c, state->responder_seed, OBFUSCATE_SEED_LENGTH);
   if (shared_seed_nonzero(state->secret_seed))
     digest_update(c, state->secret_seed, SHARED_SECRET_LENGTH);
   digest_update(c, (uchar*)keytype, strlen(keytype));
   digest_getdigest(c, buf, sizeof(buf));

   if (shared_seed_nonzero(state->secret_seed)) {
     digest_t *d;
     int i;
     for (i=0; i < OBFUSCATE_HASH_ITERATIONS; i++) {
       d = digest_new();
       digest_update(d, buf, sizeof(buf));
       digest_getdigest(d, buf, sizeof(buf));
       digest_free(d);
     }
   }
 }}}

 Spec says:

 {{{
    Optionally, if the client and server share a secret value SECRET,
    they can replace the MAC function with:

       MAC(s,x) = H^n(s | x | H(SECRET) | s)

    where n = HASH_ITERATIONS.
 }}}

 but the above code actually does `n+1` hash iterations.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/8223>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online