[tor-bugs] #8217 [Obfsproxy]: obfsproxy: obfs2: When deriving padding keys we truncate the shared-secret hash

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#8217: obfsproxy: obfs2: When deriving padding keys we truncate the shared-secret
hash
-----------------------+----------------------------------------------------
 Reporter:  asn        |          Owner:  asn             
     Type:  defect     |         Status:  new             
 Priority:  normal     |      Milestone:                  
Component:  Obfsproxy  |        Version:  Obfsproxy: 0.1.4
 Keywords:             |         Parent:                  
   Points:             |   Actualpoints:                  
-----------------------+----------------------------------------------------
 In `derive_padding_key()` we do:
 {{{
   if (shared_seed_nonzero(state->secret_seed))
     digest_update(c, state->secret_seed, OBFUSCATE_SEED_LENGTH);
 }}}

 `OBFUSCATE_SEED_LENGTH` should read `SHARED_SECRET_LENGTH`, similarly to
 how it is in `derive_key()`.

 This is a bug in obfsproxy `master`, and it will break compatibility with
 any correct obfs2 implementations.

 Good thing is that obfsproxy is going obsolete these days, so most (all?)
 obfs2 shared-secret users will probably be using pyobfsproxy.

-- 
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/8217>
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