[tor-bugs] #26860 [Core Tor/Tor]: Spec: decryption order appears to be wrongly specified

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#26860: Spec: decryption order appears to be wrongly specified
------------------------------+----------------------
     Reporter:  dmr           |      Owner:  (none)
         Type:  defect        |     Status:  new
     Priority:  Medium        |  Milestone:
    Component:  Core Tor/Tor  |    Version:
     Severity:  Normal        |   Keywords:  tor-spec
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
------------------------------+----------------------
 catalyst noted in `#tor-dev` that the decryption algorithm in 5.5:
 * `won't work unless decryption is commutative (which it is for plain AES-
 CTR, but not for the proposal)`
 * `doesn't look like it'll work to handle a relay cell arriving at the OP
 that originates from an intermediate node`

 I had also noted the latter, so I postulated `is the order of that line
 backwards? that is... should it be "For I=1...N?"` and asked `what does
 little-t tor do?`

 And asn noted:
 > in relay_decrypt_cell() it does:
 > {{{
 >    if (CIRCUIT_IS_ORIGIN(circ)) { /* We're at the beginning of the
 circuit.
 >                                     * We'll want to do layered decrypts.
 */
 >       crypt_path_t *thishop, *cpath = TO_ORIGIN_CIRCUIT(circ)->cpath;
 >       thishop = cpath;
 > }}}
 > i think this is I=1...N like you say
 > (for 1 being the hop closest to the OP)

 So it looks like this is an error in the spec, rather than a problem with
 `tor`.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26860>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online

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