[tor-bugs] #26228 [Core Tor/Tor]: Clarify/determine specification for padding bytes, PADDING cell

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#26228: Clarify/determine specification for padding bytes, PADDING cell
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     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:                |
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 ==== Background
 I was trying to interpret the tor-spec for padding bytes, and ending up
 asking nickm for some clarification over IRC.
 nickm suggested most of the cc'd for the ticket - I added atagar, too.

 ==== Unclear areas
 Here are the points that need clarification / specification:
 * spec for padding bytes does not clearly say what senders `MUST` or
 `SHOULD` do, [[https://gitweb.torproject.org/torspec.git/tree/tor-
 spec.txt?id=f6e93d9751002d970614662c8173ff2fa5b7c193#n412|only mentioning
 that padding is with 0 bytes]] or
 [[https://gitweb.torproject.org/torspec.git/tree/tor-
 spec.txt?id=f6e93d9751002d970614662c8173ff2fa5b7c193#n1480|(elsewhere) NUL
 bytes]]
 * spec for padding bytes does not say what receivers `MUST` or `SHOULD`
 do, when receiving non-zero bytes in the Cell (e.g. warn? ignore?)
 * spec is a bit inconsistent with `PADDING` cells ^^[1^^]^^[2^^]

 ==== Discussion: padding bytes
 For the padding bytes that are not part of `PADDING` cells, nickm offered
 the following as a non-exhaustive set of possible forward-compatible
 options:
 * "the [padding] bytes SHOULD be zero, and that implementations MUST
 ignore them"
 * "The first 8 padding bytes MUST be zero; all subsequent padding bytes
 SHOULD be randomized. Implementations MUST ignore padding bytes"
 * "All padding bytes should be randomized; implemenations MUST ignore
 unrecognized padding bytes"
 ... and mentioned that "[he doesn't] know enough of the argument in favor
 of randomization to have a very strong preference"

 ==== Inconsistency: `PADDING` cell payload
 (see bullet above)

 These references highlight the inconsistency:

 ^^[1^^] `PADDING: Payload is unused.` per
 [[https://gitweb.torproject.org/torspec.git/tree/tor-
 spec.txt?id=f6e93d9751002d970614662c8173ff2fa5b7c193#n469|sec 3 "Cell
 Packet format"]].
   implies 0 bytes of payload, so the rest should be padded per that
 section
 ^^[2^^] `The contents of a PADDING, VPADDING, or DROP cell SHOULD be
 chosen randomly, and MUST be ignored.` per
 [[https://gitweb.torproject.org/torspec.git/tree/tor-
 spec.txt?id=f6e93d9751002d970614662c8173ff2fa5b7c193#n1723|sec 7.2 "Link
 padding"]].
   implies the payload of a `PADDING` cell actually is the rest of the size
 of the cell, and that it SHOULD be chosen randomly

 The `PADDING` cells were mentioned in IRC but not discussed.
 I think a simple change to make the spec consistent between the two
 sections would be this:
 {{{
 PADDING: Payload contains random data. (See Sec 7.2)
 }}}

 However, given the other points here, is that correct?

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26228>
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