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

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#26228: Clarify/determine specification for padding bytes, (formerly also PADDING
cell)
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 Reporter:  dmr           |          Owner:  dmr
     Type:  defect        |         Status:  needs_revision
 Priority:  Medium        |      Milestone:  Tor: 0.3.5.x-final
Component:  Core Tor/Tor  |        Version:
 Severity:  Normal        |     Resolution:
 Keywords:  tor-spec      |  Actual Points:
Parent ID:  #26869        |         Points:
 Reviewer:                |        Sponsor:
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Changes (by teor):

 * status:  needs_review => needs_revision


Comment:

 Oops. VERSIONS cells can't have (more than 1 byte of) random padding,
 because it will be interpreted as an extra version.

 All the other variable-length cells can have extra bytes of padding,
 because their formats contain an internal length (VPADDING is a degenerate
 case, it has Length bytes of padding). We should explicitly say that these
 bytes MUST be ignored.

 All other fixed-length cells can have extra bytes of padding. If we make
 the padding random, we won't be able to extend the cell formats as easily.
 (With zero padding, we can extend a cell format by calling some of the
 remaining zero bytes a "number of extensions" or "flags that default to
 zero" field.)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26228#comment:12>
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