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Re: [tor-bugs] #26228 [Core Tor/Tor]: Clarify/determine specification for padding bytes, (formerly also PADDING cell)
#26228: Clarify/determine specification for padding bytes, (formerly also PADDING
cell)
--------------------------+------------------------------------
Reporter: dmr | Owner: dmr
Type: defect | Status: needs_review
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_revision => needs_review
Comment:
Replying to [comment:13 teor]:
> Actually, we should distinguish between padding relay cells, which need
randomness because they are relayed, and link cells, which don't need
randomness because:
> a) they don't have any digests, so randomisation doesn't gain us
anything
> b) TLS should protect their payloads from modification
I made this change in the spec, and included the relay cell padding
randomisation from #26871.
Replying to [comment:14 dmr]:
> Replying to [comment:12 teor]:
> > Oops. VERSIONS cells can't have (more than 1 byte of) random padding,
because it will be interpreted as an extra version.
>
> Actually, well-formed VERSIONS cells can't have //any// (fully) random
padding.
>
> The spec [[https://gitweb.torproject.org/torspec.git/tree/tor-
spec.txt?id=ce1d4a8b7e1e5eb714068a8cf44fdc5ae24460d9#n556|says]]:
> > Either party MUST close the connection if the versions cell is not
well-formed (for example, if it contains an odd number of bytes).
Thanks!
> It's unclear in the spec if you can pad VERSIONS cells by repeating
(pairs of) version bytes. The algorithms described in the spec make it
sound possible, but "not well-formed" is a bit loosely defined.
The implementation would interpret padding as extra versions, so let's ban
it.
Please see my branch 26228-padding-bytes on
https://github.com/teor2345/torspec.git , which also fixes #26870 and
contains the spec for #26871.
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/26228#comment:15>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online
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