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Re: [tor-bugs] #30716 [Circumvention/Obfs4]: Improve the obfs4 obfuscation protocol
#30716: Improve the obfs4 obfuscation protocol
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Reporter: phw | Owner: phw
Type: task | Status:
| assigned
Priority: High | Milestone:
Component: Circumvention/Obfs4 | Version:
Severity: Normal | Resolution:
Keywords: sponsor28, anti-censorship-roadmap- | Actual Points:
august |
Parent ID: | Points: 20
Reviewer: | Sponsor:
| Sponsor28-must
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Comment (by cohosh):
Nice! Thanks for sharing the update on this!
I have a few questions and comments about the website fingerprinting
approach. The observation that these two problems have some overlap is a
good one, but it's also worth noting that it's not necessarily the case
that implementing website fingerprinting defences will be strictly better
for a pluggable transport from an obfuscation point of view.
The main goal for a website fingerprinting defence is to prevent an
attacker from learning which site you're visiting, not from learning that
you are using Tor or even that you are using a website fingerprinting
defence. And while the website fingerprinting portion is good to have, I
think the current understanding of website fingerprinting of Tor traffic
is that it makes more sense to apply to Onion Services at the moment and
isn't urgent for the rest of Tor traffic yet? This is also being actively
worked on I believe. Of course PTs don't have to only be used with Tor and
this would make them better if used for other anti-censorship tools.
> - We need to build an evaluation framework to understand what works and
what doesn't.
So my concern here is that even though the packet breaking is
probabilistic (and eventually variable, etc.), can we do it without adding
a fingerprintable feature to the obfuscation traffic? I suppose this is
related to the "long tail" argument that we have, where hopefully by
making the sharknado traffic look less and less like any regular thing,
we're increasing the uncertainty a censor would have in blocking it.
And it brings us to your bullet point above. It would be an interesting
research question perhaps to take a look at existing website
fingerprinting work and see how identifiable the defences are as they are
implemented in those works given that the adversary knows the client is
using Tor.
> - We should find a way to make obfs4's packet sequences server-specific
by incorporating the server's shared secret into the sequence generation
process, just like it's done for packet lengths and inter-arrival times.
Was the purpose of this to maintain some consistency for censors who are
observing all traffic to a specific bridge IP address (which might make it
less suspicious)? Or to try to introduce some variability for different
obfs bridges in order to add to the confusion?
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/30716#comment:11>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online
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