[tor-bugs] #30716 [Circumvention/Obfs4]: Improve the obfs4 obfuscation protocol
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Mon Jun 24 21:54:46 UTC 2019
#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:
Parent ID: | Points: 20
Reviewer: | Sponsor:
| Sponsor28-must
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Description changed by phw:
Old description:
> As part of our work for Sponsor 28, we will evaluate and improve the
> obfs4 obfuscation protocol.
>
> Roger started the discussion [https://lists.torproject.org/pipermail
> /anti-censorship-team/2019-May/000015.html on our anti-censorship-team
> mailing list]. Relevant reading is the CCS'15 paper
> [https://censorbib.nymity.ch/#Wang2015a Seeing through Network-Protocol
> Obfuscation] and the S&P'16 paper
> [https://censorbib.nymity.ch/#Tschantz2016a SoK: Towards Grounding
> CensorshipCircumvention in Empiricism].
>
> Let's use this ticket to keep track of this effort.
>
> Suggestions for improvement:
> * [https://trac.torproject.org/projects/tor/ticket/30716#comment:1
> yawning writes] that obfs4 doesn't easily support backward incompatible
> protocol alterations.
> * [https://trac.torproject.org/projects/tor/ticket/30716#comment:3
> yawning writes] that the framing could use better cryptography.
> * [https://trac.torproject.org/projects/tor/ticket/30716#comment:2 dcf
> writes] that during the handshake, the client needs to wait for the
> server before it can send more data. A
> [https://lists.torproject.org/pipermail/tor-dev/2017-June/012310.html
> tor-dev@] post has more detail.
> * Each obfs4 server has a unique flow signature. Can we make packet
> payload unique to each server too? For example, can we automatically
> derive a formal language, so not all obfs4 instances send high-entropy
> data?
New description:
As part of our work for Sponsor 28, we will evaluate and improve the obfs4
obfuscation protocol, which may result in obfs5.
Roger started the discussion [https://lists.torproject.org/pipermail/anti-
censorship-team/2019-May/000015.html on our anti-censorship-team mailing
list]. Relevant reading is the CCS'15 paper
[https://censorbib.nymity.ch/#Wang2015a Seeing through Network-Protocol
Obfuscation] and the S&P'16 paper
[https://censorbib.nymity.ch/#Tschantz2016a SoK: Towards Grounding
CensorshipCircumvention in Empiricism].
Let's use this ticket to keep track of this effort. Below is a list of
ideas that we may or may not want to incorporate in obfs5.
== Randomisation
Obfs4 already implements randomisation for packet lengths and inter-
arrival times but there are other protocol aspects that we can randomise.
Note that the adoption of these strategies may complicate censorship
analysis: if obfs5 instance X looks very different from obfs5 instance Y,
then X may end up getting blocked while Y still works. Instead of saying
"obfs5 is blocked," one may then have to be more specific and say "the
obfs5 instances that rely on UDP are blocked."
* **Payload**: All bytes that obfs4 writes to the wire are randomly
distributed. These high-entropy packets may or may not be common on the
Internet. We could evade a "high-entropy filter" by having obfs4 servers
derive a formal language from the shared secret. This language could, say,
use dummy clear-text headers.
* **Cover traffic**: [https://lists.torproject.org/pipermail/tor-
dev/2017-June/012310.html dcf] explains that obfs4 only sends data when
it's given data to send. To improve on this, as dcf suggests, we could
make obfs5 send data even when the application has nothing to send.
* **Packet directions**: An obfs4 flow begins with the client sending data
to the server. We could randomise packet directions and have, say, the
server talk first with a server-specific probability.
* **Transport protocol**: An obfs4 server could talk either TCP or UDP or
SCTP. This may very well not be worth the effort.
== Lessons learned from [https://censorbib.nymity.ch/#Wang2015a CCS'15
paper]
* DPI boxes tend to classify flows by only inspecting the first N packets
of a flow. Keeping state is expensive, after all. We could exploit this by
relaxing our obfuscation techniques after N packets to increase
throughput.
* The paper's data set may not be representative of what countries or ISPs
would see:
* It's "only" a university uplink. Universities typically have policies
that prohibit file sharing such as BitTorrent. BitTorrent's "message
stream encryption" may look similar to obfs3 and obfs4.
* The data sets are from 2014, 2012, and 2010, respectively. That's a
long time in Internet years.
* The detectors' false positive rates are non-trivial and, as the
authors point out themselves, would be problematic for a censor given that
non-obfuscated traffic significantly outweighs obfuscated traffic.
* Does the data set only contain one obfs4 server instance? This may
have affected their results.
== Miscellaneous
* [https://trac.torproject.org/projects/tor/ticket/30716#comment:1 yawning
writes] that obfs4 doesn't easily support backward incompatible protocol
alterations.
* [https://trac.torproject.org/projects/tor/ticket/30716#comment:3 yawning
writes] that the framing could use better cryptography.
* Crazy idea: Use a modified TCP stack that ignores RST and FIN segments,
so the GFW's on-path devices cannot tear down the connection. Instead, the
obfs5 protocol could signal the end of the connection in an authenticated
control frame. We could ignore RST and FIN segments by using firewall
rules, or to get more crazy, by shipping a user space TCP stack (this may
be easy to fingerprint, though).
--
--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/30716#comment:5>
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