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04-07-2013 Slide 1 of 18
Björgvin Ragnarsson and Pieter Westein University of Amsterdam
Research Project 2
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Slide 2 of 18 04-07-2013
Introduction Research Question
Git overview
Design overview Demo
Performance measurements Prototype evaluation
Conclusion and Future work
Questions
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Slide 3 of 18 04-07-2013
Source: https://www.torproject.org/about/overview.html.en
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Slide 4 of 18 04-07-2013
Tor relays are public, easy to block
Introduction of Bridges
Scanners actively trying to reach Bridges
Introduction of Pluggable Transports
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Slide 5 of 18 04-07-2013
Modules for obfsproxy framework
Can be used for other purposes than Tor ◦ as a SOCKS proxy
Existing transports ◦ Obfs2,Obfs3, Skype, ScrambleSuit, Dust,
StegoTorus, flashproxy
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Slide 6 of 18 04-07-2013
Is it possible to shape Tor traffic in such a way that it looks identical to the Git protocol?
How could a censor identify Tor bridges and users using such an obfuscated protocol?
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Slide 7 of 18 04-07-2013
Version control system
Push and pull mechanism
Four transports protocols ◦ SSH, Git, HTTP, HTTPS
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Slide 8 of 18 04-07-2013
Client Server
git-receive-pack
Have: List of references
Want: List of references
Sending Objects
FIN-ACK
ACK
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Slide 9 of 18 04-07-2013
Client Server
Have: List of references
Want: List of references
Sending Objects
FIN-ACK
ACK
git-upload-pack
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Slide 10 of 18 04-07-2013
Files compressed and stored in the Git database
SHA1 hash of the content used as reference
File Git Object Git Database
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Slide 11 of 18 04-07-2013
TCP stream is stored as files in Git
The Git program does the transfer ◦ Makes it harder to fingerprint
◦ Provides four transports in one:
(SSH/Git/HTTP/HTTPS)
Client initiates send/receive
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Slide 12 of 18 04-07-2013
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04-07-2013 Slide 13 of 18
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Slide 14 of 18 04-07-2013
Downloading a 10MB file using git over ssh ◦ 7 KB/s over ssh through Tor
◦ 166 KB/s over ssh without Tor
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Slide 15 of 18 04-07-2013
The frequency of pushes and pulls
Tor data is compressed (not hidden)
Git traces on disk
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Slide 16 of 18 04-07-2013
Tor usage can be obfuscated as Git traffic ◦ or any other VCS
Prototype is slow, compared to normal Tor
Polling and disk writes are weak points
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Slide 17 of 18 04-07-2013
Using publicly available Git servers for relaying
Layered obfuscation
Eliminate disk writes
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04-07-2013 Slide 18 of 18
Thank you for your attention
Track development at: https://trac.torproject.org/projects/tor/ticket/9192