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Tor:
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Next-generation Onion Routing
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Roger Dingledine
Nick Mathewson
Paul Syverson
The Free Haven Project
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http://freehaven.net/
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Low-latency anonymity system
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Deployed: 20 nodes, hundreds (?) of users
Many improvements on earlier design
Free software -- available source code
Design is not covered by earlier onion routing
patent
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Talk Overview
A bit about Onion Routing
Improvements we've made
Some related work
Some lessons learned
Ask me questions
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Anonymity: Who needs it?
Private citizens
advocacy, counseling, whistleblowing, reporting, ...
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Higher-level protocols
voting, e-cash, auctions
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Government applications
research, law enforcement
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Business applications
hide relationships and volumes of communication
Who is visiting job sites?
Which groups are talking to patent lawyers?
Who are your suppliers and customers?
Is the CEO talking to a buyout partner?
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Anonymity is a network effect
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Onion Routing is...
An overlay network
Users build virtual circuits through the network
One layer of encryption at each hop
Fixed-size cells
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Tor's goal
Conservative design (minimize new design work needed)
Support testing of future research
Design for deployment; deploy for use
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Threat model
Protect against curious Bob
Protect against somebody watching Alice
Protect against a few curious nodes in the middle
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Differences / limitations
We're TCP-only, not all IP (but we're user-space and very portable)
Not peer-to-peer
No protocol normalization
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Perfect forward secrecy
Telescoping circuit
negotiates keys at each hop
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%%Separation from "protocol cleaning"
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No mixing, padding, traffic shaping (yet)
Please show us they're worth the usability tradeoff
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%%Many TCP streams can share one circuit
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Congestion control
Simple rate limiting
Plus have to keep internal nodes from overflowing
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Directory servers
Approve new servers
Tell clients who's up right now
plus their keys, location, etc
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Variable exit policies
Each server allows different outgoing connections
E.g. no servers allow outgoing mail currently
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End-to-end integrity checking
In previous onion routing, an insider could change
the text being transmitted:
"dir" => "rm *"
Even an external adversary could do this!
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Rendezvous points
allow hidden services
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Related work
c/n vs c^2/n^2 vs 2
freedom, peekabooty, jap
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Future work
Threshold directory agreement
Restricted-route (non-clique) topology
Morphmix/p2p extensions?
Location-hidden servers via rendezvous points
Make it work better
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We have working code
Plus a design document,
and a byte-level specification
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http://freehaven.net/tor/
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Privacy Enhancing Technologies workshop
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http://petworkshop.org/
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