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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

%%Not unobservable

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Perfect forward secrecy


Telescoping circuit

	negotiates keys at each hop

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No mixing, padding, traffic shaping (yet)


Please show us they're worth the usability tradeoff

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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/