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Remove legacy files from main source distribution 

The AUTHORS file was pretty outdated. Its contents moved onto the
people page. Design paper and roadmaps moved to the projects
directory in svn.
This commit is contained in:
Sebastian Hahn 2010-02-07 07:39:00 +01:00
parent 3a7e7f1be4
commit f164a76f72
41 changed files with 4 additions and 19165 deletions
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41
AUTHORS
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@ -1,41 +0,0 @@
This file lists the authors for Tor,
a free software project to provide anonymity on the Internet.
For more information about Tor, see https://www.torproject.org/.
If you got this file as a part of a larger bundle,
there are probably other authors that you should be aware of.
Main authors:
-------------
Roger Dingledine <arma@freehaven.net> overhauled all of the code, did
a bunch of new design work, etc.
Nick Mathewson <nickm@freehaven.net> wrote lots of stuff too, in
particular the router and descriptor parsing, and the crypto and tls
wrappers.
Matej Pfajfar <badbytes@freehaven.net> wrote the first version of the code
(called OR) in 2001-2002.
Contributors:
-------------
John Bashinski <jbash@velvet.com> contributed the initial rpm spec file.
Christian Grothoff <grothoff@cs.purdue.edu> contributed better daemonizing
behavior.
Steven Hazel <sah@thalassocracy.org> made 'make install' do the right
thing.
Jason Holt <jason@lunkwill.org> contributed patches to the instructions
and the man page.
Peter Palfrader <peter@palfrader.org> maintains everything that's
debian-specific, and has written other useful features.
Aaron Turner <aturner@netscreen.com> contributed the first version of
the tor.sh initscripts shell script.

4
ChangeLog
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@ -25,6 +25,10 @@ Changes in version 0.2.2.9-alpha - 2010-??-??
- Generate our manpage and HTML documentation using Asciidoc. This
should make it easier to maintain the documentation, and produce
nicer HTML.
- Removed some unnecessary files from the source distribution. The
AUTHORS file had its content merged into the people page on the
website. The roadmaps and design doc can now be found in the
projects directory in svn.
o Removed features:
- Stop shipping parts of the website and the design paper in the

73
contrib/privoxy-tor-toggle
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#!/bin/sh
# A script to turn Tor SOCKS4a in Privoxy on or off.
CONFFILE=/etc/privoxy/config # privoxy config file.
TOR_REG="forward.*localhost:9050" # Regular expression to find Tor in privoxy
PRIVOXY="/etc/init.d/privoxy restart" # command to reload privoxy config file.
SED="/bin/sed" # sed command, of course.
GREP="/bin/grep" # grep command.
usage () {
echo "\
privoxy-tor-toggle: Change Privoxy's configuration to use/not use Tor.
Usage:
privoxy.tor <-- Switch Tor on or off.
privoxy.tor [on|off] <-- Set Tor on or off.
privoxy.tor status <-- Display Tor's current status.
privoxy.tor [-h|--help|-?] <-- Print usage.
"
}
# Find out the current status of tor. Set $tor_status
get_status () {
gret=`$GREP -l -e "^$TOR_REG" $CONFFILE`
if [ x$gret = x ] ; then
tor_status=off;
else
tor_status=on;
fi
return
}
# Turn tor on/off according to $1
set_tor () {
tor_gate=$1
get_status
if [ $tor_status = $tor_gate ] ; then
echo "Tor is already $1."
return
elif [ $tor_gate = flip ] ; then
if [ $tor_status = on ] ; then
tor_gate=off
elif [ $tor_status = off ] ; then
tor_gate=on
fi
fi
echo "Turning Tor $tor_gate..."
if [ $tor_gate = on ] ; then
reg=s/^#\($TOR_REG\)/\\1/
$SED -i.bak -r "$reg" $CONFFILE
else
reg=s/^\($TOR_REG\)/#\\1/
$SED -i.bak -r "$reg" $CONFFILE
fi
$PRIVOXY
return 0;
}
if [ x$1 = x ] ; then
set_tor flip
elif [ $1 = on ] ; then
set_tor on
elif [ $1 = off ] ; then
set_tor off
elif [ $1 = status ] ; then
get_status
echo "Tor is $tor_status"
elif [ $1 = --help ] || [ $1 = -h ] || [ $1 = "-?" ] ; then
usage
exit 0
else
echo "Unrecognized option: \"$1\""
fi

52
contrib/proxy-some-domains
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@ -1,52 +0,0 @@
Subject:
Re: Anonymous/Nonymous Communication Coexisting?
From:
Kristian Köhntopp <kris@xn--khntopp-90a.de>
Date:
Fri, 10 Jun 2005 08:56:19 +0200
To:
or-talk@freehaven.net
On Wednesday 08 June 2005 04:20, yancm@sdf.lonestar.org wrote:
>> Is it possible to have a single application, such as a web
>> browser or a p2p client behave normally with normal url's but
>> use tor if the url is an xyz.onion address? Or is it
>> everything or nothing?
This is basically a question of using your proxy or not. You can
control the behaviour of your browser in great detail writing a
proxy.pac program in Javascript and setting that program as the
proxy autoconfiguration URL in your browser.
An example:
kris@jordan01:~> cat /srv/www/htdocs/proxy.pac
function FindProxyForURL(url, host)
{
var proxy_yes = "PROXY jordan01.int.cinetic.de:3128";
var proxy_no = "DIRECT";
// Redirect all accesses to mlan hosts to the mlan proxy
if (dnsDomainIs(host, ".mlan.cinetic.de")) {
return proxy_yes;
}
// Everything else is direct
return proxy_no;
}
So here the program checks if the destination is a mlan-Host, and
if so, uses the appropriate proxy on jordan for the access,
while all other accesses are direct.
You could do a similar thing with .onion accesses with a trivial
modification.
Docs:
http://wp.netscape.com/eng/mozilla/2.0/relnotes/demo/proxy-live.html
Kristian

41
contrib/tor-0.1.2.17.tar.gz.metalink.in
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@ -1,41 +0,0 @@
<?xml version="1.0" encoding="utf-8"?>
<metalink version="3.0" generator="Metalink Editor version 1.1.0" xmlns="http://www.metalinker.org/">
<publisher>
<name>The Tor Project</name>
<url>https://www.torproject.org</url>
</publisher>
<license>
<name>BSD</name>
<url>http://opensource.org/licenses/bsd-license.php</url>
</license>
<identity>Tor</identity>
<version>@VERSION@</version>
<copyright>2007 The Tor Project, Inc.</copyright>
<description>Anonymity Online</description>
<files>
<file name="tor-@VERSION@.tar.gz">
<size>1251636</size>
<language>en</language>
<os>Source</os>
<verification>
<hash type="md5">ef8fc7f45d167875c337063d437c9832</hash>
<hash type="sha1">01092fb75c407b5c1d7f33db069cf7641973d94d</hash>
<hash type="sha256">fc0fb0c2891ae09854a69512c6b4988964f2eaf62ce80ed6644cb21f87f6056a</hash>
<pieces type="sha1" length="262144">
<hash piece="0">c778dd01e05734d57f769082545f9802386e42bb</hash>
<hash piece="1">39b172ed8b9290884c7bd129db633a79e28d5ae9</hash>
<hash piece="2">28d708e7489a1e9951e757443672535aedfa3abe</hash>
<hash piece="3">a7623e07081819a37300de0511bbdda0bdc960bd</hash>
<hash piece="4">f246021e55affe320a1f86eac5b049dd0caad828</hash>
</pieces>
</verification>
<resources>
<url type="http" location="at">http://tor.cypherpunks.at/dist/</url>
<url type="http" location="ca">http://tor.depthstrike.com/dist/</url>
<url type="http" location="ca">http://tor.hermetix.org/dist/</url>
<url type="http" location="ch">http://tor.boinc.ch/dist/</url>
<url type="http" location="cn">http://tor.anonymity.cn/dist/</url>
</resources>
</file>
</files>
</metalink>

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doc/codecon04.mgp
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%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%deffont "standard" xfont "comic sans ms-medium-r"
%%deffont "thick" xfont "arial black-medium-r"
%%deffont "typewriter" xfont "courier new-bold-r"
%%deffont "type2writer" xfont "arial narrow-bold-r"
%%deffont "standard" tfont "standard.ttf", tmfont "kochi-mincho.ttf"
%%deffont "thick" tfont "thick.ttf", tmfont "goth.ttf"
%%deffont "typewriter" tfont "typewriter.ttf", tmfont "goth.ttf"
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%deffont "italic" xfont "helvetica-italic-r", tfont "ariali.ttf", tmfont "hoso6.ttf"
%deffont "typewriter" xfont "courier-medium-r", tfont "typewriter.ttf", tmfont "hoso6.ttf"
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% Default settings per each line numbers.
%%
%default 1 leftfill, size 8, fore "black", back "white", font "thick", hgap 1
%default 2 size 8, vgap 10, prefix " ", ccolor "black"
%default 3 size 6, bar "gray70", vgap 0
%default 4 size 6, fore "black", vgap 0, prefix " ", font "standard"
%%
%%default 1 area 90 90, leftfill, size 9, fore "yellow", back "blue", font "thick"
%%default 2 size 9, vgap 10, prefix " "
%%default 3 size 7, bar "gray70", vgap 10
%%default 4 size 7, vgap 30, prefix " ", font "standard"
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%
%% Default settings that are applied to TAB-indented lines.
%%
%tab 1 size 5, vgap 40, prefix " ", icon arc "red" 50
%tab 2 size 4, vgap 35, prefix " ", icon delta3 "blue" 40
%tab 3 size 3, vgap 35, prefix " ", icon dia "DarkViolet" 40
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
%nodefault
%center, size 9, font "thick", back "white", fore "black"
Tor:
%size 8
Next-generation Onion Routing
%size 7
Roger Dingledine
Nick Mathewson
Paul Syverson
The Free Haven Project
%font "typewriter", fore "blue"
http://freehaven.net/
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Low-latency anonymity system
%leftfill
Deployed: 20 nodes, hundreds (?) of users
Many improvements on earlier design
Free software -- modified BSD license
Design is not covered by earlier onion routing
patent
Uses SOCKS to interface with client apps
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
We have working code
(14 kloc of C)
and a design document,
and a byte-level specification,
and a Debian package (in Unstable)
Works on Linux, BSD, OSX, Cygwin, ...
User-space, doesn't need kernel mods or root
%size 9
http://freehaven.net/tor/
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%page
%%
%%Talk Overview
%%
%%A bit about Onion Routing
%%
%%Improvements we've made
%%
%%Some related work
%%
%%Ask me questions
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Anonymity: Who needs it?
Private citizens
advocacy, counseling, whistleblowing, reporting, ...
%size 6
Higher-level protocols
voting, e-cash, auctions
%size 6
Government applications
research, law enforcement
%size 6
Business applications
%size 5
(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?
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Anonymity is a network effect
Systems need traffic (many low-sensitivity users) to attract the high-sensitivity users
Most users do not value anonymity much
Weak security (fast system) can mean more users
which can mean
%cont, font "italic"
stronger
%cont, font "standard"
anonymity
High-sensitivity agents have incentive to run nodes
so they can be certain first node in their path is good
to attract traffic for their messages
There can be an optimal level of free-riding
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Onion Routing is...
An overlay network
Users build virtual circuits through the network
One layer of encryption at each hop
Fixed-size cells
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Tor's goals
Conservative design
minimize new design work needed
%size 6
Support testing of future research
Design for deployment; deploy for use
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Threat model -- what we aim for
Protect against somebody watching Alice
Protect against curious Bob
Protect against `some' curious nodes in the middle
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Differences / limitations
We're TCP-only, not all IP (but we're user-space and very portable)
Not as strong as high-latency systems (Mixmaster, Mixminion)
Not peer-to-peer
No protocol normalization
Not unobservable (no steg, etc)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Perfect forward secrecy
Telescoping circuit
negotiates keys at each hop
no more need for replay detection
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
No mixing, padding, traffic shaping (yet)
Please show us they're worth the usability tradeoff
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%page
%%
%%Many TCP streams can share one circuit
%%
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Many TCP streams share a circuit
Previous designs built a new circuit for each stream
lots of public key ops per request
plus anonymity dangers from making so many circuits
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Leaky-pipe circuit topology
Alice can direct cells to any node in her circuit
So we can support long-range padding,
have multiple streams exiting at different places in the circuit
etc
%size 6
Unclear whether this is dangerous or useful
More research needed
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Congestion control
Simple rate limiting
Plus have to keep internal nodes from overflowing
(Can't use global state or inter-node control)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Directory servers
To solve the `introduction' problem
Approve new servers
Tell clients who's up right now
plus their keys, location, etc
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Variable exit policies
Each server allows different outgoing connections
E.g. no servers allow outgoing mail currently
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
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!
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Rendezvous points
allow hidden services
don't need (brittle) reply onions
Access-controlled: Bob can control who he talks to
Robust: Bob's service is available even when some Tor nodes go down
Smear-resistant: Evil service can't frame a rendezvous router
Application-transparent: Don't need to modify Bob's apache
%size 6
(Not implemented yet)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
How do we compare security?
Assume adversary owns c of n nodes
can choose which
%size 6
What's the chance for a random Alice and Bob that he wins?
Freedom, Tor: (c/n)^2
Peekabooty, six-four, etc: c/n
Jap (if no padding): 1 if c>1
Anonymizer: 1 if c>0
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
Future work
Threshold directory agreement
Scalability: Morphmix/p2p extensions?
Restricted-route (non-clique topology)
Non-TCP transport
Implement rendezvous points
Make it work better
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%page
We have working code
Plus a design document,
and a byte-level specification
and a Debian package (in Unstable)
%size 9
http://freehaven.net/tor/
%size 6
Privacy Enhancing Technologies workshop
%size 9
http://petworkshop.org/

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doc/design-paper/Makefile
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cell-struct.eps: cell-struct.fig
fig2dev -L eps $< $@
interaction.eps: interaction.fig
fig2dev -L eps $< $@
cell-struct.pdf: cell-struct.fig
fig2dev -L pdf $< $@
interaction.pdf: interaction.fig
fig2dev -L pdf $< $@
tor-design.ps: cell-struct.eps interaction.eps tor-design.bib tor-design.tex usenix.sty latex8.bst
latex tor-design.tex
bibtex tor-design
latex tor-design.tex
latex tor-design.tex
dvips -o $@ tor-design.dvi
tor-design.pdf: cell-struct.pdf interaction.pdf tor-design.bib tor-design.tex usenix.sty latex8.bst
pdflatex tor-design.tex
bibtex tor-design
pdflatex tor-design.tex
pdflatex tor-design.tex

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doc/design-paper/blocking.html
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doc/design-paper/blocking.tex
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doc/design-paper/cell-struct.eps
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\documentclass{llncs}
\usepackage{url}
\usepackage{amsmath}
\usepackage{epsfig}
\setlength{\textwidth}{5.9in}
\setlength{\textheight}{8.4in}
\setlength{\topmargin}{.5cm}
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% \setlength{\labelsep}{0mm}
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\newcommand{\workingnote}[1]{} % The version that hides the note.
%\newcommand{\workingnote}[1]{(**#1)} % The version that makes the note visible.
\begin{document}
\title{Design challenges and social factors in deploying low-latency anonymity}
% Could still use a better title -PFS
\author{Roger Dingledine\inst{1} \and
Nick Mathewson\inst{1} \and
Paul Syverson\inst{2}}
\institute{The Tor Project \email{<\{arma,nickm\}@torproject.org>} \and
Naval Research Laboratory \email{<syverson@itd.nrl.navy.mil>}}
\maketitle
\pagestyle{plain}
\begin{abstract}
There are many unexpected or unexpectedly difficult obstacles to
deploying anonymous communications. We describe Tor (\emph{the}
onion routing), how to use it, our design philosophy, and some of
the challenges that we have faced and continue to face in building,
deploying, and sustaining a scalable, distributed, low-latency
anonymity network.
\end{abstract}
\section{Introduction}
This article describes Tor, a widely-used low-latency general-purpose
anonymous communication system, and discusses some unexpected
challenges arising from our experiences deploying Tor. We will tell
you how to use it, who uses it, how it works, why we designed it the
way we did, and why this makes it usable and stable.
Tor is an overlay network for anonymizing TCP streams over the
Internet~\cite{tor-design}. Tor works on the real-world Internet,
requires no special privileges or kernel modifications, requires
little synchronization or coordination between nodes, and provides a
reasonable trade-off between anonymity, usability, and efficiency.
Since deployment in October 2003 the public Tor network has grown to
about a thousand volunteer-operated nodes worldwide and over 110
megabytes average traffic per second from hundreds of thousands of
concurrent users.
\section{Tor Design and Design Philosophy: Distributed Trust and Usability}
Tor enables users to connect to Internet sites without revealing their
logical or physical locations to those sites or to observers. It
enables hosts to be publicly accessible yet have similar protection
against location through its \emph{location-hidden services}.
To connect to a remote server via Tor the client software first learns
a %signed
list of Tor nodes from several central \emph{directory servers} via a
voting protocol (to avoid dependence on or complete trust in any one
of these servers). It then incrementally creates a private pathway or
\emph{circuit} across the network. This circuit consists of
encrypted connections through authenticated Tor nodes
whose public keys were obtained from the directory servers. The client
software negotiates a separate set of encryption keys for each hop along the
circuit. The nodes in the circuit are chosen at random by the client
subject to a preference for higher performing nodes to allocate
resources effectively and with a client-chosen preferred set of first
nodes called \emph{entry guards} to complicate profiling attacks by
internal adversaries~\cite{hs-attack}.
The circuit is extended one node at a time, tunneling extensions
through already established portions of the circuit, and each node
along the way knows only the immediately previous and following nodes
in the circuit, so no individual Tor node knows the complete path that
each fixed-sized data packet (or \emph{cell}) will take. Thus,
neither an eavesdropper nor a compromised node can see both the
connection's source and destination. Later requests use a new
circuit to complicate long-term linkability between different actions
by a single user.
Tor attempts to anonymize the transport layer, not the application
layer. Thus, applications such as SSH can provide
authenticated communication that is hidden by Tor from outside observers.
When anonymity from communication partners is desired,
application-level protocols that transmit identifying
information need additional scrubbing proxies, such as
Privoxy~\cite{privoxy} for HTTP\@. Furthermore, Tor does not relay
arbitrary IP packets; it only anonymizes TCP streams and DNS requests.
Tor, the third generation of deployed onion-routing
designs~\cite{or-ih96,or-jsac98,tor-design}, was researched, developed,
and deployed by the Naval Research Laboratory and the Free Haven
Project under ONR and DARPA funding for secure government
communications. In 2005, continuing work by Free Haven was funded by
the Electronic Frontier Foundation for maintaining civil liberties of
ordinary citizens online. In 2006, The Tor Project incorporated as a
non-profit and has received continued funding from the Omidyar Network,
the U.S. International Broadcasting Bureau, and other groups to combat
blocking and censorship on the Internet. This diversity of funding fits
Tor's overall philosophy: a wide variety of interests helps maintain
both the stability and the security of the network.
Usability is also a central goal. Downloading and installing Tor is
easy. Simply go to\\
http://www.torproject.org/ and download. Tor comes with install
wizards and a GUI for major operating systems: GNU/Linux, OS X, and
Windows. It also runs on various flavors of BSD and UNIX\@. Basic
instructions, documentation, FAQs, etc.\ are available in many
languages. The Tor GUI Vidalia makes server configuration easy, e.g.,
choosing how much bandwidth to allocate to Tor, exit policy choices,
etc. And, the GUI Torbutton allows Firefox users a one-click toggle of
whether browsing goes through Tor or not. Tor is easily configured by
a site administrator to run at either individual desktops or just at a
site firewall or combinations of these.
The ideal Tor network would be practical, useful and anonymous. When
trade-offs arise between these properties, Tor's research strategy has
been to remain useful enough to attract many users, and practical
enough to support them. Only subject to these constraints do we try
to maximize anonymity. Tor thus differs from other deployed systems
for traffic analysis resistance in its security and flexibility. Mix
networks such as
% Mixmaster~\cite{mixmaster-spec} or its successor
Mixminion~\cite{minion-design} gain the highest degrees of practical
anonymity at the expense of introducing highly variable delays, making
them unsuitable for applications such as web browsing. Commercial
single-hop proxies~\cite{anonymizer} can provide good performance, but
a single-point compromise can expose all users' traffic, and a
single-point eavesdropper can perform traffic analysis on the entire
network. Also, their proprietary implementations place any
infrastructure that depends on these single-hop solutions at the mercy
of their providers' financial health as well as network security.
There are numerous other designs for distributed anonymous low-latency
communication~\cite{crowds-tissec,web-mix,freedom21-security,i2p,tarzan:ccs02,morphmix:fc04}.
Some have been deployed or even commercialized; some exist only on
paper. Though each has something unique to offer, we feel Tor has
advantages over each of them that make it a superior choice for most
users and applications. For example, unlike purely P2P designs we
neither limit ordinary users to content and services available only
within our network nor require them to take on responsibility for
connections outside the network, unless they separately choose to run
server nodes. Nonetheless because we support low-latency interactive
communications, end-to-end \emph{traffic correlation}
attacks~\cite{danezis:pet2004,defensive-dropping,SS03,hs-attack,bauer:tr2007}
allow an attacker who can observe both ends of a communication to
correlate packet timing and volume, quickly linking the initiator to
her destination.
Our defense lies in having a diverse enough set of nodes to prevent
most real-world adversaries from being in the right places to attack
users, by distributing each transaction over several nodes in the
network. This ``distributed trust'' approach means the Tor network
can be safely operated and used by a wide variety of mutually
distrustful users, providing sustainability and security.
The Tor network has a broad range of users, making it difficult for
eavesdroppers to track them or profile interests. These include
ordinary citizens concerned about their privacy, corporations who
don't want to reveal information to their competitors, and law
enforcement and government intelligence agencies who need to do
operations on the Internet without being noticed. Naturally,
organizations will not want to depend on others for their security.
If most participating providers are reliable, Tor tolerates some
hostile infiltration of the network.
This distribution of trust is central to the Tor philosophy and
pervades Tor at all levels: Onion routing has been open source since
the mid-nineties (mistrusting users can inspect the code themselves);
Tor is free software (anyone could take up the development of Tor from
the current team); anyone can use Tor without license or charge (which
encourages a broad user base with diverse interests); Tor is designed to be
usable (also promotes a large, diverse user base) and configurable (so
users can easily set up and run server nodes); the Tor
infrastructure is run by volunteers (it is not dependent on the
economic viability or business strategy of any company) who are
scattered around the globe (not completely under the jurisdiction of
any single country); ongoing development and deployment has been
funded by diverse sources (development does not fully depend on
funding from any one source or even funding for any one primary
purpose or sources in any one jurisdiction). All of these contribute
to Tor's resilience and sustainability.
\section{Social challenges}
Many of the issues the Tor project needs to address extend beyond
system design and technology development. In particular, the Tor
project's \emph{image} with respect to its users and the rest of the
Internet impacts the security it can provide. With this image issue
in mind, this section discusses the Tor user base and Tor's
interaction with other services on the Internet.
\subsection{Communicating security}
Usability for anonymity systems contributes to their security, because
usability affects the possible anonymity set~\cite{econymics,back01}.
Conversely, an unusable system attracts few users and thus can't
provide much anonymity.
This phenomenon has a second-order effect: knowing this, users should
choose which anonymity system to use based in part on how usable and
secure \emph{others} will find it, in order to get the protection of a
larger anonymity set. Thus we might supplement the adage ``usability
is a security parameter''~\cite{back01} with a new one: ``perceived
usability is a security parameter.''~\cite{usability-network-effect}.
\subsection{Reputability and perceived social value}
Another factor impacting the network's security is its reputability,
the perception of its social value based on its current user base. If
Alice is the only user who has ever downloaded the software, it might
be socially accepted, but she's not getting much anonymity. Add a
thousand activists, and she's anonymous, but everyone thinks she's an
activist too. Add a thousand diverse citizens (cancer survivors,
people concerned about identity theft, law enforcement agents, and so
on) and now she's harder to profile.
Furthermore, the network's reputability affects its operator base:
more people are willing to run a service if they believe it will be
used by human rights workers than if they believe it will be used
exclusively for disreputable ends. This effect becomes stronger if
node operators themselves think they will be associated with their
users' ends.
So the more cancer survivors on Tor, the better for the human rights
activists. The more malicious hackers, the worse for the normal
users. Thus, reputability is an anonymity issue for two
reasons. First, it impacts the sustainability of the network: a
network that's always about to be shut down has difficulty attracting
and keeping adequate nodes. Second, a disreputable network is more
vulnerable to legal and political attacks, since it will attract fewer
supporters.
Reputability becomes even more tricky in the case of privacy networks,
since the good uses of the network (such as publishing by journalists
in dangerous countries, protecting road warriors from profiling and
potential physical harm, tracking of criminals by law enforcement,
protecting corporate research interests, etc.) are typically kept private,
whereas network abuses or other problems tend to be more widely
publicized.
\subsection{Abuse}
\label{subsec:tor-and-blacklists}
For someone willing to be antisocial or even break the law, Tor is
usually a poor choice to hide bad behavior. For example, Tor nodes are
publicly identified, unlike the million-node botnets that are now
common on the Internet. Nonetheless, we always expected that,
alongside legitimate users, Tor would also attract troublemakers who
exploit Tor to abuse services on the Internet with vandalism, rude
mail, and so on. \emph{Exit policies} have allowed individual nodes
to block access to specific IP/port ranges. This approach aims to
make operators more willing to run Tor by allowing them to prevent
their nodes from being used for abusing particular services. For
example, by default Tor nodes block SMTP (port 25), to avoid the issue
of spam.
Exit policies are useful but insufficient: if not all nodes block a
given service, that service may try to block Tor instead. While being
blockable is important to being good netizens, we would like to
encourage services to allow anonymous access. Services should not need
to decide between blocking legitimate anonymous use and allowing
unlimited abuse. Nonetheless, blocking IP addresses is a
course-grained solution~\cite{netauth}: entire apartment buildings,
campuses, and even countries sometimes share a single IP address.
Also, whether intended or not, such blocking supports repression of
free speech. In many locations where Internet access of various kinds
is censored or even punished by imprisonment, Tor is a path both to
the outside world and to others inside. Blocking posts from Tor makes
the job of censoring authorities easier. This is a loss for both Tor
and services that block, such as Wikipedia: we don't want to compete
for (or divvy up) the NAT-protected entities of the world. This is
also unfortunate because there are relatively simple technical
solutions~\cite{nym}. Various schemes for escrowing anonymous posts
until they are reviewed by editors would both prevent abuse and remove
incentives for attempts to abuse. Further, pseudonymous reputation
tracking of posters through Tor would allow those who establish
adequate reputation to post without escrow~\cite{nym,nymble}.
We stress that as far as we can tell, most Tor uses are not
abusive. Most services have not complained, and others are actively
working to find ways besides banning to cope with the abuse. For
example, the Freenode IRC network had a problem with a coordinated
group of abusers joining channels and subtly taking over the
conversation; but when they labelled all users coming from Tor IP
addresses as ``anonymous users,'' removing the ability of the abusers
to blend in, the abusers stopped using Tor. This is an illustration of
how simple
technical mechanisms can remove the ability to abuse anonymously
without undermining the ability to communicate anonymously and can
thus remove the incentive to attempt abusing in this way.
\section{The Future}
\label{sec:conclusion}
Tor is the largest and most diverse low-latency anonymity network
available, but we are still in the early stages. Several major
questions remain.
First, will our volunteer-based approach to sustainability continue to
work as well in the long term as it has the first several years?
Besides node operation, Tor research, deployment, maintainance, and
development is increasingly done by volunteers: package maintenance
for various OSes, document translation, GUI design and implementation,
live CDs, specification of new design changes, etc.\
%
Second, Tor is only one of many components that preserve privacy
online. For applications where it is desirable to keep identifying
information out of application traffic, someone must build more and
better protocol-aware proxies that are usable by ordinary people.
%
Third, we need to maintain a reputation for social good, and learn how to
coexist with the variety of Internet services and their established
authentication mechanisms. We can't just keep escalating the blacklist
standoff forever.
%
Fourth, the current Tor architecture hardly scales even to handle
current user demand. We must deploy designs and incentives to further
encourage clients to relay traffic too, without thereby trading away
too much anonymity or other properties.
These are difficult and open questions. Yet choosing not to solve them
means leaving most users to a less secure network or no anonymizing
network at all.\\
\noindent{\bf Acknowledgment:} Thanks to Matt Edman for many
helpful comments on a draft of this article.
\bibliographystyle{plain} \bibliography{tor-design}
\end{document}

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\begin{document}
\title{Tor Development Roadmap: Wishlist for Nov 2006--Dec 2007}
\author{Roger Dingledine \and Nick Mathewson \and Shava Nerad}
\maketitle
\pagestyle{plain}
% TO DO:
% add cites
% add time estimates
\section{Introduction}
%Hi, Roger! Hi, Shava. This paragraph should get deleted soon. Right now,
%this document goes into about as much detail as I'd like to go into for a
%technical audience, since that's the audience I know best. It doesn't have
%time estimates everywhere. It isn't well prioritized, and it doesn't
%distinguish well between things that need lots of research and things that
%don't. The breakdowns don't all make sense. There are lots of things where
%I don't make it clear how they fit into larger goals, and lots of larger
%goals that don't break down into little things. It isn't all stuff we can do
%for sure, and it isn't even all stuff we can do for sure in 2007. The
%tmp\{\} macro indicates stuff I haven't said enough about. That said, here
%plangoes...
Tor (the software) and Tor (the overall software/network/support/document
suite) are now experiencing all the crises of success. Over the next year,
we're probably going to grow more in terms of users, developers, and funding
than before. This gives us the opportunity to perform long-neglected
maintenance tasks.
\section{Code and design infrastructure}
\subsection{Protocol revision}
To maintain backward compatibility, we've postponed major protocol
changes and redesigns for a long time. Because of this, there are a number
of sensible revisions we've been putting off until we could deploy several of
them at once. To do each of these, we first need to discuss design
alternatives with other cryptographers and outside collaborators to
make sure that our choices are secure.
First of all, our protocol needs better {\bf versioning support} so that we
can make backward-incompatible changes to our core protocol. There are
difficult anonymity issues here, since many naive designs would make it easy
to tell clients apart (and then track them) based on their supported versions.
With protocol versioning support would come the ability to {\bf future-proof
our ciphersuites}. For example, not only our OR protocol, but also our
directory protocol, is pretty firmly tied to the SHA-1 hash function, which
though not yet known to be insecure for our purposes, has begun to show
its age. We should
remove assumptions throughout our design based on the assumption that public
keys, secret keys, or digests will remain any particular size indefinitely.
Our OR {\bf authentication protocol}, though provably
secure\cite{tap:pet2006}, relies more on particular aspects of RSA and our
implementation thereof than we had initially believed. To future-proof
against changes, we should replace it with a less delicate approach.
\plan{For all the above: 2 person-months to specify, spread over several
months with time for interaction with external participants. One
person-month to implement. Start specifying in early 2007.}
We might design a {\bf stream migration} feature so that streams tunneled
over Tor could be more resilient to dropped connections and changed IPs.
\plan{Not in 2007.}
A new protocol could support {\bf multiple cell sizes}. Right now, all data
passes through the Tor network divided into 512-byte cells. This is
efficient for high-bandwidth protocols, but inefficient for protocols
like SSH or AIM that send information in small chunks. Of course, we need to
investigate the extent to which multiple sizes could make it easier for an
adversary to fingerprint a traffic pattern. \plan{Not in 2007.}
As a part of our design, we should investigate possible {\bf cipher modes}
other than counter mode. For example, a mode with built-in integrity
checking, error propagation, and random access could simplify our protocol
significantly. Sadly, many of these are patented and unavailable for us.
\plan{Not in 2007.}
\subsection{Scalability}
\subsubsection{Improved directory efficiency}
Right now, clients download a statement of the {\bf network status} made by
each directory authority. We could reduce network bandwidth significantly by
having the authorities jointly sign a statement reflecting their vote on the
current network status. This would save clients up to 160K per hour, and
make their view of the network more uniform. Of course, we'd need to make
sure the voting process was secure and resilient to failures in the
network.\plan{Must do; specify in 2006. 2 weeks to specify, 3-4 weeks to
implement.}
We should {\bf shorten router descriptors}, since the current format includes
a great deal of information that's only of interest to the directory
authorities, and not of interest to clients. We can do this by having each
router upload a short-form and a long-form signed descriptor, and having
clients download only the short form. Even a naive version of this would
save about 40\% of the bandwidth currently spent by clients downloading
descriptors.\plan{Must do; specify in 2006. 3-4 weeks.}
We should {\bf have routers upload their descriptors even less often}, so
that clients do not need to download replacements every 18 hours whether any
information has changed or not. (As of Tor 0.1.2.3-alpha, clients tolerate
routers that don't upload often, but routers still upload at least every 18
hours to support older clients.) \plan{Must do, but not until 0.1.1.x is
deprecated in mid 2007. 1 week.}
\subsubsection{Non-clique topology}
Our current network design achieves a certain amount of its anonymity by
making clients act like each other through the simple expedient of making
sure that all clients know all servers, and that any server can talk to any
other server. But as the number of servers increases to serve an
ever-greater number of clients, these assumptions become impractical.
At worst, if these scalability issues become troubling before a solution is
found, we can design and build a solution to {\bf split the network into
multiple slices} until a better solution comes along. This is not ideal,
since rather than looking like all other users from a point of view of path
selection, users would ``only'' look like 200,000--300,000 other
users.\plan{Not unless needed.}
We are in the process of designing {\bf improved schemes for network
scalability}. Some approaches focus on limiting what an adversary can know
about what a user knows; others focus on reducing the extent to which an
adversary can exploit this knowledge. These are currently in their infancy,
and will probably not be needed in 2007, but they must be designed in 2007 if
they are to be deployed in 2008.\plan{Design in 2007; unknown difficulty.
Write a paper.}
\subsubsection{Relay incentives}
To support more users on the network, we need to get more servers. So far,
we've relied on volunteerism to attract server operators, and so far it's
served us well. But in the long run, we need to {\bf design incentives for
users to run servers} and relay traffic for others. Most obviously, we
could try to build the network so that servers offered improved service for
other servers, but we would need to do so without weakening anonymity and
making it obvious which connections originate from users running servers. We
have some preliminary designs~\cite{incentives-txt,tor-challenges},
but need to perform
some more research to make sure they would be safe and effective.\plan{Write
a draft paper; 2 person-months.}
\subsection{Portability}
Our {\bf Windows implementation}, though much improved, continues to lag
behind Unix and Mac OS X, especially when running as a server. We hope to
merge promising patches from Mike Chiussi to address this point, and bring
Windows performance on par with other platforms.\plan{Do in 2007; 1.5 months
to integrate not counting Mike's work.}
We should have {\bf better support for portable devices}, including modes of
operation that require less RAM, and that write to disk less frequently (to
avoid wearing out flash RAM).\plan{Optional; 2 weeks.}
We should {\bf stop using socketpair on Windows}; instead, we can use
in-memory structures to communicate between cpuworkers and the main thread,
and between connections.\plan{Optional; 1 week.}
\subsection{Performance: resource usage}
We've been working on {\bf using less RAM}, especially on servers. This has
paid off a lot for directory caches in the 0.1.2, which in some cases are
using 90\% less memory than they used to require. But we can do better,
especially in the area around our buffer management algorithms, by using an
approach more like the BSD and Linux kernels use instead of our current ring
buffer approach. (For OR connections, we can just use queues of cell-sized
chunks produced with a specialized allocator.) This could potentially save
around 25 to 50\% of the memory currently allocated for network buffers, and
make Tor a more attractive proposition for restricted-memory environments
like old computers, mobile devices, and the like.\plan{Do in 2007; 2-3 weeks
plus one week measurement.}
We should improve our {\bf bandwidth limiting}. The current system has been
crucial in making users willing to run servers: nobody is willing to run a
server if it might use an unbounded amount of bandwidth, especially if they
are charged for their usage. We can make our system better by letting users
configure bandwidth limits independently for their own traffic and traffic
relayed for others; and by adding write limits for users running directory
servers.\plan{Do in 2006; 2-3 weeks.}
On many hosts, sockets are still in short supply, and will be until we can
migrate our protocol to UDP. We can {\bf use fewer sockets} by making our
self-to-self connections happen internally to the code rather than involving
the operating system's socket implementation.\plan{Optional; 1 week.}
\subsection{Performance: network usage}
We know too little about how well our current path
selection algorithms actually spread traffic around the network in practice.
We should {\bf research the efficacy of our traffic allocation} and either
assure ourselves that it is close enough to optimal as to need no improvement
(unlikely) or {\bf identify ways to improve network usage}, and get more
users' traffic delivered faster. Performing this research will require
careful thought about anonymity implications.
We should also {\bf examine the efficacy of our congestion control
algorithm}, and see whether we can improve client performance in the
presence of a congested network through dynamic `sendme' window sizes or
other means. This will have anonymity implications too if we aren't careful.
\plan{For both of the above: research, design and write
a measurement tool in 2007: 1 month. See if we can interest a graduate
student.}
We should work on making Tor's cell-based protocol perform better on
networks with low bandwidth
and high packet loss.\plan{Do in 2007 if we're funded to do it; 4-6 weeks.}
\subsection{Performance scenario: one Tor client, many users}
We should {\bf improve Tor's performance when a single Tor handles many
clients}. Many organizations want to manage a single Tor client on their
firewall for many users, rather than having each user install a separate
Tor client. We haven't optimized for this scenario, and it is likely that
there are some code paths in the current implementation that become
inefficient when a single Tor is servicing hundreds or thousands of client
connections. (Additionally, it is likely that such clients have interesting
anonymity requirements the we should investigate.) We should profile Tor
under appropriate loads, identify bottlenecks, and fix them.\plan{Do in 2007
if we're funded to do it; 4-8 weeks.}
\subsection{Tor servers on asymmetric bandwidth}
Tor should work better on servers that have asymmetric connections like cable
or DSL. Because Tor has separate TCP connections between each
hop, if the incoming bytes are arriving just fine and the outgoing bytes are
all getting dropped on the floor, the TCP push-back mechanisms don't really
transmit this information back to the incoming streams.\plan{Do in 2007 since
related to bandwidth limiting. 3-4 weeks.}
\subsection{Running Tor as both client and server}
Many performance tradeoffs and balances that might need more attention.
We first need to track and fix whatever bottlenecks emerge; but we also
need to invent good algorithms for prioritizing the client's traffic
without starving the server's traffic too much.\plan{No idea; try
profiling and improving things in 2007.}
\subsection{Protocol redesign for UDP}
Tor has relayed only TCP traffic since its first versions, and has used
TLS-over-TCP to do so. This approach has proved reliable and flexible, but
in the long term we will need to allow UDP traffic on the network, and switch
some or all of the network to using a UDP transport. {\bf Supporting UDP
traffic} will make Tor more suitable for protocols that require UDP, such
as many VOIP protocols. {\bf Using a UDP transport} could greatly reduce
resource limitations on servers, and make the network far less interruptible
by lossy connections. Either of these protocol changes would require a great
deal of design work, however. We hope to be able to enlist the aid of a few
talented graduate students to assist with the initial design and
specification, but the actual implementation will require significant testing
of different reliable transport approaches.\plan{Maybe do a design in 2007 if
we find an interested academic. Ian or Ben L might be good partners here.}
\section{Blocking resistance}
\subsection{Design for blocking resistance}
We have written a design document explaining our general approach to blocking
resistance. We should workshop it with other experts in the field to get
their ideas about how we can improve Tor's efficacy as an anti-censorship
tool.
\subsection{Implementation: client-side and bridges-side}
Our anticensorship design calls for some nodes to act as ``bridges''
that are outside a national firewall, and others inside the firewall to
act as pure clients. This part of the design is quite clear-cut; we're
probably ready to begin implementing it. To {\bf implement bridges}, we
need to have servers publish themselves as limited-availability relays
to a special bridge authority if they judge they'd make good servers.
We will also need to help provide documentation for port forwarding,
and an easy configuration tool for running as a bridge.
To {\bf implement clients}, we need to provide a flexible interface to
learn about bridges and to act on knowledge of bridges. We also need
to teach them how to know to use bridges as their first hop, and how to
fetch directory information from both classes of directory authority.
Clients also need to {\bf use the encrypted directory variant} added in Tor
0.1.2.3-alpha. This will let them retrieve directory information over Tor
once they've got their initial bridges. We may want to get the rest of the
Tor user base to begin using this encrypted directory variant too, to
provide cover.
Bridges will want to be able to {\bf listen on multiple addresses and ports}
if they can, to give the adversary more ports to block.
\subsection{Research: anonymity implications from becoming a bridge}
\subsection{Implementation: bridge authority}
The design here is also reasonably clear-cut: we need to run some
directory authorities with a slightly modified protocol that doesn't leak
the entire list of bridges. Thus users can learn up-to-date information
for bridges they already know about, but they can't learn about arbitrary
new bridges.
\subsection{Normalizing the Tor protocol on the wire}
Additionally, we should {\bf resist content-based filters}. Though an
adversary can't see what users are saying, some aspects of our protocol are
easy to fingerprint {\em as} Tor. We should correct this where possible.
Look like Firefox; or look like nothing?
Future research: investigate timing similarities with other protocols.
\subsection{Access control for bridges}
Design/impl: password-protecting bridges, in light of above.
And/or more general access control.
\subsection{Research: scanning-resistance}
\subsection{Research/Design/Impl: how users discover bridges}
Our design anticipates an arms race between discovery methods and censors.
We need to begin the infrastructure on our side quickly, preferably in a
flexible language like Python, so we can adapt quickly to censorship.
phase one: personal bridges
phase two: families of personal bridges
phase three: more structured social network
phase four: bag of tricks
Research: phase five...
Integration with Psiphon, etc?
\subsection{Document best practices for users}
Document best practices for various activities common among
blocked users (e.g. WordPress use).
\subsection{Research: how to know if a bridge has been blocked?}
\subsection{GeoIP maintenance, and "private" user statistics}
How to know if the whole idea is working?
\subsection{Research: hiding whether the user is reading or publishing?}
\subsection{Research: how many bridges do you need to know to maintain
reachability?}
\subsection{Resisting censorship of the Tor website, docs, and mirrors}
We should take some effort to consider {\bf initial distribution of Tor and
related information} in countries where the Tor website and mirrors are
censored. (Right now, most countries that block access to Tor block only the
main website and leave mirrors and the network itself untouched.) Falling
back on word-of-mouth is always a good last resort, but we should also take
steps to make sure it's relatively easy for users to get ahold of a copy.
\section{Security}
\subsection{Security research projects}
We should investigate approaches with some promise to help Tor resist
end-to-end traffic correlation attacks. It's an open research question
whether (and to what extent) {\bf mixed-latency} networks, {\bf low-volume
long-distance padding}, or other approaches can resist these attacks, which
are currently some of the most effective against careful Tor users. We
should research these questions and perform simulations to identify
opportunities for strengthening our design without dropping performance to
unacceptable levels. %Cite something
\plan{Start doing this in 2007; write a paper. 8-16 weeks.}
We've got some preliminary results suggesting that {\bf a topology-aware
routing algorithm}~\cite{feamster:wpes2004} could reduce Tor users'
vulnerability against local or ISP-level adversaries, by ensuring that they
are never in a position to watch both ends of a connection. We need to
examine the effects of this approach in more detail and consider side-effects
on anonymity against other kinds of adversaries. If the approach still looks
promising, we should investigate ways for clients to implement it (or an
approximation of it) without having to download routing tables for the whole
Internet. \plan{Not in 2007 unless a graduate student wants to do it.}
%\tmp{defenses against end-to-end correlation} We don't expect any to work
%right now, but it would be useful to learn that one did. Alternatively,
%proving that one didn't would free up researchers in the field to go work on
%other things.
%
% See above; I think I got this.
We should research the efficacy of {\bf website fingerprinting} attacks,
wherein an adversary tries to match the distinctive traffic and timing
pattern of the resources constituting a given website to the traffic pattern
of a user's client. These attacks work great in simulations, but in
practice we hear they don't work nearly as well. We should get some actual
numbers to investigate the issue, and figure out what's going on. If we
resist these attacks, or can improve our design to resist them, we should.
% add cites
\plan{Possibly part of end-to-end correlation paper. Otherwise, not in 2007
unless a graduate student is interested.}
\subsection{Implementation security}
Right now, each Tor node stores its keys unencrypted. We should {\bf encrypt
more Tor keys} so that Tor authorities can require a startup password. We
should look into adding intermediary medium-term ``signing keys'' between
identity keys and onion keys, so that a password could be required to replace
a signing key, but not to start Tor. This would improve Tor's long-term
security, especially in its directory authority infrastructure.\plan{Design this
as a part of the revised ``v2.1'' directory protocol; implement it in
2007. 3-4 weeks.}
We should also {\bf mark RAM that holds key material as non-swappable} so
that there is no risk of recovering key material from a hard disk
compromise. This would require submitting patches upstream to OpenSSL, where
support for marking memory as sensitive is currently in a very preliminary
state.\plan{Nice to do, but not in immediate Tor scope.}
There are numerous tools for identifying trouble spots in code (such as
Coverity or even VS2005's code analysis tool) and we should convince somebody
to run some of them against the Tor codebase. Ideally, we could figure out a
way to get our code checked periodically rather than just once.\plan{Almost
no time once we talk somebody into it.}
We should try {\bf protocol fuzzing} to identify errors in our
implementation.\plan{Not in 2007 unless we find a grad student or
undergraduate who wants to try.}
Our guard nodes help prevent an attacker from being able to become a chosen
client's entry point by having each client choose a few favorite entry points
as ``guards'' and stick to them. We should implement a {\bf directory
guards} feature to keep adversaries from enumerating Tor users by acting as
a directory cache.\plan{Do in 2007; 2 weeks.}
\subsection{Detect corrupt exits and other servers}
With the success of our network, we've attracted servers in many locations,
operated by many kinds of people. Unfortunately, some of these locations
have compromised or defective networks, and some of these people are
untrustworthy or incompetent. Our current design relies on authority
administrators to identify bad nodes and mark them as nonfunctioning. We
should {\bf automate the process of identifying malfunctioning nodes} as
follows:
We should create a generic {\bf feedback mechanism for add-on tools} like
Mike Perry's ``Snakes on a Tor'' to report failing nodes to authorities.
\plan{Do in 2006; 1-2 weeks.}
We should write tools to {\bf detect more kinds of innocent node failure},
such as nodes whose network providers intercept SSL, nodes whose network
providers censor popular websites, and so on. We should also try to detect
{\bf routers that snoop traffic}; we could do this by launching connections
to throwaway accounts, and seeing which accounts get used.\plan{Do in 2007;
ask Mike Perry if he's interested. 4-6 weeks.}
We should add {\bf an efficient way for authorities to mark a set of servers
as probably collaborating} though not necessarily otherwise dishonest.
This happens when an administrator starts multiple routers, but doesn't mark
them as belonging to the same family.\plan{Do during v2.1 directory protocol
redesign; 1-2 weeks to implement.}
To avoid attacks where an adversary claims good performance in order to
attract traffic, we should {\bf have authorities measure node performance}
(including stability and bandwidth) themselves, and not simply believe what
they're told. Measuring stability can be done by tracking MTBF. Measuring
bandwidth can be tricky, since it's hard to distinguish between a server with
low capacity, and a high-capacity server with most of its capacity in
use.\plan{Do ``Stable'' in 2007; 2-3 weeks. ``Fast'' will be harder; do it
if we can interest a grad student.}
{\bf Operating a directory authority should be easier.} We rely on authority
operators to keep the network running well, but right now their job involves
too much busywork and administrative overhead. A better interface for them
to use could free their time to work on exception cases rather than on
adding named nodes to the network.\plan{Do in 2007; 4-5 weeks.}
\subsection{Protocol security}
In addition to other protocol changes discussed above,
% And should we move some of them down here? -NM
we should add {\bf hooks for denial-of-service resistance}; we have some
preliminary designs, but we shouldn't postpone them until we really need them.
If somebody tries a DDoS attack against the Tor network, we won't want to
wait for all the servers and clients to upgrade to a new
version.\plan{Research project; do this in 2007 if funded.}
\section{Development infrastructure}
\subsection{Build farm}
We've begun to deploy a cross-platform distributed build farm of hosts
that build and test the Tor source every time it changes in our development
repository.
We need to {\bf get more participants}, so that we can test a larger variety
of platforms. (Previously, we've only found out when our code had broken on
obscure platforms when somebody got around to building it.)
We need also to {\bf add our dependencies} to the build farm, so that we can
ensure that libraries we need (especially libevent) do not stop working on
any important platform between one release and the next.
\plan{This is ongoing as more buildbots arrive.}
\subsection{Improved testing harness}
Currently, our {\bf unit tests} cover only about 20\% of the code base. This
is uncomfortably low; we should write more and switch to a more flexible
testing framework.\plan{Ongoing basis, time permitting.}
We should also write flexible {\bf automated single-host deployment tests} so
we can more easily verify that the current codebase works with the
network.\plan{Worthwhile in 2007; would save lots of time. 2-4 weeks.}
We should build automated {\bf stress testing} frameworks so we can see which
realistic loads cause Tor to perform badly, and regularly profile Tor against
these loads. This would give us {\it in vitro} performance values to
supplement our deployment experience.\plan{Worthwhile in 2007; 2-6 weeks.}
We should improve our memory profiling code.\plan{...}
\subsection{Centralized build system}
We currently rely on a separate packager to maintain the packaging system and
to build Tor on each platform for which we distribute binaries. Separate
package maintainers is sensible, but separate package builders has meant
long turnaround times between source releases and package releases. We
should create the necessary infrastructure for us to produce binaries for all
major packages within an hour or so of source release.\plan{We should
brainstorm this at least in 2007.}
\subsection{Improved metrics}
We need a way to {\bf measure the network's health, capacity, and degree of
utilization}. Our current means for doing this are ad hoc and not
completely accurate
We need better ways to {\bf tell which countries are users are coming from,
and how many there are}. A good perspective of the network helps us
allocate resources and identify trouble spots, but our current approaches
will work less and less well as we make it harder for adversaries to
enumerate users. We'll probably want to shift to a smarter, statistical
approach rather than our current ``count and extrapolate'' method.
\plan{All of this in 2007 if funded; 4-8 weeks}
% \tmp{We'd like to know how much of the network is getting used.}
% I think this is covered above -NM
\subsection{Controller library}
We've done lots of design and development on our controller interface, which
allows UI applications and other tools to interact with Tor. We could
encourage the development of more such tools by releasing a {\bf
general-purpose controller library}, ideally with API support for several
popular programming languages.\plan{2006 or 2007; 1-2 weeks.}
\section{User experience}
\subsection{Get blocked less, get blocked less broadly}
Right now, some services block connections from the Tor network because
they don't have a better
way to keep vandals from abusing them than blocking IP addresses associated
with vandalism. Our approach so far has been to educate them about better
solutions that currently exist, but we should also {\bf create better
solutions for limiting vandalism by anonymous users} like credential and
blind-signature based implementations, and encourage their use. Other
promising starting points including writing a patch and explanation for
Wikipedia, and helping Freenode to document, maintain, and expand its
current Tor-friendly position.\plan{Do a writeup here in 2007; 1-2 weeks.}
Those who do block Tor users also block overbroadly, sometimes blacklisting
operators of Tor servers that do not permit exit to their services. We could
obviate innocent reasons for doing so by designing a {\bf narrowly-targeted Tor
RBL service} so that those who wanted to overblock Tor could no longer
plead incompetence.\plan{Possibly in 2007 if we decide it's a good idea; 3
weeks.}
\subsection{All-in-one bundle}
We need a well-tested, well-documented bundle of Tor and supporting
applications configured to use it correctly. We have an initial
implementation well under way, but it will need additional work in
identifying requisite Firefox extensions, identifying security threats,
improving user experience, and so on. This will need significantly more work
before it's ready for a general public release.
\subsection{LiveCD Tor}
We need a nice bootable livecd containing a minimal OS and a few applications
configured to use it correctly. The Anonym.OS project demonstrated that this
is quite feasible, but their project is not currently maintained.
\subsection{A Tor client in a VM}
\tmp{a.k.a JanusVM} which is quite related to the firewall-level deployment
section below. JanusVM is a Linux kernel running in VMWare. It gets an IP
address from the network, and serves as a DHCP server for its host Windows
machine. It intercepts all outgoing traffic and redirects it into Privoxy,
Tor, etc. This Linux-in-Windows approach may help us with scalability in
the short term, and it may also be a good long-term solution rather than
accepting all security risks in Windows.
%\subsection{Interface improvements}
%\tmp{Allow controllers to manipulate server status.}
% (Why is this in the User Experience section?) -RD
% I think it's better left to a generic ``make controller iface better'' item.
\subsection{Firewall-level deployment}
Another useful deployment mode for some users is using {\bf Tor in a firewall
configuration}, and directing all their traffic through Tor. This can be a
little tricky to set up currently, but it's an effective way to make sure no
traffic leaves the host un-anonymized. To achieve this, we need to {\bf
improve and port our new TransPort} feature which allows Tor to be used
without SOCKS support; to {\bf add an anonymizing DNS proxy} feature to Tor;
and to {\bf construct a recommended set of firewall configurations} to redirect
traffic to Tor.
This is an area where {\bf deployment via a livecd}, or an installation
targeted at specialized home routing hardware, could be useful.
\subsection{Assess software and configurations for anonymity risks}
Right now, users and packagers are more or less on their own when selecting
Firefox extensions. We should {\bf assemble a recommended list of browser
extensions} through experiment, and include this in the application bundles
we distribute.
We should also describe {\bf best practices for using Tor with each class of
application}. For example, Ethan Zuckerman has written a detailed
tutorial on how to use Tor, Firefox, GMail, and Wordpress to blog with
improved safety. There are many other cases on the Internet where anonymity
would be helpful, and there are a lot of ways to screw up using Tor.
The Foxtor and Torbutton extensions serve similar purposes; we should pick a
favorite, and merge in the useful features of the other.
%\tmp{clean up our own bundled software:
%E.g. Merge the good features of Foxtor into Torbutton}
%
% What else did you have in mind? -NM
\subsection{Localization}
Right now, most of our user-facing code is internationalized. We need to
internationalize the last few hold-outs (like the Tor expert installer), and get
more translations for the parts that are already internationalized.
Also, we should look into a {\bf unified translator's solution}. Currently,
since different tools have been internationalized using the
framework-appropriate method, different tools require translators to localize
them via different interfaces. Inasmuch as possible, we should make
translators only need to use a single tool to translate the whole Tor suite.
\section{Support}
It would be nice to set up some {\bf user support infrastructure} and
{\bf contributor support infrastructure}, especially focusing on server
operators and on coordinating volunteers.
This includes intuitive and easy ticket systems for bug reports and
feature suggestions (not just mailing lists with a half dozen people
and no clear roles for who answers what), but it also includes a more
personalized and efficient framework for interaction so we keep the
attention and interest of the contributors, and so we make them feel
helpful and wanted.
\section{Documentation}
\subsection{Unified documentation scheme}
We need to {\bf inventory our documentation.} Our documentation so far has
been mostly produced on an {\it ad hoc} basis, in response to particular
needs and requests. We should figure out what documentation we have, which of
it (if any) should get priority, and whether we can't put it all into a
single format.
We could {\bf unify the docs} into a single book-like thing. This will also
help us identify what sections of the ``book'' are missing.
\subsection{Missing technical documentation}
We should {\bf revise our design paper} to reflect the new decisions and
research we've made since it was published in 2004. This will help other
researchers evaluate and suggest improvements to Tor's current design.
Other projects sometimes implement the client side of our protocol. We
encourage this, but we should write {\bf a document about how to avoid
excessive resource use}, so we don't need to worry that they will do so
without regard to the effect of their choices on server resources.
\subsection{Missing user documentation}
Our documentation falls into two broad categories: some is `discoursive' and
explains in detail why users should take certain actions, and other
documentation is `comprehensive' and describes all of Tor's features. Right
now, we have no document that is both deep, readable, and thorough. We
should correct this by identifying missing spots in our design.
\bibliographystyle{plain} \bibliography{tor-design}
\end{document}

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\documentclass{article}
\usepackage{url}
\usepackage{fullpage}
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\setlength{\itemsep}{0mm}
\setlength{\parsep}{0mm}
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\newcommand{\tmp}[1]{{\bf #1} [......] \\}
\newcommand{\plan}[1]{ {\bf (#1)}}
\begin{document}
\title{Tor Development Roadmap: Wishlist for 2008 and beyond}
\author{Roger Dingledine \and Nick Mathewson}
\date{}
\maketitle
\pagestyle{plain}
\section{Introduction}
Tor (the software) and Tor (the overall software/network/support/document
suite) are now experiencing all the crises of success. Over the next
years, we're probably going to grow even more in terms of users, developers,
and funding than before. This document attempts to lay out all the
well-understood next steps that Tor needs to take. We should periodically
reorganize it to reflect current and intended priorities.
\section{Everybody can be a relay}
We've made a lot of progress towards letting an ordinary Tor client also
serve as a Tor relay. But these issues remain.
\subsection{UPNP}
We should teach Vidalia how to speak UPNP to automatically open and
forward ports on common (e.g. Linksys) routers. There are some promising
Qt-based UPNP libs out there, and in any case there are others (e.g. in
Perl) that we can base it on.
\subsection{``ORPort auto'' to look for a reachable port}
Vidalia defaults to port 443 on Windows and port 8080 elsewhere. But if
that port is already in use, or the ISP filters incoming connections
on that port (some cablemodem providers filter 443 inbound), the user
needs to learn how to notice this, and then pick a new one and type it
into Vidalia.
We should add a new option ``auto'' that cycles through a set of preferred
ports, testing bindability and reachability for each of them, and only
complains to the user once it's given up on the common choices.
\subsection{Incentives design}
Roger has been working with researchers at Rice University to simulate
and analyze a new design where the directory authorities assign gold
stars to well-behaving relays, and then all the relays give priority
to traffic from gold-starred relays. The great feature of the design is
that not only does it provide the (explicit) incentive to run a relay,
but it also aims to grow the overall capacity of the network, so even
non-relays will benefit.
It needs more analysis, and perhaps more design work, before we try
deploying it.
\subsection{Windows libevent}
Tor relays still don't work well or reliably on Windows XP or Windows
Vista, because we don't use the Windows-native ``overlapped IO''
approach. Christian King made a good start at teaching libevent about
overlapped IO during Google Summer of Code 2007, and next steps are
to a) finish that, b) teach Tor to do openssl calls on buffers rather
than directly to the network, and c) teach Tor to use the new libevent
buffers approach.
\subsection{Network scaling}
If we attract many more relays, we will need to handle the growing pains
in terms of getting all the directory information to all the users.
The first piece of this issue is a practical question: since the
directory size scales linearly with more relays, at some point it
will no longer be practical for every client to learn about every
relay. We can try to reduce the amount of information each client needs
to fetch (e.g. based on fetching less information preemptively as in
Section~\ref{subsec:fewer-descriptor-fetches} below), but eventually
clients will need to learn about only a subset of the network, and we
will need to design good ways to divide up the network information.
The second piece is an anonymity question that arises from this
partitioning: if Tor's security comes from having all the clients
behaving in similar ways, yet we are now giving different clients
different directory information, how can we minimize the new anonymity
attacks we introduce?
\subsection{Using fewer sockets}
Since in the current network every Tor relay can reach every other Tor
relay, and we have many times more users than relays, pretty much every
possible link in the network is in use. That is, the current network
is a clique in practice.
And since each of these connections requires a TCP socket, it's going
to be hard for the network to grow much larger: many systems come with
a default of 1024 file descriptors allowed per process, and raising
that ulimit is hard for end users. Worse, many low-end gateway/firewall
routers can't handle this many connections in their routing table.
One approach is a restricted-route topology~\cite{danezis:pet2003}:
predefine which relays can reach which other relays, and communicate
these restrictions to the relays and the clients. We need to compute
which links are acceptable in a way that's decentralized yet scalable,
and in a way that achieves a small-worlds property; and we
need an efficient (compact) way to characterize the topology information
so all the users could keep up to date.
Another approach would be to switch to UDP-based transport between
relays, so we don't need to keep the TCP sockets open at all. Needs more
investigation too.
\subsection{Auto bandwidth detection and rate limiting, especially for
asymmetric connections.}
\subsection{Better algorithms for giving priority to local traffic}
Proposal 111 made a lot of progress at separating local traffic from
relayed traffic, so Tor users can rate limit the relayed traffic at a
stricter level. But since we want to pass both traffic classes over the
same TCP connection, we can't keep them entirely separate. The current
compromise is that we treat all bytes to/from a given connectin as
local traffic if any of the bytes within the past N seconds were local
bytes. But a) we could use some more intelligent heuristics, and b)
this leaks information to an active attacker about when local traffic
was sent/received.
\subsection{Tolerate absurdly wrong clocks, even for relays}
Many of our users are on Windows, running with a clock several days or
even several years off from reality. Some of them are even intentionally
in this state so they can run software that will only run in the past.
Before Tor 0.1.1.x, Tor clients would still function if their clock was
wildly off --- they simply got a copy of the directory and believed it.
Starting in Tor 0.1.1.x (and even moreso in Tor 0.2.0.x), the clients
only use networkstatus documents that they believe to be recent, so
clients with extremely wrong clocks no longer work. (This bug has been
an unending source of vague and confusing bug reports.)
The first step is for clients to recognize when all the directory material
they're fetching has roughly the same offset from their current time,
and then automatically correct for it.
Once that's working well, clients who opt to become bridge relays should
be able to use the same approach to serve accurate directory information
to their bridge users.
\subsection{Risks from being a relay}
Three different research
papers~\cite{back01,clog-the-queue,attack-tor-oak05} describe ways to
identify the nodes in a circuit by running traffic through candidate nodes
and looking for dips in the traffic while the circuit is active. These
clogging attacks are not that scary in the Tor context so long as relays
are never clients too. But if we're trying to encourage more clients to
turn on relay functionality too (whether as bridge relays or as normal
relays), then we need to understand this threat better and learn how to
mitigate it.
One promising research direction is to investigate the RelayBandwidthRate
feature that lets Tor rate limit relayed traffic differently from local
traffic. Since the attacker's ``clogging'' traffic is not in the same
bandwidth class as the traffic initiated by the user, it may be harder
to detect interference. Or it may not be.
\subsection{First a bridge, then a public relay?}
Once enough of the items in this section are done, I want all clients
to start out automatically detecting their reachability and opting
to be bridge relays.
Then if they realize they have enough consistency and bandwidth, they
should automatically upgrade to being non-exit relays.
What metrics should we use for deciding when we're fast enough
and stable enough to switch? Given that the list of bridge relays needs
to be kept secret, it doesn't make much sense to switch back.
\section{Tor on low resources / slow links}
\subsection{Reducing directory fetches further}
\label{subsec:fewer-descriptor-fetches}
\subsection{AvoidDiskWrites}
\subsection{Using less ram}
\subsection{Better DoS resistance for tor servers / authorities}
\section{Blocking resistance}
\subsection{Better bridge-address-distribution strategies}
\subsection{Get more volunteers running bridges}
\subsection{Handle multiple bridge authorities}
\subsection{Anonymity for bridge users: second layer of entry guards, etc?}
\subsection{More TLS normalization}
\subsection{Harder to block Tor software distribution}
\subsection{Integration with Psiphon}
\section{Packaging}
\subsection{Switch Privoxy out for Polipo}
- Make Vidalia able to launch more programs itself
\subsection{Continue Torbutton improvements}
especially better docs
\subsection{Vidalia and stability (especially wrt ongoing Windows problems)}
learn how to get useful crash reports (tracebacks) from Windows users
\subsection{Polipo support on Windows}
\subsection{Auto update for Tor, Vidalia, others}
\subsection{Tor browser bundle for USB and standalone use}
\subsection{LiveCD solution}
\subsection{VM-based solution}
\subsection{Tor-on-enclave-firewall configuration}
\subsection{General tutorials on what common applications are Tor-friendly}
\subsection{Controller libraries (torctl) plus documentation}
\subsection{Localization and translation (Vidalia, Torbutton, web pages)}
\section{Interacting better with Internet sites}
\subsection{Make tordnsel (tor exitlist) better and more well-known}
\subsection{Nymble}
\subsection{Work with Wikipedia, Slashdot, Google(, IRC networks)}
\subsection{IPv6 support for exit destinations}
\section{Network health}
\subsection{torflow / soat to detect bad relays}
\subsection{make authorities more automated}
\subsection{torstatus pages and better trend tracking}
\subsection{better metrics for assessing network health / growth}
- geoip usage-by-country reporting and aggregation
(Once that's working, switch to Directory guards)
\section{Performance research}
\subsection{Load balance better}
\subsection{Improve our congestion control algorithms}
\subsection{Two-hops vs Three-hops}
\subsection{Transport IP packets end-to-end}
\section{Outreach and user education}
\subsection{"Who uses Tor" use cases}
\subsection{Law enforcement contacts}
- "Was this IP address a Tor relay recently?" database
\subsection{Commercial/enterprise outreach. Help them use Tor well and
not fear it.}
\subsection{NGO outreach and training.}
- "How to be a safe blogger"
\subsection{More activist coordinators, more people to answer user questions}
\subsection{More people to hold hands of server operators}
\subsection{Teaching the media about Tor}
\subsection{The-dangers-of-plaintext awareness}
\subsection{check.torproject.org and other "privacy checkers"}
\subsection{Stronger legal FAQ for US}
\subsection{Legal FAQs for other countries}
\section{Anonymity research}
\subsection{estimate relay bandwidth more securely}
\subsection{website fingerprinting attacks}
\subsection{safer e2e defenses}
\subsection{Using Tor when you really need anonymity. Can you compose it
with other steps, like more trusted guards or separate proxies?}
\subsection{Topology-aware routing; routing-zones, steven's pet2007 paper.}
\subsection{Exactly what do guard nodes provide?}
Entry guards seem to defend against all sorts of attacks. Can we work
through all the benefits they provide? Papers like Nikita's CCS 2007
paper make me think their value is not well-understood by the research
community.
\section{Organizational growth and stability}
\subsection{A contingency plan if Roger gets hit by a bus}
- Get a new executive director
\subsection{More diversity of funding}
- Don't rely on any one funder as much
- Don't rely on any sector or funder category as much
\subsection{More Tor-funded people who are skilled at peripheral apps like
Vidalia, Torbutton, Polipo, etc}
\subsection{More coordinated media handling and strategy}
\subsection{Clearer and more predictable trademark behavior}
\subsection{More outside funding for internships, etc e.g. GSoC.}
\section{Hidden services}
\subsection{Scaling: how to handle many hidden services}
\subsection{Performance: how to rendezvous with them quickly}
\subsection{Authentication/authorization: how to tolerate DoS / load}
\section{Tor as a general overlay network}
\subsection{Choose paths / exit by country}
\subsection{Easier to run your own private servers and have Tor use them
anywhere in the path}
\subsection{Easier to run an independent Tor network}
\section{Code security/correctness}
\subsection{veracode}
\subsection{code audit}
\subsection{more fuzzing tools}
\subsection{build farm, better testing harness}
\subsection{Long-overdue code refactoring and cleanup}
\section{Protocol security}
\subsection{safer circuit handshake}
\subsection{protocol versioning for future compatibility}
\subsection{cell sizes}
\subsection{adapt to new key sizes, etc}
\bibliographystyle{plain} \bibliography{tor-design}
\end{document}
\section{Code and design infrastructure}
\subsection{Protocol revision}
To maintain backward compatibility, we've postponed major protocol
changes and redesigns for a long time. Because of this, there are a number
of sensible revisions we've been putting off until we could deploy several of
them at once. To do each of these, we first need to discuss design
alternatives with other cryptographers and outside collaborators to
make sure that our choices are secure.
First of all, our protocol needs better {\bf versioning support} so that we
can make backward-incompatible changes to our core protocol. There are
difficult anonymity issues here, since many naive designs would make it easy
to tell clients apart (and then track them) based on their supported versions.
With protocol versioning support would come the ability to {\bf future-proof
our ciphersuites}. For example, not only our OR protocol, but also our
directory protocol, is pretty firmly tied to the SHA-1 hash function, which
though not yet known to be insecure for our purposes, has begun to show
its age. We should
remove assumptions throughout our design based on the assumption that public
keys, secret keys, or digests will remain any particular size indefinitely.
Our OR {\bf authentication protocol}, though provably
secure\cite{tap:pet2006}, relies more on particular aspects of RSA and our
implementation thereof than we had initially believed. To future-proof
against changes, we should replace it with a less delicate approach.
\plan{For all the above: 2 person-months to specify, spread over several
months with time for interaction with external participants. One
person-month to implement. Start specifying in early 2007.}
We might design a {\bf stream migration} feature so that streams tunneled
over Tor could be more resilient to dropped connections and changed IPs.
\plan{Not in 2007.}
A new protocol could support {\bf multiple cell sizes}. Right now, all data
passes through the Tor network divided into 512-byte cells. This is
efficient for high-bandwidth protocols, but inefficient for protocols
like SSH or AIM that send information in small chunks. Of course, we need to
investigate the extent to which multiple sizes could make it easier for an
adversary to fingerprint a traffic pattern. \plan{Not in 2007.}
As a part of our design, we should investigate possible {\bf cipher modes}
other than counter mode. For example, a mode with built-in integrity
checking, error propagation, and random access could simplify our protocol
significantly. Sadly, many of these are patented and unavailable for us.
\plan{Not in 2007.}
\subsection{Scalability}
\subsubsection{Improved directory efficiency}
We should {\bf have routers upload their descriptors even less often}, so
that clients do not need to download replacements every 18 hours whether any
information has changed or not. (As of Tor 0.1.2.3-alpha, clients tolerate
routers that don't upload often, but routers still upload at least every 18
hours to support older clients.) \plan{Must do, but not until 0.1.1.x is
deprecated in mid 2007. 1 week.}
\subsubsection{Non-clique topology}
Our current network design achieves a certain amount of its anonymity by
making clients act like each other through the simple expedient of making
sure that all clients know all servers, and that any server can talk to any
other server. But as the number of servers increases to serve an
ever-greater number of clients, these assumptions become impractical.
At worst, if these scalability issues become troubling before a solution is
found, we can design and build a solution to {\bf split the network into
multiple slices} until a better solution comes along. This is not ideal,
since rather than looking like all other users from a point of view of path
selection, users would ``only'' look like 200,000--300,000 other
users.\plan{Not unless needed.}
We are in the process of designing {\bf improved schemes for network
scalability}. Some approaches focus on limiting what an adversary can know
about what a user knows; others focus on reducing the extent to which an
adversary can exploit this knowledge. These are currently in their infancy,
and will probably not be needed in 2007, but they must be designed in 2007 if
they are to be deployed in 2008.\plan{Design in 2007; unknown difficulty.
Write a paper.}
\subsubsection{Relay incentives}
To support more users on the network, we need to get more servers. So far,
we've relied on volunteerism to attract server operators, and so far it's
served us well. But in the long run, we need to {\bf design incentives for
users to run servers} and relay traffic for others. Most obviously, we
could try to build the network so that servers offered improved service for
other servers, but we would need to do so without weakening anonymity and
making it obvious which connections originate from users running servers. We
have some preliminary designs~\cite{incentives-txt,tor-challenges},
but need to perform
some more research to make sure they would be safe and effective.\plan{Write
a draft paper; 2 person-months.}
(XXX we did that)
\subsection{Portability}
Our {\bf Windows implementation}, though much improved, continues to lag
behind Unix and Mac OS X, especially when running as a server. We hope to
merge promising patches from Christian King to address this point, and bring
Windows performance on par with other platforms.\plan{Do in 2007; 1.5 months
to integrate not counting Mike's work.}
We should have {\bf better support for portable devices}, including modes of
operation that require less RAM, and that write to disk less frequently (to
avoid wearing out flash RAM).\plan{Optional; 2 weeks.}
\subsection{Performance: resource usage}
We've been working on {\bf using less RAM}, especially on servers. This has
paid off a lot for directory caches in the 0.1.2, which in some cases are
using 90\% less memory than they used to require. But we can do better,
especially in the area around our buffer management algorithms, by using an
approach more like the BSD and Linux kernels use instead of our current ring
buffer approach. (For OR connections, we can just use queues of cell-sized
chunks produced with a specialized allocator.) This could potentially save
around 25 to 50\% of the memory currently allocated for network buffers, and
make Tor a more attractive proposition for restricted-memory environments
like old computers, mobile devices, and the like.\plan{Do in 2007; 2-3 weeks
plus one week measurement.} (XXX We did this, but we need to do something
more/else.)
\subsection{Performance: network usage}
We know too little about how well our current path
selection algorithms actually spread traffic around the network in practice.
We should {\bf research the efficacy of our traffic allocation} and either
assure ourselves that it is close enough to optimal as to need no improvement
(unlikely) or {\bf identify ways to improve network usage}, and get more
users' traffic delivered faster. Performing this research will require
careful thought about anonymity implications.
We should also {\bf examine the efficacy of our congestion control
algorithm}, and see whether we can improve client performance in the
presence of a congested network through dynamic `sendme' window sizes or
other means. This will have anonymity implications too if we aren't careful.
\plan{For both of the above: research, design and write
a measurement tool in 2007: 1 month. See if we can interest a graduate
student.}
We should work on making Tor's cell-based protocol perform better on
networks with low bandwidth
and high packet loss.\plan{Do in 2007 if we're funded to do it; 4-6 weeks.}
\subsection{Performance scenario: one Tor client, many users}
We should {\bf improve Tor's performance when a single Tor handles many
clients}. Many organizations want to manage a single Tor client on their
firewall for many users, rather than having each user install a separate
Tor client. We haven't optimized for this scenario, and it is likely that
there are some code paths in the current implementation that become
inefficient when a single Tor is servicing hundreds or thousands of client
connections. (Additionally, it is likely that such clients have interesting
anonymity requirements the we should investigate.) We should profile Tor
under appropriate loads, identify bottlenecks, and fix them.\plan{Do in 2007
if we're funded to do it; 4-8 weeks.}
\subsection{Tor servers on asymmetric bandwidth}
Tor should work better on servers that have asymmetric connections like cable
or DSL. Because Tor has separate TCP connections between each
hop, if the incoming bytes are arriving just fine and the outgoing bytes are
all getting dropped on the floor, the TCP push-back mechanisms don't really
transmit this information back to the incoming streams.\plan{Do in 2007 since
related to bandwidth limiting. 3-4 weeks.}
\subsection{Running Tor as both client and server}
Many performance tradeoffs and balances that might need more attention.
We first need to track and fix whatever bottlenecks emerge; but we also
need to invent good algorithms for prioritizing the client's traffic
without starving the server's traffic too much.\plan{No idea; try
profiling and improving things in 2007.}
\subsection{Protocol redesign for UDP}
Tor has relayed only TCP traffic since its first versions, and has used
TLS-over-TCP to do so. This approach has proved reliable and flexible, but
in the long term we will need to allow UDP traffic on the network, and switch
some or all of the network to using a UDP transport. {\bf Supporting UDP
traffic} will make Tor more suitable for protocols that require UDP, such
as many VOIP protocols. {\bf Using a UDP transport} could greatly reduce
resource limitations on servers, and make the network far less interruptible
by lossy connections. Either of these protocol changes would require a great
deal of design work, however. We hope to be able to enlist the aid of a few
talented graduate students to assist with the initial design and
specification, but the actual implementation will require significant testing
of different reliable transport approaches.\plan{Maybe do a design in 2007 if
we find an interested academic. Ian or Ben L might be good partners here.}
\section{Blocking resistance}
\subsection{Design for blocking resistance}
We have written a design document explaining our general approach to blocking
resistance. We should workshop it with other experts in the field to get
their ideas about how we can improve Tor's efficacy as an anti-censorship
tool.
\subsection{Implementation: client-side and bridges-side}
Bridges will want to be able to {\bf listen on multiple addresses and ports}
if they can, to give the adversary more ports to block.
\subsection{Research: anonymity implications from becoming a bridge}
see arma's bridge proposal; e.g. should bridge users use a second layer of
entry guards?
\subsection{Implementation: bridge authority}
we run some
directory authorities with a slightly modified protocol that doesn't leak
the entire list of bridges. Thus users can learn up-to-date information
for bridges they already know about, but they can't learn about arbitrary
new bridges.
we need a design for distributing the bridge authority over more than one
server
\subsection{Normalizing the Tor protocol on the wire}
Additionally, we should {\bf resist content-based filters}. Though an
adversary can't see what users are saying, some aspects of our protocol are
easy to fingerprint {\em as} Tor. We should correct this where possible.
Look like Firefox; or look like nothing?
Future research: investigate timing similarities with other protocols.
\subsection{Research: scanning-resistance}
\subsection{Research/Design/Impl: how users discover bridges}
Our design anticipates an arms race between discovery methods and censors.
We need to begin the infrastructure on our side quickly, preferably in a
flexible language like Python, so we can adapt quickly to censorship.
phase one: personal bridges
phase two: families of personal bridges
phase three: more structured social network
phase four: bag of tricks
Research: phase five...
Integration with Psiphon, etc?
\subsection{Document best practices for users}
Document best practices for various activities common among
blocked users (e.g. WordPress use).
\subsection{Research: how to know if a bridge has been blocked?}
\subsection{GeoIP maintenance, and "private" user statistics}
How to know if the whole idea is working?
\subsection{Research: hiding whether the user is reading or publishing?}
\subsection{Research: how many bridges do you need to know to maintain
reachability?}
\subsection{Resisting censorship of the Tor website, docs, and mirrors}
We should take some effort to consider {\bf initial distribution of Tor and
related information} in countries where the Tor website and mirrors are
censored. (Right now, most countries that block access to Tor block only the
main website and leave mirrors and the network itself untouched.) Falling
back on word-of-mouth is always a good last resort, but we should also take
steps to make sure it's relatively easy for users to get ahold of a copy.
\section{Security}
\subsection{Security research projects}
We should investigate approaches with some promise to help Tor resist
end-to-end traffic correlation attacks. It's an open research question
whether (and to what extent) {\bf mixed-latency} networks, {\bf low-volume
long-distance padding}, or other approaches can resist these attacks, which
are currently some of the most effective against careful Tor users. We
should research these questions and perform simulations to identify
opportunities for strengthening our design without dropping performance to
unacceptable levels. %Cite something
\plan{Start doing this in 2007; write a paper. 8-16 weeks.}
We've got some preliminary results suggesting that {\bf a topology-aware
routing algorithm}~\cite{feamster:wpes2004} could reduce Tor users'
vulnerability against local or ISP-level adversaries, by ensuring that they
are never in a position to watch both ends of a connection. We need to
examine the effects of this approach in more detail and consider side-effects
on anonymity against other kinds of adversaries. If the approach still looks
promising, we should investigate ways for clients to implement it (or an
approximation of it) without having to download routing tables for the whole
Internet. \plan{Not in 2007 unless a graduate student wants to do it.}
%\tmp{defenses against end-to-end correlation} We don't expect any to work
%right now, but it would be useful to learn that one did. Alternatively,
%proving that one didn't would free up researchers in the field to go work on
%other things.
%
% See above; I think I got this.
We should research the efficacy of {\bf website fingerprinting} attacks,
wherein an adversary tries to match the distinctive traffic and timing
pattern of the resources constituting a given website to the traffic pattern
of a user's client. These attacks work great in simulations, but in
practice we hear they don't work nearly as well. We should get some actual
numbers to investigate the issue, and figure out what's going on. If we
resist these attacks, or can improve our design to resist them, we should.
% add cites
\plan{Possibly part of end-to-end correlation paper. Otherwise, not in 2007
unless a graduate student is interested.}
\subsection{Implementation security}
We should also {\bf mark RAM that holds key material as non-swappable} so
that there is no risk of recovering key material from a hard disk
compromise. This would require submitting patches upstream to OpenSSL, where
support for marking memory as sensitive is currently in a very preliminary
state.\plan{Nice to do, but not in immediate Tor scope.}
There are numerous tools for identifying trouble spots in code (such as
Coverity or even VS2005's code analysis tool) and we should convince somebody
to run some of them against the Tor codebase. Ideally, we could figure out a
way to get our code checked periodically rather than just once.\plan{Almost
no time once we talk somebody into it.}
We should try {\bf protocol fuzzing} to identify errors in our
implementation.\plan{Not in 2007 unless we find a grad student or
undergraduate who wants to try.}
Our guard nodes help prevent an attacker from being able to become a chosen
client's entry point by having each client choose a few favorite entry points
as ``guards'' and stick to them. We should implement a {\bf directory
guards} feature to keep adversaries from enumerating Tor users by acting as
a directory cache.\plan{Do in 2007; 2 weeks.}
\subsection{Detect corrupt exits and other servers}
With the success of our network, we've attracted servers in many locations,
operated by many kinds of people. Unfortunately, some of these locations
have compromised or defective networks, and some of these people are
untrustworthy or incompetent. Our current design relies on authority
administrators to identify bad nodes and mark them as nonfunctioning. We
should {\bf automate the process of identifying malfunctioning nodes} as
follows:
We should create a generic {\bf feedback mechanism for add-on tools} like
Mike Perry's ``Snakes on a Tor'' to report failing nodes to authorities.
\plan{Do in 2006; 1-2 weeks.}
We should write tools to {\bf detect more kinds of innocent node failure},
such as nodes whose network providers intercept SSL, nodes whose network
providers censor popular websites, and so on. We should also try to detect
{\bf routers that snoop traffic}; we could do this by launching connections
to throwaway accounts, and seeing which accounts get used.\plan{Do in 2007;
ask Mike Perry if he's interested. 4-6 weeks.}
We should add {\bf an efficient way for authorities to mark a set of servers
as probably collaborating} though not necessarily otherwise dishonest.
This happens when an administrator starts multiple routers, but doesn't mark
them as belonging to the same family.\plan{Do during v2.1 directory protocol
redesign; 1-2 weeks to implement.}
To avoid attacks where an adversary claims good performance in order to
attract traffic, we should {\bf have authorities measure node performance}
(including stability and bandwidth) themselves, and not simply believe what
they're told. We also measure stability by tracking MTBF. Measuring
bandwidth will be tricky, since it's hard to distinguish between a server with
low capacity, and a high-capacity server with most of its capacity in
use. See also Nikita's NDSS 2008 paper.\plan{Do it if we can interest
a grad student.}
{\bf Operating a directory authority should be easier.} We rely on authority
operators to keep the network running well, but right now their job involves
too much busywork and administrative overhead. A better interface for them
to use could free their time to work on exception cases rather than on
adding named nodes to the network.\plan{Do in 2007; 4-5 weeks.}
\subsection{Protocol security}
In addition to other protocol changes discussed above,
% And should we move some of them down here? -NM
we should add {\bf hooks for denial-of-service resistance}; we have some
preliminary designs, but we shouldn't postpone them until we really need them.
If somebody tries a DDoS attack against the Tor network, we won't want to
wait for all the servers and clients to upgrade to a new
version.\plan{Research project; do this in 2007 if funded.}
\section{Development infrastructure}
\subsection{Build farm}
We've begun to deploy a cross-platform distributed build farm of hosts
that build and test the Tor source every time it changes in our development
repository.
We need to {\bf get more participants}, so that we can test a larger variety
of platforms. (Previously, we've only found out when our code had broken on
obscure platforms when somebody got around to building it.)
We need also to {\bf add our dependencies} to the build farm, so that we can
ensure that libraries we need (especially libevent) do not stop working on
any important platform between one release and the next.
\plan{This is ongoing as more buildbots arrive.}
\subsection{Improved testing harness}
Currently, our {\bf unit tests} cover only about 20\% of the code base. This
is uncomfortably low; we should write more and switch to a more flexible
testing framework.\plan{Ongoing basis, time permitting.}
We should also write flexible {\bf automated single-host deployment tests} so
we can more easily verify that the current codebase works with the
network.\plan{Worthwhile in 2007; would save lots of time. 2-4 weeks.}
We should build automated {\bf stress testing} frameworks so we can see which
realistic loads cause Tor to perform badly, and regularly profile Tor against
these loads. This would give us {\it in vitro} performance values to
supplement our deployment experience.\plan{Worthwhile in 2007; 2-6 weeks.}
We should improve our memory profiling code.\plan{...}
\subsection{Centralized build system}
We currently rely on a separate packager to maintain the packaging system and
to build Tor on each platform for which we distribute binaries. Separate
package maintainers is sensible, but separate package builders has meant
long turnaround times between source releases and package releases. We
should create the necessary infrastructure for us to produce binaries for all
major packages within an hour or so of source release.\plan{We should
brainstorm this at least in 2007.}
\subsection{Improved metrics}
We need a way to {\bf measure the network's health, capacity, and degree of
utilization}. Our current means for doing this are ad hoc and not
completely accurate
We need better ways to {\bf tell which countries are users are coming from,
and how many there are}. A good perspective of the network helps us
allocate resources and identify trouble spots, but our current approaches
will work less and less well as we make it harder for adversaries to
enumerate users. We'll probably want to shift to a smarter, statistical
approach rather than our current ``count and extrapolate'' method.
\plan{All of this in 2007 if funded; 4-8 weeks}
% \tmp{We'd like to know how much of the network is getting used.}
% I think this is covered above -NM
\subsection{Controller library}
We've done lots of design and development on our controller interface, which
allows UI applications and other tools to interact with Tor. We could
encourage the development of more such tools by releasing a {\bf
general-purpose controller library}, ideally with API support for several
popular programming languages.\plan{2006 or 2007; 1-2 weeks.}
\section{User experience}
\subsection{Get blocked less, get blocked less broadly}
Right now, some services block connections from the Tor network because
they don't have a better
way to keep vandals from abusing them than blocking IP addresses associated
with vandalism. Our approach so far has been to educate them about better
solutions that currently exist, but we should also {\bf create better
solutions for limiting vandalism by anonymous users} like credential and
blind-signature based implementations, and encourage their use. Other
promising starting points including writing a patch and explanation for
Wikipedia, and helping Freenode to document, maintain, and expand its
current Tor-friendly position.\plan{Do a writeup here in 2007; 1-2 weeks.}
Those who do block Tor users also block overbroadly, sometimes blacklisting
operators of Tor servers that do not permit exit to their services. We could
obviate innocent reasons for doing so by designing a {\bf narrowly-targeted Tor
RBL service} so that those who wanted to overblock Tor could no longer
plead incompetence.\plan{Possibly in 2007 if we decide it's a good idea; 3
weeks.}
\subsection{All-in-one bundle}
We need a well-tested, well-documented bundle of Tor and supporting
applications configured to use it correctly. We have an initial
implementation well under way, but it will need additional work in
identifying requisite Firefox extensions, identifying security threats,
improving user experience, and so on. This will need significantly more work
before it's ready for a general public release.
\subsection{LiveCD Tor}
We need a nice bootable livecd containing a minimal OS and a few applications
configured to use it correctly. The Anonym.OS project demonstrated that this
is quite feasible, but their project is not currently maintained.
\subsection{A Tor client in a VM}
\tmp{a.k.a JanusVM} which is quite related to the firewall-level deployment
section below. JanusVM is a Linux kernel running in VMWare. It gets an IP
address from the network, and serves as a DHCP server for its host Windows
machine. It intercepts all outgoing traffic and redirects it into Privoxy,
Tor, etc. This Linux-in-Windows approach may help us with scalability in
the short term, and it may also be a good long-term solution rather than
accepting all security risks in Windows.
%\subsection{Interface improvements}
%\tmp{Allow controllers to manipulate server status.}
% (Why is this in the User Experience section?) -RD
% I think it's better left to a generic ``make controller iface better'' item.
\subsection{Firewall-level deployment}
Another useful deployment mode for some users is using {\bf Tor in a firewall
configuration}, and directing all their traffic through Tor. This can be a
little tricky to set up currently, but it's an effective way to make sure no
traffic leaves the host un-anonymized. To achieve this, we need to {\bf
improve and port our new TransPort} feature which allows Tor to be used
without SOCKS support; to {\bf add an anonymizing DNS proxy} feature to Tor;
and to {\bf construct a recommended set of firewall configurations} to redirect
traffic to Tor.
This is an area where {\bf deployment via a livecd}, or an installation
targeted at specialized home routing hardware, could be useful.
\subsection{Assess software and configurations for anonymity risks}
Right now, users and packagers are more or less on their own when selecting
Firefox extensions. We should {\bf assemble a recommended list of browser
extensions} through experiment, and include this in the application bundles
we distribute.
We should also describe {\bf best practices for using Tor with each class of
application}. For example, Ethan Zuckerman has written a detailed
tutorial on how to use Tor, Firefox, GMail, and Wordpress to blog with
improved safety. There are many other cases on the Internet where anonymity
would be helpful, and there are a lot of ways to screw up using Tor.
The Foxtor and Torbutton extensions serve similar purposes; we should pick a
favorite, and merge in the useful features of the other.
%\tmp{clean up our own bundled software:
%E.g. Merge the good features of Foxtor into Torbutton}
%
% What else did you have in mind? -NM
\subsection{Localization}
Right now, most of our user-facing code is internationalized. We need to
internationalize the last few hold-outs (like the Tor expert installer), and get
more translations for the parts that are already internationalized.
Also, we should look into a {\bf unified translator's solution}. Currently,
since different tools have been internationalized using the
framework-appropriate method, different tools require translators to localize
them via different interfaces. Inasmuch as possible, we should make
translators only need to use a single tool to translate the whole Tor suite.
\section{Support}
It would be nice to set up some {\bf user support infrastructure} and
{\bf contributor support infrastructure}, especially focusing on server
operators and on coordinating volunteers.
This includes intuitive and easy ticket systems for bug reports and
feature suggestions (not just mailing lists with a half dozen people
and no clear roles for who answers what), but it also includes a more
personalized and efficient framework for interaction so we keep the
attention and interest of the contributors, and so we make them feel
helpful and wanted.
\section{Documentation}
\subsection{Unified documentation scheme}
We need to {\bf inventory our documentation.} Our documentation so far has
been mostly produced on an {\it ad hoc} basis, in response to particular
needs and requests. We should figure out what documentation we have, which of
it (if any) should get priority, and whether we can't put it all into a
single format.
We could {\bf unify the docs} into a single book-like thing. This will also
help us identify what sections of the ``book'' are missing.
\subsection{Missing technical documentation}
We should {\bf revise our design paper} to reflect the new decisions and
research we've made since it was published in 2004. This will help other
researchers evaluate and suggest improvements to Tor's current design.
Other projects sometimes implement the client side of our protocol. We
encourage this, but we should write {\bf a document about how to avoid
excessive resource use}, so we don't need to worry that they will do so
without regard to the effect of their choices on server resources.
\subsection{Missing user documentation}
Our documentation falls into two broad categories: some is `discoursive' and
explains in detail why users should take certain actions, and other
documentation is `comprehensive' and describes all of Tor's features. Right
now, we have no document that is both deep, readable, and thorough. We
should correct this by identifying missing spots in our design.
\bibliographystyle{plain} \bibliography{tor-design}
\end{document}

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Tor:
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Next-generation Onion Routing
%size 7
Roger Dingledine
Nick Mathewson
Paul Syverson
%%The Free Haven Project
%%%font "typewriter", fore "blue"
%%http://freehaven.net/
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Low-latency anonymity system
%leftfill
Deployed: 19 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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Perfect forward secrecy
Telescoping circuit
negotiates keys at each hop
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%%page
%%
%%Separation from "protocol cleaning"
%%
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%page
No mixing, padding, traffic shaping (yet)
Please show us they're worth the usability tradeoff
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%%page
%%
%%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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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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We have working code
Plus a design document,
and a byte-level specification
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http://freehaven.net/tor/