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+\newcommand{\workingnote}[1]{}        % The version that hides the note.
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+\begin{document}
+
+\title{Design challenges and social factors in deploying low-latency anonymity}
+
+\author{Roger Dingledine\inst{1} \and
+Nick Mathewson\inst{1} \and
+Paul Syverson\inst{2}}
+\institute{The Free Haven Project \email{<\{arma,nickm\}@freehaven.net>} \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 the design
+  philosophy of Tor (the third-generation onion routing network), and,
+  drawing on our experiences deploying Tor, we describe social
+  challenges and related technical issues that must be faced in
+  building, deploying, and sustaining a scalable, distributed,
+  low-latency anonymity network.
+\end{abstract}
+
+\section{Introduction}
+% Your network is not practical unless it is sustainable and distributed.
+Anonymous communication is full of surprises.  This article describes
+Tor, a low-latency general-purpose anonymous communication system, and
+discusses some unexpected challenges arising from our experiences
+deploying Tor.  We will discuss
+some of the difficulties we have experienced and how we have met them (or how
+we plan to meet them, if we know).
+%  We also discuss some less
+% troublesome open problems that we must nevertheless eventually address.
+%We will describe both those future challenges that we intend to explore and
+%those that we have decided not to explore and why.
+
+Tor is an overlay network for anonymizing TCP streams over the
+Internet~\cite{tor-design}.  It addresses limitations in earlier Onion
+Routing designs~\cite{or-ih96,or-jsac98,or-discex00,or-pet00} by adding
+perfect forward secrecy, congestion control, directory servers, data
+integrity, 
+%configurable exit policies, Huh? That was part of the gen. 1 design -PFS
+and a revised design for location-hidden services using
+rendezvous points.  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.
+
+We deployed the public Tor network in October 2003; since then it has
+grown to over nine hundred volunteer-operated nodes worldwide
+and over 100 megabytes average traffic per second from hundreds of
+thousands of concurrent users.
+Tor's research strategy has focused on deploying
+a network to as many users as possible; thus, we have resisted designs that
+would compromise deployability by imposing high resource demands on node
+operators, and designs that would compromise usability by imposing
+unacceptable restrictions on which applications we support.  Although this
+strategy has drawbacks (including a weakened threat model, as
+discussed below), it has made it possible for Tor to serve many
+hundreds of thousands of users and attract funding from diverse
+sources whose goals range from security on a national scale down to
+individual liberties.
+
+In~\cite{tor-design} we gave an overall view of Tor's design and
+goals.  Here we review that design at a higher level and describe
+some policy and social issues that we face as
+we continue deployment. Though we will discuss technical responses to
+these, we do not in this article discuss purely technical challenges
+facing Tor (e.g., transport protocol, resource scaling issues, moving
+to non-clique topologies, performance, etc.), nor do we even cover
+all of the social issues: we simply touch on some of the most salient of these.
+Also, rather than providing complete solutions to every problem, we
+instead lay out the challenges and constraints that we have observed while
+deploying Tor.  In doing so, we aim to provide a research agenda
+of general interest to projects attempting to build
+and deploy practical, usable anonymity networks in the wild.
+
+%While the Tor design paper~\cite{tor-design} gives an overall view its
+%design and goals,
+%this paper describes the policy and technical issues that Tor faces as
+%we continue deployment. Rather than trying to provide complete solutions
+%to every problem here, we lay out the assumptions and constraints
+%that we have observed through deploying Tor in the wild. In doing so, we
+%aim to create a research agenda for others to
+%help in addressing these issues.
+% Section~\ref{sec:what-is-tor} gives an
+%overview of the Tor
+%design and ours goals. Sections~\ref{sec:crossroads-policy}
+%and~\ref{sec:crossroads-design} go on to describe the practical challenges,
+%both policy and technical respectively,
+%that stand in the way of moving
+%from a practical useful network to a practical useful anonymous network.
+
+%\section{What Is Tor}
+\section{Background}
+Here we give a basic overview of the Tor design and its properties, and
+compare Tor to other low-latency anonymity designs.
+
+\subsection{Tor, threat models, and distributed trust}
+\label{sec:what-is-tor}
+
+%Here we give a basic overview of the Tor design and its properties. For
+%details on the design, assumptions, and security arguments, we refer
+%the reader to the Tor design paper~\cite{tor-design}.
+
+Tor provides \emph{forward privacy}, so that users can connect to
+Internet sites without revealing their logical or physical locations
+to those sites or to observers.  It also provides \emph{location-hidden
+services}, so that servers can support authorized users without
+giving an effective vector for physical or online attackers.
+Tor provides these protections even when a portion of its
+infrastructure is compromised.
+
+To connect to a remote server via Tor, the client software learns a signed
+list of Tor nodes from one of several central \emph{directory servers}, and
+incrementally creates a private pathway or \emph{circuit} of encrypted
+connections through authenticated Tor nodes on the network, negotiating a
+separate set of encryption keys for each hop along the circuit.  The circuit
+is extended one node at a time, 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.
+%Because each node sees no more than one hop in the
+%circuit,
+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 also helps servers hide their locations while
+providing services such as web publishing or instant
+messaging.  Using ``rendezvous points'', other Tor users can
+connect to these authenticated hidden services, neither one learning the
+other's network identity.
+
+Tor attempts to anonymize the transport layer, not the application layer.
+This approach is useful for applications such as SSH
+where authenticated communication is desired. However, when anonymity from
+those with whom we communicate is desired,
+application protocols that include personally identifying information need
+additional application-level 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.
+%, and only supports
+%connections via SOCKS
+%(but see Section~\ref{subsec:tcp-vs-ip}).
+
+Most node operators do not want to allow arbitrary TCP traffic. % to leave
+%their server.
+To address this, Tor provides \emph{exit policies} so
+each exit node can block the IP addresses and ports it is unwilling to allow.
+Tor nodes advertise their exit policies to the directory servers, so that
+client can tell which nodes will support their connections.
+
+As of this writing, the Tor network has grown to around nine hundred nodes
+on four continents, with a total average load exceeding 100 MB/s and
+a total capacity exceeding %1Gbit/s. 
+\\***What's the current capacity? -PFS***\\
+%Appendix A
+%shows a graph of the number of working nodes over time, as well as a
+%graph of the number of bytes being handled by the network over time.
+%The network is now sufficiently diverse for further development
+%and testing; but of course we always encourage new nodes
+%to join.
+
+Building from earlier versions of onion routing developed at NRL,
+Tor was researched and developed by NRL and FreeHaven under
+funding by ONR and DARPA for use in securing government
+communications. Continuing development and deployment has also been
+funded by the Omidyar Network, the Electronic Frontier Foundation for use
+in maintaining civil liberties for ordinary citizens online, and the
+International Broadcasting Bureau and Reporters without Borders to combat
+blocking and censorship on the Internet. As we will see below,
+this wide variety of interests helps maintain both the stability and
+the security of the network.
+
+% The Tor
+%protocol was chosen
+%for the anonymizing layer in the European Union's PRIME directive to
+%help maintain privacy in Europe.
+%The AN.ON project in Germany
+%has integrated an independent implementation of the Tor protocol into
+%their popular Java Anon Proxy anonymizing client.
+
+\medskip
+\noindent
+{\bf Threat models and design philosophy.}
+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.\footnote{This is not the only possible
+direction in anonymity research: designs exist that provide more anonymity
+than Tor at the expense of significantly increased resource requirements, or
+decreased flexibility in application support (typically because of increased
+latency).  Such research does not typically abandon aspirations toward
+deployability or utility, but instead tries to maximize deployability and
+utility subject to a certain degree of structural anonymity (structural because
+usability and practicality affect usage which affects the actual anonymity
+provided by the network \cite{econymics,back01}).}
+%{We believe that these
+%approaches can be promising and useful, but that by focusing on deploying a
+%usable system in the wild, Tor helps us experiment with the actual parameters
+%of what makes a system ``practical'' for volunteer operators and ``useful''
+%for home users, and helps illuminate undernoticed issues which any deployed
+%volunteer anonymity network will need to address.}
+Because of our strategy, Tor has a weaker threat model than many designs in
+the literature.  In particular, because we
+support interactive communications without impractically expensive padding,
+we fall prey to a variety
+of intra-network~\cite{back01,attack-tor-oak05,flow-correlation04,hs-attack}
+and
+end-to-end~\cite{danezis:pet2004,SS03} anonymity-breaking attacks.
+
+Tor does not attempt to defend against a global observer.  In general, an
+attacker who can measure both ends of a connection through the Tor network
+% I say 'measure' rather than 'observe', to encompass murdoch-danezis
+% style attacks. -RD
+can correlate the timing and volume of data on that connection as it enters
+and leaves the network, and so link communication partners.
+Known solutions to this attack would seem to require introducing a
+prohibitive degree of traffic padding between the user and the network, or
+introducing an unacceptable degree of latency.
+Also, it is not clear that these methods would
+work at all against a minimally active adversary who could introduce timing
+patterns or additional traffic.  Thus, Tor only attempts to defend against
+external observers who cannot observe both sides of a user's connections.
+
+Against internal attackers who sign up Tor nodes, the situation is more
+complicated.  In the simplest case, if an adversary has compromised $c$ of
+$n$ nodes on the Tor network, then the adversary will be able to compromise
+a random circuit with probability $\frac{c^2}{n^2}$~\cite{or-pet00}
+(since the circuit
+initiator chooses hops randomly).  But there are
+complicating factors:
+(1)~If the user continues to build random circuits over time, an adversary
+  is pretty certain to see a statistical sample of the user's traffic, and
+  thereby can build an increasingly accurate profile of her behavior. 
+(2)~An adversary who controls a popular service outside the Tor network
+  can be certain to observe all connections to that service; he
+  can therefore trace connections to that service with probability
+  $\frac{c}{n}$.
+(3)~Users do not in fact choose nodes with uniform probability; they
+  favor nodes with high bandwidth or uptime, and exit nodes that
+  permit connections to their favorite services.
+We demonstrated the severity of these problems in experiments on the
+live Tor network in 2006~\cite{hsattack} and introduced \emph{entry
+  guards} as a means to curtail them.  By choosing entry nodes from
+a small persistent subset, it becomes difficult for an adversary to
+increase the number of circuits observed entering the network from any
+given client simply by causing
+numerous connections or by watching compromised nodes over time.% (See
+%also Section~\ref{subsec:routing-zones} for discussion of larger
+%adversaries and our dispersal goals.)
+
+
+% I'm trying to make this paragraph work without reference to the
+% analysis/confirmation distinction, which we haven't actually introduced
+% yet, and which we realize isn't very stable anyway.  Also, I don't want to
+% deprecate these attacks if we can't demonstrate that they don't work, since
+% in case they *do* turn out to work well against Tor, we'll look pretty
+% foolish. -NM
+More powerful attacks may exist. In \cite{hintz-pet02} it was
+shown that an attacker who can catalog data volumes of popular
+responder destinations (say, websites with consistent data volumes) may not
+need to
+observe both ends of a stream to learn source-destination links for those
+responders. Entry guards should complicate such attacks as well.
+Similarly, latencies of going through various routes can be
+cataloged~\cite{back01} to connect endpoints.
+% Also, \cite{kesdogan:pet2002} takes the
+% attack another level further, to narrow down where you could be
+% based on an intersection attack on subpages in a website. -RD
+It has not yet been shown whether these attacks will succeed or fail
+in the presence of the variability and volume quantization introduced by the
+Tor network, but it seems likely that these factors will at best delay
+the time and data needed for success
+rather than prevent the attacks completely.
+
+\workingnote{
+Along similar lines, the same paper suggests a ``clogging
+attack'' in which the throughput on a circuit is observed to slow
+down when an adversary clogs the right nodes with his own traffic.
+To determine the nodes in a circuit this attack requires the ability
+to continuously monitor the traffic exiting the network on a circuit
+that is up long enough to probe all network nodes in binary fashion.
+% Though somewhat related, clogging and interference are really different
+% attacks with different assumptions about adversary distribution and
+% capabilities as well as different techniques. -pfs
+Murdoch and Danezis~\cite{attack-tor-oak05} show a practical
+interference attack against portions of
+the fifty node Tor network as deployed in mid 2004.
+An outside attacker can actively trace a circuit through the Tor network
+by observing changes in the latency of his
+own traffic sent through various Tor nodes. This can be done
+simultaneously at multiple nodes; however, like clogging,
+this attack only reveals
+the Tor nodes in the circuit, not initiator and responder addresses,
+so it is still necessary to discover the endpoints to complete an
+effective attack. The the size and diversity of the Tor network have
+increased many fold since then, and it is unknown if the attacks
+can scale to the current Tor network.
+}
+
+
+%discuss $\frac{c^2}{n^2}$, except how in practice the chance of owning
+%the last hop is not $c/n$ since that doesn't take the destination (website)
+%into account. so in cases where the adversary does not also control the
+%final destination we're in good shape, but if he *does* then we'd be better
+%off with a system that lets each hop choose a path.
+%
+%Isn't it more accurate to say ``If the adversary _always_ controls the final
+% dest, we would be just as well off with such as system.'' ?  If not, why
+% not? -nm
+% Sure. In fact, better off, since they seem to scale more easily. -rd
+
+%Murdoch and Danezis describe an attack
+%\cite{attack-tor-oak05} that lets an attacker determine the nodes used
+%in a circuit; yet s/he cannot identify the initiator or responder,
+%e.g., client or web server, through this attack. So the endpoints
+%remain secure, which is the goal. It is conceivable that an
+%adversary could attack or set up observation of all connections
+%to an arbitrary Tor node in only a few minutes.  If such an adversary
+%were to exist, s/he could use this probing to remotely identify a node
+%for further attack.  Of more likely immediate practical concern
+%an adversary with active access to the responder traffic
+%wants to keep a circuit alive long enough to attack an identified
+%node. Thus it is important to prevent the responding end of the circuit
+%from keeping it open indefinitely. 
+%Also, someone could identify nodes in this way and if in their
+%jurisdiction, immediately get a subpoena (if they even need one)
+%telling the node operator(s) that she must retain all the active
+%circuit data she now has.
+%Further, the enclave model, which had previously looked to be the most
+%generally secure, seems particularly threatened by this attack, since
+%it identifies endpoints when they're also nodes in the Tor network:
+%see Section~\ref{subsec:helper-nodes} for discussion of some ways to
+%address this issue.
+
+\medskip
+\noindent
+{\bf Distributed trust.}
+In practice Tor's threat model is based on
+dispersal and diversity.
+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.
+%than some previous attempts at anonymizing networks.
+
+No organization can achieve this security on its own.  If a single
+corporation or government agency were to build a private network to
+protect its operations, any connections entering or leaving that network
+would be obviously linkable to the controlling organization.  The members
+and operations of that agency would be easier, not harder, to distinguish.
+
+Instead, to protect our networks from traffic analysis, we must
+collaboratively blend the traffic from many organizations and private
+citizens, so that an eavesdropper can't tell which users are which,
+and who is looking for what information.  %By bringing more users onto
+%the network, all users become more secure~\cite{econymics}.
+%[XXX I feel uncomfortable saying this last sentence now. -RD]
+%[So, I took it out. I think we can do without it. -PFS]
+The Tor network has a broad range of users, including 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.  For maximum protection,
+the Tor design includes an enclave approach that lets data be encrypted
+(and authenticated) end-to-end, so high-sensitivity users can be sure it
+hasn't been read or modified.  This even works for Internet services that
+don't have built-in encryption and authentication, such as unencrypted
+HTTP or chat, and it requires no modification of those services.
+
+%\subsection{Related work}
+Tor 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 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 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.
+The Java
+Anon Proxy (JAP)~\cite{web-mix} provides similar functionality to Tor but
+handles only web browsing rather than all TCP\@. Because all traffic
+passes through fixed ``cascades'' for which the endpoints are predictable,
+an adversary can know where to watch for traffic analysis from particular
+clients or to particular web servers. The design calls for padding to
+complicate this, although it does not appear to be implemented.
+%Some peer-to-peer file-sharing overlay networks such as
+%Freenet~\cite{freenet} and Mute~\cite{mute}
+The Freedom 
+network from Zero-Knowledge Systems~\cite{freedom21-security}
+was even more flexible than Tor in
+transporting arbitrary IP packets, and also supported
+pseudonymity in addition to anonymity; but it had
+a different approach to sustainability (collecting money from users
+and paying ISPs to run Tor nodes), and was eventually shut down due to financial
+load.  Finally, %potentially more scalable
+% [I had added 'potentially' because the scalability of these designs
+% is not established, and I am uncomfortable making the
+% bolder unmodified assertion. Roger took 'potentially' out.
+% Here's an attempt at more neutral wording -pfs]
+peer-to-peer designs that are intended to be more scalable,
+for example Tarzan~\cite{tarzan:ccs02} and
+MorphMix~\cite{morphmix:fc04}, have been proposed in the literature but
+have not been fielded. These systems differ somewhat
+in threat model and presumably practical resistance to threats.
+Note that MorphMix differs from Tor only in
+node discovery and circuit setup; so Tor's architecture is flexible
+enough to contain a MorphMix experiment. Recently, 
+Tor has adopted from MorphMix the approach of making it harder to
+own both ends of a circuit by requiring that nodes be chosen from
+different /16 subnets. This requires
+an adversary to own nodes in multiple address ranges to even have the
+possibility of observing both ends of a circuit.  We direct the
+interested reader to~\cite{tor-design} for a more in-depth review of
+related work.
+
+%XXXX six-four. crowds. i2p.
+
+%XXXX
+%have a serious discussion of morphmix's assumptions, since they would
+%seem to be the direct competition. in fact tor is a flexible architecture
+%that would encompass morphmix, and they're nearly identical except for
+%path selection and node discovery. and the trust system morphmix has
+%seems overkill (and/or insecure) based on the threat model we've picked.
+% this para should probably move to the scalability / directory system. -RD
+% Nope. Cut for space, except for small comment added above -PFS
+
+\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.'' From here we can better understand the effects
+of publicity on security: the more convincing your
+advertising, the more likely people will believe you have users, and thus
+the more users you will attract. Perversely, over-hyped systems (if they
+are not too broken) may be a better choice than modestly promoted ones,
+if the hype attracts more users~\cite{usability-network-effect}.
+
+%So it follows that we should come up with ways to accurately communicate
+%the available security levels to the user, so she can make informed
+%decisions.
+%JAP aims to do this by including a
+%comforting `anonymity meter' dial in the software's graphical interface,
+%giving the user an impression of the level of protection for her current
+%traffic.
+
+However, there's a catch. For users to share the same anonymity set,
+they need to act like each other. An attacker who can distinguish
+a given user's traffic from the rest of the traffic will not be
+distracted by anonymity set size. For high-latency systems like
+Mixminion, where the threat model is based on mixing messages with each
+other, there's an arms race between end-to-end statistical attacks and
+counter-strategies~\cite{statistical-disclosure,minion-design,e2e-traffic,trickle02}.
+But for low-latency systems like Tor, end-to-end \emph{traffic
+correlation} attacks~\cite{danezis:pet2004,defensive-dropping,SS03,hs-attack}
+allow an attacker who can observe both ends of a communication
+to correlate packet timing and volume, quickly linking
+the initiator to her destination.
+
+\workingnote{
+Like Tor, the current JAP implementation does not pad connections
+apart from using small fixed-size cells for transport. In fact,
+JAP's cascade-based network topology may be more vulnerable to these
+attacks, because its network has fewer edges. JAP was born out of
+the ISDN mix design~\cite{isdn-mixes}, where padding made sense because
+every user had a fixed bandwidth allocation and altering the timing
+pattern of packets could be immediately detected. But in its current context
+as an Internet web anonymizer, adding sufficient padding to JAP
+would probably be prohibitively expensive and ineffective against a
+minimally active attacker.\footnote{Even if JAP could
+fund higher-capacity nodes indefinitely, our experience
+suggests that many users would not accept the increased per-user
+bandwidth requirements, leading to an overall much smaller user base.}
+Therefore, since under this threat
+model the number of concurrent users does not seem to have much impact
+on the anonymity provided, we suggest that JAP's anonymity meter is not
+accurately communicating security levels to its users.
+}
+
+On the other hand, while the number of active concurrent users may not
+matter as much as we'd like, it still helps to have some other users
+on the network, in particular different types of users.
+We investigate this issue next.
+
+\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, privacy enthusiasts, 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' disreputable 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.
+
+While people therefore have an incentive for the network to be used for
+``more reputable'' activities than their own, there are still trade-offs
+involved when it comes to anonymity. To follow the above example, a
+network used entirely by cancer survivors might welcome file sharers
+onto the network, though of course they'd prefer a wider
+variety of users.
+
+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) are typically kept private, whereas network abuses
+or other problems tend to be more widely publicized.
+
+The impact of public perception on security is especially important
+during the bootstrapping phase of the network, where the first few
+widely publicized uses of the network can dictate the types of users it
+attracts next.
+As an example, some U.S.~Department of Energy
+penetration testing engineers are tasked with compromising DoE computers
+from the outside. They only have a limited number of ISPs from which to
+launch their attacks, and they found that the defenders were recognizing
+attacks because they came from the same IP space. These engineers wanted
+to use Tor to hide their tracks. First, from a technical standpoint,
+Tor does not support the variety of IP packets one would like to use in
+such attacks.% (see Section~\ref{subsec:tcp-vs-ip}).
+But aside from this, we also decided that it would probably be poor
+precedent to encourage such use---even legal use that improves
+national security---and managed to dissuade them.
+
+%% "outside of academia, jap has just lost, permanently".  (That is,
+%% even though the crime detection issues are resolved and are unlikely
+%% to go down the same way again, public perception has not been kind.)
+
+\subsection{Sustainability and incentives}
+One of the unsolved problems in low-latency anonymity designs is
+how to keep the nodes running.  ZKS's Freedom network
+depended on paying third parties to run its servers; the JAP project's
+bandwidth depends on grants to pay for its bandwidth and
+administrative expenses.  In Tor, bandwidth and administrative costs are
+distributed across the volunteers who run Tor nodes, so we at least have
+reason to think that the Tor network could survive without continued research
+funding.\footnote{It also helps that Tor is implemented with free and open
+  source software that can be maintained by anybody with the ability and
+  inclination.}  But why are these volunteers running nodes, and what can we
+do to encourage more volunteers to do so?
+
+We have not formally surveyed Tor node operators to learn why they are
+running nodes, but
+from the information they have provided, it seems that many of them run Tor
+nodes for reasons of personal interest in privacy issues.  It is possible
+that others are running Tor nodes to protect their own
+anonymity, but of course they are
+hardly likely to tell us specifics if they are.
+%Significantly, Tor's threat model changes the anonymity incentives for running
+%a node.  In a high-latency mix network, users can receive additional
+%anonymity by running their own node, since doing so obscures when they are
+%injecting messages into the network.  But, anybody observing all I/O to a Tor
+%node can tell when the node is generating traffic that corresponds to
+%none of its incoming traffic.
+%
+%I didn't buy the above for reason's subtle enough that I just cut it -PFS
+Tor exit node operators do attain a degree of
+``deniability'' for traffic that originates at that exit node.  For
+  example, it is likely in practice that HTTP requests from a Tor node's IP
+  will be assumed to be from the Tor network.
+  More significantly, people and organizations who use Tor for
+  anonymity depend on the
+  continued existence of the Tor network to do so; running a node helps to
+  keep the network operational.
+%\item Local Tor entry and exit nodes allow users on a network to run in an
+%  `enclave' configuration.  [XXXX need to resolve this. They would do this
+%   for E2E encryption + auth?]
+
+
+%We must try to make the costs of running a Tor node easily minimized.
+Since Tor is run by volunteers, the most crucial software usability issue is
+usability by operators: when an operator leaves, the network becomes less
+usable by everybody.  To keep operators pleased, we must try to keep Tor's
+resource and administrative demands as low as possible.
+
+Because of ISP billing structures, many Tor operators have underused capacity
+that they are willing to donate to the network, at no additional monetary
+cost to them.  Features to limit bandwidth have been essential to adoption.
+Also useful has been a ``hibernation'' feature that allows a Tor node that
+wants to provide high bandwidth, but no more than a certain amount in a
+giving billing cycle, to become dormant once its bandwidth is exhausted, and
+to reawaken at a random offset into the next billing cycle.  This feature has
+interesting policy implications, however; see
+the next section below.
+Exit policies help to limit administrative costs by limiting the frequency of
+abuse complaints (see Section~\ref{subsec:tor-and-blacklists}).
+% We discuss
+%technical incentive mechanisms in Section~\ref{subsec:incentives-by-design}.
+
+%[XXXX say more.  Why else would you run a node? What else can we do/do we
+%  already do to make running a node more attractive?]
+%[We can enforce incentives; see Section 6.1. We can rate-limit clients.
+%  We can put "top bandwidth nodes lists" up a la seti@home.]
+
+\workingnote{
+\subsection{Bandwidth and file-sharing}
+\label{subsec:bandwidth-and-file-sharing}
+%One potentially problematical area with deploying Tor has been our response
+%to file-sharing applications.
+Once users have configured their applications to work with Tor, the largest
+remaining usability issue is performance.  Users begin to suffer
+when websites ``feel slow.''
+Clients currently try to build their connections through nodes that they
+guess will have enough bandwidth.  But even if capacity is allocated
+optimally, it seems unlikely that the current network architecture will have
+enough capacity to provide every user with as much bandwidth as she would
+receive if she weren't using Tor, unless far more nodes join the network.
+
+%Limited capacity does not destroy the network, however.  Instead, usage tends
+%towards an equilibrium: when performance suffers, users who value performance
+%over anonymity tend to leave the system, thus freeing capacity until the
+%remaining users on the network are exactly those willing to use that capacity
+%there is.
+
+Much of Tor's recent bandwidth difficulties have come from file-sharing
+applications.  These applications provide two challenges to
+any anonymizing network: their intensive bandwidth requirement, and the
+degree to which they are associated (correctly or not) with copyright
+infringement.
+
+High-bandwidth protocols can make the network unresponsive,
+but tend to be somewhat self-correcting as lack of bandwidth drives away
+users who need it.  Issues of copyright violation,
+however, are more interesting.  Typical exit node operators want to help
+people achieve private and anonymous speech, not to help people (say) host
+Vin Diesel movies for download; and typical ISPs would rather not
+deal with customers who draw menacing letters
+from the MPAA\@.  While it is quite likely that the operators are doing nothing
+illegal, many ISPs have policies of dropping users who get repeated legal
+threats regardless of the merits of those threats, and many operators would
+prefer to avoid receiving even meritless legal threats.
+So when letters arrive, operators are likely to face
+pressure to block file-sharing applications entirely, in order to avoid the
+hassle.
+
+But blocking file-sharing is not easy: popular
+protocols have evolved to run on non-standard ports to
+get around other port-based bans.  Thus, exit node operators who want to
+block file-sharing would have to find some way to integrate Tor with a
+protocol-aware exit filter.  This could be a technically expensive
+undertaking, and one with poor prospects: it is unlikely that Tor exit nodes
+would succeed where so many institutional firewalls have failed.  Another
+possibility for sensitive operators is to run a restrictive node that
+only permits exit connections to a restricted range of ports that are
+not frequently associated with file sharing.  There are increasingly few such
+ports.
+
+Other possible approaches might include rate-limiting connections, especially
+long-lived connections or connections to file-sharing ports, so that
+high-bandwidth connections do not flood the network.  We might also want to
+give priority to cells on low-bandwidth connections to keep them interactive,
+but this could have negative anonymity implications.
+
+For the moment, it seems that Tor's bandwidth issues have rendered it
+unattractive for bulk file-sharing traffic; this may continue to be so in the
+future.  Nevertheless, Tor will likely remain attractive for limited use in
+file-sharing protocols that have separate control and data channels.
+
+%[We should say more -- but what?  That we'll see a similar
+%  equilibriating effect as with bandwidth, where sensitive ops switch to
+%  middleman, and we become less useful for file-sharing, so the file-sharing
+%  people back off, so we get more ops since there's less file-sharing, so the
+%  file-sharers come back, etc.]
+
+%XXXX
+%in practice, plausible deniability is hypothetical and doesn't seem very
+%convincing. if ISPs find the activity antisocial, they don't care *why*
+%your computer is doing that behavior.
+}
+
+\subsection{Tor and blacklists}
+\label{subsec:tor-and-blacklists}
+
+It was long 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.
+Our initial answer to this situation was to use ``exit policies''
+to allow individual Tor 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. Note that for spammers, Tor would be 
+a step back, a much less effective means of distributing spam than
+those currently available. This is thus primarily an unmistakable
+answer to those confused about Internet communication who might raise
+spam as an issue.
+
+Exit policies are useful, but they are 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. For the time being, blocking by IP address is
+an expedient strategy, even if it undermines Internet stability and
+functionality in the long run~\cite{netauth}
+
+This is potentially a bigger problem than it may appear.
+On the one hand, services should be allowed to refuse connections from
+sources of possible abuse.
+But when a Tor node administrator decides whether he prefers to be able
+to post to Wikipedia from his IP address, or to allow people to read
+Wikipedia anonymously through his Tor node, he is making the decision
+for others as well. (For a while, Wikipedia
+blocked all posting from all Tor nodes based on IP addresses.) If
+the Tor node shares an address with a campus or corporate NAT,
+then the decision can prevent the entire population from posting.
+Similarly, 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 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.
+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. Software to support pseudonymous access via Tor designed precisely
+to interact with Wikipedia's access mechanism has even been developed
+and proposed to Wikimedia by Jason Holt~\cite{nym}, but has not been taken up.
+
+
+Perhaps worse, many IP blacklists are coarse-grained: they ignore Tor's exit
+policies, partly because it's easier to implement and partly
+so they can punish
+all Tor nodes. One IP blacklist even bans
+every class C network that contains a Tor node, and recommends banning SMTP
+from these networks even though Tor does not allow SMTP at all.  This
+strategic decision aims to discourage the
+operation of anything resembling an open proxy by encouraging its neighbors
+to shut it down to get unblocked themselves. This pressure even
+affects Tor nodes running in middleman mode (disallowing all exits) when
+those nodes are blacklisted too.
+% Perception of Tor as an abuse vector
+%is also partly driven by multiple base-rate fallacies~\cite{axelsson00}.
+
+Problems of abuse occur mainly with services such as IRC networks and
+Wikipedia, which rely on IP blocking to ban abusive users.  While at first
+blush this practice might seem to depend on the anachronistic assumption that
+each IP is an identifier for a single user, it is actually more reasonable in
+practice: it assumes that non-proxy IPs are a costly resource, and that an
+abuser can not change IPs at will.  By blocking IPs which are used by Tor
+nodes, open proxies, and service abusers, these systems hope to make
+ongoing abuse difficult.  Although the system is imperfect, it works
+tolerably well for them in practice.
+
+Of course, we would prefer that legitimate anonymous users be able to
+access abuse-prone services.  One conceivable approach would require
+would-be IRC users, for instance, to register accounts if they want to
+access the IRC network from Tor.  In practice this would not
+significantly impede abuse if creating new accounts were easily automatable;
+this is why services use IP blocking.  To deter abuse, pseudonymous
+identities need to require a significant switching cost in resources or human
+time.  Some popular webmail applications
+impose cost with Reverse Turing Tests, but this step may not deter all
+abusers.  Freedom used blind signatures to limit
+the number of pseudonyms for each paying account, but Tor has neither the
+ability nor the desire to collect payment.
+
+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 IPs as ``anonymous users,''
+removing the ability of the abusers to blend in, the abuse stopped.
+This is an illustration of how simple technical mechanisms can remove
+the ability to abuse anonymously without undermining the ability
+to communicate anonymous and can thus remove the incentive to attempt
+abusing in this way.
+
+%The use of squishy IP-based ``authentication'' and ``authorization''
+%has not broken down even to the level that SSNs used for these
+%purposes have in commercial and public record contexts. Externalities
+%and misplaced incentives cause a continued focus on fighting identity
+%theft by protecting SSNs rather than developing better authentication
+%and incentive schemes \cite{price-privacy}. Similarly we can expect a
+%continued use of identification by IP number as long as there is no
+%workable alternative.
+
+%[XXX Mention correct DNS-RBL implementation. -NM]
+
+\workingnote{
+\section{Design choices}
+
+In addition to social issues, Tor also faces some design trade-offs that must
+be investigated as the network develops.
+
+\subsection{Transporting the stream vs transporting the packets}
+\label{subsec:stream-vs-packet}
+\label{subsec:tcp-vs-ip}
+
+Tor transports streams; it does not tunnel packets.
+It has often been suggested that like the old Freedom
+network~\cite{freedom21-security}, Tor should
+``obviously'' anonymize IP traffic
+at the IP layer. Before this could be done, many issues need to be resolved:
+
+\begin{enumerate}
+\setlength{\itemsep}{0mm}
+\setlength{\parsep}{0mm}
+\item \emph{IP packets reveal OS characteristics.}  We would still need to do
+IP-level packet normalization, to stop things like TCP fingerprinting
+attacks. %There likely exist libraries that can help with this.
+This is unlikely to be a trivial task, given the diversity and complexity of
+TCP stacks.
+\item \emph{Application-level streams still need scrubbing.} We still need
+Tor to be easy to integrate with user-level application-specific proxies
+such as Privoxy. So it's not just a matter of capturing packets and
+anonymizing them at the IP layer.
+\item \emph{Certain protocols will still leak information.} For example, we
+must rewrite DNS requests so they are delivered to an unlinkable DNS server
+rather than the DNS server at a user's ISP; thus, we must understand the
+protocols we are transporting.
+\item \emph{The crypto is unspecified.} First we need a block-level encryption
+approach that can provide security despite
+packet loss and out-of-order delivery. Freedom allegedly had one, but it was
+never publicly specified.
+Also, TLS over UDP is not yet implemented or
+specified, though some early work has begun~\cite{dtls}.
+\item \emph{We'll still need to tune network parameters.} Since the above
+encryption system will likely need sequence numbers (and maybe more) to do
+replay detection, handle duplicate frames, and so on, we will be reimplementing
+a subset of TCP anyway---a notoriously tricky path.
+\item \emph{Exit policies for arbitrary IP packets mean building a secure
+IDS\@.}  Our node operators tell us that exit policies are one of
+the main reasons they're willing to run Tor.
+Adding an Intrusion Detection System to handle exit policies would
+increase the security complexity of Tor, and would likely not work anyway,
+as evidenced by the entire field of IDS and counter-IDS papers. Many
+potential abuse issues are resolved by the fact that Tor only transports
+valid TCP streams (as opposed to arbitrary IP including malformed packets
+and IP floods), so exit policies become even \emph{more} important as
+we become able to transport IP packets. We also need to compactly
+describe exit policies so clients can predict
+which nodes will allow which packets to exit.
+\item \emph{The Tor-internal name spaces would need to be redesigned.} We
+support hidden service {\tt{.onion}} addresses (and other special addresses,
+like {\tt{.exit}} which lets the user request a particular exit node),
+by intercepting the addresses when they are passed to the Tor client.
+Doing so at the IP level would require a more complex interface between
+Tor and the local DNS resolver.
+\end{enumerate}
+
+This list is discouragingly long, but being able to transport more
+protocols obviously has some advantages. It would be good to learn which
+items are actual roadblocks and which are easier to resolve than we think.
+
+To be fair, Tor's stream-based approach has run into
+stumbling blocks as well. While Tor supports the SOCKS protocol,
+which provides a standardized interface for generic TCP proxies, many
+applications do not support SOCKS\@. For them we already need to
+replace the networking system calls with SOCKS-aware
+versions, or run a SOCKS tunnel locally, neither of which is
+easy for the average user. %---even with good instructions.
+Even when applications can use SOCKS, they often make DNS requests
+themselves before handing an IP address to Tor, which advertises
+where the user is about to connect.
+We are still working on more usable solutions.
+
+%So to actually provide good anonymity, we need to make sure that
+%users have a practical way to use Tor anonymously.  Possibilities include
+%writing wrappers for applications to anonymize them automatically; improving
+%the applications' support for SOCKS; writing libraries to help application
+%writers use Tor properly; and implementing a local DNS proxy to reroute DNS
+%requests to Tor so that applications can simply point their DNS resolvers at
+%localhost and continue to use SOCKS for data only.
+
+\subsection{Mid-latency}
+\label{subsec:mid-latency}
+
+Some users need to resist traffic correlation attacks.  Higher-latency
+mix-networks introduce variability into message
+arrival times: as timing variance increases, timing correlation attacks
+require increasingly more data~\cite{e2e-traffic}. Can we improve Tor's
+resistance without losing too much usability?
+
+We need to learn whether we can trade a small increase in latency
+for a large anonymity increase, or if we'd end up trading a lot of
+latency for only a minimal security gain. A trade-off might be worthwhile
+even if we
+could only protect certain use cases, such as infrequent short-duration
+transactions. % To answer this question
+We might adapt the techniques of~\cite{e2e-traffic} to a lower-latency mix
+network, where the messages are batches of cells in temporally clustered
+connections. These large fixed-size batches can also help resist volume
+signature attacks~\cite{hintz-pet02}. We could also experiment with traffic
+shaping to get a good balance of throughput and security.
+%Other padding regimens might supplement the
+%mid-latency option; however, we should continue the caution with which
+%we have always approached padding lest the overhead cost us too much
+%performance or too many volunteers.
+
+We must keep usability in mind too. How much can latency increase
+before we drive users away? We've already been forced to increase
+latency slightly, as our growing network incorporates more DSL and
+cable-modem nodes and more nodes in distant continents. Perhaps we can
+harness this increased latency to improve anonymity rather than just
+reduce usability. Further, if we let clients label certain circuits as
+mid-latency as they are constructed, we could handle both types of traffic
+on the same network, giving users a choice between speed and security---and
+giving researchers a chance to experiment with parameters to improve the
+quality of those choices.
+
+\subsection{Enclaves and helper nodes}
+\label{subsec:helper-nodes}
+
+It has long been thought that users can improve their anonymity by
+running their own node~\cite{tor-design,or-ih96,or-pet00}, and using
+it in an \emph{enclave} configuration, where all their circuits begin
+at the node under their control. Running Tor clients or servers at
+the enclave perimeter is useful when policy or other requirements
+prevent individual machines within the enclave from running Tor
+clients~\cite{or-jsac98,or-discex00}.
+
+Of course, Tor's default path length of
+three is insufficient for these enclaves, since the entry and/or exit
+% [edit war: without the ``and/'' the natural reading here
+% is aut rather than vel. And the use of the plural verb does not work -pfs]
+themselves are sensitive. Tor thus increments path length by one
+for each sensitive endpoint in the circuit.
+Enclaves also help to protect against end-to-end attacks, since it's
+possible that traffic coming from the node has simply been relayed from
+elsewhere. However, if the node has recognizable behavior patterns,
+an attacker who runs nodes in the network can triangulate over time to
+gain confidence that it is in fact originating the traffic. Wright et
+al.~\cite{wright03} introduce the notion of a \emph{helper node}---a
+single fixed entry node for each user---to combat this \emph{predecessor
+attack}.
+
+However, the attack in~\cite{attack-tor-oak05} shows that simply adding
+to the path length, or using a helper node, may not protect an enclave
+node. A hostile web server can send constant interference traffic to
+all nodes in the network, and learn which nodes are involved in the
+circuit (though at least in the current attack, he can't learn their
+order). Using randomized path lengths may help some, since the attacker
+will never be certain he has identified all nodes in the path unless
+he probes the entire network, but as
+long as the network remains small this attack will still be feasible.
+
+Helper nodes also aim to help Tor clients, because choosing entry and exit
+points
+randomly and changing them frequently allows an attacker who controls
+even a few nodes to eventually link some of their destinations. The goal
+is to take the risk once and for all about choosing a bad entry node,
+rather than taking a new risk for each new circuit. (Choosing fixed
+exit nodes is less useful, since even an honest exit node still doesn't
+protect against a hostile website.) But obstacles remain before
+we can implement helper nodes.
+For one, the literature does not describe how to choose helpers from a list
+of nodes that changes over time.  If Alice is forced to choose a new entry
+helper every $d$ days and $c$ of the $n$ nodes are bad, she can expect
+to choose a compromised node around
+every $dc/n$ days. Statistically over time this approach only helps
+if she is better at choosing honest helper nodes than at choosing
+honest nodes.  Worse, an attacker with the ability to DoS nodes could
+force users to switch helper nodes more frequently, or remove
+other candidate helpers.
+
+%Do general DoS attacks have anonymity implications? See e.g. Adam
+%Back's IH paper, but I think there's more to be pointed out here. -RD
+% Not sure what you want to say here. -NM
+
+%Game theory for helper nodes: if Alice offers a hidden service on a
+%server (enclave model), and nobody ever uses helper nodes, then against
+%George+Steven's attack she's totally nailed. If only Alice uses a helper
+%node, then she's still identified as the source of the data. If everybody
+%uses a helper node (including Alice), then the attack identifies the
+%helper node and also Alice, and knows which one is which. If everybody
+%uses a helper node (but not Alice), then the attacker figures the real
+%source was a client that is using Alice as a helper node. [How's my
+%logic here?] -RD
+%
+% Not sure about the logic.  For the attack to work with helper nodes, the
+%attacker needs to guess that Alice is running the hidden service, right?
+%Otherwise, how can he know to measure her traffic specifically? -NM
+%
+% In the Murdoch-Danezis attack, the adversary measures all servers. -RD
+
+%point to routing-zones section re: helper nodes to defend against
+%big stuff.
+
+\subsection{Location-hidden services}
+\label{subsec:hidden-services}
+
+% This section is first up against the wall when the revolution comes.
+
+Tor's \emph{rendezvous points}
+let users provide TCP services to other Tor users without revealing
+the service's location. Since this feature is relatively recent, we describe
+here
+a couple of our early observations from its deployment.
+
+First, our implementation of hidden services seems less hidden than we'd
+like, since they build a different rendezvous circuit for each user,
+and an external adversary can induce them to
+produce traffic. This insecurity means that they may not be suitable as
+a building block for Free Haven~\cite{freehaven-berk} or other anonymous
+publishing systems that aim to provide long-term security, though helper
+nodes, as discussed above, would seem to help.
+
+\emph{Hot-swap} hidden services, where more than one location can
+provide the service and loss of any one location does not imply a
+change in service, would help foil intersection and observation attacks
+where an adversary monitors availability of a hidden service and also
+monitors whether certain users or servers are online. The design
+challenges in providing such services without otherwise compromising
+the hidden service's anonymity remain an open problem;
+however, see~\cite{move-ndss05}.
+
+In practice, hidden services are used for more than just providing private
+access to a web server or IRC server. People are using hidden services
+as a poor man's VPN and firewall-buster. Many people want to be able
+to connect to the computers in their private network via secure shell,
+and rather than playing with dyndns and trying to pierce holes in their
+firewall, they run a hidden service on the inside and then rendezvous
+with that hidden service externally.
+
+News sites like Bloggers Without Borders (www.b19s.org) are advertising
+a hidden-service address on their front page. Doing this can provide
+increased robustness if they use the dual-IP approach we describe
+in~\cite{tor-design},
+but in practice they do it to increase visibility
+of the Tor project and their support for privacy, and to offer
+a way for their users, using unmodified software, to get end-to-end
+encryption and authentication to their website.
+
+\subsection{Location diversity and ISP-class adversaries}
+\label{subsec:routing-zones}
+
+Anonymity networks have long relied on diversity of node location for
+protection against attacks---typically an adversary who can observe a
+larger fraction of the network can launch a more effective attack. One
+way to achieve dispersal involves growing the network so a given adversary
+sees less. Alternately, we can arrange the topology so traffic can enter
+or exit at many places (for example, by using a free-route network
+like Tor rather than a cascade network like JAP). Lastly, we can use
+distributed trust to spread each transaction over multiple jurisdictions.
+But how do we decide whether two nodes are in related locations?
+
+Feamster and Dingledine defined a \emph{location diversity} metric
+in~\cite{feamster:wpes2004}, and began investigating a variant of location
+diversity based on the fact that the Internet is divided into thousands of
+independently operated networks called {\em autonomous systems} (ASes).
+The key insight from their paper is that while we typically think of a
+connection as going directly from the Tor client to the first Tor node,
+actually it traverses many different ASes on each hop. An adversary at
+any of these ASes can monitor or influence traffic. Specifically, given
+plausible initiators and recipients, and given random path selection,
+some ASes in the simulation were able to observe 10\% to 30\% of the
+transactions (that is, learn both the origin and the destination) on
+the deployed Tor network (33 nodes as of June 2004).
+
+The paper concludes that for best protection against the AS-level
+adversary, nodes should be in ASes that have the most links to other ASes:
+Tier-1 ISPs such as AT\&T and Abovenet. Further, a given transaction
+is safest when it starts or ends in a Tier-1 ISP\@. Therefore, assuming
+initiator and responder are both in the U.S., it actually \emph{hurts}
+our location diversity to use far-flung nodes in
+continents like Asia or South America.
+% it's not just entering or exiting from them. using them as the middle
+% hop reduces your effective path length, which you presumably don't
+% want because you chose that path length for a reason.
+%
+% Not sure I buy that argument. Two end nodes in the right ASs to
+% discourage linking are still not known to each other. If some
+% adversary in a single AS can bridge the middle node, it shouldn't
+% therefore be able to identify initiator or responder; although it could
+% contribute to further attacks given more assumptions.
+% Nonetheless, no change to the actual text for now.
+
+Many open questions remain. First, it will be an immense engineering
+challenge to get an entire BGP routing table to each Tor client, or to
+summarize it sufficiently. Without a local copy, clients won't be
+able to safely predict what ASes will be traversed on the various paths
+through the Tor network to the final destination. Tarzan~\cite{tarzan:ccs02}
+and MorphMix~\cite{morphmix:fc04} suggest that we compare IP prefixes to
+determine location diversity; but the above paper showed that in practice
+many of the Mixmaster nodes that share a single AS have entirely different
+IP prefixes. When the network has scaled to thousands of nodes, does IP
+prefix comparison become a more useful approximation? % Alternatively, can
+%relevant parts of the routing tables be summarized centrally and delivered to
+%clients in a less verbose format?
+%% i already said "or to summarize is sufficiently" above. is that not
+%% enough? -RD
+%
+Second, we can take advantage of caching certain content at the
+exit nodes, to limit the number of requests that need to leave the
+network at all. What about taking advantage of caches like Akamai or
+Google~\cite{shsm03}? (Note that they're also well-positioned as global
+adversaries.)
+%
+Third, if we follow the recommendations in~\cite{feamster:wpes2004}
+ and tailor path selection
+to avoid choosing endpoints in similar locations, how much are we hurting
+anonymity against larger real-world adversaries who can take advantage
+of knowing our algorithm?
+%
+Fourth, can we use this knowledge to figure out which gaps in our network
+most affect our robustness to this class of attack, and go recruit
+new nodes with those ASes in mind?
+
+%Tor's security relies in large part on the dispersal properties of its
+%network. We need to be more aware of the anonymity properties of various
+%approaches so we can make better design decisions in the future.
+
+\subsection{The Anti-censorship problem}
+\label{subsec:china}
+
+Citizens in a variety of countries, such as most recently China and
+Iran, are blocked from accessing various sites outside
+their country. These users try to find any tools available to allow
+them to get-around these firewalls. Some anonymity networks, such as
+Six-Four~\cite{six-four}, are designed specifically with this goal in
+mind; others like the Anonymizer~\cite{anonymizer} are paid by sponsors
+such as Voice of America to encourage Internet
+freedom. Even though Tor wasn't
+designed with ubiquitous access to the network in mind, thousands of
+users across the world are now using it for exactly this purpose.
+% Academic and NGO organizations, peacefire, \cite{berkman}, etc
+
+Anti-censorship networks hoping to bridge country-level blocks face
+a variety of challenges. One of these is that they need to find enough
+exit nodes---servers on the `free' side that are willing to relay
+traffic from users to their final destinations. Anonymizing
+networks like Tor are well-suited to this task since we have
+already gathered a set of exit nodes that are willing to tolerate some
+political heat.
+
+The other main challenge is to distribute a list of reachable relays
+to the users inside the country, and give them software to use those relays,
+without letting the censors also enumerate this list and block each
+relay. Anonymizer solves this by buying lots of seemingly-unrelated IP
+addresses (or having them donated), abandoning old addresses as they are
+`used up,' and telling a few users about the new ones. Distributed
+anonymizing networks again have an advantage here, in that we already
+have tens of thousands of separate IP addresses whose users might
+volunteer to provide this service since they've already installed and use
+the software for their own privacy~\cite{koepsell:wpes2004}. Because
+the Tor protocol separates routing from network discovery \cite{tor-design},
+volunteers could configure their Tor clients
+to generate node descriptors and send them to a special directory
+server that gives them out to dissidents who need to get around blocks.
+
+Of course, this still doesn't prevent the adversary
+from enumerating and preemptively blocking the volunteer relays.
+Perhaps a tiered-trust system could be built where a few individuals are
+given relays' locations. They could then recommend other individuals
+by telling them
+those addresses, thus providing a built-in incentive to avoid letting the
+adversary intercept them. Max-flow trust algorithms~\cite{advogato}
+might help to bound the number of IP addresses leaked to the adversary. Groups
+like the W3C are looking into using Tor as a component in an overall system to
+help address censorship; we wish them success.
+
+%\cite{infranet}
+
+
+\section{Scaling}
+\label{sec:scaling}
+
+Tor is running today with hundreds of nodes and hundreds of thousands of
+users, but it will certainly not scale to millions.
+Scaling Tor involves four main challenges. First, to get a
+large set of nodes, we must address incentives for
+users to carry traffic for others. Next is safe node discovery, both
+while bootstrapping (Tor clients must robustly find an initial
+node list) and later (Tor clients must learn about a fair sample
+of honest nodes and not let the adversary control circuits).
+We must also detect and handle node speed and reliability as the network
+becomes increasingly heterogeneous: since the speed and reliability
+of a circuit is limited by its worst link, we must learn to track and
+predict performance. Finally, we must stop assuming that all points on
+the network can connect to all other points.
+
+\subsection{Incentives by Design}
+\label{subsec:incentives-by-design}
+
+There are three behaviors we need to encourage for each Tor node: relaying
+traffic; providing good throughput and reliability while doing it;
+and allowing traffic to exit the network from that node.
+
+We encourage these behaviors through \emph{indirect} incentives: that
+is, by designing the system and educating users in such a way that users
+with certain goals will choose to relay traffic.  One
+main incentive for running a Tor node is social: volunteers
+altruistically donate their bandwidth and time.  We encourage this with
+public rankings of the throughput and reliability of nodes, much like
+seti@home.  We further explain to users that they can get
+deniability for any traffic emerging from the same address as a Tor
+exit node, and they can use their own Tor node
+as an entry or exit point with confidence that it's not run by an adversary.
+Further, users may run a node simply because they need such a network
+to be persistently available and usable, and the value of supporting this
+exceeds any countervening costs.
+Finally, we can encourage operators by improving the usability and feature
+set of the software:
+rate limiting support and easy packaging decrease the hassle of
+maintaining a node, and our configurable exit policies allow each
+operator to advertise a policy describing the hosts and ports to which
+he feels comfortable connecting.
+
+To date these incentives appear to have been adequate. As the system scales
+or as new issues emerge, however, we may also need to provide
+ \emph{direct} incentives:
+providing payment or other resources in return for high-quality service.
+Paying actual money is problematic: decentralized e-cash systems are
+not yet practical, and a centralized collection system not only reduces
+robustness, but also has failed in the past (the history of commercial
+anonymizing networks is littered with failed attempts).  A more promising
+option is to use a tit-for-tat incentive scheme, where nodes provide better
+service to nodes that have provided good service for them.
+
+Unfortunately, such an approach introduces new anonymity problems.
+There are many surprising ways for nodes to game the incentive and
+reputation system to undermine anonymity---such systems are typically
+designed to encourage fairness in storage or bandwidth usage, not
+fairness of provided anonymity. An adversary can attract more traffic
+by performing well or can target individual users by selectively
+performing, to undermine their anonymity. Typically a user who
+chooses evenly from all nodes is most resistant to an adversary
+targeting him, but that approach hampers the efficient use
+of heterogeneous nodes.
+
+%When a node (call him Steve) performs well for Alice, does Steve gain
+%reputation with the entire system, or just with Alice? If the entire
+%system, how does Alice tell everybody about her experience in a way that
+%prevents her from lying about it yet still protects her identity? If
+%Steve's behavior only affects Alice's behavior, does this allow Steve to
+%selectively perform only for Alice, and then break her anonymity later
+%when somebody (presumably Alice) routes through his node?
+
+A possible solution is a simplified approach to the tit-for-tat
+incentive scheme based on two rules: (1) each node should measure the
+service it receives from adjacent nodes, and provide service relative
+to the received service, but (2) when a node is making decisions that
+affect its own security (such as building a circuit for its own
+application connections), it should choose evenly from a sufficiently
+large set of nodes that meet some minimum service
+threshold~\cite{casc-rep}.  This approach allows us to discourage
+bad service
+without opening Alice up as much to attacks.  All of this requires
+further study.
+
+
+\subsection{Trust and discovery}
+\label{subsec:trust-and-discovery}
+
+The published Tor design is deliberately simplistic in how
+new nodes are authorized and how clients are informed about Tor
+nodes and their status.
+All nodes periodically upload a signed description
+of their locations, keys, and capabilities to each of several well-known {\it
+  directory servers}.  These directory servers construct a signed summary
+of all known Tor nodes (a ``directory''), and a signed statement of which
+nodes they
+believe to be operational then (a ``network status'').  Clients
+periodically download a directory to learn the latest nodes and
+keys, and more frequently download a network status to learn which nodes are
+likely to be running.  Tor nodes also operate as directory caches, to
+lighten the bandwidth on the directory servers.
+
+To prevent Sybil attacks (wherein an adversary signs up many
+purportedly independent nodes to increase her network view),
+this design
+requires the directory server operators to manually
+approve new nodes.  Unapproved nodes are included in the directory,
+but clients
+do not use them at the start or end of their circuits.  In practice,
+directory administrators perform little actual verification, and tend to
+approve any Tor node whose operator can compose a coherent email.
+This procedure
+may prevent trivial automated Sybil attacks, but will do little
+against a clever and determined attacker.
+
+There are a number of flaws in this system that need to be addressed as we
+move forward. First,
+each directory server represents an independent point of failure: any
+compromised directory server could start recommending only compromised
+nodes.
+Second, as more nodes join the network, %the more unreasonable it
+%becomes to expect clients to know about them all.
+directories
+become infeasibly large, and downloading the list of nodes becomes
+burdensome.
+Third, the validation scheme may do as much harm as it does good.  It 
+does not prevent clever attackers from mounting Sybil attacks,
+and it may deter node operators from joining the network---if
+they expect the validation process to be difficult, or they do not share
+any languages in common with the directory server operators.
+
+We could try to move the system in several directions, depending on our
+choice of threat model and requirements.  If we did not need to increase
+network capacity to support more users, we could simply
+ adopt even stricter validation requirements, and reduce the number of
+nodes in the network to a trusted minimum.  
+But, we can only do that if can simultaneously make node capacity
+scale much more than we anticipate to be feasible soon, and if we can find
+entities willing to run such nodes, an equally daunting prospect.
+
+In order to address the first two issues, it seems wise to move to a system
+including a number of semi-trusted directory servers, no one of which can
+compromise a user on its own.  Ultimately, of course, we cannot escape the
+problem of a first introducer: since most users will run Tor in whatever
+configuration the software ships with, the Tor distribution itself will
+remain a single point of failure so long as it includes the seed
+keys for directory servers, a list of directory servers, or any other means
+to learn which nodes are on the network.  But omitting this information
+from the Tor distribution would only delegate the trust problem to each
+individual user. %, most of whom are presumably less informed about how to make
+%trust decisions than the Tor developers.
+A well publicized, widely available, authoritatively and independently
+endorsed and signed list of initial directory servers and their keys
+is a possible solution. But, setting that up properly is itself a large 
+bootstrapping task.
+
+%Network discovery, sybil, node admission, scaling. It seems that the code
+%will ship with something and that's our trust root. We could try to get
+%people to build a web of trust, but no. Where we go from here depends
+%on what threats we have in mind. Really decentralized if your threat is
+%RIAA; less so if threat is to application data or individuals or...
+
+
+\subsection{Measuring performance and capacity}
+\label{subsec:performance}
+
+One of the paradoxes with engineering an anonymity network is that we'd like
+to learn as much as we can about how traffic flows so we can improve the
+network, but we want to prevent others from learning how traffic flows in
+order to trace users' connections through the network.  Furthermore, many
+mechanisms that help Tor run efficiently
+require measurements about the network.
+
+Currently, nodes try to deduce their own available bandwidth (based on how
+much traffic they have been able to transfer recently) and include this
+information in the descriptors they upload to the directory. Clients
+choose servers weighted by their bandwidth, neglecting really slow
+servers and capping the influence of really fast ones.
+%
+This is, of course, eminently cheatable.  A malicious node can get a
+disproportionate amount of traffic simply by claiming to have more bandwidth
+than it does.  But better mechanisms have their problems.  If bandwidth data
+is to be measured rather than self-reported, it is usually possible for
+nodes to selectively provide better service for the measuring party, or
+sabotage the measured value of other nodes.  Complex solutions for
+mix networks have been proposed, but do not address the issues
+completely~\cite{mix-acc,casc-rep}.
+
+Even with no cheating, network measurement is complex.  It is common
+for views of a node's latency and/or bandwidth to vary wildly between
+observers.  Further, it is unclear whether total bandwidth is really
+the right measure; perhaps clients should instead be considering nodes
+based on unused bandwidth or observed throughput.
+%How to measure performance without letting people selectively deny service
+%by distinguishing pings. Heck, just how to measure performance at all. In
+%practice people have funny firewalls that don't match up to their exit
+%policies and Tor doesn't deal.
+%
+%Network investigation: Is all this bandwidth publishing thing a good idea?
+%How can we collect stats better? Note weasel's smokeping, at
+%http://seppia.noreply.org/cgi-bin/smokeping.cgi?target=Tor
+%which probably gives george and steven enough info to break tor?
+%
+And even if we can collect and use this network information effectively,
+we must ensure
+that it is not more useful to attackers than to us.  While it
+seems plausible that bandwidth data alone is not enough to reveal
+sender-recipient connections under most circumstances, it could certainly
+reveal the path taken by large traffic flows under low-usage circumstances.
+
+\subsection{Non-clique topologies}
+
+Tor's comparatively weak threat model may allow easier scaling than
+other
+designs.  High-latency mix networks need to avoid partitioning attacks, where
+network splits let an attacker distinguish users in different partitions.
+Since Tor assumes the adversary cannot cheaply observe nodes at will,
+a network split may not decrease protection much.
+Thus, one option when the scale of a Tor network
+exceeds some size is simply to split it. Nodes could be allocated into
+partitions while hampering collaborating hostile nodes from taking over
+a single partition~\cite{casc-rep}.
+Clients could switch between
+networks, even on a per-circuit basis.
+%Future analysis may uncover
+%other dangers beyond those affecting mix-nets.
+
+More conservatively, we can try to scale a single Tor network. Likely
+problems with adding more servers to a single Tor network include an
+explosion in the number of sockets needed on each server as more servers
+join, and increased coordination overhead to keep each users' view of
+the network consistent. As we grow, we will also have more instances of
+servers that can't reach each other simply due to Internet topology or
+routing problems.
+
+%include restricting the number of sockets and the amount of bandwidth
+%used by each node.  The number of sockets is determined by the network's
+%connectivity and the number of users, while bandwidth capacity is determined
+%by the total bandwidth of nodes on the network.  The simplest solution to
+%bandwidth capacity is to add more nodes, since adding a Tor node of any
+%feasible bandwidth will increase the traffic capacity of the network.  So as
+%a first step to scaling, we should focus on making the network tolerate more
+%nodes, by reducing the interconnectivity of the nodes; later we can reduce
+%overhead associated with directories, discovery, and so on.
+
+We can address these points by reducing the network's connectivity.
+Danezis~\cite{danezis:pet2003} considers
+the anonymity implications of restricting routes on mix networks and
+recommends an approach based on expander graphs (where any subgraph is likely
+to have many neighbors).  It is not immediately clear that this approach will
+extend to Tor, which has a weaker threat model but higher performance
+requirements: instead of analyzing the
+probability of an attacker's viewing whole paths, we will need to examine the
+attacker's likelihood of compromising the endpoints.
+%
+Tor may not need an expander graph per se: it
+may be enough to have a single central subnet that is highly connected, like
+an Internet backbone. %  As an
+%example, assume fifty nodes of relatively high traffic capacity.  This
+%\emph{center} forms a clique.  Assume each center node can
+%handle 200 connections to other nodes (including the other ones in the
+%center). Assume every noncenter node connects to three nodes in the
+%center and anyone out of the center that they want to.  Then the
+%network easily scales to c. 2500 nodes with commensurate increase in
+%bandwidth.
+There are many open questions: how to distribute connectivity information
+(presumably nodes will learn about the central nodes
+when they download Tor), whether central nodes
+will need to function as a `backbone', and so on. As above,
+this could reduce the amount of anonymity available from a mix-net,
+but for a low-latency network where anonymity derives largely from
+the edges, it may be feasible.
+
+%In a sense, Tor already has a non-clique topology.
+%Individuals can set up and run Tor nodes without informing the
+%directory servers. This allows groups to run a
+%local Tor network of private nodes that connects to the public Tor
+%network. This network is hidden behind the Tor network, and its
+%only visible connection to Tor is at those points where it connects.
+%As far as the public network, or anyone observing it, is concerned,
+%they are running clients.
+}
+
+\section{The Future}
+\label{sec:conclusion}
+
+Tor is the largest and most diverse low-latency anonymity network
+available, but we are still in the beginning stages of deployment. Several
+major questions remain.
+
+First, will our volunteer-based approach to sustainability work in the
+long term? As we add more features and destabilize the network, the
+developers spend a lot of time keeping the server operators happy. Even
+though Tor is free software, the network would likely stagnate and die at
+this stage if the developers stopped actively working on it. We may get
+an unexpected boon from the fact that we're a general-purpose overlay
+network: as Tor grows more popular, other groups who need an overlay
+network on the Internet are starting to adapt Tor to their needs.
+%
+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 gain 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 does not scale even to handle current user demand. We must
+find designs and incentives to let some clients relay traffic too, without
+sacrificing too much anonymity.
+
+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.
+
+\bibliographystyle{plain} \bibliography{tor-design}
+
+\end{document}
+
+\clearpage
+\appendix
+
+\begin{figure}[t]
+%\unitlength=1in
+\centering
+%\begin{picture}(6.0,2.0)
+%\put(3,1){\makebox(0,0)[c]{\epsfig{figure=graphnodes,width=6in}}}
+%\end{picture}
+\mbox{\epsfig{figure=graphnodes,width=5in}}
+\caption{Number of Tor nodes over time, through January 2005. Lowest
+line is number of exit
+nodes that allow connections to port 80. Middle line is total number of
+verified (registered) Tor nodes. The line above that represents nodes
+that are running but not yet registered.}
+\label{fig:graphnodes}
+\end{figure}
+
+\begin{figure}[t]
+\centering
+\mbox{\epsfig{figure=graphtraffic,width=5in}}
+\caption{The sum of traffic reported by each node over time, through
+January 2005. The bottom
+pair show average throughput, and the top pair represent the largest 15
+minute burst in each 4 hour period.}
+\label{fig:graphtraffic}
+\end{figure}
+
+
+
+%Making use of nodes with little bandwidth, or high latency/packet loss.
+
+%Running Tor nodes behind NATs, behind great-firewalls-of-China, etc.
+%Restricted routes. How to propagate to everybody the topology? BGP
+%style doesn't work because we don't want just *one* path. Point to
+%Geoff's stuff.
+