[tor-commits] [tech-reports/master] Add unchanged blocking tech report sources.

karsten at torproject.org karsten at torproject.org
Fri Aug 31 10:35:38 UTC 2012

commit 59a46d6edd7bea6df8fb5cb9779054340ac1a408
Author: Karsten Loesing <karsten.loesing at gmx.net>
Date:   Thu Aug 30 11:26:49 2012 +0200

    Add unchanged blocking tech report sources.
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+  \setlength{\itemsep}{0mm}
+    \setlength{\parsep}{0mm}
+    %  \setlength{\labelsep}{0mm}
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+\newcommand{\workingnote}[1]{}        % The version that hides the note.
+%\newcommand{\workingnote}[1]{(**#1)}   % makes the note visible.
+\title{Design of a blocking-resistant anonymity system\\Tor Project technical report, Nov 2006}
+%\author{Roger Dingledine\inst{1} \and Nick Mathewson\inst{1}}
+\author{Roger Dingledine \\ The Tor Project \\ arma at torproject.org \and
+Nick Mathewson \\ The Tor Project \\ nickm at torproject.org}
+Internet censorship is on the rise as websites around the world are
+increasingly blocked by government-level firewalls.  Although popular
+anonymizing networks like Tor were originally designed to keep attackers from
+tracing people's activities, many people are also using them to evade local
+censorship.  But if the censor simply denies access to the Tor network
+itself, blocked users can no longer benefit from the security Tor offers.
+Here we describe a design that builds upon the current Tor network
+to provide an anonymizing network that resists blocking
+by government-level attackers. % We have implemented and deployed this
+%design, and talk briefly about early use.
+Anonymizing networks like Tor~\cite{tor-design} bounce traffic around a
+network of encrypting relays.  Unlike encryption, which hides only {\it what}
+is said, these networks also aim to hide who is communicating with whom, which
+users are using which websites, and so on.  These systems have a
+broad range of users, including ordinary citizens who want to avoid being
+profiled for targeted advertisements, 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
+Historical anonymity research has focused on an
+attacker who monitors the user (call her Alice) and tries to discover her
+activities, yet lets her reach any piece of the network. In more modern
+threat models such as Tor's, the adversary is allowed to perform active
+attacks such as modifying communications to trick Alice
+into revealing her destination, or intercepting some connections
+to run a man-in-the-middle attack. But these systems still assume that
+Alice can eventually reach the anonymizing network.
+An increasing number of users are using the Tor software
+less for its anonymity properties than for its censorship
+resistance properties---if they use Tor to access Internet sites like
+and Blogspot, they are no longer affected by local censorship
+and firewall rules. In fact, an informal user study
+%(described in Appendix~\ref{app:geoip})
+showed that a few hundred thousand users people access the Tor network
+each day, with about 20\% of them coming from China. %~\cite{something}.
+The current Tor design is easy to block if the attacker controls Alice's
+connection to the Tor network---by blocking the directory authorities,
+by blocking all the relay IP addresses in the directory, or by filtering
+based on the network fingerprint of the Tor TLS handshake. Here we
+describe an
+extended design that builds upon the current Tor network to provide an
+network that resists censorship as well as anonymity-breaking attacks.
+In section~\ref{sec:adversary} we discuss our threat model---that is,
+the assumptions we make about our adversary. Section~\ref{sec:current-tor}
+describes the components of the current Tor design and how they can be
+leveraged for a new blocking-resistant design. Section~\ref{sec:related}
+explains the features and drawbacks of the currently deployed solutions.
+In sections~\ref{sec:bridges} through~\ref{sec:discovery}, we explore the
+components of our designs in detail.  Section~\ref{sec:security} considers
+security implications and Section~\ref{sec:reachability} presents other
+issues with maintaining connectivity and sustainability for the design.
+%Section~\ref{sec:future} speculates about future more complex designs,
+Finally section~\ref{sec:conclusion} summarizes our next steps and
+% The other motivation is for places where we're concerned they will
+% try to enumerate a list of Tor users. So even if they're not blocking
+% the Tor network, it may be smart to not be visible as connecting to it.
+%And adding more different classes of users and goals to the Tor network
+%improves the anonymity for all Tor users~\cite{econymics,usability:weis2006}.
+% Adding use classes for countering blocking as well as anonymity has
+% benefits too. Should add something about how providing undetected
+% access to Tor would facilitate people talking to, e.g., govt. authorities
+% about threats to public safety etc. in an environment where Tor use
+% is not otherwise widespread and would make one stand out.
+\section{Adversary assumptions}
+To design an effective anti-censorship tool, we need a good model for the
+goals and resources of the censors we are evading.  Otherwise, we risk
+spending our effort on keeping the adversaries from doing things they have no
+interest in doing, and thwarting techniques they do not use.
+The history of blocking-resistance designs is littered with conflicting
+assumptions about what adversaries to expect and what problems are
+in the critical path to a solution. Here we describe our best
+understanding of the current situation around the world.
+In the traditional security style, we aim to defeat a strong
+attacker---if we can defend against this attacker, we inherit protection
+against weaker attackers as well.  After all, we want a general design
+that will work for citizens of China, Thailand, and other censored
+countries; for
+whistleblowers in firewalled corporate networks; and for people in
+unanticipated oppressive situations. In fact, by designing with
+a variety of adversaries in mind, we can take advantage of the fact that
+adversaries will be in different stages of the arms race at each location,
+so an address blocked in one locale can still be useful in others.
+We focus on an attacker with somewhat complex goals:
+\item The attacker would like to restrict the flow of certain kinds of
+  information, particularly when this information is seen as embarrassing to
+  those in power (such as information about rights violations or corruption),
+  or when it enables or encourages others to oppose them effectively (such as
+  information about opposition movements or sites that are used to organize
+  protests).
+\item As a second-order effect, censors aim to chill citizens' behavior by
+  creating an impression that their online activities are monitored.
+\item In some cases, censors make a token attempt to block a few sites for
+  obscenity, blasphemy, and so on, but their efforts here are mainly for
+  show. In other cases, they really do try hard to block such content.
+\item Complete blocking (where nobody at all can ever download censored
+  content) is not a
+  goal. Attackers typically recognize that perfect censorship is not only
+  impossible, it is unnecessary: if ``undesirable'' information is known only
+  to a small few, further censoring efforts can be focused elsewhere.
+\item Similarly, the censors do not attempt to shut down or block {\it
+  every} anti-censorship tool---merely the tools that are popular and
+  effective (because these tools impede the censors' information restriction
+  goals) and those tools that are highly visible (thus making the censors
+  look ineffectual to their citizens and their bosses).
+\item Reprisal against {\it most} passive consumers of {\it most} kinds of
+  blocked information is also not a goal, given the broadness of most
+  censorship regimes. This seems borne out by fact.\footnote{So far in places
+  like China, the authorities mainly go after people who publish materials
+  and coordinate organized movements~\cite{mackinnon-personal}.
+  If they find that a
+  user happens to be reading a site that should be blocked, the typical
+  response is simply to block the site. Of course, even with an encrypted
+  connection, the adversary may be able to distinguish readers from
+  publishers by observing whether Alice is mostly downloading bytes or mostly
+  uploading them---we discuss this issue more in
+  Section~\ref{subsec:upload-padding}.}
+\item Producers and distributors of targeted information are in much
+  greater danger than consumers; the attacker would like to not only block
+  their work, but identify them for reprisal.
+\item The censors (or their governments) would like to have a working, useful
+  Internet. There are economic, political, and social factors that prevent
+  them from ``censoring'' the Internet by outlawing it entirely, or by
+  blocking access to all but a tiny list of sites.
+  Nevertheless, the censors {\it are} willing to block innocuous content
+  (like the bulk of a newspaper's reporting) in order to censor other content
+  distributed through the same channels (like that newspaper's coverage of
+  the censored country).
+We assume there are three main technical network attacks in use by censors
+\item Block a destination or type of traffic by automatically searching for
+  certain strings or patterns in TCP packets.  Offending packets can be
+  dropped, or can trigger a response like closing the
+  connection.
+\item Block certain IP addresses or destination ports at a
+  firewall or other routing control point.
+\item Intercept DNS requests and give bogus responses for certain
+  destination hostnames.
+We assume the network firewall has limited CPU and memory per
+connection~\cite{clayton:pet2006}.  Against an adversary who could carefully
+examine the contents of every packet and correlate the packets in every
+stream on the network, we would need some stronger mechanism such as
+steganography, which introduces its own
+problems~\cite{active-wardens,tcpstego}.  But we make a ``weak
+steganography'' assumption here: to remain unblocked, it is necessary to
+remain unobservable only by computational resources on par with a modern
+router, firewall, proxy, or IDS.
+We assume that while various different regimes can coordinate and share
+notes, there will be a time lag between one attacker learning how to overcome
+a facet of our design and other attackers picking it up.  (The most common
+vector of transmission seems to be commercial providers of censorship tools:
+once a provider adds a feature to meet one country's needs or requests, the
+feature is available to all of the provider's customers.)  Conversely, we
+assume that insider attacks become a higher risk only after the early stages
+of network development, once the system has reached a certain level of
+success and visibility.
+We do not assume that government-level attackers are always uniform
+across the country. For example, users of different ISPs in China
+experience different censorship policies and mechanisms. %~\cite{china-ccs07}.
+%there is no single centralized place in China
+%that coordinates its specific censorship decisions and steps.
+We assume that the attacker may be able to use political and economic
+resources to secure the cooperation of extraterritorial or multinational
+corporations and entities in investigating information sources.
+For example, the censors can threaten the service providers of
+troublesome blogs with economic reprisals if they do not reveal the
+authors' identities.
+We assume that our users have control over their hardware and
+software---they don't have any spyware installed, there are no
+cameras watching their screens, etc. Unfortunately, in many situations
+these threats are real~\cite{zuckerman-threatmodels}; yet
+software-based security systems like ours are poorly equipped to handle
+a user who is entirely observed and controlled by the adversary. See
+Section~\ref{subsec:cafes-and-livecds} for more discussion of what little
+we can do about this issue.
+Similarly, we assume that the user will be able to fetch a genuine
+version of Tor, rather than one supplied by the adversary; see
+Section~\ref{subsec:trust-chain} for discussion on helping the user
+confirm that he has a genuine version and that he can connect to the
+real Tor network.
+\section{Adapting the current Tor design to anti-censorship}
+Tor is popular and sees a lot of use---it's the largest anonymity
+network of its kind, and has
+attracted more than 1500 volunteer-operated routers from around the
+world.  Tor protects each user by routing their traffic through a multiply
+encrypted ``circuit'' built of a few randomly selected relay, each of which
+can remove only a single layer of encryption.  Each relay sees only the step
+before it and the step after it in the circuit, and so no single relay can
+learn the connection between a user and her chosen communication partners.
+In this section, we examine some of the reasons why Tor has become popular,
+with particular emphasis to how we can take advantage of these properties
+for a blocking-resistance design.
+Tor aims to provide three security properties:
+\item 1. A local network attacker can't learn, or influence, your
+\item 2. No single router in the Tor network can link you to your
+\item 3. The destination, or somebody watching the destination,
+can't learn your location.
+For blocking-resistance, we care most clearly about the first
+property. But as the arms race progresses, the second property
+will become important---for example, to discourage an adversary
+from volunteering a relay in order to learn that Alice is reading
+or posting to certain websites. The third property helps keep users safe from
+collaborating websites: consider websites and other Internet services 
+that have been pressured
+recently into revealing the identity of bloggers
+or treating clients differently depending on their network
+The Tor design provides other features as well that are not typically
+present in manual or ad hoc circumvention techniques.
+First, Tor has a well-analyzed and well-understood way to distribute
+information about relay.
+Tor directory authorities automatically aggregate, test,
+and publish signed summaries of the available Tor routers. Tor clients
+can fetch these summaries to learn which routers are available and
+which routers are suitable for their needs. Directory information is cached
+throughout the Tor network, so once clients have bootstrapped they never
+need to interact with the authorities directly. (To tolerate a minority
+of compromised directory authorities, we use a threshold trust scheme---
+see Section~\ref{subsec:trust-chain} for details.)
+Second, the list of directory authorities is not hard-wired.
+Clients use the default authorities if no others are specified,
+but it's easy to start a separate (or even overlapping) Tor network just
+by running a different set of authorities and convincing users to prefer
+a modified client. For example, we could launch a distinct Tor network
+inside China; some users could even use an aggregate network made up of
+both the main network and the China network. (But we should not be too
+quick to create other Tor networks---part of Tor's anonymity comes from
+users behaving like other users, and there are many unsolved anonymity
+questions if different users know about different pieces of the network.)
+Third, in addition to automatically learning from the chosen directories
+which Tor routers are available and working, Tor takes care of building
+paths through the network and rebuilding them as needed. So the user
+never has to know how paths are chosen, never has to manually pick
+working proxies, and so on. More generally, at its core the Tor protocol
+is simply a tool that can build paths given a set of routers. Tor is
+quite flexible about how it learns about the routers and how it chooses
+the paths. Harvard's Blossom project~\cite{blossom-thesis} makes this
+flexibility more concrete: Blossom makes use of Tor not for its security
+properties but for its reachability properties. It runs a separate set
+of directory authorities, its own set of Tor routers (called the Blossom
+network), and uses Tor's flexible path-building to let users view Internet
+resources from any point in the Blossom network.
+Fourth, Tor separates the role of \emph{internal relay} from the
+role of \emph{exit relay}. That is, some volunteers choose just to relay
+traffic between Tor users and Tor routers, and others choose to also allow
+connections to external Internet resources. Because we don't force all
+volunteers to play both roles, we end up with more relays. This increased
+diversity in turn is what gives Tor its security: the more options the
+user has for her first hop, and the more options she has for her last hop,
+the less likely it is that a given attacker will be watching both ends
+of her circuit~\cite{tor-design}. As a bonus, because our design attracts
+more internal relays that want to help out but don't want to deal with
+being an exit relay, we end up providing more options for the first
+hop---the one most critical to being able to reach the Tor network.
+Fifth, Tor is sustainable. Zero-Knowledge Systems offered the commercial
+but now defunct Freedom Network~\cite{freedom21-security}, a design with
+security comparable to Tor's, but its funding model relied on collecting
+money from users to pay relay operators. Modern commercial proxy systems
+need to keep collecting money to support their infrastructure. On the
+other hand, Tor has built a self-sustaining community of volunteers who
+donate their time and resources. This community trust is rooted in Tor's
+open design: we tell the world exactly how Tor works, and we provide all
+the source code. Users can decide for themselves, or pay any security
+expert to decide, whether it is safe to use. Further, Tor's modularity
+as described above, along with its open license, mean that its impact
+will continue to grow.
+Sixth, Tor has an established user base of hundreds of
+thousands of people from around the world. This diversity of
+users contributes to sustainability as above: Tor is used by
+ordinary citizens, activists, corporations, law enforcement, and
+even government and military users,
+and they can
+only achieve their security goals by blending together in the same
+network~\cite{econymics,usability:weis2006}. This user base also provides
+something else: hundreds of thousands of different and often-changing
+addresses that we can leverage for our blocking-resistance design.
+Finally and perhaps most importantly, Tor provides anonymity and prevents any
+single relay from linking users to their communication partners.  Despite
+initial appearances, {\it distributed-trust anonymity is critical for
+anti-censorship efforts}.  If any single relay can expose dissident bloggers
+or compile a list of users' behavior, the censors can profitably compromise
+that relay's operator, perhaps by applying economic pressure to their
+breaking into their computer, pressuring their family (if they have relatives
+in the censored area), or so on.  Furthermore, in designs where any relay can
+expose its users, the censors can spread suspicion that they are running some
+of the relays and use this belief to chill use of the network.
+We discuss and adapt these components further in
+Section~\ref{sec:bridges}. But first we examine the strengths and
+weaknesses of other blocking-resistance approaches, so we can expand
+our repertoire of building blocks and ideas.
+\section{Current proxy solutions}
+Relay-based blocking-resistance schemes generally have two main
+components: a relay component and a discovery component. The relay part
+encompasses the process of establishing a connection, sending traffic
+back and forth, and so on---everything that's done once the user knows
+where she's going to connect. Discovery is the step before that: the
+process of finding one or more usable relays.
+For example, we can divide the pieces of Tor in the previous section
+into the process of building paths and sending
+traffic over them (relay) and the process of learning from the directory
+authorities about what routers are available (discovery).  With this
+in mind, we now examine several categories of relay-based schemes.
+\subsection{Centrally-controlled shared proxies}
+Existing commercial anonymity solutions (like Anonymizer.com) are based
+on a set of single-hop proxies. In these systems, each user connects to
+a single proxy, which then relays traffic between the user and her
+destination. These public proxy
+systems are typically characterized by two features: they control and
+operate the proxies centrally, and many different users get assigned
+to each proxy.
+In terms of the relay component, single proxies provide weak security
+compared to systems that distribute trust over multiple relays, since a
+compromised proxy can trivially observe all of its users' actions, and
+an eavesdropper only needs to watch a single proxy to perform timing
+correlation attacks against all its users' traffic and thus learn where
+everyone is connecting. Worse, all users
+need to trust the proxy company to have good security itself as well as
+to not reveal user activities.
+On the other hand, single-hop proxies are easier to deploy, and they
+can provide better performance than distributed-trust designs like Tor,
+since traffic only goes through one relay. They're also more convenient
+from the user's perspective---since users entirely trust the proxy,
+they can just use their web browser directly.
+Whether public proxy schemes are more or less scalable than Tor is
+still up for debate: commercial anonymity systems can use some of their
+revenue to provision more bandwidth as they grow, whereas volunteer-based
+anonymity systems can attract thousands of fast relays to spread the load.
+The discovery piece can take several forms. Most commercial anonymous
+proxies have one or a handful of commonly known websites, and their users
+log in to those websites and relay their traffic through them. When
+these websites get blocked (generally soon after the company becomes
+popular), if the company cares about users in the blocked areas, they
+start renting lots of disparate IP addresses and rotating through them
+as they get blocked. They notify their users of new addresses (by email,
+for example). It's an arms race, since attackers can sign up to receive the
+email too, but operators have one nice trick available to them: because they
+have a list of paying subscribers, they can notify certain subscribers
+about updates earlier than others.
+Access control systems on the proxy let them provide service only to
+users with certain characteristics, such as paying customers or people
+from certain IP address ranges.
+Discovery in the face of a government-level firewall is a complex and
+topic, and we're stuck in this same arms race ourselves; we explore it
+in more detail in Section~\ref{sec:discovery}. But first we examine the
+other end of the spectrum---getting volunteers to run the proxies,
+and telling only a few people about each proxy.
+\subsection{Independent personal proxies}
+Personal proxies such as Circumventor~\cite{circumventor} and
+CGIProxy~\cite{cgiproxy} use the same technology as the public ones as
+far as the relay component goes, but they use a different strategy for
+discovery. Rather than managing a few centralized proxies and constantly
+getting new addresses for them as the old addresses are blocked, they
+aim to have a large number of entirely independent proxies, each managing
+its own (much smaller) set of users.
+As the Circumventor site explains, ``You don't
+actually install the Circumventor \emph{on} the computer that is blocked
+from accessing Web sites. You, or a friend of yours, has to install the
+Circumventor on some \emph{other} machine which is not censored.''
+This tactic has great advantages in terms of blocking-resistance---recall
+our assumption in Section~\ref{sec:adversary} that the attention
+a system attracts from the attacker is proportional to its number of
+users and level of publicity. If each proxy only has a few users, and
+there is no central list of proxies, most of them will never get noticed by
+the censors.
+On the other hand, there's a huge scalability question that so far has
+prevented these schemes from being widely useful: how does the fellow
+in China find a person in Ohio who will run a Circumventor for him? In
+some cases he may know and trust some people on the outside, but in many
+cases he's just out of luck. Just as hard, how does a new volunteer in
+Ohio find a person in China who needs it?
+% another key feature of a proxy run by your uncle is that you
+% self-censor, so you're unlikely to bring abuse complaints onto
+% your uncle. self-censoring clearly has a downside too, though.
+This challenge leads to a hybrid design---centrally-distributed
+personal proxies---which we will investigate in more detail in
+\subsection{Open proxies}
+Yet another currently used approach to bypassing firewalls is to locate
+open and misconfigured proxies on the Internet. A quick Google search
+for ``open proxy list'' yields a wide variety of freely available lists
+of HTTP, HTTPS, and SOCKS proxies. Many small companies have sprung up
+providing more refined lists to paying customers.
+There are some downsides to using these open proxies though. First,
+the proxies are of widely varying quality in terms of bandwidth and
+stability, and many of them are entirely unreachable. Second, unlike
+networks of volunteers like Tor, the legality of routing traffic through
+these proxies is questionable: it's widely believed that most of them
+don't realize what they're offering, and probably wouldn't allow it if
+they realized. Third, in many cases the connection to the proxy is
+unencrypted, so firewalls that filter based on keywords in IP packets
+will not be hindered. Fourth, in many countries (including China), the
+firewall authorities hunt for open proxies as well, to preemptively
+block them. And last, many users are suspicious that some
+open proxies are a little \emph{too} convenient: are they run by the
+adversary, in which case they get to monitor all the user's requests
+just as single-hop proxies can?
+A distributed-trust design like Tor resolves each of these issues for
+the relay component, but a constantly changing set of thousands of open
+relays is clearly a useful idea for a discovery component. For example,
+users might be able to make use of these proxies to bootstrap their
+first introduction into the Tor network.
+\subsection{Blocking resistance and JAP}
+K\"{o}psell and Hilling's Blocking Resistance
+design~\cite{koepsell:wpes2004} is probably
+the closest related work, and is the starting point for the design in this
+paper.  In this design, the JAP anonymity system~\cite{web-mix} is used
+as a base instead of Tor.  Volunteers operate a large number of access
+points that relay traffic to the core JAP
+network, which in turn anonymizes users' traffic.  The software to run these
+relays is, as in our design, included in the JAP client software and enabled
+only when the user decides to enable it.  Discovery is handled with a
+CAPTCHA-based mechanism; users prove that they aren't an automated process,
+and are given the address of an access point.  (The problem of a determined
+attacker with enough manpower to launch many requests and enumerate all the
+access points is not considered in depth.)  There is also some suggestion
+that information about access points could spread through existing social
+The Infranet design~\cite{infranet} uses one-hop relays to deliver web
+content, but disguises its communications as ordinary HTTP traffic.  Requests
+are split into multiple requests for URLs on the relay, which then encodes
+its responses in the content it returns.  The relay needs to be an actual
+website with plausible content and a number of URLs which the user might want
+to access---if the Infranet software produced its own cover content, it would
+be far easier for censors to identify.  To keep the censors from noticing
+that cover content changes depending on what data is embedded, Infranet needs
+the cover content to have an innocuous reason for changing frequently: the
+paper recommends watermarked images and webcams.
+The attacker and relay operators in Infranet's threat model are significantly
+different than in ours.  Unlike our attacker, Infranet's censor can't be
+bypassed with encrypted traffic (presumably because the censor blocks
+encrypted traffic, or at least considers it suspicious), and has more
+computational resources to devote to each connection than ours (so it can
+notice subtle patterns over time).  Unlike our bridge operators, Infranet's
+operators (and users) have more bandwidth to spare; the overhead in typical
+steganography schemes is far higher than Tor's.
+The Infranet design does not include a discovery element.  Discovery,
+however, is a critical point: if whatever mechanism allows users to learn
+about relays also allows the censor to do so, he can trivially discover and
+block their addresses, even if the steganography would prevent mere traffic
+observation from revealing the relays' addresses.
+\subsection{RST-evasion and other packet-level tricks}
+In their analysis of China's firewall's content-based blocking, Clayton,
+Murdoch and Watson discovered that rather than blocking all packets in a TCP
+streams once a forbidden word was noticed, the firewall was simply forging
+RST packets to make the communicating parties believe that the connection was
+closed~\cite{clayton:pet2006}. They proposed altering operating systems
+to ignore forged RST packets. This approach might work in some cases, but
+in practice it appears that many firewalls start filtering by IP address
+once a sufficient number of RST packets have been sent.
+Other packet-level responses to filtering include splitting
+sensitive words across multiple TCP packets, so that the censors'
+firewalls can't notice them without performing expensive stream
+reconstruction~\cite{ptacek98insertion}. This technique relies on the
+same insight as our weak steganography assumption.
+%\subsection{Internal caching networks}
+%Freenet~\cite{freenet-pets00} is an anonymous peer-to-peer data store.
+%Analyzing Freenet's security can be difficult, as its design is in flux as
+%new discovery and routing mechanisms are proposed, and no complete
+%specification has (to our knowledge) been written.  Freenet servers relay
+%requests for specific content (indexed by a digest of the content)
+%``toward'' the server that hosts it, and then cache the content as it
+%follows the same path back to
+%the requesting user.  If Freenet's routing mechanism is successful in
+%allowing nodes to learn about each other and route correctly even as some
+%node-to-node links are blocked by firewalls, then users inside censored areas
+%can ask a local Freenet server for a piece of content, and get an answer
+%without having to connect out of the country at all.  Of course, operators of
+%servers inside the censored area can still be targeted, and the addresses of
+%external servers can still be blocked.
+%The popular Skype voice-over-IP software uses multiple techniques to tolerate
+%restrictive networks, some of which allow it to continue operating in the
+%presence of censorship.  By switching ports and using encryption, Skype
+%attempts to resist trivial blocking and content filtering.  Even if no
+%encryption were used, it would still be expensive to scan all voice
+%traffic for sensitive words.  Also, most current keyloggers are unable to
+%store voice traffic.  Nevertheless, Skype can still be blocked, especially at
+%its central login server.
+%*sjmurdoch* "we consider the login server to be the only central component in
+%the Skype p2p network."
+%*sjmurdoch* http://www1.cs.columbia.edu/~salman/publications/skype1_4.pdf
+%-> *sjmurdoch* ok. what is the login server's role?
+%-> *sjmurdoch* and do you need to reach it directly to use skype?
+%*sjmurdoch* It checks the username and password
+%*sjmurdoch* It is necessary in the current implementation, but I don't know if
+%it is a fundemental limitation of the architecture
+\subsection{Tor itself}
+And last, we include Tor itself in the list of current solutions
+to firewalls. Tens of thousands of people use Tor from countries that
+routinely filter their Internet. Tor's website has been blocked in most
+of them. But why hasn't the Tor network been blocked yet?
+We have several theories. The first is the most straightforward: tens of
+thousands of people are simply too few to matter. It may help that Tor is
+perceived to be for experts only, and thus not worth attention yet. The
+more subtle variant on this theory is that we've positioned Tor in the
+public eye as a tool for retaining civil liberties in more free countries,
+so perhaps blocking authorities don't view it as a threat. (We revisit
+this idea when we consider whether and how to publicize a Tor variant
+that improves blocking-resistance---see Section~\ref{subsec:publicity}
+for more discussion.)
+The broader explanation is that the maintenance of most government-level
+filters is aimed at stopping widespread information flow and appearing to be
+in control, not by the impossible goal of blocking all possible ways to bypass
+censorship. Censors realize that there will always
+be ways for a few people to get around the firewall, and as long as Tor
+has not publically threatened their control, they see no urgent need to
+block it yet.
+We should recognize that we're \emph{already} in the arms race. These
+constraints can give us insight into the priorities and capabilities of
+our various attackers.
+\section{The relay component of our blocking-resistant design}
+Section~\ref{sec:current-tor} describes many reasons why Tor is
+well-suited as a building block in our context, but several changes will
+allow the design to resist blocking better. The most critical changes are
+to get more relay addresses, and to distribute them to users differently.
+%We need to address three problems:
+%- adapting the relay component of Tor so it resists blocking better.
+%- Discovery.
+%- Tor's network fingerprint.
+%Here we describe the new pieces we need to add to the current Tor design.
+\subsection{Bridge relays}
+Today, Tor relays operate on a few thousand distinct IP addresses;
+an adversary
+could enumerate and block them all with little trouble.  To provide a
+means of ingress to the network, we need a larger set of entry points, most
+of which an adversary won't be able to enumerate easily.  Fortunately, we
+have such a set: the Tor users.
+Hundreds of thousands of people around the world use Tor. We can leverage
+our already self-selected user base to produce a list of thousands of
+frequently-changing IP addresses. Specifically, we can give them a little
+button in the GUI that says ``Tor for Freedom'', and users who click
+the button will turn into \emph{bridge relays} (or just \emph{bridges}
+for short). They can rate limit relayed connections to 10 KB/s (almost
+nothing for a broadband user in a free country, but plenty for a user
+who otherwise has no access at all), and since they are just relaying
+bytes back and forth between blocked users and the main Tor network, they
+won't need to make any external connections to Internet sites. Because
+of this separation of roles, and because we're making use of software
+that the volunteers have already installed for their own use, we expect
+our scheme to attract and maintain more volunteers than previous schemes.
+As usual, there are new anonymity and security implications from running a
+bridge relay, particularly from letting people relay traffic through your
+Tor client; but we leave this discussion for Section~\ref{sec:security}.
+%...need to outline instructions for a Tor config that will publish
+%to an alternate directory authority, and for controller commands
+%that will do this cleanly.
+\subsection{The bridge directory authority}
+How do the bridge relays advertise their existence to the world? We
+introduce a second new component of the design: a specialized directory
+authority that aggregates and tracks bridges. Bridge relays periodically
+publish relay descriptors (summaries of their keys, locations, etc,
+signed by their long-term identity key), just like the relays in the
+``main'' Tor network, but in this case they publish them only to the
+bridge directory authorities.
+The main difference between bridge authorities and the directory
+authorities for the main Tor network is that the main authorities provide
+a list of every known relay, but the bridge authorities only give
+out a relay descriptor if you already know its identity key. That is,
+you can keep up-to-date on a bridge's location and other information
+once you know about it, but you can't just grab a list of all the bridges.
+The identity key, IP address, and directory port for each bridge
+authority ship by default with the Tor software, so the bridge relays
+can be confident they're publishing to the right location, and the
+blocked users can establish an encrypted authenticated channel. See
+Section~\ref{subsec:trust-chain} for more discussion of the public key
+infrastructure and trust chain.
+Bridges use Tor to publish their descriptors privately and securely,
+so even an attacker monitoring the bridge directory authority's network
+can't make a list of all the addresses contacting the authority.
+Bridges may publish to only a subset of the
+authorities, to limit the potential impact of an authority compromise.
+%\subsection{A simple matter of engineering}
+%Although we've described bridges and bridge authorities in simple terms
+%above, some design modifications and features are needed in the Tor
+%codebase to add them. We describe the four main changes here.
+%Firstly, we need to get smarter about rate limiting:
+%Bandwidth classes
+%Secondly, while users can in fact configure which directory authorities
+%they use, we need to add a new type of directory authority and teach
+%bridges to fetch directory information from the main authorities while
+%publishing relay descriptors to the bridge authorities. We're most of
+%the way there, since we can already specify attributes for directory
+%add a separate flag named ``blocking''.
+%Thirdly, need to build paths using bridges as the first
+%hop. One more hole in the non-clique assumption.
+%Lastly, since bridge authorities don't answer full network statuses,
+%we need to add a new way for users to learn the current status for a
+%single relay or a small set of relays---to answer such questions as
+%``is it running?'' or ``is it behaving correctly?'' We describe in
+%Section~\ref{subsec:enclave-dirs} a way for the bridge authority to
+%publish this information without resorting to signing each answer
+\subsection{Putting them together}
+If a blocked user knows the identity keys of a set of bridge relays, and
+he has correct address information for at least one of them, he can use
+that one to make a secure connection to the bridge authority and update
+his knowledge about the other bridge relays. He can also use it to make
+secure connections to the main Tor network and directory authorities, so he
+can build circuits and connect to the rest of the Internet. All of these
+updates happen in the background: from the blocked user's perspective,
+he just accesses the Internet via his Tor client like always.
+So now we've reduced the problem from how to circumvent the firewall
+for all transactions (and how to know that the pages you get have not
+been modified by the local attacker) to how to learn about a working
+bridge relay.
+There's another catch though. We need to make sure that the network
+traffic we generate by simply connecting to a bridge relay doesn't stand
+out too much.
+%The following section describes ways to bootstrap knowledge of your first
+%bridge relay, and ways to maintain connectivity once you know a few
+%bridge relays.
+% (See Section~\ref{subsec:first-bridge} for a discussion
+%of exactly what information is sufficient to characterize a bridge relay.)
+\section{Hiding Tor's network fingerprint}
+Currently, Tor uses two protocols for its network communications. The
+main protocol uses TLS for encrypted and authenticated communication
+between Tor instances. The second protocol is standard HTTP, used for
+fetching directory information. All Tor relays listen on their ``ORPort''
+for TLS connections, and some of them opt to listen on their ``DirPort''
+as well, to serve directory information. Tor relays choose whatever port
+numbers they like; the relay descriptor they publish to the directory
+tells users where to connect.
+One format for communicating address information about a bridge relay is
+its IP address and DirPort. From there, the user can ask the bridge's
+directory cache for an up-to-date copy of its relay descriptor, and
+learn its current circuit keys, its ORPort, and so on.
+However, connecting directly to the directory cache involves a plaintext
+HTTP request. A censor could create a network fingerprint (known as a
+\emph{signature} in the intrusion detection field) for the request
+and/or its response, thus preventing these connections. To resolve this
+vulnerability, we've modified the Tor protocol so that users can connect
+to the directory cache via the main Tor port---they establish a TLS
+connection with the bridge as normal, and then send a special ``begindir''
+relay command to establish an internal connection to its directory cache.
+Therefore a better way to summarize a bridge's address is by its IP
+address and ORPort, so all communications between the client and the
+bridge will use ordinary TLS. But there are other details that need
+more investigation.
+What port should bridges pick for their ORPort? We currently recommend
+that they listen on port 443 (the default HTTPS port) if they want to
+be most useful, because clients behind standard firewalls will have
+the best chance to reach them. Is this the best choice in all cases,
+or should we encourage some fraction of them pick random ports, or other
+ports commonly permitted through firewalls like 53 (DNS) or 110
+(POP)?  Or perhaps we should use other ports where TLS traffic is
+expected, like 993 (IMAPS) or 995 (POP3S).  We need more research on our
+potential users, and their current and anticipated firewall restrictions.
+Furthermore, we need to look at the specifics of Tor's TLS handshake.
+Right now Tor uses some predictable strings in its TLS handshakes. For
+example, it sets the X.509 organizationName field to ``Tor'', and it puts
+the Tor relay's nickname in the certificate's commonName field. We
+should tweak the handshake protocol so it doesn't rely on any unusual details
+in the certificate, yet it remains secure; the certificate itself
+should be made to resemble an ordinary HTTPS certificate.  We should also try
+to make our advertised cipher-suites closer to what an ordinary web server
+would support.
+Tor's TLS handshake uses two-certificate chains: one certificate
+contains the self-signed identity key for
+the router, and the second contains a current TLS key, signed by the
+identity key. We use these to authenticate that we're talking to the right
+router, and to limit the impact of TLS-key exposure.  Most (though far from
+all) consumer-oriented HTTPS services provide only a single certificate.
+These extra certificates may help identify Tor's TLS handshake; instead,
+bridges should consider using only a single TLS key certificate signed by
+their identity key, and providing the full value of the identity key in an
+early handshake cell.  More significantly, Tor currently has all clients
+present certificates, so that clients are harder to distinguish from relays.
+But in a blocking-resistance environment, clients should not present
+certificates at all.
+Last, what if the adversary starts observing the network traffic even
+more closely? Even if our TLS handshake looks innocent, our traffic timing
+and volume still look different than a user making a secure web connection
+to his bank. The same techniques used in the growing trend to build tools
+to recognize encrypted Bittorrent traffic
+could be used to identify Tor communication and recognize bridge
+relays. Rather than trying to look like encrypted web traffic, we may be
+better off trying to blend with some other encrypted network protocol. The
+first step is to compare typical network behavior for a Tor client to
+typical network behavior for various other protocols. This statistical
+cat-and-mouse game is made more complex by the fact that Tor transports a
+variety of protocols, and we'll want to automatically handle web browsing
+differently from, say, instant messaging.
+% Tor cells are 512 bytes each. So TLS records will be roughly
+% multiples of this size? How bad is this? -RD
+% Look at ``Inferring the Source of Encrypted HTTP Connections''
+% by Marc Liberatore and Brian Neil Levine (CCS 2006)
+% They substantially flesh out the numbers for the  web fingerprinting
+% attack. -PS
+% Yes, but I meant detecting the fingerprint of Tor traffic itself, not
+% learning what websites we're going to. I wouldn't be surprised to
+% learn that these are related problems, but it's not obvious to me. -RD
+\subsection{Identity keys as part of addressing information}
+We have described a way for the blocked user to bootstrap into the
+network once he knows the IP address and ORPort of a bridge. What about
+local spoofing attacks? That is, since we never learned an identity
+key fingerprint for the bridge, a local attacker could intercept our
+connection and pretend to be the bridge we had in mind. It turns out
+that giving false information isn't that bad---since the Tor client
+ships with trusted keys for the bridge directory authority and the Tor
+network directory authorities, the user can learn whether he's being
+given a real connection to the bridge authorities or not. (After all,
+if the adversary intercepts every connection the user makes and gives
+him a bad connection each time, there's nothing we can do.)
+What about anonymity-breaking attacks from observing traffic, if the
+blocked user doesn't start out knowing the identity key of his intended
+bridge? The vulnerabilities aren't so bad in this case either---the
+adversary could do similar attacks just by monitoring the network
+% cue paper by steven and george
+Once the Tor client has fetched the bridge's relay descriptor, it should
+remember the identity key fingerprint for that bridge relay. Thus if
+the bridge relay moves to a new IP address, the client can query the
+bridge directory authority to look up a fresh relay descriptor using
+this fingerprint.
+So we've shown that it's \emph{possible} to bootstrap into the network
+just by learning the IP address and ORPort of a bridge, but are there
+situations where it's more convenient or more secure to learn the bridge's
+identity fingerprint as well as instead, while bootstrapping? We keep
+that question in mind as we next investigate bootstrapping and discovery.
+\section{Discovering working bridge relays}
+Tor's modular design means that we can develop a better relay component
+independently of developing the discovery component. This modularity's
+great promise is that we can pick any discovery approach we like; but the
+unfortunate fact is that we have no magic bullet for discovery. We're
+in the same arms race as all the other designs we described in
+In this section we describe a variety of approaches to adding discovery
+components for our design.
+\subsection{Bootstrapping: finding your first bridge.}
+In Section~\ref{subsec:relay-together}, we showed that a user who knows
+a working bridge address can use it to reach the bridge authority and
+to stay connected to the Tor network. But how do new users reach the
+bridge authority in the first place? After all, the bridge authority
+will be one of the first addresses that a censor blocks.
+First, we should recognize that most government firewalls are not
+perfect. That is, they may allow connections to Google cache or some
+open proxy servers, or they let file-sharing traffic, Skype, instant
+messaging, or World-of-Warcraft connections through. Different users will
+have different mechanisms for bypassing the firewall initially. Second,
+we should remember that most people don't operate in a vacuum; users will
+hopefully know other people who are in other situations or have other
+resources available. In the rest of this section we develop a toolkit
+of different options and mechanisms, so that we can enable users in a
+diverse set of contexts to bootstrap into the system.
+(For users who can't use any of these techniques, hopefully they know
+a friend who can---for example, perhaps the friend already knows some
+bridge relay addresses. If they can't get around it at all, then we
+can't help them---they should go meet more people or learn more about
+the technology running the firewall in their area.)
+By deploying all the schemes in the toolkit at once, we let bridges and
+blocked users employ the discovery approach that is most appropriate
+for their situation.
+\subsection{Independent bridges, no central discovery}
+The first design is simply to have no centralized discovery component at
+all. Volunteers run bridges, and we assume they have some blocked users
+in mind and communicate their address information to them out-of-band
+(for example, through Gmail). This design allows for small personal
+bridges that have only one or a handful of users in mind, but it can
+also support an entire community of users. For example, Citizen Lab's
+upcoming Psiphon single-hop proxy tool~\cite{psiphon} plans to use this
+\emph{social network} approach as its discovery component.
+There are several ways to do bootstrapping in this design. In the simple
+case, the operator of the bridge informs each chosen user about his
+bridge's address information and/or keys. A different approach involves
+blocked users introducing new blocked users to the bridges they know.
+That is, somebody in the blocked area can pass along a bridge's address to
+somebody else they trust. This scheme brings in appealing but complex game
+theoretic properties: the blocked user making the decision has an incentive
+only to delegate to trustworthy people, since an adversary who learns
+the bridge's address and filters it makes it unavailable for both of them.
+Also, delegating known bridges to members of your social network can be
+dangerous: an the adversary who can learn who knows which bridges may
+be able to reconstruct the social network.
+Note that a central set of bridge directory authorities can still be
+compatible with a decentralized discovery process. That is, how users
+first learn about bridges is entirely up to the bridges, but the process
+of fetching up-to-date descriptors for them can still proceed as described
+in Section~\ref{sec:bridges}. Of course, creating a central place that
+knows about all the bridges may not be smart, especially if every other
+piece of the system is decentralized. Further, if a user only knows
+about one bridge and he loses track of it, it may be quite a hassle to
+reach the bridge authority. We address these concerns next.
+\subsection{Families of bridges, no central discovery}
+Because the blocked users are running our software too, we have many
+opportunities to improve usability or robustness. Our second design builds
+on the first by encouraging volunteers to run several bridges at once
+(or coordinate with other bridge volunteers), such that some
+of the bridges are likely to be available at any given time.
+The blocked user's Tor client would periodically fetch an updated set of
+recommended bridges from any of the working bridges. Now the client can
+learn new additions to the bridge pool, and can expire abandoned bridges
+or bridges that the adversary has blocked, without the user ever needing
+to care. To simplify maintenance of the community's bridge pool, each
+community could run its own bridge directory authority---reachable via
+the available bridges, and also mirrored at each bridge.
+\subsection{Public bridges with central discovery}
+What about people who want to volunteer as bridges but don't know any
+suitable blocked users? What about people who are blocked but don't
+know anybody on the outside? Here we describe how to make use of these
+\emph{public bridges} in a way that still makes it hard for the attacker
+to learn all of them.
+The basic idea is to divide public bridges into a set of pools based on
+identity key. Each pool corresponds to a \emph{distribution strategy}:
+an approach to distributing its bridge addresses to users. Each strategy
+is designed to exercise a different scarce resource or property of
+the user.
+How do we divide bridges between these strategy pools such that they're
+evenly distributed and the allocation is hard to influence or predict,
+but also in a way that's amenable to creating more strategies later
+on without reshuffling all the pools? We assign a given bridge
+to a strategy pool by hashing the bridge's identity key along with a
+secret that only the bridge authority knows: the first $n$ bits of this
+hash dictate the strategy pool number, where $n$ is a parameter that
+describes how many strategy pools we want at this point. We choose $n=3$
+to start, so we divide bridges between 8 pools; but as we later invent
+new distribution strategies, we can increment $n$ to split the 8 into
+16. Since a bridge can't predict the next bit in its hash, it can't
+anticipate which identity key will correspond to a certain new pool
+when the pools are split. Further, since the bridge authority doesn't
+provide any feedback to the bridge about which strategy pool it's in,
+an adversary who signs up bridges with the goal of filling a certain
+pool~\cite{casc-rep} will be hindered.
+% This algorithm is not ideal. When we split pools, each existing
+% pool is cut in half, where half the bridges remain with the
+% old distribution policy, and half will be under what the new one
+% is. So the new distribution policy inherits a bunch of blocked
+% bridges if the old policy was too loose, or a bunch of unblocked
+% bridges if its policy was still secure. -RD
+% I think it should be more chordlike.
+% Bridges are allocated to wherever on the ring which is divided
+% into arcs (buckets).
+% If a bucket gets too full, you can just split it.
+% More on this below. -PFS
+The first distribution strategy (used for the first pool) publishes bridge
+addresses in a time-release fashion. The bridge authority divides the
+available bridges into partitions, and each partition is deterministically
+available only in certain time windows. That is, over the course of a
+given time slot (say, an hour), each requester is given a random bridge
+from within that partition. When the next time slot arrives, a new set
+of bridges from the pool are available for discovery. Thus some bridge
+address is always available when a new
+user arrives, but to learn about all bridges the attacker needs to fetch
+all new addresses at every new time slot. By varying the length of the
+time slots, we can make it harder for the attacker to guess when to check
+back. We expect these bridges will be the first to be blocked, but they'll
+help the system bootstrap until they \emph{do} get blocked. Further,
+remember that we're dealing with different blocking regimes around the
+world that will progress at different rates---so this pool will still
+be useful to some users even as the arms races progress.
+The second distribution strategy publishes bridge addresses based on the IP
+address of the requesting user. Specifically, the bridge authority will
+divide the available bridges in the pool into a bunch of partitions
+(as in the first distribution scheme), hash the requester's IP address
+with a secret of its own (as in the above allocation scheme for creating
+pools), and give the requester a random bridge from the appropriate
+partition. To raise the bar, we should discard the last octet of the
+IP address before inputting it to the hash function, so an attacker
+who only controls a single ``/24'' network only counts as one user. A
+large attacker like China will still be able to control many addresses,
+but the hassle of establishing connections from each network (or spoofing
+TCP connections) may still slow them down. Similarly, as a special case,
+we should treat IP addresses that are Tor exit nodes as all being on
+the same network.
+The third strategy combines the time-based and location-based
+strategies to further constrain and rate-limit the available bridge
+addresses. Specifically, the bridge address provided in a given time
+slot to a given network location is deterministic within the partition,
+rather than chosen randomly each time from the partition. Thus, repeated
+requests during that time slot from a given network are given the same
+bridge address as the first request.
+The fourth strategy is based on Circumventor's discovery strategy.
+The Circumventor project, realizing that its adoption will remain limited
+if it has no central coordination mechanism, has started a mailing list to
+distribute new proxy addresses every few days. From experimentation it
+seems they have concluded that sending updates every three or four days
+is sufficient to stay ahead of the current attackers.
+The fifth strategy provides an alternative approach to a mailing list:
+users provide an email address and receive an automated response
+listing an available bridge address. We could limit one response per
+email address. To further rate limit queries, we could require a CAPTCHA
+in each case too. In fact, we wouldn't need to
+implement the CAPTCHA on our side: if we only deliver bridge addresses
+to Yahoo or GMail addresses, we can leverage the rate-limiting schemes
+that other parties already impose for account creation.
+The sixth strategy ties in the social network design with public
+bridges and a reputation system. We pick some seeds---trusted people in
+blocked areas---and give them each a few dozen bridge addresses and a few
+\emph{delegation tokens}. We run a website next to the bridge authority,
+where users can log in (they connect via Tor, and they don't need to
+provide actual identities, just persistent pseudonyms). Users can delegate
+trust to other people they know by giving them a token, which can be
+exchanged for a new account on the website. Accounts in ``good standing''
+then accrue new bridge addresses and new tokens. As usual, reputation
+schemes bring in a host of new complexities~\cite{rep-anon}: how do we
+decide that an account is in good standing? We could tie reputation
+to whether the bridges they're told about have been blocked---see
+Section~\ref{subsec:geoip} below for initial thoughts on how to discover
+whether bridges have been blocked. We could track reputation between
+accounts (if you delegate to somebody who screws up, it impacts you too),
+or we could use blinded delegation tokens~\cite{chaum-blind} to prevent
+the website from mapping the seeds' social network. We put off deeper
+discussion of the social network reputation strategy for future work.
+Pools seven and eight are held in reserve, in case our currently deployed
+tricks all fail at once and the adversary blocks all those bridges---so
+we can adapt and move to new approaches quickly, and have some bridges
+immediately available for the new schemes. New strategies might be based
+on some other scarce resource, such as relaying traffic for others or
+other proof of energy spent. (We might also worry about the incentives
+for bridges that sign up and get allocated to the reserve pools: will they
+be unhappy that they're not being used? But this is a transient problem:
+if Tor users are bridges by default, nobody will mind not being used yet.
+See also Section~\ref{subsec:incentives}.)
+%Is it useful to load balance which bridges are handed out? The above
+%pool concept makes some bridges wildly popular and others less so.
+%But I guess that's the point.
+\subsection{Public bridges with coordinated discovery}
+We presented the above discovery strategies in the context of a single
+bridge directory authority, but in practice we will want to distribute the
+operations over several bridge authorities---a single point of failure
+or attack is a bad move. The first answer is to run several independent
+bridge directory authorities, and bridges gravitate to one based on
+their identity key. The better answer would be some federation of bridge
+authorities that work together to provide redundancy but don't introduce
+new security issues. We could even imagine designs where the bridge
+authorities have encrypted versions of the bridge's relay descriptors,
+and the users learn a decryption key that they keep private when they
+first hear about the bridge---this way the bridge authorities would not
+be able to learn the IP address of the bridges.
+We leave this design question for future work.
+\subsection{Assessing whether bridges are useful}
+Learning whether a bridge is useful is important in the bridge authority's
+decision to include it in responses to blocked users. For example, if
+we end up with a list of thousands of bridges and only a few dozen of
+them are reachable right now, most blocked users will not end up knowing
+about working bridges.
+There are three components for assessing how useful a bridge is. First,
+is it reachable from the public Internet? Second, what proportion of
+the time is it available? Third, is it blocked in certain jurisdictions?
+The first component can be tested just as we test reachability of
+ordinary Tor relays. Specifically, the bridges do a self-test---connect
+to themselves via the Tor network---before they are willing to
+publish their descriptor, to make sure they're not obviously broken or
+misconfigured. Once the bridges publish, the bridge authority also tests
+reachability to make sure they're not confused or outright lying.
+The second component can be measured and tracked by the bridge authority.
+By doing periodic reachability tests, we can get a sense of how often the
+bridge is available. More complex tests will involve bandwidth-intensive
+checks to force the bridge to commit resources in order to be counted as
+available. We need to evaluate how the relationship of uptime percentage
+should weigh into our choice of which bridges to advertise. We leave
+this to future work.
+The third component is perhaps the trickiest: with many different
+adversaries out there, how do we keep track of which adversaries have
+blocked which bridges, and how do we learn about new blocks as they
+occur? We examine this problem next.
+\subsection{How do we know if a bridge relay has been blocked?}
+There are two main mechanisms for testing whether bridges are reachable
+from inside each blocked area: active testing via users, and passive
+testing via bridges.
+In the case of active testing, certain users inside each area
+sign up as testing relays. The bridge authorities can then use a
+Blossom-like~\cite{blossom-thesis} system to build circuits through them
+to each bridge and see if it can establish the connection. But how do
+we pick the users? If we ask random users to do the testing (or if we
+solicit volunteers from the users), the adversary should sign up so he
+can enumerate the bridges we test. Indeed, even if we hand-select our
+testers, the adversary might still discover their location and monitor
+their network activity to learn bridge addresses.
+Another answer is not to measure directly, but rather let the bridges
+report whether they're being used.
+%If they periodically report to their
+%bridge directory authority how much use they're seeing, perhaps the
+%authority can make smart decisions from there.
+Specifically, bridges should install a GeoIP database such as the public
+IP-To-Country list~\cite{ip-to-country}, and then periodically report to the
+bridge authorities which countries they're seeing use from. This data
+would help us track which countries are making use of the bridge design,
+and can also let us learn about new steps the adversary has taken in
+the arms race. (The compressed GeoIP database is only several hundred
+kilobytes, and we could even automate the update process by serving it
+from the bridge authorities.)
+More analysis of this passive reachability
+testing design is needed to resolve its many edge cases: for example,
+if a bridge stops seeing use from a certain area, does that mean the
+bridge is blocked or does that mean those users are asleep?
+There are many more problems with the general concept of detecting whether
+bridges are blocked. First, different zones of the Internet are blocked
+in different ways, and the actual firewall jurisdictions do not match
+country borders. Our bridge scheme could help us map out the topology
+of the censored Internet, but this is a huge task. More generally,
+if a bridge relay isn't reachable, is that because of a network block
+somewhere, because of a problem at the bridge relay, or just a temporary
+outage somewhere in between? And last, an attacker could poison our
+bridge database by signing up already-blocked bridges. In this case,
+if we're stingy giving out bridge addresses, users in that country won't
+learn working bridges.
+All of these issues are made more complex when we try to integrate this
+testing into our social network reputation system above.
+Since in that case we punish or reward users based on whether bridges
+get blocked, the adversary has new attacks to trick or bog down the
+reputation tracking. Indeed, the bridge authority doesn't even know
+what zone the blocked user is in, so do we blame him for any possible
+censored zone, or what?
+Clearly more analysis is required. The eventual solution will probably
+involve a combination of passive measurement via GeoIP and active
+measurement from trusted testers.  More generally, we can use the passive
+feedback mechanism to track usage of the bridge network as a whole---which
+would let us respond to attacks and adapt the design, and it would also
+let the general public track the progress of the project.
+%Worry: the adversary could choose not to block bridges but just record
+%connections to them. So be it, I guess.
+\subsection{Advantages of deploying all solutions at once}
+For once, we're not in the position of the defender: we don't have to
+defend against every possible filtering scheme; we just have to defend
+against at least one. On the flip side, the attacker is forced to guess
+how to allocate his resources to defend against each of these discovery
+strategies. So by deploying all of our strategies at once, we not only
+increase our chances of finding one that the adversary has difficulty
+blocking, but we actually make \emph{all} of the strategies more robust
+in the face of an adversary with limited resources.
+%\subsection{Remaining unsorted notes}
+%In the first subsection we describe how to find a first bridge.
+%Going to be an arms race. Need a bag of tricks. Hard to say
+%which ones will work. Don't spend them all at once.
+%Some techniques are sufficient to get us an IP address and a port,
+%and others can get us IP:port:key. Lay out some plausible options
+%for how users can bootstrap into learning their first bridge.
+%\section{The account / reputation system}
+%\section{Social networks with directory-side support}
+%One answer is to measure based on whether the bridge addresses
+%we give it end up blocked. But how do we decide if they get blocked?
+%Perhaps each bridge should be known by a single bridge directory
+%authority. This makes it easier to trace which users have learned about
+%it, so easier to blame or reward. It also makes things more brittle,
+%since loss of that authority means its bridges aren't advertised until
+%they switch, and means its bridge users are sad too.
+%(Need a slick hash algorithm that will map our identity key to a
+%bridge authority, in a way that's sticky even when we add bridge
+%directory authorities, but isn't sticky when our authority goes
+%away. Does this exist?)
+%   [[Ian says: What about just using something like hash table chaining?
+%   This should work, so long as the client knows which authorities currently
+%   exist.]]
+%\subsection{Discovery based on social networks}
+%A token that can be exchanged at the bridge authority (assuming you
+%can reach it) for a new bridge address.
+%The account server runs as a Tor controller for the bridge authority.
+%Users can establish reputations, perhaps based on social network
+%connectivity, perhaps based on not getting their bridge relays blocked,
+%Probably the most critical lesson learned in past work on reputation
+%systems in privacy-oriented environments~\cite{rep-anon} is the need for
+%verifiable transactions. That is, the entity computing and advertising
+%reputations for participants needs to actually learn in a convincing
+%way that a given transaction was successful or unsuccessful.
+%(Lesson from designing reputation systems~\cite{rep-anon}: easy to
+%reward good behavior, hard to punish bad behavior.
+\section{Security considerations}
+\subsection{Possession of Tor in oppressed areas}
+Many people speculate that installing and using a Tor client in areas with
+particularly extreme firewalls is a high risk---and the risk increases
+as the firewall gets more restrictive. This notion certainly has merit, but
+a counter pressure as well: as the firewall gets more restrictive, more
+ordinary people behind it end up using Tor for more mainstream activities,
+such as learning
+about Wall Street prices or looking at pictures of women's ankles. So
+as the restrictive firewall pushes up the number of Tor users, the
+``typical'' Tor user becomes more mainstream, and therefore mere
+use or possession of the Tor software is not so surprising.
+It's hard to say which of these pressures will ultimately win out,
+but we should keep both sides of the issue in mind.
+%Nick, want to rewrite/elaborate on this section?
+%Ian suggests: 
+%  Possession of Tor: this is totally of-the-cuff, and there are lots of
+% security issues to think about, but what about an ActiveX version of
+% Tor?  The magic you learn (as opposed to a bridge address) is a plain
+% old HTTPS server, which feeds you an ActiveX applet pre-configured with
+% some bridge address (possibly on the same machine).  For bonus points,
+% somehow arrange that (a) the applet is signed in some way the user can
+% reliably check, but (b) don't end up with anything like an incriminating
+% long-term cert stored on the user's computer.  This may be marginally
+% useful in some Internet-cafe situations as well, though (a) is even
+% harder to get right there.
+\subsection{Observers can tell who is publishing and who is reading}
+Tor encrypts traffic on the local network, and it obscures the eventual
+destination of the communication, but it doesn't do much to obscure the
+traffic volume. In particular, a user publishing a home video will have a
+different network fingerprint than a user reading an online news article.
+Based on our assumption in Section~\ref{sec:adversary} that users who
+publish material are in more danger, should we work to improve Tor's
+security in this situation?
+In the general case this is an extremely challenging task:
+effective \emph{end-to-end traffic confirmation attacks}
+are known where the adversary observes the origin and the
+destination of traffic and confirms that they are part of the
+same communication~\cite{danezis:pet2004,e2e-traffic}. Related are
+\emph{website fingerprinting attacks}, where the adversary downloads
+a few hundred popular websites, makes a set of "fingerprints" for each
+site, and then observes the target Tor client's traffic to look for
+a match~\cite{pet05-bissias,defensive-dropping}. But can we do better
+against a limited adversary who just does coarse-grained sweeps looking
+for unusually prolific publishers?
+One answer is for bridge users to automatically send bursts of padding
+traffic periodically. (This traffic can be implemented in terms of
+long-range drop cells, which are already part of the Tor specification.)
+Of course, convincingly simulating an actual human publishing interesting
+content is a difficult arms race, but it may be worthwhile to at least
+start the race. More research remains.
+\subsection{Anonymity effects from acting as a bridge relay}
+Against some attacks, relaying traffic for others can improve
+anonymity. The simplest example is an attacker who owns a small number
+of Tor relays. He will see a connection from the bridge, but he won't
+be able to know whether the connection originated there or was relayed
+from somebody else. More generally, the mere uncertainty of whether the
+traffic originated from that user may be helpful.
+There are some cases where it doesn't seem to help: if an attacker can
+watch all of the bridge's incoming and outgoing traffic, then it's easy
+to learn which connections were relayed and which started there. (In this
+case he still doesn't know the final destinations unless he is watching
+them too, but in this case bridges are no better off than if they were
+an ordinary client.)
+There are also some potential downsides to running a bridge. First, while
+we try to make it hard to enumerate all bridges, it's still possible to
+learn about some of them, and for some people just the fact that they're
+running one might signal to an attacker that they place a higher value
+on their anonymity. Second, there are some more esoteric attacks on Tor
+relays that are not as well-understood or well-tested---for example, an
+attacker may be able to ``observe'' whether the bridge is sending traffic
+even if he can't actually watch its network, by relaying traffic through
+it and noticing changes in traffic timing~\cite{attack-tor-oak05}. On
+the other hand, it may be that limiting the bandwidth the bridge is
+willing to relay will allow this sort of attacker to determine if it's
+being used as a bridge but not easily learn whether it is adding traffic
+of its own.
+We also need to examine how entry guards fit in. Entry guards
+(a small set of nodes that are always used for the first
+step in a circuit) help protect against certain attacks
+where the attacker runs a few Tor relays and waits for
+the user to choose these relays as the beginning and end of her
+If the blocked user doesn't use the bridge's entry guards, then the bridge
+doesn't gain as much cover benefit. On the other hand, what design changes
+are needed for the blocked user to use the bridge's entry guards without
+learning what they are (this seems hard), and even if we solve that,
+do they then need to use the guards' guards and so on down the line?
+It is an open research question whether the benefits of running a bridge
+outweigh the risks. A lot of the decision rests on which attacks the
+users are most worried about. For most users, we don't think running a
+bridge relay will be that damaging, and it could help quite a bit.
+\subsection{Trusting local hardware: Internet cafes and LiveCDs}
+Assuming that users have their own trusted hardware is not
+always reasonable.
+For Internet cafe Windows computers that let you attach your own USB key,
+a USB-based Tor image would be smart. There's Torpark, and hopefully
+there will be more thoroughly analyzed and trustworthy options down the
+road. Worries remain about hardware or software keyloggers and other
+spyware, as well as physical surveillance.
+If the system lets you boot from a CD or from a USB key, you can gain
+a bit more security by bringing a privacy LiveCD with you. (This
+approach isn't foolproof either of course, since hardware
+keyloggers and physical surveillance are still a worry).
+In fact, LiveCDs are also useful if it's your own hardware, since it's
+easier to avoid leaving private data and logs scattered around the
+%\subsection{Forward compatibility and retiring bridge authorities}
+%Eventually we'll want to change the identity key and/or location
+%of a bridge authority. How do we do this mostly cleanly?
+\subsection{The trust chain}
+Tor's ``public key infrastructure'' provides a chain of trust to
+let users verify that they're actually talking to the right relays.
+There are four pieces to this trust chain.
+First, when Tor clients are establishing circuits, at each step
+they demand that the next Tor relay in the path prove knowledge of
+its private key~\cite{tor-design}. This step prevents the first node
+in the path from just spoofing the rest of the path. Second, the
+Tor directory authorities provide a signed list of relays along with
+their public keys---so unless the adversary can control a threshold
+of directory authorities, he can't trick the Tor client into using other
+Tor relays. Third, the location and keys of the directory authorities,
+in turn, is hard-coded in the Tor source code---so as long as the user
+got a genuine version of Tor, he can know that he is using the genuine
+Tor network. And last, the source code and other packages are signed
+with the GPG keys of the Tor developers, so users can confirm that they
+did in fact download a genuine version of Tor.
+In the case of blocked users contacting bridges and bridge directory
+authorities, the same logic applies in parallel: the blocked users fetch
+information from both the bridge authorities and the directory authorities
+for the `main' Tor network, and they combine this information locally.
+How can a user in an oppressed country know that he has the correct
+key fingerprints for the developers? As with other security systems, it
+ultimately comes down to human interaction. The keys are signed by dozens
+of people around the world, and we have to hope that our users have met
+enough people in the PGP web of trust
+that they can learn
+the correct keys. For users that aren't connected to the global security
+community, though, this question remains a critical weakness.
+%\subsection{Security through obscurity: publishing our design}
+%Many other schemes like dynaweb use the typical arms race strategy of
+%not publishing their plans. Our goal here is to produce a design---a
+%framework---that can be public and still secure. Where's the tradeoff?
+%\section{Performance improvements}
+%\subsection{Fetch relay descriptors just-in-time}
+%I guess we should encourage most places to do this, so blocked
+%users don't stand out.
+%network-status and directory optimizations. caching better. partitioning
+\section{Maintaining reachability}
+\subsection{How many bridge relays should you know about?}
+The strategies described in Section~\ref{sec:discovery} talked about
+learning one bridge address at a time. But if most bridges are ordinary
+Tor users on cable modem or DSL connection, many of them will disappear
+and/or move periodically. How many bridge relays should a blocked user
+know about so that she is likely to have at least one reachable at any
+given point? This is already a challenging problem if we only consider
+natural churn: the best approach is to see what bridges we attract in
+reality and measure their churn. We may also need to factor in a parameter
+for how quickly bridges get discovered and blocked by the attacker;
+we leave this for future work after we have more deployment experience.
+A related question is: if the bridge relays change IP addresses
+periodically, how often does the blocked user need to fetch updates in
+order to keep from being cut out of the loop?
+Once we have more experience and intuition, we should explore technical
+solutions to this problem too. For example, if the discovery strategies
+give out $k$ bridge addresses rather than a single bridge address, perhaps
+we can improve robustness from the user perspective without significantly
+aiding the adversary. Rather than giving out a new random subset of $k$
+addresses at each point, we could bind them together into \emph{bridge
+families}, so all users that learn about one member of the bridge family
+are told about the rest as well.
+This scheme may also help defend against attacks to map the set of
+bridges. That is, if all blocked users learn a random subset of bridges,
+the attacker should learn about a few bridges, monitor the country-level
+firewall for connections to them, then watch those users to see what
+other bridges they use, and repeat. By segmenting the bridge address
+space, we can limit the exposure of other users.
+\subsection{Cablemodem users don't usually provide important websites}
+Another attacker we might be concerned about is that the attacker could
+just block all DSL and cablemodem network addresses, on the theory that
+they don't run any important services anyway. If most of our bridges
+are on these networks, this attack could really hurt.
+The first answer is to aim to get volunteers both from traditionally
+``consumer'' networks and also from traditionally ``producer'' networks.
+Since bridges don't need to be Tor exit nodes, as we improve our usability
+it seems quite feasible to get a lot of websites helping out.
+The second answer (not as practical) would be to encourage more use of
+consumer networks for popular and useful Internet services. 
+%(But P2P exists;
+%minor websites exist; gaming exists; IM exists; ...)
+A related attack we might worry about is based on large countries putting
+economic pressure on companies that want to expand their business. For
+example, what happens if Verizon wants to sell services in China, and
+China pressures Verizon to discourage its users in the free world from
+running bridges?
+\subsection{Scanning resistance: making bridges more subtle}
+If it's trivial to verify that a given address is operating as a bridge,
+and most bridges run on a predictable port, then it's conceivable our
+attacker could scan the whole Internet looking for bridges. (In fact,
+he can just concentrate on scanning likely networks like cablemodem
+and DSL services---see Section~\ref{subsec:block-cable} above for
+related attacks.) It would be nice to slow down this attack. It would
+be even nicer to make it hard to learn whether we're a bridge without
+first knowing some secret. We call this general property \emph{scanning
+resistance}, and it goes along with normalizing Tor's TLS handshake and
+network fingerprint.
+We could provide a password to the blocked user, and she (or her Tor
+client) provides a nonced hash of this password when she connects. We'd
+need to give her an ID key for the bridge too (in addition to the IP
+address and port---see Section~\ref{subsec:id-address}), and wait to
+present the password until we've finished the TLS handshake, else it
+would look unusual. If Alice can authenticate the bridge before she
+tries to send her password, we can resist an adversary who pretends
+to be the bridge and launches a man-in-the-middle attack to learn the
+password. But even if she can't, we still resist against widespread
+How should the bridge behave if accessed without the correct
+authorization? Perhaps it should act like an unconfigured HTTPS server
+(``welcome to the default Apache page''), or maybe it should mirror
+and act like common websites, or websites randomly chosen from Google.
+We might assume that the attacker can recognize HTTPS connections that
+use self-signed certificates. (This process would be resource-intensive
+but not out of the realm of possibility.) But even in this case, many
+popular websites around the Internet use self-signed or just plain broken
+SSL certificates.
+%to unknown servers. It can then attempt to connect to them and block
+%connections to servers that seem suspicious. It may be that password
+%protected web sites will not be suspicious in general, in which case
+%that may be the easiest way to give controlled access to the bridge.
+%If such sites that have no other overt features are automatically
+%blocked when detected, then we may need to be more subtle.
+%Possibilities include serving an innocuous web page if a TLS encrypted
+%request is received without the authorization needed to access the Tor
+%network and only responding to a requested access to the Tor network
+%of proper authentication is given. If an unauthenticated request to
+%access the Tor network is sent, the bridge should respond as if
+%it has received a message it does not understand (as would be the
+%case were it not a bridge).
+% Ian suggests a ``socialist millionaires'' protocol here, for something.
+% Did we once mention knocking here?  it's a good idea, but we should clarify
+% what we mean.  Ian also notes that knocking itself is very fingerprintable,
+% and we should beware.
+\subsection{How to motivate people to run bridge relays}
+One of the traditional ways to get people to run software that benefits
+others is to give them motivation to install it themselves.  An often
+suggested approach is to install it as a stunning screensaver so everybody
+will be pleased to run it. We take a similar approach here, by leveraging
+the fact that these users are already interested in protecting their
+own Internet traffic, so they will install and run the software.
+Eventually, we may be able to make all Tor users become bridges if they
+pass their self-reachability tests---the software and installers need
+more work on usability first, but we're making progress.
+In the mean time, we can make a snazzy network graph with
+Vidalia\footnote{\url{http://vidalia-project.net/}} that
+emphasizes the connections the bridge user is currently relaying.
+%anonymity implications, but hey.) (In many cases there won't be much
+%activity, so this may backfire. Or it may be better suited to full-fledged
+%Tor relay.)
+% Also consider everybody-a-relay. Many of the scalability questions
+% are easier when you're talking about making everybody a bridge.
+%\subsection{What if the clients can't install software?}
+%[this section should probably move to the related work section,
+%or just disappear entirely.]
+%Bridge users without Tor software
+%Bridge relays could always open their socks proxy. This is bad though,
+%because bridges learn the bridge users' destinations, and second because
+%we've learned that open socks proxies tend to attract abusive users who
+%have no idea they're using Tor.
+%Bridges could require passwords in the socks handshake (not supported
+%by most software including Firefox). Or they could run web proxies
+%that require authentication and then pass the requests into Tor. This
+%approach is probably a good way to help bootstrap the Psiphon network,
+%if one of its barriers to deployment is a lack of volunteers willing
+%to exit directly to websites. But it clearly drops some of the nice
+%anonymity and security features Tor provides.
+%A hybrid approach where the user gets his anonymity from Tor but his
+%software-less use from a web proxy running on a trusted machine on the
+%free side.
+\subsection{Publicity attracts attention}
+Many people working on this field want to publicize the existence
+and extent of censorship concurrently with the deployment of their
+circumvention software. The easy reason for this two-pronged push is
+to attract volunteers for running proxies in their systems; but in many
+cases their main goal is not to focus on getting more users signed up,
+but rather to educate the rest of the world about the
+censorship. The media also tries to do its part by broadcasting the
+existence of each new circumvention system.
+But at the same time, this publicity attracts the attention of the
+censors. We can slow down the arms race by not attracting as much
+attention, and just spreading by word of mouth. If our goal is to
+establish a solid social network of bridges and bridge users before
+the adversary gets involved, does this extra attention work to our
+\subsection{The Tor website: how to get the software}
+One of the first censoring attacks against a system like ours is to
+block the website and make the software itself hard to find. Our system
+should work well once the user is running an authentic
+copy of Tor and has found a working bridge, but to get to that point
+we rely on their individual skills and ingenuity.
+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 a copy,
+such as publicizing the mirrors more and making copies available through
+other media. We might also mirror the latest version of the software on
+each bridge, so users who hear about an honest bridge can get a good
+See Section~\ref{subsec:first-bridge} for more discussion.
+% Ian suggests that we have every tor relay distribute a signed copy of the
+% software.
+\section{Next Steps}
+Technical solutions won't solve the whole censorship problem. After all,
+the firewalls in places like China are \emph{socially} very
+successful, even if technologies and tricks exist to get around them.
+However, having a strong technical solution is still necessary as one
+important piece of the puzzle.
+In this paper, we have shown that Tor provides a great set of building
+blocks to start from. The next steps are to deploy prototype bridges and
+bridge authorities, implement some of the proposed discovery strategies,
+and then observe the system in operation and get more intuition about
+the actual requirements and adversaries we're up against.
+\bibliographystyle{plain} \bibliography{tor-design}
+%\section{Counting Tor users by country}
+\section{Future designs}
+\subsection{Bridges inside the blocked network too}
+Assuming actually crossing the firewall is the risky part of the
+operation, can we have some bridge relays inside the blocked area too,
+and more established users can use them as relays so they don't need to
+communicate over the firewall directly at all? A simple example here is
+to make new blocked users into internal bridges also---so they sign up
+on the bridge authority as part of doing their query, and we give out
+their addresses
+rather than (or along with) the external bridge addresses. This design
+is a lot trickier because it brings in the complexity of whether the
+internal bridges will remain available, can maintain reachability with
+the outside world, etc.
+More complex future designs involve operating a separate Tor network
+inside the blocked area, and using \emph{hidden service bridges}---bridges
+that can be accessed by users of the internal Tor network but whose
+addresses are not published or findable, even by these users---to get
+from inside the firewall to the rest of the Internet. But this design
+requires directory authorities to run inside the blocked area too,
+and they would be a fine target to take down the network.
+% Hidden services as bridge directory authorities.
+ship geoip db to bridges. they look up users who tls to them in the db,
+and upload a signed list of countries and number-of-users each day. the
+bridge authority aggregates them and publishes stats.
+bridge relays have buddies
+they ask a user to test the reachability of their buddy.
+leaks O(1) bridges, but not O(n).
+we should not be blockable by ordinary cisco censorship features.
+that is, if they want to block our new design, they will need to
+add a feature to block exactly this.
+strategically speaking, this may come in handy.
+Bridges come in clumps of 4 or 8 or whatever. If you know one bridge
+in a clump, the authority will tell you the rest. Now bridges can
+ask users to test reachability of their buddies.
+Giving out clumps helps with dynamic IP addresses too. Whether it
+should be 4 or 8 depends on our churn.
+the account server. let's call it a database, it doesn't have to
+be a thing that human interacts with.
+so how do we reward people for being good?
+\subsubsection{Public Bridges with Coordinated Discovery}
+****Pretty much this whole subsubsection will probably need to be
+deferred until ``later'' and moved to after end document, but I'm leaving
+it here for now in case useful.******
+Rather than be entirely centralized, we can have a coordinated
+collection of bridge authorities, analogous to how Tor network
+directory authorities now work.
+Key components
+``Authorities'' will distribute caches of what they know to overlapping
+collections of nodes so that no one node is owned by one authority.
+Also so that it is impossible to DoS info maintained by one authority
+simply by making requests to it.
+Where a bridge gets assigned is not predictable by the bridge?
+If authorities don't know the IP addresses of the bridges they
+are responsible for, they can't abuse that info (or be attacked for
+having it). But, they also can't, e.g., control being sent massive
+lists of nodes that were never good. This raises another question.
+We generally decry use of IP address for location, etc. but we
+need to do that to limit the introduction of functional but useless
+IP addresses because, e.g., they are in China and the adversary
+owns massive chunks of the IP space there.
+We don't want an arbitrary someone to be able to contact the
+authorities and say an IP address is bad because it would be easy
+for an adversary to take down all the suspicious bridges
+even if they provide good cover websites, etc. Only the bridge
+itself and/or the directory authority can declare a bridge blocked
+from somewhere.
+9. Bridge directories must not simply be a handful of nodes that
+provide the list of bridges. They must flood or otherwise distribute
+information out to other Tor nodes as mirrors. That way it becomes
+difficult for censors to flood the bridge directory authorities with
+requests, effectively denying access for others. But, there's lots of
+churn and a much larger size than Tor directories.  We are forced to
+handle the directory scaling problem here much sooner than for the
+network in general. Authorities can pass their bridge directories
+(and policy info) to some moderate number of unidentified Tor nodes.
+Anyone contacting one of those nodes can get bridge info. the nodes
+must remain somewhat synched to prevent the adversary from abusing,
+e.g., a timed release policy or the distribution to those nodes must
+be resilient even if they are not coordinating.
+I think some kind of DHT like scheme would work here. A Tor node is
+assigned a chunk of the directory.  Lookups in the directory should be
+via hashes of keys (fingerprints) and that should determine the Tor
+nodes responsible. Ordinary directories can publish lists of Tor nodes
+responsible for fingerprint ranges.  Clients looking to update info on
+some bridge will make a Tor connection to one of the nodes responsible
+for that address.  Instead of shutting down a circuit after getting
+info on one address, extend it to another that is responsible for that
+address (the node from which you are extending knows you are doing so
+anyway). Keep going.  This way you can amortize the Tor connection.
+10. We need some way to give new identity keys out to those who need
+them without letting those get immediately blocked by authorities. One
+way is to give a fingerprint that gets you more fingerprints, as
+already described. These are meted out/updated periodically but allow
+us to keep track of which sources are compromised: if a distribution
+fingerprint repeatedly leads to quickly blocked bridges, it should be
+suspect, dropped, etc. Since we're using hashes, there shouldn't be a
+correlation with bridge directory mirrors, bridges, portions of the
+network observed, etc. It should just be that the authorities know
+about that key that leads to new addresses.
+This last point is very much like the issues in the valet nodes paper,
+which is essentially about blocking resistance wrt exiting the Tor network,
+while this paper is concerned with blocking the entering to the Tor network.
+In fact the tickets used to connect to the IPo (Introduction Point),
+could serve as an example, except that instead of authorizing
+a connection to the Hidden Service, it's authorizing the downloading
+of more fingerprints.
+Also, the fingerprints can follow the hash(q + '1' + cookie) scheme of
+that paper (where q = hash(PK + salt) gave the q.onion address).  This
+allows us to control and track which fingerprint was causing problems.
+Note that, unlike many settings, the reputation problem should not be
+hard here. If a bridge says it is blocked, then it might as well be.
+If an adversary can say that the bridge is blocked wrt
+$\mathit{censor}_i$, then it might as well be, since
+$\mathit{censor}_i$ can presumably then block that bridge if it so
+11. How much damage can the adversary do by running nodes in the Tor
+network and watching for bridge nodes connecting to it?  (This is
+analogous to an Introduction Point watching for Valet Nodes connecting
+to it.) What percentage of the network do you need to own to do how
+much damage. Here the entry-guard design comes in helpfully.  So we
+need to have bridges use entry-guards, but (cf. 3 above) not use
+bridges as entry-guards. Here's a serious tradeoff (again akin to the
+ratio of valets to IPos) the more bridges/client the worse the
+anonymity of that client. The fewer bridges/client the worse the 
+blocking resistance of that client.
diff --git a/2006/blocking/tor-design.bib b/2006/blocking/tor-design.bib
new file mode 100644
index 0000000..981761e
--- /dev/null
+++ b/2006/blocking/tor-design.bib
@@ -0,0 +1,1493 @@
+% hs-attack
+ at inproceedings{hs-attack,
+  title = {Locating Hidden Servers},
+  author = {Lasse {\O}verlier and Paul Syverson}, 
+  booktitle = {Proceedings of the 2006 IEEE Symposium on Security and Privacy}, 
+  year = {2006}, 
+  month = {May}, 
+  publisher = {IEEE CS},
+ at TechReport{bauer:tr2007,
+  author = 	 {Kevin Bauer and Damon McCoy and Dirk Grunwald and Tadayoshi Kohno and Douglas Sicker},
+  title = 	 {Low-Resource Routing Attacks Against Anonymous Systems},
+  institution =  {University of Colorado at Boulder},
+  year = 	 2007,
+  number = 	 {CU-CS-1025-07}
+ at inproceedings{bauer:wpes2007,
+  title = {Low-Resource Routing Attacks Against Tor},
+  author = {Kevin Bauer and Damon McCoy and Dirk Grunwald and Tadayoshi Kohno and Douglas Sicker},
+  booktitle = {{Proceedings of the Workshop on Privacy in the Electronic Society (WPES 2007)}},
+  year = {2007},
+  month = {October},
+  address = {Washington, DC, USA},
+% fix me
+ at misc{tannenbaum96,
+  author = "Andrew Tannenbaum",
+  title = "Computer Networks",
+  year = "1996",
+  publisher = "Prentice Hall, 3rd edition",
+ at article{ meadows96,
+    author = "Catherine Meadows",
+    title = "The {NRL} Protocol Analyzer: An Overview",
+    journal = "Journal of Logic Programming",
+    volume = "26",
+    number = "2",
+    pages = "113--131",
+    year = "1996",
+ at inproceedings{kesdogan:pet2002,
+  title = {Unobservable Surfing on the World Wide Web: Is Private Information Retrieval an
+        alternative to the MIX based Approach?},
+  author = {Dogan Kesdogan and Mark Borning and Michael Schmeink},
+  booktitle = {Privacy Enhancing Technologies (PET 2002)},
+  year = {2002},
+  month = {April},
+  editor = {Roger Dingledine and Paul Syverson},
+  publisher = {Springer-Verlag, LNCS 2482},
+ at inproceedings{statistical-disclosure,
+  title = {Statistical Disclosure Attacks},
+  author = {George Danezis},
+  booktitle = {Security and Privacy in the Age of Uncertainty ({SEC2003})},
+  organization = {{IFIP TC11}},
+  year = {2003},
+  month = {May},
+  address = {Athens},
+  pages = {421--426},
+  publisher = {Kluwer},
+ at inproceedings{limits-open,
+  title = {Limits of Anonymity in Open Environments},
+  author = {Dogan Kesdogan and Dakshi Agrawal and Stefan Penz},
+  booktitle = {Information Hiding Workshop (IH 2002)},
+  year = {2002},
+  month = {October},
+  editor = {Fabien Petitcolas},
+  publisher = {Springer-Verlag, LNCS 2578},
+ at inproceedings{isdn-mixes,
+  title = {{ISDN-mixes: Untraceable communication with very small bandwidth overhead}},
+  author = {Andreas Pfitzmann and Birgit Pfitzmann and Michael Waidner},
+  booktitle = {GI/ITG Conference on Communication in Distributed Systems},
+  year = {1991},
+  month = {February},
+  pages = {451-463},
+ at Article{jerichow-jsac98,
+  author = 	 {Anja Jerichow and Jan M\"{u}ller and Andreas
+                  Pfitzmann and Birgit Pfitzmann and Michael Waidner},
+  title = 	 {Real-Time Mixes: A Bandwidth-Efficient Anonymity Protocol},
+  journal = 	 {IEEE Journal on Selected Areas in Communications},
+  year = 	 1998,
+  volume =	 16,
+  number =	 4,
+  pages =	 {495--509},
+  month =	 {May}
+ at inproceedings{tarzan:ccs02,
+  title = {Tarzan: A Peer-to-Peer Anonymizing Network Layer},
+  author = {Michael J. Freedman and Robert Morris},
+  booktitle = {9th {ACM} {C}onference on {C}omputer and {C}ommunications
+        {S}ecurity ({CCS 2002})},
+  year = {2002},
+  month = {November},
+  address = {Washington, DC},
+ at inproceedings{cebolla,
+  title = {{Cebolla: Pragmatic IP Anonymity}},
+  author = {Zach Brown},
+  booktitle = {Ottawa Linux Symposium},
+  year = {2002},
+  month = {June},
+ at inproceedings{eax,
+  author = "M. Bellare and P. Rogaway and D. Wagner",
+  title = {The {EAX} Mode of Operation: A Two-Pass Authenticated-Encryption Scheme Optimized for Simplicity and Efficiency},
+  booktitle = {Fast Software Encryption 2004},
+  month = {February},
+  year = {2004},
+ at misc{darkside,
+  title = {{The Dark Side of the Web: An Open Proxy's View}},
+  author = {Vivek S. Pai and Limin Wang and KyoungSoo Park and Ruoming Pang and Larry Peterson},
+  note = {\newline \url{http://codeen.cs.princeton.edu/}},
+%  note = {Submitted to HotNets-II. \url{http://codeen.cs.princeton.edu/}},
+ at Misc{anonymizer,
+  key =          {anonymizer},
+  title =        {The {Anonymizer}},
+  note =         {\url{http://anonymizer.com/}}
+ at Misc{privoxy,
+  key =          {privoxy},
+  title =        {{Privoxy}},
+  note =         {\url{http://www.privoxy.org/}}
+ at Misc{i2p,
+  key =          {i2p},
+  title =        {{I2P}},
+  note =         {\url{http://www.i2p.net/}}
+ at Misc{nym,
+  author = 	 {Jason Holt},
+  title = 	 {nym: practical pseudonymity for anonymous networks},
+  note = 	 {Paper and source code at \url{http://www.lunkwill.org/src/nym/}}
+ at InProceedings{nymble,
+  author = 	 {Peter C. Johnson and Apu Kapadia and Patrick P. Tsang and Sean W. Smith},
+  title = 	 {Nymble: Anonymous {IP}-address Blocking},
+  booktitle = {Privacy Enhancing Technologies (PET 2007)},
+  year = 	 2007,
+  publisher = {Springer-Verlag, LNCS 4776}
+ at inproceedings{anonnet,
+  title = {{Analysis of an Anonymity Network for Web Browsing}},
+  author = {Marc Rennhard and Sandro Rafaeli and Laurent Mathy and Bernhard Plattner and
+        David Hutchison},
+  booktitle = {{IEEE 7th Intl. Workshop on Enterprise Security (WET ICE
+        2002)}},
+  year = {2002},
+  month = {June},
+  address = {Pittsburgh, USA},
+%  pages = {49--54},
+ at inproceedings{econymics,
+  title = {On the Economics of Anonymity},
+  author = {Alessandro Acquisti and Roger Dingledine and Paul Syverson},
+  booktitle = {Financial Cryptography},
+  year = {2003},
+  editor = {Rebecca N. Wright},
+  publisher = {Springer-Verlag, LNCS 2742},
+ at inproceedings{defensive-dropping,
+  title = {Timing Analysis in Low-Latency Mix-Based Systems},
+  author = {Brian N. Levine and Michael K. Reiter and Chenxi Wang and Matthew Wright},
+  booktitle = {Financial Cryptography},
+  year = {2004},
+  editor = {Ari Juels},
+  publisher = {Springer-Verlag, LNCS (forthcoming)},
+ at inproceedings{morphmix:fc04,
+  title = {Practical Anonymity for the Masses with MorphMix},
+  author = {Marc Rennhard and Bernhard Plattner},
+  booktitle = {Financial Cryptography},
+  year = {2004},
+  editor = {Ari Juels},
+  publisher = {Springer-Verlag, LNCS (forthcoming)},
+ at inproceedings{eternity,
+  title = {The Eternity Service},
+  author = {Ross Anderson},
+  booktitle = {Pragocrypt '96},
+  year = {1996},
+  %note =  {\url{http://www.cl.cam.ac.uk/users/rja14/eternity/eternity.html}},
+ at inproceedings{minion-design,
+  title = {Mixminion: Design of a Type {III} Anonymous Remailer Protocol},
+  author = {George Danezis and Roger Dingledine and Nick Mathewson},
+  booktitle = {2003 IEEE Symposium on Security and Privacy},
+  year = {2003},
+  month = {May},
+  publisher = {IEEE CS},
+  pages = {2--15},
+  %note = {\url{http://mixminion.net/minion-design.pdf}},
+ at inproceedings{ rao-pseudonymity,
+    author = "Josyula R. Rao and Pankaj Rohatgi",
+    title = "Can Pseudonymity Really Guarantee Privacy?",
+    booktitle = "Proceedings of the Ninth USENIX Security Symposium",
+    year = {2000},
+    month = Aug,
+    publisher = {USENIX},
+    pages = "85--96",
+    %note = {\url{http://www.usenix.org/publications/library/proceedings/sec2000/
+ at InProceedings{pfitzmann90how,
+    author = "Birgit Pfitzmann and Andreas Pfitzmann",
+    title = "How to Break the Direct {RSA}-Implementation of {MIXes}",
+    booktitle = {Eurocrypt 89},
+    publisher = {Springer-Verlag, LNCS 434},
+    year = {1990},
+    note = {\url{http://citeseer.nj.nec.com/pfitzmann90how.html}},
+ at Misc{tor-spec,
+   author =      {Roger Dingledine and Nick Mathewson},
+   title =       {Tor Protocol Specifications},
+   note =        {\url{https://www.torproject.org/svn/trunk/doc/tor-spec.txt}},
+ at Misc{incentives-txt,
+   author =      {Roger Dingledine and Nick Mathewson},
+   title =       {Tor Incentives Design Brainstorms},
+   note =        {\url{https://www.torproject.org/svn/trunk/doc/incentives.txt}},
+ at InProceedings{BM:mixencrypt,
+  author =       {M{\"o}ller, Bodo},
+  title =        {Provably Secure Public-Key Encryption for Length-Preserving Chaumian Mixes},
+  booktitle =    {{CT-RSA} 2003},
+  publisher =    {Springer-Verlag, LNCS 2612},
+  year =         2003,
+ at InProceedings{back01,
+  author =       {Adam Back and Ulf M\"oller and Anton Stiglic},
+  title =        {Traffic Analysis Attacks and Trade-Offs in Anonymity Providing Systems},
+  booktitle =    {Information Hiding (IH 2001)},
+  pages =        {245--257},
+  year =         2001,
+  editor =       {Ira S. Moskowitz},
+  publisher =    {Springer-Verlag, LNCS 2137},
+  %note =         {\newline \url{http://www.cypherspace.org/adam/pubs/traffic.pdf}},
+ at InProceedings{rackoff93cryptographic,
+   author =      {Charles Rackoff and Daniel R. Simon},
+   title =       {Cryptographic Defense Against Traffic Analysis},
+   booktitle =   {{ACM} Symposium on Theory of Computing},
+   pages =       {672--681},
+   year =        {1993},
+   %note =        {\url{http://research.microsoft.com/crypto/dansimon/me.htm}},
+ at InProceedings{freehaven-berk,
+   author =      {Roger Dingledine and Michael J. Freedman and David Molnar},
+   title =       {The Free Haven Project: Distributed Anonymous Storage Service},
+   booktitle =   {Designing Privacy Enhancing Technologies: Workshop
+                  on Design Issue in Anonymity and Unobservability},
+   year =        2000,
+   month =       {July},
+   editor =      {H. Federrath},
+   publisher =   {Springer-Verlag, LNCS 2009},
+   @InProceedings{move-ndss05,
+  author = 	 {Angelos Stavrou and Angelos D. Keromytis and Jason Nieh and Vishal Misra and Dan Rubenstein},
+  title = 	 {MOVE: An End-to-End Solution To Network Denial of Service},
+  booktitle = 	 {{ISOC Network and Distributed System Security Symposium (NDSS05)}},
+  year =	 2005,
+  month =	 {February},
+  publisher =	 {Internet Society}
+%note =        {\url{http://freehaven.net/papers.html}},
+ at InProceedings{raymond00,
+  author =       {J. F. Raymond},
+  title =        {{Traffic Analysis: Protocols, Attacks, Design Issues,
+                  and Open Problems}},
+  booktitle =    {Designing Privacy Enhancing Technologies: Workshop
+                  on Design Issue in Anonymity and Unobservability},
+  year =         2000,
+  month =        {July},
+  pages =        {10-29},
+  editor =       {H. Federrath},
+  publisher =    {Springer-Verlag, LNCS 2009},
+ at InProceedings{sybil,
+  author = "John Douceur",
+  title = {{The Sybil Attack}},
+  booktitle = "Proceedings of the 1st International Peer To Peer Systems Workshop (IPTPS)",
+  month = Mar,
+  year = 2002,
+ at InCollection{price-privacy,
+  author =	 {Paul Syverson and Adam Shostack},
+  editor =	 {L. Jean Camp and Stephen Lewis},
+  title = 	 {What Price Privacy? (and why identity theft is about neither identity nor theft)},
+  booktitle =	 {Economics of Information Security},
+  chapter = 	 10,
+  publisher = 	 {Kluwer},
+  year = 	 2004,
+  pages =	 {129--142}
+ at InProceedings{trickle02,
+  author =       {Andrei Serjantov and Roger Dingledine and Paul Syverson},
+  title =        {From a Trickle to a Flood: Active Attacks on Several
+                  Mix Types},
+  booktitle =    {Information Hiding (IH 2002)},
+  year =         {2002},
+  editor =       {Fabien Petitcolas},
+  publisher =    {Springer-Verlag, LNCS 2578},
+ at InProceedings{langos02,
+  author =      {Oliver Berthold and Heinrich Langos},
+  title =       {Dummy Traffic Against Long Term Intersection Attacks},
+  booktitle =    {Privacy Enhancing Technologies (PET 2002)},
+  year =         {2002},
+  editor =       {Roger Dingledine and Paul Syverson},
+  publisher =    {Springer-Verlag, LNCS 2482}
+ at InProceedings{hintz-pet02,
+  author = 	 {Andrew Hintz},
+  title = 	 {Fingerprinting Websites Using Traffic Analysis},
+  booktitle = 	 {Privacy Enhancing Technologies (PET 2002)},
+  pages =	 {171--178},
+  year =	 2002,
+  editor =	 {Roger Dingledine and Paul Syverson},
+  publisher =	 {Springer-Verlag, LNCS 2482}
+ at InProceedings{or-discex00,
+   author =      {Paul Syverson and Michael Reed and David Goldschlag},
+   title =       {{O}nion {R}outing Access Configurations},
+   booktitle =   {DARPA Information Survivability Conference and
+                  Exposition (DISCEX 2000)},
+   year =        {2000},
+   publisher =   {IEEE CS Press},
+   pages =       {34--40},
+   volume =      {1},
+   %note =        {\newline \url{http://www.onion-router.net/Publications.html}},
+ at Inproceedings{or-pet00,
+  title =        {{Towards an Analysis of Onion Routing Security}},
+  author =       {Paul Syverson and Gene Tsudik and Michael Reed and
+                  Carl Landwehr},
+  booktitle =    {Designing Privacy Enhancing Technologies: Workshop
+                  on Design Issue in Anonymity and Unobservability},
+  year =         2000,
+  month =        {July},
+  pages =        {96--114},
+  editor =       {H. Federrath},
+  publisher =    {Springer-Verlag, LNCS 2009},
+  %note =         {\url{http://www.onion-router.net/Publications/WDIAU-2000.ps.gz}},
+ at Inproceedings{freenet-pets00,
+  title =        {Freenet: A Distributed Anonymous Information Storage
+    and Retrieval System},
+  author =       {Ian Clarke and Oskar Sandberg and Brandon Wiley and
+    Theodore W. Hong},
+  booktitle =    {Designing Privacy Enhancing Technologies: Workshop
+                  on Design Issue in Anonymity and Unobservability},
+  year =         2000,
+  month =        {July},
+  pages =        {46--66},
+  editor =       {H. Federrath},
+  publisher =    {Springer-Verlag, LNCS 2009},
+  %note =         {\url{http://citeseer.nj.nec.com/clarke00freenet.html}},
+ at InProceedings{or-ih96,
+  author =       {David M. Goldschlag and Michael G. Reed and Paul
+                  F. Syverson},
+  title =        {Hiding Routing Information},
+  booktitle =    {Information Hiding, First International Workshop},
+  pages =        {137--150},
+  year =         1996,
+  editor =       {R. Anderson},
+  month =        {May},
+  publisher =    {Springer-Verlag, LNCS 1174},
+ at InProceedings{federrath-ih96,
+  author = 	 {Hannes Federrath and Anja Jerichow and Andreas Pfitzmann},
+  title = 	 {{MIXes} in Mobile Communication Systems: Location
+                  Management with Privacy},
+  booktitle =    {Information Hiding, First International Workshop},
+  pages =        {121--135},
+  year =         1996,
+  editor =       {R. Anderson},
+  month =        {May},
+  publisher =    {Springer-Verlag, LNCS 1174},
+ at InProceedings{reed-protocols97,
+  author = 	 {Michael G. Reed and Paul F. Syverson and David
+                  M. Goldschlag},
+  title = 	 {Protocols Using Anonymous Connections: Mobile Applications},
+  booktitle = 	 {Security Protocols: 5th International Workshop},
+  pages =	 {13--23},
+  year =	 1997,
+  editor =	 {Bruce Christianson and Bruno Crispo and Mark Lomas
+                  and Michael Roe},
+  month =	 {April},
+  publisher =	 {Springer-Verlag, LNCS 1361}
+ at Article{or-jsac98,
+  author =       {Michael G. Reed and Paul F. Syverson and David
+                  M. Goldschlag},
+  title =        {Anonymous Connections and Onion Routing},
+  journal =      {IEEE Journal on Selected Areas in Communications},
+  year =         1998,
+  volume =       16,
+  number =       4,
+  pages =        {482--494},
+  month =        {May},
+  %note =         {\url{http://www.onion-router.net/Publications/JSAC-1998.ps.gz}}
+ at Misc{TLS,
+   author =      {T. Dierks and C. Allen},
+   title =       {The {TLS} {P}rotocol --- {V}ersion 1.0},
+   howpublished = {IETF RFC 2246},
+   month =       {January},
+   year =        {1999},
+%note =        {\url{http://www.rfc-editor.org/rfc/rfc2246.txt}},
+ at Misc{SMTP,
+   author =      {J. Postel},
+   title =       {Simple {M}ail {T}ransfer {P}rotocol},
+   howpublished = {IETF RFC 2821 (also STD0010)},
+   month =       {April},
+   year =        {2001},
+   note =        {\url{http://www.rfc-editor.org/rfc/rfc2821.txt}},
+ at Misc{IMAP,
+   author =      {M. Crispin},
+   title =       {Internet {M}essage {A}ccess {P}rotocol --- {V}ersion 4rev1},
+   howpublished = {IETF RFC 2060},
+   month =       {December},
+   year =        {1996},
+   note =        {\url{http://www.rfc-editor.org/rfc/rfc2060.txt}},
+ at misc{pipenet,
+  title = {PipeNet 1.1},
+  author = {Wei Dai},
+  year = 1996,
+  month = {August},
+  howpublished = {Usenet post},
+  note = {\url{http://www.eskimo.com/~weidai/pipenet.txt} First mentioned
+      in a post to the cypherpunks list, Feb.\ 1995.},
+ at Misc{POP3,
+   author =      {J. Myers and M. Rose},
+   title =       {Post {O}ffice {P}rotocol --- {V}ersion 3},
+   howpublished = {IETF RFC 1939 (also STD0053)},
+   month =       {May},
+   year =        {1996},
+   note =        {\url{http://www.rfc-editor.org/rfc/rfc1939.txt}},
+ at InProceedings{shuffle,
+   author =      {C. Andrew Neff},
+   title =       {A Verifiable Secret Shuffle and its Application to E-Voting},
+   booktitle =    {8th ACM Conference on Computer and Communications
+                  Security (CCS-8)},
+   pages =       {116--125},
+   year =        2001,
+   editor =      {P. Samarati},
+   month =       {November},
+   publisher =   {ACM Press},
+   %note =        {\url{http://www.votehere.net/ada_compliant/ourtechnology/
+   %                    technicaldocs/shuffle.pdf}},
+ at InProceedings{dolev91,
+   author =      {Danny Dolev and Cynthia Dwork and Moni Naor},
+   title =       {Non-Malleable Cryptography},
+   booktitle =   {23rd ACM Symposium on the Theory of Computing (STOC)},
+   pages =       {542--552},
+   year =        1991,
+   note =        {Updated version at
+                  \url{http://citeseer.nj.nec.com/dolev00nonmalleable.html}},
+ at TechReport{rsw96,
+   author =      {Ronald L. Rivest and Adi Shamir and David A. Wagner},
+   title =       {Time-lock puzzles and timed-release Crypto},
+   year =        1996,
+   type =        {MIT LCS technical memo},
+   number =      {MIT/LCS/TR-684},
+   month =       {February},
+   note =        {\newline \url{http://citeseer.nj.nec.com/rivest96timelock.html}},
+ at InProceedings{web-mix,
+   author =      {Oliver Berthold and Hannes Federrath and Stefan K\"opsell},
+   title =       {Web {MIX}es: A system for anonymous and unobservable
+                  {I}nternet access},
+  booktitle =    {Designing Privacy Enhancing Technologies: Workshop
+                  on Design Issue in Anonymity and Unobservability},
+   editor =       {H. Federrath},
+   publisher =    {Springer-Verlag, LNCS 2009},
+   year =        {2000},
+%   pages =       {115--129},
+ at InProceedings{disad-free-routes,
+   author =      {Oliver Berthold and Andreas Pfitzmann and Ronny Standtke},
+   title =       {The disadvantages of free {MIX} routes and how to overcome
+                  them},
+   booktitle =   {Designing Privacy Enhancing Technologies: Workshop
+                  on Design Issue in Anonymity and Unobservability},
+   pages =       {30--45},
+   year =        2000,
+   editor =       {H. Federrath},
+   publisher =    {Springer-Verlag, LNCS 2009},
+   %note =        {\url{http://www.tik.ee.ethz.ch/~weiler/lehre/netsec/Unterlagen/anon/
+   %                    disadvantages_berthold.pdf}},
+ at InProceedings{boneh00,
+   author =      {Dan Boneh and Moni Naor},
+   title =       {Timed Commitments},
+   booktitle =   {Advances in Cryptology -- {CRYPTO} 2000},
+   pages =       {236--254},
+   year =        2000,
+   publisher =   {Springer-Verlag, LNCS 1880},
+   note =        {\newline \url{http://crypto.stanford.edu/~dabo/abstracts/timedcommit.html}},
+ at InProceedings{goldschlag98,
+   author =      {David M. Goldschlag and Stuart G. Stubblebine},
+   title =       {Publicly Verifiable Lotteries: Applications of
+                  Delaying Functions},
+   booktitle =   {Financial Cryptography},
+   pages =       {214--226},
+   year =        1998,
+   publisher =   {Springer-Verlag, LNCS 1465},
+   note =        {\newline \url{http://citeseer.nj.nec.com/goldschlag98publicly.html}},
+ at InProceedings{syverson98,
+   author =      {Paul Syverson},
+   title =       {Weakly Secret Bit Commitment: Applications to
+                  Lotteries and Fair Exchange},
+   booktitle =   {Computer Security Foundations Workshop (CSFW11)},
+   pages =       {2--13},
+   year =        1998,
+   address =     {Rockport Massachusetts},
+   month =       {June},
+   publisher =   {IEEE CS Press},
+   note =        {\newline \url{http://chacs.nrl.navy.mil/publications/CHACS/1998/}},
+ at Misc{shoup-iso,
+   author =      {Victor Shoup},
+   title =       {A Proposal for an {ISO} {S}tandard for Public Key Encryption (version 2.1)},
+   note =        {Revised December 20, 2001. \url{http://www.shoup.net/papers/}},
+ at Misc{shoup-oaep,
+   author =      {Victor Shoup},
+   title =       {{OAEP} Reconsidered},
+   howpublished = {{IACR} e-print 2000/060},
+   note =        {\newline \url{http://eprint.iacr.org/2000/060/}},
+ at Misc{oaep-still-alive,
+   author =      {E. Fujisaki and D. Pointcheval and T. Okamoto and J. Stern},
+   title =       {{RSA}-{OAEP} is Still Alive!},
+   howpublished = {{IACR} e-print 2000/061},
+   note =        {\newline \url{http://eprint.iacr.org/2000/061/}},
+ at misc{echolot,
+  author = {Peter Palfrader},
+  title = {Echolot: a pinger for anonymous remailers},
+  note = {\url{http://www.palfrader.org/echolot/}},
+ at Misc{mixmaster-attacks,
+   author =      {Lance Cottrell},
+   title =       {Mixmaster and Remailer Attacks},
+   note =        {\url{http://www.obscura.com/~loki/remailer/remailer-essay.html}},
+ at Misc{mixmaster-spec,
+   author =      {Ulf M{\"o}ller and Lance Cottrell and Peter
+                  Palfrader and Len Sassaman},
+   title =       {Mixmaster {P}rotocol --- {V}ersion 2},
+   year =        {2003},
+   month =       {July},
+   howpublished = {Draft},
+   note =        {\url{http://www.abditum.com/mixmaster-spec.txt}},
+ at InProceedings{puzzles-tls,
+    author = "Drew Dean and Adam Stubblefield",
+    title = {{Using Client Puzzles to Protect TLS}},
+    booktitle = "Proceedings of the 10th USENIX Security Symposium",
+    year = {2001},
+    month = Aug,
+    publisher = {USENIX},
+ at InProceedings{breadpudding,
+  author =       {Markus Jakobsson and Ari Juels},
+  title =        {Proofs of Work and Bread Pudding Protocols},
+  booktitle =    {Proceedings of the IFIP TC6 and TC11 Joint Working
+                  Conference on Communications and Multimedia Security
+                  (CMS '99)},
+  year =         1999,
+  month =        {September},
+  publisher =    {Kluwer}
+ at Misc{hashcash,
+   author =      {Adam Back},
+   title =       {Hash cash},
+   note =        {\newline \url{http://www.cypherspace.org/~adam/hashcash/}},
+ at InProceedings{oreilly-acc,
+   author =      {Roger Dingledine and Michael J. Freedman and David Molnar},
+   title =       {Accountability},
+   booktitle =   {Peer-to-peer: Harnessing the Benefits of a Disruptive
+                  Technology},
+   year =        {2001},
+   publisher =   {O'Reilly and Associates},
+ at InProceedings{han,
+   author =      {Yongfei Han},
+   title =       {Investigation of non-repudiation protocols},
+   booktitle =   {ACISP '96},
+   year =        1996,
+   publisher =   {Springer-Verlag},
+ at Misc{socks5,
+  key =          {socks5},
+  title =        {{SOCKS} {P}rotocol {V}ersion 5},
+  howpublished=  {IETF RFC 1928},
+  month =        {March},
+  year =         1996,
+  note =         {\url{http://www.ietf.org/rfc/rfc1928.txt}}
+ at InProceedings{abe,
+   author =      {Masayuki Abe},
+   title =       {Universally Verifiable {MIX} With Verification Work Independent of
+                  The Number of {MIX} Servers},
+   booktitle =   {{EUROCRYPT} 1998},
+   year =        {1998},
+   publisher =   {Springer-Verlag, LNCS 1403},
+ at InProceedings{desmedt,
+   author =      {Yvo Desmedt and Kaoru Kurosawa},
+   title =       {How To Break a Practical {MIX} and Design a New One},
+   booktitle =   {{EUROCRYPT} 2000},
+   year =        {2000},
+   publisher =   {Springer-Verlag, LNCS 1803},
+   note =        {\url{http://citeseer.nj.nec.com/447709.html}},
+ at InProceedings{mitkuro,
+   author =      {M. Mitomo and K. Kurosawa},
+   title =       {{Attack for Flash MIX}},
+   booktitle =   {{ASIACRYPT} 2000},
+   year =        {2000},
+   publisher =   {Springer-Verlag, LNCS 1976},
+   note =        {\newline \url{http://citeseer.nj.nec.com/450148.html}},
+ at InProceedings{hybrid-mix,
+   author =      {M. Ohkubo and M. Abe},
+   title =       {A {L}ength-{I}nvariant {H}ybrid {MIX}},
+   booktitle =   {Advances in Cryptology - {ASIACRYPT} 2000},
+   year =        {2000},
+   publisher =   {Springer-Verlag, LNCS 1976},
+ at InProceedings{PShuffle,
+  author = {Jun Furukawa and Kazue Sako},
+  title = {An Efficient Scheme for Proving a Shuffle},
+  editor    = {Joe Kilian},
+  booktitle = {CRYPTO 2001},
+  year      = {2001},
+  publisher = {Springer-Verlag, LNCS 2139},
+ at InProceedings{jakobsson-optimally,
+  author = "Markus Jakobsson and Ari Juels",
+  title = "An Optimally Robust Hybrid Mix Network (Extended Abstract)",
+  booktitle =   {Principles of Distributed Computing - {PODC} '01},
+  year = "2001",
+  publisher =   {ACM Press},
+  note = {\url{http://citeseer.nj.nec.com/492015.html}},
+ at InProceedings{kesdogan,
+   author =      {D. Kesdogan and M. Egner and T. B\"uschkes},
+   title =       {Stop-and-Go {MIX}es Providing Probabilistic Anonymity in an Open
+                  System},
+   booktitle =   {Information Hiding (IH 1998)},
+   year =        {1998},
+   publisher =   {Springer-Verlag, LNCS 1525},
+   %note =        {\url{http://www.cl.cam.ac.uk/~fapp2/ihw98/ihw98-sgmix.pdf}},
+ at InProceedings{socks4,
+  author =       {David Koblas and Michelle R. Koblas},
+  title =        {{SOCKS}},
+  booktitle =    {UNIX Security III Symposium (1992 USENIX Security
+                  Symposium)},
+  pages =        {77--83},
+  year =         1992,
+  publisher =    {USENIX},
+ at InProceedings{flash-mix,
+   author =      {Markus Jakobsson},
+   title =       {Flash {M}ixing},
+   booktitle =   {Principles of Distributed Computing - {PODC} '99},
+   year =        {1999},
+   publisher =   {ACM Press},
+   note =        {\newline \url{http://citeseer.nj.nec.com/jakobsson99flash.html}},
+ at InProceedings{SK,
+   author =      {Joe Kilian and Kazue Sako},
+   title =       {Receipt-Free {MIX}-Type Voting Scheme - A Practical Solution to
+                  the Implementation of a Voting Booth},
+   booktitle =   {EUROCRYPT '95},
+   year =        {1995},
+   publisher =   {Springer-Verlag},
+ at InProceedings{OAEP,
+   author =      {M. Bellare and P. Rogaway},
+   year =        {1994},
+   booktitle =   {EUROCRYPT '94},
+   title =       {Optimal {A}symmetric {E}ncryption {P}adding : How To Encrypt With
+                  {RSA}},
+   publisher =   {Springer-Verlag},
+   note =        {\newline \url{http://www-cse.ucsd.edu/users/mihir/papers/oaep.html}},
+ at inproceedings{babel,
+  title = {Mixing {E}-mail With {B}abel},
+  author = {Ceki G\"ulc\"u and Gene Tsudik},
+  booktitle = {{Network and Distributed Security Symposium (NDSS 96)}},
+  year = 1996,
+  month = {February},
+  pages = {2--16},
+  publisher = {IEEE},
+  %note = {\url{http://citeseer.nj.nec.com/2254.html}},
+ at Misc{rprocess,
+   author =      {RProcess},
+   title =       {Selective Denial of Service Attacks},
+   note =        {\newline \url{http://www.eff.org/pub/Privacy/Anonymity/1999\_09\_DoS\_remail\_vuln.html}},
+ at Article{remailer-history,
+  author = {Sameer Parekh},
+  title = {Prospects for Remailers},
+  journal = {First Monday},
+  volume = {1},
+  number = {2},
+  month = {August},
+  year = {1996},
+  note = {\url{http://www.firstmonday.dk/issues/issue2/remailers/}},
+ at Article{chaum-mix,
+   author =      {David Chaum},
+   title =       {Untraceable electronic mail, return addresses, and digital pseudo-nyms},
+   journal =     {Communications of the ACM},
+   year =        {1981},
+   volume =      {4},
+   number =      {2},
+   month =       {February},
+   %note =        {\url{http://www.eskimo.com/~weidai/mix-net.txt}},
+ at InProceedings{nym-alias-net,
+  author =       {David Mazi\`{e}res and M. Frans Kaashoek},
+  title =        {{The Design, Implementation and Operation of an Email
+                  Pseudonym Server}},
+  booktitle =    {$5^{th}$ ACM Conference on Computer and
+                  Communications Security (CCS'98)},
+  year =         1998,
+  publisher =    {ACM Press},
+  %note =        {\newline \url{http://www.scs.cs.nyu.edu/~dm/}},
+ at InProceedings{tangler,
+  author =       {Marc Waldman and David Mazi\`{e}res},
+  title =        {Tangler: A Censorship-Resistant Publishing System
+                  Based on Document Entanglements},
+  booktitle =    {$8^{th}$ ACM Conference on Computer and
+                  Communications Security (CCS-8)},
+  pages =        {86--135},
+  year =         2001,
+  publisher =    {ACM Press},
+  %note =         {\url{http://www.scs.cs.nyu.edu/~dm/}}
+ at misc{neochaum,
+   author =      {Tim May},
+   title =       {Payment mixes for anonymity},
+   howpublished = {E-mail archived at
+                  \url{http://\newline www.inet-one.com/cypherpunks/dir.2000.02.28-2000.03.05/msg00334.html}},
+ at misc{helsingius,
+   author =      {J. Helsingius},
+   title =       {{\tt anon.penet.fi} press release},
+   note =        {\newline \url{http://www.penet.fi/press-english.html}},
+ at InProceedings{garay97secure,
+   author =      {J. Garay and R. Gennaro and C. Jutla and T. Rabin},
+   title =       {Secure distributed storage and retrieval},
+   booktitle =   {11th International Workshop, WDAG '97},
+   pages =       {275--289},
+   year =        {1997},
+   publisher =   {Springer-Verlag, LNCS 1320},
+   note =        {\newline \url{http://citeseer.nj.nec.com/garay97secure.html}},
+ at InProceedings{PIK,
+   author =      {C. Park and K. Itoh and K. Kurosawa},
+   title =       {Efficient anonymous channel and all/nothing election scheme},
+   booktitle =   {Advances in Cryptology -- {EUROCRYPT} '93},
+   pages =       {248--259},
+   publisher =   {Springer-Verlag, LNCS 765},
+ at Misc{pgpfaq,
+   key =         {PGP},
+   title =       {{PGP} {FAQ}},
+   note =        {\newline \url{http://www.faqs.org/faqs/pgp-faq/}},
+ at Article{riordan-schneier,
+   author =      {James Riordan and Bruce Schneier},
+   title =       {A Certified E-mail Protocol with No Trusted Third Party},
+   journal =     {13th Annual Computer Security Applications Conference},
+   month =       {December},
+   year =        {1998},
+   note =        {\newline \url{http://www.counterpane.com/certified-email.html}},
+ at Article{crowds-tissec,
+  author =       {Michael K. Reiter and Aviel D. Rubin},
+  title =        {Crowds: Anonymity for Web Transactions},
+  journal =      {ACM TISSEC},
+  year =         1998,
+  volume =       1,
+  number =       1,
+  pages =        {66--92},
+  month =        {June},
+  %note =         {\url{http://citeseer.nj.nec.com/284739.html}}
+ at Article{crowds-dimacs,
+   author =      {Michael K. Reiter and Aviel D. Rubin},
+   title =       {Crowds: Anonymity for Web Transactions},
+   journal =     {{DIMACS} Technical Report (Revised)},
+   volume =      {97},
+   number =      {15},
+   month =       {August},
+   year =        {1997},
+ at Misc{advogato,
+   author =      {Raph Levien},
+   title =       {Advogato's Trust Metric},
+   note =        {\newline \url{http://www.advogato.org/trust-metric.html}},
+ at InProceedings{publius,
+   author =      {Marc Waldman and Aviel Rubin and Lorrie Cranor},
+   title =       {Publius: {A} robust, tamper-evident, censorship-resistant and
+                  source-anonymous web publishing system},
+   booktitle =   {Proc. 9th USENIX Security Symposium},
+   pages =       {59--72},
+   year =        {2000},
+   month =       {August},
+   %note =        {\newline \url{http://citeseer.nj.nec.com/waldman00publius.html}},
+ at Misc{freedom-nyms,
+   author =      {Russell Samuels},
+   title =       {Untraceable Nym Creation on the {F}reedom {N}etwork},
+   year =        {1999},
+   month =       {November},
+   day =         {21},
+   note =        {\newline \url{http://www.freedom.net/products/whitepapers/white11.html}},
+ at techreport{freedom2-arch,
+  title = {Freedom Systems 2.0 Architecture},
+  author = {Philippe Boucher and Adam Shostack and Ian Goldberg},
+  institution = {Zero Knowledge Systems, {Inc.}},
+  year = {2000},
+  month = {December},
+  type = {White Paper},
+  day = {18},
+ at techreport{freedom21-security,
+  title = {Freedom Systems 2.1 Security Issues and Analysis},
+  author = {Adam Back and Ian Goldberg and Adam Shostack},
+  institution = {Zero Knowledge Systems, {Inc.}},
+  year = {2001},
+  month = {May},
+  type = {White Paper},
+ at inproceedings{cfs:sosp01,
+  title = {Wide-area cooperative storage with {CFS}},
+  author = {Frank Dabek and M. Frans Kaashoek and David Karger and Robert Morris and Ion Stoica},
+  booktitle = {18th {ACM} {S}ymposium on {O}perating {S}ystems {P}rinciples ({SOSP} '01)},
+  year = {2001},
+  month = {October},
+  address = {Chateau Lake Louise, Banff, Canada},
+ at inproceedings{SS03,
+  title = {Passive Attack Analysis for Connection-Based Anonymity Systems},
+  author = {Andrei Serjantov and Peter Sewell},
+  booktitle = {Computer Security -- ESORICS 2003},
+  publisher =   {Springer-Verlag, LNCS 2808},
+  year = {2003},
+  month = {October},
+  %note = {\url{http://www.cl.cam.ac.uk/users/aas23/papers_aas/conn_sys.ps}},
+ at Misc{pk-relations,
+   author =      {M. Bellare and A. Desai and D. Pointcheval and P. Rogaway},
+   title =       {Relations Among Notions of Security for Public-Key Encryption
+                  Schemes},
+   howpublished = {
+                  Extended abstract in {\em Advances in Cryptology - CRYPTO '98}, LNCS Vol. 1462.
+                  Springer-Verlag, 1998.
+                  Full version available from \newline \url{http://www-cse.ucsd.edu/users/mihir/}},
+ at InProceedings{mix-acc,
+  author =      {Roger Dingledine and Michael J. Freedman and David
+                  Hopwood and David Molnar},
+  title =       {{A Reputation System to Increase MIX-net
+                  Reliability}},
+  booktitle =   {Information Hiding (IH 2001)},
+  pages =       {126--141},
+  year =        2001,
+  editor =      {Ira S. Moskowitz},
+  publisher =   {Springer-Verlag, LNCS 2137},
+  %note =        {\url{http://www.freehaven.net/papers.html}},
+ at InProceedings{casc-rep,
+   author =      {Roger Dingledine and Paul Syverson},
+   title =       {{Reliable MIX Cascade Networks through Reputation}},
+  booktitle =    {Financial Cryptography},
+  year =         2002,
+  editor =       {Matt Blaze},
+  publisher =    {Springer-Verlag, LNCS 2357},
+  %note =        {\newline \url{http://www.freehaven.net/papers.html}},
+ at InProceedings{zhou96certified,
+   author =      {Zhou and Gollmann},
+   title =       {Certified Electronic Mail},
+   booktitle =   {{ESORICS: European Symposium on Research in Computer
+                  Security}},
+   publisher =   {Springer-Verlag, LNCS 1146},
+   year =        {1996},
+   note =        {\newline \url{http://citeseer.nj.nec.com/zhou96certified.html}},
+ at Misc{realtime-mix,
+   author =      {Anja Jerichow and Jan M\"uller and Andreas Pfitzmann and
+                  Birgit Pfitzmann and Michael Waidner},
+   title =       {{Real-Time MIXes: A Bandwidth-Efficient Anonymity Protocol}},
+   howpublished = {IEEE Journal on Selected Areas in Communications, 1998.},
+   note =        {\url{http://www.zurich.ibm.com/security/publications/1998.html}},
+ at InProceedings{danezis:pet2003,
+  author =       {George Danezis},
+  title =        {Mix-networks with Restricted Routes},
+  booktitle =    {Privacy Enhancing Technologies (PET 2003)},
+  year =         2003,
+  editor =       {Roger Dingledine},
+  publisher =    {Springer-Verlag LNCS 2760}
+ at InProceedings{gap-pets03,
+  author =       {Krista Bennett and Christian Grothoff},
+  title =        {{GAP} -- practical anonymous networking},
+  booktitle =    {Privacy Enhancing Technologies (PET 2003)},
+  year =         2003,
+  editor =       {Roger Dingledine},
+  publisher =    {Springer-Verlag LNCS 2760}
+ at Article{hordes-jcs,
+  author =       {Brian Neal Levine and Clay Shields},
+  title =        {Hordes: A Multicast-Based Protocol for Anonymity},
+  journal =      {Journal of Computer Security},
+  year =         2002,
+  volume =       10,
+  number =       3,
+  pages =        {213--240}
+ at TechReport{herbivore,
+  author =       {Sharad Goel and Mark Robson and Milo Polte and Emin G\"{u}n Sirer},
+  title =        {Herbivore: A Scalable and Efficient Protocol for Anonymous Communication},
+  institution =  {Cornell University Computing and Information Science},
+  year =         2003,
+  type =         {Technical Report},
+  number =       {TR2003-1890},
+  month =        {February}
+ at InProceedings{p5,
+  author =       {Rob Sherwood and Bobby Bhattacharjee and Aravind Srinivasan},
+  title =        {$P^5$: A Protocol for Scalable Anonymous Communication},
+  booktitle =    {IEEE Symposium on Security and Privacy},
+  pages =        {58--70},
+  year =         2002,
+  publisher =    {IEEE CS}
+ at phdthesis{ian-thesis,
+  title = {A Pseudonymous Communications Infrastructure for the Internet},
+  author = {Ian Goldberg},
+  school = {UC Berkeley},
+  year = {2000},
+  month = {Dec},
+ at Article{taz,
+  author =       {Ian Goldberg and David Wagner},
+  title =        {TAZ Servers and the Rewebber Network: Enabling
+                  Anonymous Publishing on the World Wide Web},
+  journal =      {First Monday},
+  year =         1998,
+  volume =       3,
+  number =       4,
+  month =        {August},
+  note =         {\url{http://www.firstmonday.dk/issues/issue3_4/goldberg/}}
+ at Misc{tcp-over-tcp-is-bad,
+  key =          {tcp-over-tcp-is-bad},
+  title =        {Why {TCP} Over {TCP} Is A Bad Idea},
+  author =       {Olaf Titz},
+  note =         {\url{http://sites.inka.de/sites/bigred/devel/tcp-tcp.html}}
+ at inproceedings{wright02,
+  title = {An Analysis of the Degradation of Anonymous Protocols},
+  author = {Matthew Wright and Micah Adler and Brian Neil Levine and Clay Shields},
+  booktitle = {{Network and Distributed Security Symposium (NDSS 02)}},
+  year = {2002},
+  month = {February},
+  publisher = {IEEE},
+ at inproceedings{wright03,
+  title = {Defending Anonymous Communication Against Passive Logging Attacks},
+  author = {Matthew Wright and Micah Adler and Brian Neil Levine and Clay Shields},
+  booktitle = {IEEE Symposium on Security and Privacy},
+  pages= {28--41},
+  year = {2003},
+  month = {May},
+  publisher = {IEEE CS},
+ at InProceedings{attack-tor-oak05,
+  author = 	 {Steven J. Murdoch and George Danezis},
+  title = 	 {Low-cost Traffic Analysis of {T}or},
+  booktitle = 	 {IEEE Symposium on Security and Privacy},
+  year =	 2005,
+  month =	 {May},
+  publisher =	 {IEEE CS}
+ at Misc{jap-backdoor,
+  author={{The AN.ON Project}},
+  howpublished={Press release},
+  year={2003},
+  month={September},
+  title={German Police proceeds against anonymity service},
+  note={\url{http://www.datenschutzzentrum.de/material/themen/presse/anon-bka_e.htm}}
+ at article{shsm03,
+  title = {Using Caching for Browsing Anonymity},
+  author = {Anna Shubina and Sean Smith},
+  journal = {ACM SIGEcom Exchanges},
+  volume = {4},
+  number = {2},
+  year = {2003},
+  month = {Sept},
+  note = {\url{http://www.acm.org/sigs/sigecom/exchanges/volume_4_(03)/4.2-Shubina.pdf}},
+ at inproceedings{tor-design,
+  title = {Tor: The Second-Generation Onion Router},
+  author = {Roger Dingledine and Nick Mathewson and Paul Syverson},
+  booktitle = {Proceedings of the 13th USENIX Security Symposium},
+  year = {2004},
+  month = {August},
+  note = {\url{https://www.torproject.org/tor-design.pdf}}
+ at inproceedings{flow-correlation04,
+  title = {On Flow Correlation Attacks and Countermeasures in Mix Networks},
+  author = {Ye Zhu and Xinwen Fu and Bryan Graham and Riccardo Bettati and Wei Zhao},
+  booktitle = {Proceedings of Privacy Enhancing Technologies workshop (PET 2004)},
+  year = {2004},
+  month = {May},
+  series = {LNCS},
+  note = {\url{http://students.cs.tamu.edu/xinwenfu/paper/PET04.pdf}},
+ at InProceedings{danezis:pet2004,
+  author = "George Danezis",
+  title = "The Traffic Analysis of Continuous-Time Mixes",
+  booktitle= {Privacy Enhancing Technologies (PET 2004)},
+  editor = {David Martin and Andrei Serjantov},
+  month = {May},
+  year = {2004},
+  series = {LNCS},
+  note = {\url{http://www.cl.cam.ac.uk/users/gd216/cmm2.pdf}},
+ at inproceedings{feamster:wpes2004,
+  title = {Location Diversity in Anonymity Networks},
+  author = {Nick Feamster and Roger Dingledine},
+  booktitle = {{Proceedings of the Workshop on Privacy in the Electronic Society (WPES 2004)}},
+  year = {2004},
+  month = {October},
+  address = {Washington, DC, USA},
+  note = {\url{http://freehaven.net/doc/routing-zones/routing-zones.ps}},
+ at inproceedings{koepsell:wpes2004,
+  title = {How to Achieve Blocking Resistance for Existing Systems Enabling Anonymous Web Surfing},
+  author = {Stefan K\"opsell and Ulf Hilling},
+  booktitle = {{Proceedings of the Workshop on Privacy in the Electronic Society (WPES 2004)}},
+  year = {2004},
+  month = {October},
+  address = {Washington, DC, USA},
+  note = {\url{http://freehaven.net/anonbib/papers/p103-koepsell.pdf}},
+ at inproceedings{sync-batching,
+  title = {Synchronous Batching: From Cascades to Free Routes},
+  author = {Roger Dingledine and Vitaly Shmatikov and Paul Syverson},
+  booktitle = {Proceedings of Privacy Enhancing Technologies workshop (PET 2004)},
+  editor = {David Martin and Andrei Serjantov},
+  year = {2004},
+  month = {May},
+  series = {LNCS},
+  note = {\url{http://freehaven.net/doc/sync-batching/sync-batching.pdf}},
+ at InProceedings{e2e-traffic,
+  author = "Nick Mathewson and Roger Dingledine",
+  title = "Practical Traffic Analysis: Extending and Resisting Statistical Disclosure",
+  booktitle= {Privacy Enhancing Technologies (PET 2004)},
+  editor = {David Martin and Andrei Serjantov},
+  month = {May},
+  year = {2004},
+  series = {LNCS},
+  note = {\url{http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf}},
+ at Misc{dtls,
+   author =      {E. Rescorla and N. Modadugu},
+   title =       {{Datagram Transport Layer Security}},
+   howpublished = {IETF Draft},
+   month =       {December},
+   year =        {2003},
+   note =        {\url{http://www.ietf.org/internet-drafts/draft-rescorla-dtls-02.txt}},
+ at InProceedings{usability-network-effect,
+   author={Roger Dingledine and Nick Mathewson},
+   title={Anonymity Loves Company: Usability and the Network Effect},
+   booktitle = {Designing Security Systems That People Can Use},
+   year = {2005},
+   publisher = {O'Reilly Media},
+ at inproceedings{usability:weis2006,
+  title = {Anonymity Loves Company: Usability and the Network Effect},
+  author = {Roger Dingledine and Nick Mathewson},
+  booktitle = {Proceedings of the Fifth Workshop on the Economics of Information Security
+        (WEIS 2006)},
+  year = {2006},
+  month = {June},
+  address = {Cambridge, UK},
+  bookurl = {http://weis2006.econinfosec.org/},
+  note = {\url{http://freehaven.net/doc/wupss04/usability.pdf}},
+ at Misc{six-four,
+  key =          {six-four},
+  title =        {{The Six/Four System}},
+  note =         {\url{http://sourceforge.net/projects/sixfour/}}
+ at inproceedings{clayton:pet2006,
+  title = {Ignoring the Great Firewall of China},
+  author = {Richard Clayton and Steven J. Murdoch and Robert N. M. Watson},
+  booktitle = {Proceedings of the Sixth Workshop on Privacy Enhancing Technologies (PET 2006)},
+  year = {2006},
+  month = {June},
+  address = {Cambridge, UK},
+  publisher = {Springer},
+  bookurl = {http://petworkshop.org/2006/},
+  note = {\url{http://www.cl.cam.ac.uk/~rnc1/ignoring.pdf}},
+ at Misc{zuckerman-threatmodels,
+  key =          {zuckerman-threatmodels},
+  title =        {We've got to adjust some of our threat models},
+  author =       {Ethan Zuckerman},
+  note =         {\url{http://www.ethanzuckerman.com/blog/?p=1019}}
+ at Misc{cgiproxy,
+  key =          {cgiproxy},
+  title =        {{CGIProxy: HTTP/FTP Proxy in a CGI Script}},
+  author =       {James Marshall},
+  note =         {\url{http://www.jmarshall.com/tools/cgiproxy/}}
+ at Misc{circumventor,
+  key =          {circumventor},
+  title =        {{How to install the Circumventor program}},
+  author =       {Bennett Haselton},
+  note =         {\url{http://www.peacefire.org/circumventor/simple-circumventor-instructions.html}}
+ at Misc{psiphon,
+  key =          {psiphon},
+  title =        {Psiphon},
+  author =       {Ronald Deibert et al},
+  note =         {\url{http://psiphon.civisec.org/}}
+ at InProceedings{tcpstego,                                                                    author =       {Steven J. Murdoch and Stephen Lewis},
+  title =        {Embedding Covert Channels into {TCP/IP}},
+  booktitle = {Information Hiding: 7th International Workshop},
+  pages  =       {247--261},
+  year =         {2005},
+  editor =       {Mauro Barni and Jordi Herrera-Joancomart\'{\i} and
+Stefan Katzenbeisser and Fernando P\'{e}rez-Gonz\'{a}lez},
+  volume =       {3727},
+  series =       {LNCS},
+  address =      {Barcelona, Catalonia (Spain)},
+  month =        {June},
+  publisher = {Springer-Verlag},
+  url = {http://www.cl.cam.ac.uk/~sjm217/papers/ih05coverttcp.pdf}
+ at phdthesis{blossom-thesis,
+  title = {Perspective Access Networks},
+  author = {Geoffrey Goodell},
+  school = {Harvard University},
+  year = {2006},
+  month = {July},
+  note = {\url{http://afs.eecs.harvard.edu/~goodell/thesis.pdf}},
+ at inproceedings{tap:pet2006,
+  title = {On the Security of the Tor Authentication Protocol},
+  author = {Ian Goldberg},
+  booktitle = {Proceedings of the Sixth Workshop on Privacy Enhancing Technologies (PET 2006)},
+  year = {2006},
+  month = {June},
+  address = {Cambridge, UK},
+  publisher = {Springer},
+  bookurl = {http://petworkshop.org/2006/},
+  note = {\url{http://www.cypherpunks.ca/~iang/pubs/torsec.pdf}},
+ at inproceedings{rep-anon,
+  title = {{Reputation in P2P Anonymity Systems}},
+  author = {Roger Dingledine and Nick Mathewson and Paul Syverson},
+  booktitle = {Proceedings of Workshop on Economics of Peer-to-Peer Systems},
+  year = {2003},
+  month = {June},
+  note = {\url{http://freehaven.net/doc/econp2p03/econp2p03.pdf}},
+ at misc{tor-challenges,
+   author       = {Roger Dingledine and Nick Mathewson and Paul Syverson},
+   title        = {Challenges in deploying low-latency anonymity},
+   year         = {2005},
+   note         = {Manuscript}
+ at InProceedings{chaum-blind,
+  author =       {David Chaum},
+  title =        {Blind Signatures for Untraceable Payments},
+  booktitle =    {Advances in Cryptology: Proceedings of Crypto 82},
+  pages =        {199--203},
+  year =         1983,
+  editor =       {D. Chaum and R.L. Rivest and A.T. Sherman},
+  publisher =    {Plenum Press}
+ at Article{netauth,
+  author = {Geoffrey Goodell and Paul Syverson},
+  title = {The Right Place at the Right Time: Examining the use of network location in authentication and abuse prevention},
+  journal = 	 {Communications of the ACM},
+  year = 	 2007,
+  volume = 	 50,
+  number = 	 5,
+  pages = 	 {113--117},
+  month = 	 {May}
+ at misc{ip-to-country,
+  key = {ip-to-country},
+  title = {IP-to-country database},
+  note = {\url{http://ip-to-country.webhosting.info/}},
+ at misc{mackinnon-personal,
+  author = {Rebecca MacKinnon},
+  title = {Private communication},
+  year = {2006},
+ at inproceedings{pet05-bissias,
+  title = {Privacy Vulnerabilities in Encrypted HTTP Streams},
+  author = {George Dean Bissias and Marc Liberatore and Brian Neil Levine},
+  booktitle = {Proceedings of Privacy Enhancing Technologies workshop (PET 2005)},
+  year = {2005},
+  month = {May},
+  note = {\url{http://prisms.cs.umass.edu/brian/pubs/bissias.liberatore.pet.2005.pdf}},
+ at InProceedings{infranet,
+   author =      {Nick Feamster and Magdalena Balazinska and Greg Harfst and Hari Balakrishnan and David Karger},
+   title =       {Infranet: Circumventing Web Censorship and Surveillance},
+   booktitle =   {Proceedings of the 11th USENIX Security Symposium},
+   year =        {2002},
+   month =       {August},
+   note = {\url{http://nms.lcs.mit.edu/~feamster/papers/usenixsec2002.pdf}},
+ at techreport{ ptacek98insertion,
+  author = "Thomas H. Ptacek and Timothy N. Newsham",
+  title = "Insertion, Evasion, and Denial of Service: Eluding Network Intrusion Detection",
+  institution = "Secure Networks, Inc.",
+  address = "Suite 330, 1201 5th Street S.W, Calgary, Alberta, Canada, T2R-0Y6",
+  year = "1998",
+  url = "citeseer.ist.psu.edu/ptacek98insertion.html",
+ at inproceedings{active-wardens,
+  author = "Gina Fisk and Mike Fisk and Christos Papadopoulos and Joshua Neil",
+  title = "Eliminating Steganography in Internet Traffic with Active Wardens",
+  booktitle = {Information Hiding Workshop (IH 2002)},
+  year = {2002},
+  month = {October},
+  editor = {Fabien Petitcolas},
+  publisher = {Springer-Verlag, LNCS 2578},
+ at inproceedings{clog-the-queue,
+  title = {Don't Clog the Queue: Circuit Clogging and Mitigation in {P2P} anonymity schemes},
+  author = {Jon McLachlan and Nicholas Hopper},
+  booktitle = {Proceedings of Financial Cryptography (FC '08)},
+  year = {2008},
+  month = {January},
+ at inproceedings{snader08,
+  title = {A Tune-up for {Tor}: Improving Security and Performance in the {Tor} Network},
+  author = {Robin Snader and Nikita Borisov},
+  booktitle = {Proceedings of the Network and Distributed Security Symposium - {NDSS} '08},
+  year = {2008},
+  month = {February},
+  publisher = {Internet Society},
+ at inproceedings{murdoch-pet2008,
+  title = {Metrics for Security and Performance in Low-Latency Anonymity Networks},
+  author = {Steven J. Murdoch and Robert N. M. Watson},
+  booktitle = {Proceedings of the Eighth International Symposium on Privacy Enhancing Technologies (PETS 2008)},
+  year = {2008},
+  month = {July},
+  address = {Leuven, Belgium},
+  pages = {115--132},
+  editor = {Nikita Borisov and Ian Goldberg},
+  publisher = {Springer},
+  bookurl = {http://petsymposium.org/2008/},
+ at inproceedings{danezis-pet2008,
+  title = {Bridging and Fingerprinting: Epistemic Attacks on Route Selection},
+  author = {George Danezis and Paul Syverson},
+  booktitle = {Proceedings of the Eighth International Symposium on Privacy Enhancing Technologies (PETS 2008)},
+  year = {2008},
+  month = {July},
+  address = {Leuven, Belgium},
+  pages = {133--150},
+  editor = {Nikita Borisov and Ian Goldberg},
+  publisher = {Springer},
+  bookurl = {http://petsymposium.org/2008/},
+%%% Local Variables:
+%%% mode: latex
+%%% TeX-master: "tor-design"
+%%% End:
diff --git a/2006/blocking/usenixsubmit.cls b/2006/blocking/usenixsubmit.cls
new file mode 100644
index 0000000..743ffcf
--- /dev/null
+++ b/2006/blocking/usenixsubmit.cls
@@ -0,0 +1,7 @@
+% Created by Anil Somayaji

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