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diff --git a/doc/design-paper/blocking.html b/doc/design-paper/blocking.html deleted file mode 100644 index 6028f5dc1c..0000000000 --- a/doc/design-paper/blocking.html +++ /dev/null @@ -1,2112 +0,0 @@ -<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" - "DTD/xhtml1-transitional.dtd"> -<html> -<meta name="GENERATOR" content="TtH 3.77"> -<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"> - <style type="text/css"> div.p { margin-top: 7pt;}</style> - <style type="text/css"><!-- - td div.comp { margin-top: -0.6ex; margin-bottom: -1ex;} - td div.comb { margin-top: -0.6ex; margin-bottom: -.6ex;} - td div.hrcomp { line-height: 0.9; margin-top: -0.8ex; margin-bottom: -1ex;} - td div.norm {line-height:normal;} - span.roman {font-family: serif; font-style: normal; font-weight: normal;} - span.overacc2 {position: relative; left: .8em; top: -1.2ex;} - span.overacc1 {position: relative; left: .6em; top: -1.2ex;} --></style> - - -<title> Design of a blocking-resistant anonymity system\DRAFT</title> - -<h1 align="center">Design of a blocking-resistant anonymity system<br />DRAFT </h1> - -<div class="p"><!----></div> - -<h3 align="center">Roger Dingledine, Nick Mathewson </h3> - - -<div class="p"><!----></div> - -<h2> Abstract</h2> -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. - -<div class="p"><!----></div> -Here we describe a design that builds upon the current Tor network -to provide an anonymizing network that resists blocking -by government-level attackers. - -<div class="p"><!----></div> - - <h2><a name="tth_sEc1"> -1</a> Introduction and Goals</h2> - -<div class="p"><!----></div> -Anonymizing networks like Tor [<a href="#tor-design" name="CITEtor-design">11</a>] bounce traffic around a -network of encrypting relays. Unlike encryption, which hides only <i>what</i> -is said, these networks also aim to hide who is communicating with whom, which -users are using which websites, and similar relations. 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 -noticed. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 -Wikipedia -and Blogspot, they are no longer affected by local censorship -and firewall rules. In fact, an informal user study -showed China as the third largest user base -for Tor clients, with perhaps ten thousand people accessing the Tor -network from China each day. - -<div class="p"><!----></div> -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 server IP addresses in the directory, or by filtering -based on the fingerprint of the Tor TLS handshake. Here we describe an -extended design that builds upon the current Tor network to provide an -anonymizing -network that resists censorship as well as anonymity-breaking attacks. -In section <a href="#sec:adversary">2</a> we discuss our threat model — that is, -the assumptions we make about our adversary. Section <a href="#sec:current-tor">3</a> -describes the components of the current Tor design and how they can be -leveraged for a new blocking-resistant design. Section <a href="#sec:related">4</a> -explains the features and drawbacks of the currently deployed solutions. -In sections <a href="#sec:bridges">5</a> through <a href="#sec:discovery">7</a>, we explore the -components of our designs in detail. Section <a href="#sec:security">8</a> considers -security implications and Section <a href="#sec:reachability">9</a> presents other -issues with maintaining connectivity and sustainability for the design. -Section <a href="#sec:future">10</a> speculates about future more complex designs, -and finally Section <a href="#sec:conclusion">11</a> summarizes our next steps and -recommendations. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h2><a name="tth_sEc2"> -<a name="sec:adversary"> -2</a> Adversary assumptions</h2> -</a> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 a server blocked in one locale can still be useful in others. - -<div class="p"><!----></div> -We assume that the attackers' goals are somewhat complex. - -<dl compact="compact"> - - <dt><b></b></dt> - <dd><li>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).</dd> - <dt><b></b></dt> - <dd><li>As a second-order effect, censors aim to chill citizens' behavior by - creating an impression that their online activities are monitored.</dd> - <dt><b></b></dt> - <dd><li>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.</dd> - <dt><b></b></dt> - <dd><li>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, but unnecessary: if "undesirable" information is known only - to a small few, further censoring efforts can be focused elsewhere.</dd> - <dt><b></b></dt> - <dd><li>Similarly, the censors are not attempting to shut down or block <i> - every</i> 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).</dd> - <dt><b></b></dt> - <dd><li>Reprisal against <i>most</i> passive consumers of <i>most</i> kinds of - blocked information is also not a goal, given the broadness of most - censorship regimes. This seems borne out by fact.<a href="#tthFtNtAAB" name="tthFrefAAB"><sup>1</sup></a></dd> - <dt><b></b></dt> - <dd><li>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.</dd> - <dt><b></b></dt> - <dd><li>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 <i>are</i> 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). -</dd> -</dl> - -<div class="p"><!----></div> -We assume there are three main technical network attacks in use by censors -currently [<a href="#clayton:pet2006" name="CITEclayton:pet2006">7</a>]: - -<div class="p"><!----></div> - -<dl compact="compact"> - - <dt><b></b></dt> - <dd><li>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.</dd> - <dt><b></b></dt> - <dd><li>Block a destination by listing its IP address at a - firewall or other routing control point.</dd> - <dt><b></b></dt> - <dd><li>Intercept DNS requests and give bogus responses for certain - destination hostnames. -</dd> -</dl> - -<div class="p"><!----></div> -We assume the network firewall has limited CPU and memory per -connection [<a href="#clayton:pet2006" name="CITEclayton:pet2006">7</a>]. 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 [<a href="#active-wardens" name="CITEactive-wardens">15</a>,<a href="#tcpstego" name="CITEtcpstego">26</a>]. 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 [<a href="#zuckerman-threatmodels" name="CITEzuckerman-threatmodels">28</a>]; 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 <a href="#subsec:cafes-and-livecds">8.4</a> for more discussion of what little -we can do about this issue. - -<div class="p"><!----></div> -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 <a href="#subsec:trust-chain">8.5</a> for discussion on helping the user -confirm that he has a genuine version and that he can connect to the -real Tor network. - -<div class="p"><!----></div> - <h2><a name="tth_sEc3"> -<a name="sec:current-tor"> -3</a> Adapting the current Tor design to anti-censorship</h2> -</a> - -<div class="p"><!----></div> -Tor is popular and sees a lot of use — it's the largest anonymity -network of its kind, and has -attracted more than 800 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 servers, each of which -can remove only a single layer of encryption. Each server sees only the step -before it and the step after it in the circuit, and so no single server 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. - -<div class="p"><!----></div> -Tor aims to provide three security properties: - -<dl compact="compact"> - - <dt><b></b></dt> - <dd>1. A local network attacker can't learn, or influence, your -destination.</dd> - <dt><b></b></dt> - <dd>2. No single router in the Tor network can link you to your -destination.</dd> - <dt><b></b></dt> - <dd>3. The destination, or somebody watching the destination, -can't learn your location. -</dd> -</dl> - -<div class="p"><!----></div> -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 -location [<a href="#goodell-syverson06" name="CITEgoodell-syverson06">17</a>]. - -<div class="p"><!----></div> -The Tor design provides other features as well that are not typically -present in manual or ad hoc circumvention techniques. - -<div class="p"><!----></div> -First, Tor has a well-analyzed and well-understood way to distribute -information about servers. -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 <a href="#subsec:trust-chain">8.5</a> for details.) - -<div class="p"><!----></div> -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.) - -<div class="p"><!----></div> -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 [<a href="#blossom-thesis" name="CITEblossom-thesis">16</a>] 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. - -<div class="p"><!----></div> -Fourth, Tor separates the role of <em>internal relay</em> from the -role of <em>exit relay</em>. 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 [<a href="#tor-design" name="CITEtor-design">11</a>]. 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. - -<div class="p"><!----></div> -Fifth, Tor is sustainable. Zero-Knowledge Systems offered the commercial -but now defunct Freedom Network [<a href="#freedom21-security" name="CITEfreedom21-security">2</a>], 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 -similarly -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. - -<div class="p"><!----></div> -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 [<a href="#econymics" name="CITEeconymics">1</a>,<a href="#usability:weis2006" name="CITEusability:weis2006">9</a>]. 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. - -<div class="p"><!----></div> -Finally and perhaps most importantly, Tor provides anonymity and prevents any -single server from linking users to their communication partners. Despite -initial appearances, <i>distributed-trust anonymity is critical for -anti-censorship efforts</i>. If any single server can expose dissident bloggers -or compile a list of users' behavior, the censors can profitably compromise -that server's operator, perhaps by applying economic pressure to their -employers, -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. - -<div class="p"><!----></div> -We discuss and adapt these components further in -Section <a href="#sec:bridges">5</a>. But first we examine the strengths and -weaknesses of other blocking-resistance approaches, so we can expand -our repertoire of building blocks and ideas. - -<div class="p"><!----></div> - <h2><a name="tth_sEc4"> -<a name="sec:related"> -4</a> Current proxy solutions</h2> -</a> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 -servers about what routers are available (discovery). With this distinction -in mind, we now examine several categories of relay-based schemes. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.1"> -4.1</a> Centrally-controlled shared proxies</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -Discovery in the face of a government-level firewall is a complex and -unsolved -topic, and we're stuck in this same arms race ourselves; we explore it -in more detail in Section <a href="#sec:discovery">7</a>. 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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.2"> -4.2</a> Independent personal proxies</h3> - -<div class="p"><!----></div> -Personal proxies such as Circumventor [<a href="#circumventor" name="CITEcircumventor">18</a>] and -CGIProxy [<a href="#cgiproxy" name="CITEcgiproxy">23</a>] 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. - -<div class="p"><!----></div> -As the Circumventor site explains, "You don't -actually install the Circumventor <em>on</em> the computer that is blocked -from accessing Web sites. You, or a friend of yours, has to install the -Circumventor on some <em>other</em> machine which is not censored." - -<div class="p"><!----></div> -This tactic has great advantages in terms of blocking-resistance — recall -our assumption in Section <a href="#sec:adversary">2</a> 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. - -<div class="p"><!----></div> -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? - -<div class="p"><!----></div> - -<div class="p"><!----></div> -This challenge leads to a hybrid design-centrally — distributed -personal proxies — which we will investigate in more detail in -Section <a href="#sec:discovery">7</a>. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.3"> -4.3</a> Open proxies</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 <em>too</em> 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? - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.4"> -4.4</a> Blocking resistance and JAP</h3> - -<div class="p"><!----></div> -Köpsell and Hilling's Blocking Resistance -design [<a href="#koepsell:wpes2004" name="CITEkoepsell:wpes2004">20</a>] is probably -the closest related work, and is the starting point for the design in this -paper. In this design, the JAP anonymity system [<a href="#web-mix" name="CITEweb-mix">3</a>] 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 -networks. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.5"> -4.5</a> Infranet</h3> - -<div class="p"><!----></div> -The Infranet design [<a href="#infranet" name="CITEinfranet">14</a>] 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.6"> -4.6</a> RST-evasion and other packet-level tricks</h3> - -<div class="p"><!----></div> -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 [<a href="#clayton:pet2006" name="CITEclayton:pet2006">7</a>]. 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. - -<div class="p"><!----></div> -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 [<a href="#ptacek98insertion" name="CITEptacek98insertion">27</a>]. This technique relies on the -same insight as our weak steganography assumption. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.7"> -4.7</a> Internal caching networks</h3> - -<div class="p"><!----></div> -Freenet [<a href="#freenet-pets00" name="CITEfreenet-pets00">6</a>] 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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.8"> -4.8</a> Skype</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc4.9"> -4.9</a> Tor itself</h3> - -<div class="p"><!----></div> -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? - -<div class="p"><!----></div> -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 <a href="#subsec:publicity">9.5</a> -for more discussion.) - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -We should recognize that we're <em>already</em> in the arms race. These -constraints can give us insight into the priorities and capabilities of -our various attackers. - -<div class="p"><!----></div> - <h2><a name="tth_sEc5"> -<a name="sec:bridges"> -5</a> The relay component of our blocking-resistant design</h2> -</a> - -<div class="p"><!----></div> -Section <a href="#sec:current-tor">3</a> 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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc5.1"> -5.1</a> Bridge relays</h3> - -<div class="p"><!----></div> -Today, Tor servers operate on less than a 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. - -<div class="p"><!----></div> -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 <em>bridge relays</em> (or just <em>bridges</em> -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. - -<div class="p"><!----></div> -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 <a href="#sec:security">8</a>. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc5.2"> -5.2</a> The bridge directory authority</h3> - -<div class="p"><!----></div> -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 server 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. - -<div class="p"><!----></div> -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 server 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. - -<div class="p"><!----></div> -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 <a href="#subsec:trust-chain">8.5</a> for more discussion of the public key -infrastructure and trust chain. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc5.3"> -<a name="subsec:relay-together"> -5.3</a> Putting them together</h3> -</a> - -<div class="p"><!----></div> -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 servers, 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h2><a name="tth_sEc6"> -<a name="sec:network-fingerprint"> -<a name="subsec:enclave-dirs"> -6</a> Hiding Tor's network fingerprint</h2> -</a> -</a> - -<div class="p"><!----></div> -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 servers listen on their "ORPort" -for TLS connections, and some of them opt to listen on their "DirPort" -as well, to serve directory information. Tor servers choose whatever port -numbers they like; the server descriptor they publish to the directory -tells users where to connect. - -<div class="p"><!----></div> -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 server descriptor, and -learn its current circuit keys, its ORPort, and so on. - -<div class="p"><!----></div> -However, connecting directly to the directory cache involves a plaintext -HTTP request. A censor could create a network fingerprint (known as a -<em>signature</em> 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 server'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. - -<div class="p"><!----></div> -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 servers. -But in a blocking-resistance environment, clients should not present -certificates at all. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc6.1"> -<a name="subsec:id-address"> -6.1</a> Identity keys as part of addressing information</h3> -</a> - -<div class="p"><!----></div> -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.) - -<div class="p"><!----></div> -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 -traffic. - -<div class="p"><!----></div> -Once the Tor client has fetched the bridge's server 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 server descriptor using -this fingerprint. - -<div class="p"><!----></div> -So we've shown that it's <em>possible</em> 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. - -<div class="p"><!----></div> - <h2><a name="tth_sEc7"> -<a name="sec:discovery"> -7</a> Discovering working bridge relays</h2> -</a> - -<div class="p"><!----></div> -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 -Section <a href="#sec:related">4</a>. - -<div class="p"><!----></div> -In this section we describe a variety of approaches to adding discovery -components for our design. - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.1"> -<a name="subsec:first-bridge"> -7.1</a> Bootstrapping: finding your first bridge.</h3> -</a> - -<div class="p"><!----></div> -In Section <a href="#subsec:relay-together">5.3</a>, 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -(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.) - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.2"> -7.2</a> Independent bridges, no central discovery</h3> - -<div class="p"><!----></div> -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 [<a href="#psiphon" name="CITEpsiphon">13</a>] plans to use this -<em>social network</em> approach as its discovery component. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 <a href="#sec:bridges">5</a>. 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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.3"> -7.3</a> Families of bridges, no central discovery</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.4"> -7.4</a> Public bridges with central discovery</h3> - -<div class="p"><!----></div> -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 -<em>public bridges</em> in a way that still makes it hard for the attacker -to learn all of them. - -<div class="p"><!----></div> -The basic idea is to divide public bridges into a set of pools based on -identity key. Each pool corresponds to a <em>distribution strategy</em>: -an approach to distributing its bridge addresses to users. Each strategy -is designed to exercise a different scarce resource or property of -the user. - -<div class="p"><!----></div> -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 [<a href="#casc-rep" name="CITEcasc-rep">12</a>] will be hindered. - -<div class="p"><!----></div> - -<div class="p"><!----></div> -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 <em>do</em> 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 -solution -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. - -<div class="p"><!----></div> -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 -<em>delegation tokens</em>. 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 [<a href="#rep-anon" name="CITErep-anon">10</a>]: 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 <a href="#subsec:geoip">7.7</a> 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 [<a href="#chaum-blind" name="CITEchaum-blind">5</a>] to prevent -the website from mapping the seeds' social network. We put off deeper -discussion of the social network reputation strategy for future work. - -<div class="p"><!----></div> -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 <a href="#subsec:incentives">9.4</a>.) - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.5"> -7.5</a> Public bridges with coordinated discovery</h3> - -<div class="p"><!----></div> -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 server 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. - -<div class="p"><!----></div> -We leave this design question for future work. - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.6"> -7.6</a> Assessing whether bridges are useful</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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? - -<div class="p"><!----></div> -The first component can be tested just as we test reachability of -ordinary Tor servers. 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.7"> -<a name="subsec:geoip"> -7.7</a> How do we know if a bridge relay has been blocked?</h3> -</a> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 [<a href="#blossom-thesis" name="CITEblossom-thesis">16</a>] 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. - -<div class="p"><!----></div> -Another answer is not to measure directly, but rather let the bridges -report whether they're being used. -Specifically, bridges should install a GeoIP database such as the public -IP-To-Country list [<a href="#ip-to-country" name="CITEip-to-country">19</a>], 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? - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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? - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc7.8"> -7.8</a> Advantages of deploying all solutions at once</h3> - -<div class="p"><!----></div> -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 <em>all</em> of the strategies more robust -in the face of an adversary with limited resources. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h2><a name="tth_sEc8"> -<a name="sec:security"> -8</a> Security considerations</h2> -</a> - -<div class="p"><!----></div> - <h3><a name="tth_sEc8.1"> -8.1</a> Possession of Tor in oppressed areas</h3> - -<div class="p"><!----></div> -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 -there's -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. - -<div class="p"><!----></div> -It's hard to say which of these pressures will ultimately win out, -but we should keep both sides of the issue in mind. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc8.2"> -<a name="subsec:upload-padding"> -8.2</a> Observers can tell who is publishing and who is reading</h3> -</a> - -<div class="p"><!----></div> -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 <a href="#sec:adversary">2</a> that users who -publish material are in more danger, should we work to improve Tor's -security in this situation? - -<div class="p"><!----></div> -In the general case this is an extremely challenging task: -effective <em>end-to-end traffic confirmation attacks</em> -are known where the adversary observes the origin and the -destination of traffic and confirms that they are part of the -same communication [<a href="#danezis:pet2004" name="CITEdanezis:pet2004">8</a>,<a href="#e2e-traffic" name="CITEe2e-traffic">24</a>]. Related are -<em>website fingerprinting attacks</em>, 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 [<a href="#pet05-bissias" name="CITEpet05-bissias">4</a>,<a href="#defensive-dropping" name="CITEdefensive-dropping">21</a>]. But can we do better -against a limited adversary who just does coarse-grained sweeps looking -for unusually prolific publishers? - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc8.3"> -8.3</a> Anonymity effects from acting as a bridge relay</h3> - -<div class="p"><!----></div> -Against some attacks, relaying traffic for others can improve -anonymity. The simplest example is an attacker who owns a small number -of Tor servers. 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. - -<div class="p"><!----></div> -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.) - -<div class="p"><!----></div> -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 [<a href="#attack-tor-oak05" name="CITEattack-tor-oak05">25</a>]. 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. - -<div class="p"><!----></div> -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 servers and waits for -the user to choose these servers as the beginning and end of her -circuit<a href="#tthFtNtAAC" name="tthFrefAAC"><sup>2</sup></a>. -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? - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc8.4"> -<a name="subsec:cafes-and-livecds"> -8.4</a> Trusting local hardware: Internet cafes and LiveCDs</h3> -</a> - -<div class="p"><!----></div> -Assuming that users have their own trusted hardware is not -always reasonable. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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). - -<div class="p"><!----></div> -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 -system. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc8.5"> -<a name="subsec:trust-chain"> -8.5</a> The trust chain</h3> -</a> - -<div class="p"><!----></div> -Tor's "public key infrastructure" provides a chain of trust to -let users verify that they're actually talking to the right servers. -There are four pieces to this trust chain. - -<div class="p"><!----></div> -First, when Tor clients are establishing circuits, at each step -they demand that the next Tor server in the path prove knowledge of -its private key [<a href="#tor-design" name="CITEtor-design">11</a>]. 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 servers 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 servers. 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h2><a name="tth_sEc9"> -<a name="sec:reachability"> -9</a> Maintaining reachability</h2> -</a> - -<div class="p"><!----></div> - <h3><a name="tth_sEc9.1"> -9.1</a> How many bridge relays should you know about?</h3> - -<div class="p"><!----></div> -The strategies described in Section <a href="#sec:discovery">7</a> 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. - -<div class="p"><!----></div> -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? - -<div class="p"><!----></div> -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 <em>bridge -families</em>, so all users that learn about one member of the bridge family -are told about the rest as well. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - <h3><a name="tth_sEc9.2"> -<a name="subsec:block-cable"> -9.2</a> Cablemodem users don't usually provide important websites</h3> -</a> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -The second answer (not as practical) would be to encourage more use of -consumer networks for popular and useful Internet services. - -<div class="p"><!----></div> -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? - -<div class="p"><!----></div> - <h3><a name="tth_sEc9.3"> -9.3</a> Scanning resistance: making bridges more subtle</h3> - -<div class="p"><!----></div> -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 <a href="#subsec:block-cable">9.2</a> -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 <em>scanning -resistance</em>, and it goes along with normalizing Tor's TLS handshake and -network fingerprint. - -<div class="p"><!----></div> -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 <a href="#subsec:id-address">6.1</a>), 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 -scanning. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc9.4"> -<a name="subsec:incentives"> -9.4</a> How to motivate people to run bridge relays</h3> -</a> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -In the mean time, we can make a snazzy network graph with -Vidalia<a href="#tthFtNtAAD" name="tthFrefAAD"><sup>3</sup></a> that -emphasizes the connections the bridge user is currently relaying. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h3><a name="tth_sEc9.5"> -<a name="subsec:publicity"> -9.5</a> Publicity attracts attention</h3> -</a> - -<div class="p"><!----></div> -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 actually allowing individuals -to circumvent the firewall, but rather to educate the world about the -censorship. The media also tries to do its part by broadcasting the -existence of each new circumvention system. - -<div class="p"><!----></div> -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 -disadvantage? - -<div class="p"><!----></div> - <h3><a name="tth_sEc9.6"> -9.6</a> The Tor website: how to get the software</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -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 -copy. -See Section <a href="#subsec:first-bridge">7.1</a> for more discussion. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h2><a name="tth_sEc10"> -<a name="sec:future"> -10</a> Future designs</h2> -</a> - -<div class="p"><!----></div> - <h3><a name="tth_sEc10.1"> -10.1</a> Bridges inside the blocked network too</h3> - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> -More complex future designs involve operating a separate Tor network -inside the blocked area, and using <em>hidden service bridges</em> — 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. - -<div class="p"><!----></div> - -<div class="p"><!----></div> - <h2><a name="tth_sEc11"> -<a name="sec:conclusion"> -11</a> Next Steps</h2> -</a> - -<div class="p"><!----></div> -Technical solutions won't solve the whole censorship problem. After all, -the firewalls in places like China are <em>socially</em> 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. - -<div class="p"><!----></div> -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. - -<div class="p"><!----></div> - -<h2>References</h2> - -<dl compact="compact"> - <dt><a href="#CITEeconymics" name="econymics">[1]</a></dt><dd> -Alessandro Acquisti, Roger Dingledine, and Paul Syverson. - On the economics of anonymity. - In Rebecca N. Wright, editor, <em>Financial Cryptography</em>. - Springer-Verlag, LNCS 2742, 2003. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEfreedom21-security" name="freedom21-security">[2]</a></dt><dd> -Adam Back, Ian Goldberg, and Adam Shostack. - Freedom systems 2.1 security issues and analysis. - White paper, Zero Knowledge Systems, Inc., May 2001. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEweb-mix" name="web-mix">[3]</a></dt><dd> -Oliver Berthold, Hannes Federrath, and Stefan Köpsell. - Web MIXes: A system for anonymous and unobservable Internet - access. - In H. Federrath, editor, <em>Designing Privacy Enhancing - Technologies: Workshop on Design Issue in Anonymity and Unobservability</em>. - Springer-Verlag, LNCS 2009, 2000. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEpet05-bissias" name="pet05-bissias">[4]</a></dt><dd> -George Dean Bissias, Marc Liberatore, and Brian Neil Levine. - Privacy vulnerabilities in encrypted http streams. - In <em>Proceedings of Privacy Enhancing Technologies workshop (PET - 2005)</em>, May 2005. - - <a href="http://prisms.cs.umass.edu/brian/pubs/bissias.liberatore.pet.2005.pdf"><tt>http://prisms.cs.umass.edu/brian/pubs/bissias.liberatore.pet.2005.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEchaum-blind" name="chaum-blind">[5]</a></dt><dd> -David Chaum. - Blind signatures for untraceable payments. - In D. Chaum, R.L. Rivest, and A.T. Sherman, editors, <em>Advances in - Cryptology: Proceedings of Crypto 82</em>, pages 199-203. Plenum Press, 1983. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEfreenet-pets00" name="freenet-pets00">[6]</a></dt><dd> -Ian Clarke, Oskar Sandberg, Brandon Wiley, and Theodore W. Hong. - Freenet: A distributed anonymous information storage and retrieval - system. - In H. Federrath, editor, <em>Designing Privacy Enhancing - Technologies: Workshop on Design Issue in Anonymity and Unobservability</em>, - pages 46-66. Springer-Verlag, LNCS 2009, July 2000. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEclayton:pet2006" name="clayton:pet2006">[7]</a></dt><dd> -Richard Clayton, Steven J. Murdoch, and Robert N. M. Watson. - Ignoring the great firewall of china. - In <em>Proceedings of the Sixth Workshop on Privacy Enhancing - Technologies (PET 2006)</em>, Cambridge, UK, June 2006. Springer. - <a href="http://www.cl.cam.ac.uk/~rnc1/ignoring.pdf"><tt>http://www.cl.cam.ac.uk/~rnc1/ignoring.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEdanezis:pet2004" name="danezis:pet2004">[8]</a></dt><dd> -George Danezis. - The traffic analysis of continuous-time mixes. - In David Martin and Andrei Serjantov, editors, <em>Privacy Enhancing - Technologies (PET 2004)</em>, LNCS, May 2004. - <a href="http://www.cl.cam.ac.uk/users/gd216/cmm2.pdf"><tt>http://www.cl.cam.ac.uk/users/gd216/cmm2.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEusability:weis2006" name="usability:weis2006">[9]</a></dt><dd> -Roger Dingledine and Nick Mathewson. - Anonymity loves company: Usability and the network effect. - In <em>Proceedings of the Fifth Workshop on the Economics of - Information Security (WEIS 2006)</em>, Cambridge, UK, June 2006. - <a href="http://freehaven.net/doc/wupss04/usability.pdf"><tt>http://freehaven.net/doc/wupss04/usability.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITErep-anon" name="rep-anon">[10]</a></dt><dd> -Roger Dingledine, Nick Mathewson, and Paul Syverson. - Reputation in P2P Anonymity Systems. - In <em>Proceedings of Workshop on Economics of Peer-to-Peer - Systems</em>, June 2003. - <a href="http://freehaven.net/doc/econp2p03/econp2p03.pdf"><tt>http://freehaven.net/doc/econp2p03/econp2p03.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEtor-design" name="tor-design">[11]</a></dt><dd> -Roger Dingledine, Nick Mathewson, and Paul Syverson. - Tor: The second-generation onion router. - In <em>Proceedings of the 13th USENIX Security Symposium</em>, August - 2004. - <a href="http://tor.eff.org/tor-design.pdf"><tt>http://tor.eff.org/tor-design.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEcasc-rep" name="casc-rep">[12]</a></dt><dd> -Roger Dingledine and Paul Syverson. - Reliable MIX Cascade Networks through Reputation. - In Matt Blaze, editor, <em>Financial Cryptography</em>. Springer-Verlag, - LNCS 2357, 2002. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEpsiphon" name="psiphon">[13]</a></dt><dd> -Ronald Deibert et al. - Psiphon. - <a href="http://psiphon.civisec.org/"><tt>http://psiphon.civisec.org/</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEinfranet" name="infranet">[14]</a></dt><dd> -Nick Feamster, Magdalena Balazinska, Greg Harfst, Hari Balakrishnan, and David - Karger. - Infranet: Circumventing web censorship and surveillance. - In <em>Proceedings of the 11th USENIX Security Symposium</em>, August - 2002. - <a href="http://nms.lcs.mit.edu/~feamster/papers/usenixsec2002.pdf"><tt>http://nms.lcs.mit.edu/~feamster/papers/usenixsec2002.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEactive-wardens" name="active-wardens">[15]</a></dt><dd> -Gina Fisk, Mike Fisk, Christos Papadopoulos, and Joshua Neil. - Eliminating steganography in internet traffic with active wardens. - In Fabien Petitcolas, editor, <em>Information Hiding Workshop (IH - 2002)</em>. Springer-Verlag, LNCS 2578, October 2002. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEblossom-thesis" name="blossom-thesis">[16]</a></dt><dd> -Geoffrey Goodell. - <em>Perspective Access Networks</em>. - PhD thesis, Harvard University, July 2006. - <a href="http://afs.eecs.harvard.edu/~goodell/thesis.pdf"><tt>http://afs.eecs.harvard.edu/~goodell/thesis.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEgoodell-syverson06" name="goodell-syverson06">[17]</a></dt><dd> -Geoffrey Goodell and Paul Syverson. - The right place at the right time: The use of network location in - authentication and abuse prevention, 2006. - Submitted. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEcircumventor" name="circumventor">[18]</a></dt><dd> -Bennett Haselton. - How to install the Circumventor program. - - <a href="http://www.peacefire.org/circumventor/simple-circumventor-instructions.html"><tt>http://www.peacefire.org/circumventor/simple-circumventor-instructions.html</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEip-to-country" name="ip-to-country">[19]</a></dt><dd> -Ip-to-country database. - <a href="http://ip-to-country.webhosting.info/"><tt>http://ip-to-country.webhosting.info/</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEkoepsell:wpes2004" name="koepsell:wpes2004">[20]</a></dt><dd> -Stefan Köpsell and Ulf Hilling. - How to achieve blocking resistance for existing systems enabling - anonymous web surfing. - In <em>Proceedings of the Workshop on Privacy in the Electronic - Society (WPES 2004)</em>, Washington, DC, USA, October 2004. - <a href="http://freehaven.net/anonbib/papers/p103-koepsell.pdf"><tt>http://freehaven.net/anonbib/papers/p103-koepsell.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEdefensive-dropping" name="defensive-dropping">[21]</a></dt><dd> -Brian N. Levine, Michael K. Reiter, Chenxi Wang, and Matthew Wright. - Timing analysis in low-latency mix-based systems. - In Ari Juels, editor, <em>Financial Cryptography</em>. Springer-Verlag, - LNCS (forthcoming), 2004. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEmackinnon-personal" name="mackinnon-personal">[22]</a></dt><dd> -Rebecca MacKinnon. - Private communication, 2006. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEcgiproxy" name="cgiproxy">[23]</a></dt><dd> -James Marshall. - CGIProxy: HTTP/FTP Proxy in a CGI Script. - <a href="http://www.jmarshall.com/tools/cgiproxy/"><tt>http://www.jmarshall.com/tools/cgiproxy/</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEe2e-traffic" name="e2e-traffic">[24]</a></dt><dd> -Nick Mathewson and Roger Dingledine. - Practical traffic analysis: Extending and resisting statistical - disclosure. - In David Martin and Andrei Serjantov, editors, <em>Privacy Enhancing - Technologies (PET 2004)</em>, LNCS, May 2004. - <a href="http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf"><tt>http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf</tt></a>. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEattack-tor-oak05" name="attack-tor-oak05">[25]</a></dt><dd> -Steven J. Murdoch and George Danezis. - Low-cost traffic analysis of tor. - In <em>IEEE Symposium on Security and Privacy</em>. IEEE CS, May 2005. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEtcpstego" name="tcpstego">[26]</a></dt><dd> -Steven J. Murdoch and Stephen Lewis. - Embedding covert channels into TCP/IP. - In Mauro Barni, Jordi Herrera-Joancomartí, Stefan Katzenbeisser, - and Fernando Pérez-González, editors, <em>Information Hiding: 7th - International Workshop</em>, volume 3727 of <em>LNCS</em>, pages 247-261, - Barcelona, Catalonia (Spain), June 2005. Springer-Verlag. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEptacek98insertion" name="ptacek98insertion">[27]</a></dt><dd> -Thomas H. Ptacek and Timothy N. Newsham. - Insertion, evasion, and denial of service: Eluding network intrusion - detection. - Technical report, Secure Networks, Inc., Suite 330, 1201 5th Street - S.W, Calgary, Alberta, Canada, T2R-0Y6, 1998. - -<div class="p"><!----></div> -</dd> - <dt><a href="#CITEzuckerman-threatmodels" name="zuckerman-threatmodels">[28]</a></dt><dd> -Ethan Zuckerman. - We've got to adjust some of our threat models. - <a href="http://www.ethanzuckerman.com/blog/?p=1019"><tt>http://www.ethanzuckerman.com/blog/?p=1019</tt></a>.</dd> -</dl> - - -<div class="p"><!----></div> - -<div class="p"><!----></div> - -<div class="p"><!----></div> -<hr /><h3>Footnotes:</h3> - -<div class="p"><!----></div> -<a name="tthFtNtAAB"></a><a href="#tthFrefAAB"><sup>1</sup></a>So far in places - like China, the authorities mainly go after people who publish materials - and coordinate organized movements [<a href="#mackinnon-personal" name="CITEmackinnon-personal">22</a>]. - 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 <a href="#subsec:upload-padding">8.2</a>. -<div class="p"><!----></div> -<a name="tthFtNtAAC"></a><a href="#tthFrefAAC"><sup>2</sup></a><a href="http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ\#EntryGuards"><tt>http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ#EntryGuards</tt></a> -<div class="p"><!----></div> -<a name="tthFtNtAAD"></a><a href="#tthFrefAAD"><sup>3</sup></a><a href="http://vidalia-project.net/"><tt>http://vidalia-project.net/</tt></a> -<br /><br /><hr /><small>File translated from -T<sub><font size="-1">E</font></sub>X -by <a href="http://hutchinson.belmont.ma.us/tth/"> -T<sub><font size="-1">T</font></sub>H</a>, -version 3.77.<br />On 11 May 2007, 21:49.</small> -</html> - |