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+ Design of a blocking-resistant anonymity system\DRAFT
+
+Design of a blocking-resistant anonymity system
DRAFT
+
+
+
+Roger Dingledine, Nick Mathewson
+
+
+
+
+ Abstract
+Internet censorship is on the rise as websites around the world are
+increasingly blocked by government-level firewalls. Although popular
+anonymizing networks like Tor were originally designed to keep attackers from
+tracing people's activities, many people are also using them to evade local
+censorship. But if the censor simply denies access to the Tor network
+itself, blocked users can no longer benefit from the security Tor offers.
+
+
+Here we describe a design that builds upon the current Tor network
+to provide an anonymizing network that resists blocking
+by government-level attackers.
+
+
+
+
+1 Introduction and Goals
+
+
+Anonymizing networks like Tor [11] bounce traffic around a
+network of encrypting relays. Unlike encryption, which hides only what
+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.
+
+
+Historical anonymity research has focused on an
+attacker who monitors the user (call her Alice) and tries to discover her
+activities, yet lets her reach any piece of the network. In more modern
+threat models such as Tor's, the adversary is allowed to perform active
+attacks such as modifying communications to trick Alice
+into revealing her destination, or intercepting some connections
+to run a man-in-the-middle attack. But these systems still assume that
+Alice can eventually reach the anonymizing network.
+
+
+An increasing number of users are using the Tor software
+less for its anonymity properties than for its censorship
+resistance properties — if they use Tor to access Internet sites like
+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.
+
+
+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 2 we discuss our threat model — that is,
+the assumptions we make about our adversary. Section 3
+describes the components of the current Tor design and how they can be
+leveraged for a new blocking-resistant design. Section 4
+explains the features and drawbacks of the currently deployed solutions.
+In sections 5 through 7, we explore the
+components of our designs in detail. Section 8 considers
+security implications and Section 9 presents other
+issues with maintaining connectivity and sustainability for the design.
+Section 10 speculates about future more complex designs,
+and finally Section 11 summarizes our next steps and
+recommendations.
+
+
+
+
+
+
+
+
+
+
+2 Adversary assumptions
+
+
+
+To design an effective anti-censorship tool, we need a good model for the
+goals and resources of the censors we are evading. Otherwise, we risk
+spending our effort on keeping the adversaries from doing things they have no
+interest in doing, and thwarting techniques they do not use.
+The history of blocking-resistance designs is littered with conflicting
+assumptions about what adversaries to expect and what problems are
+in the critical path to a solution. Here we describe our best
+understanding of the current situation around the world.
+
+
+In the traditional security style, we aim to defeat a strong
+attacker — if we can defend against this attacker, we inherit protection
+against weaker attackers as well. After all, we want a general design
+that will work for citizens of China, Thailand, and other censored
+countries; for
+whistleblowers in firewalled corporate networks; and for people in
+unanticipated oppressive situations. In fact, by designing with
+a variety of adversaries in mind, we can take advantage of the fact that
+adversaries will be in different stages of the arms race at each location,
+so a server blocked in one locale can still be useful in others.
+
+
+We assume that the attackers' goals are somewhat complex.
+
+
+
+ - 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).
- As a second-order effect, censors aim to chill citizens' behavior by
+ creating an impression that their online activities are monitored.
- 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.
- 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.
- Similarly, the censors are not attempting to shut down or block
+ every anti-censorship tool — merely the tools that are popular and
+ effective (because these tools impede the censors' information restriction
+ goals) and those tools that are highly visible (thus making the censors
+ look ineffectual to their citizens and their bosses).
- Reprisal against most passive consumers of most kinds of
+ blocked information is also not a goal, given the broadness of most
+ censorship regimes. This seems borne out by fact.1
- 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.
- 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 are willing to block innocuous content
+ (like the bulk of a newspaper's reporting) in order to censor other content
+ distributed through the same channels (like that newspaper's coverage of
+ the censored country).
+
+
+
+We assume there are three main technical network attacks in use by censors
+currently [7]:
+
+
+
+
+
+ - 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.
- Block a destination by listing its IP address at a
+ firewall or other routing control point.
- Intercept DNS requests and give bogus responses for certain
+ destination hostnames.
+
+
+
+We assume the network firewall has limited CPU and memory per
+connection [7]. 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 [15,26]. But we make a "weak
+steganography" assumption here: to remain unblocked, it is necessary to
+remain unobservable only by computational resources on par with a modern
+router, firewall, proxy, or IDS.
+
+
+We assume that while various different regimes can coordinate and share
+notes, there will be a time lag between one attacker learning how to overcome
+a facet of our design and other attackers picking it up. (The most common
+vector of transmission seems to be commercial providers of censorship tools:
+once a provider adds a feature to meet one country's needs or requests, the
+feature is available to all of the provider's customers.) Conversely, we
+assume that insider attacks become a higher risk only after the early stages
+of network development, once the system has reached a certain level of
+success and visibility.
+
+
+We do not assume that government-level attackers are always uniform
+across the country. For example, users of different ISPs in China
+experience different censorship policies and mechanisms.
+
+
+We assume that the attacker may be able to use political and economic
+resources to secure the cooperation of extraterritorial or multinational
+corporations and entities in investigating information sources.
+For example, the censors can threaten the service providers of
+troublesome blogs with economic reprisals if they do not reveal the
+authors' identities.
+
+
+We assume that our users have control over their hardware and
+software — they don't have any spyware installed, there are no
+cameras watching their screens, etc. Unfortunately, in many situations
+these threats are real [28]; 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 8.4 for more discussion of what little
+we can do about this issue.
+
+
+Similarly, we assume that the user will be able to fetch a genuine
+version of Tor, rather than one supplied by the adversary; see
+Section 8.5 for discussion on helping the user
+confirm that he has a genuine version and that he can connect to the
+real Tor network.
+
+
+
+
+3 Adapting the current Tor design to anti-censorship
+
+
+
+Tor is popular and sees a lot of use — it's the largest anonymity
+network of its kind, and has
+attracted more than 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.
+
+
+Tor aims to provide three security properties:
+
+
+
+ - 1. A local network attacker can't learn, or influence, your
+destination.
+ - 2. No single router in the Tor network can link you to your
+destination.
+ - 3. The destination, or somebody watching the destination,
+can't learn your location.
+
+
+
+
+For blocking-resistance, we care most clearly about the first
+property. But as the arms race progresses, the second property
+will become important — for example, to discourage an adversary
+from volunteering a relay in order to learn that Alice is reading
+or posting to certain websites. The third property helps keep users safe from
+collaborating websites: consider websites and other Internet services
+that have been pressured
+recently into revealing the identity of bloggers
+or treating clients differently depending on their network
+location [17].
+
+
+The Tor design provides other features as well that are not typically
+present in manual or ad hoc circumvention techniques.
+
+
+First, Tor has a well-analyzed and well-understood way to distribute
+information about 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 8.5 for details.)
+
+
+Second, the list of directory authorities is not hard-wired.
+Clients use the default authorities if no others are specified,
+but it's easy to start a separate (or even overlapping) Tor network just
+by running a different set of authorities and convincing users to prefer
+a modified client. For example, we could launch a distinct Tor network
+inside China; some users could even use an aggregate network made up of
+both the main network and the China network. (But we should not be too
+quick to create other Tor networks — part of Tor's anonymity comes from
+users behaving like other users, and there are many unsolved anonymity
+questions if different users know about different pieces of the network.)
+
+
+Third, in addition to automatically learning from the chosen directories
+which Tor routers are available and working, Tor takes care of building
+paths through the network and rebuilding them as needed. So the user
+never has to know how paths are chosen, never has to manually pick
+working proxies, and so on. More generally, at its core the Tor protocol
+is simply a tool that can build paths given a set of routers. Tor is
+quite flexible about how it learns about the routers and how it chooses
+the paths. Harvard's Blossom project [16] makes this
+flexibility more concrete: Blossom makes use of Tor not for its security
+properties but for its reachability properties. It runs a separate set
+of directory authorities, its own set of Tor routers (called the Blossom
+network), and uses Tor's flexible path-building to let users view Internet
+resources from any point in the Blossom network.
+
+
+Fourth, Tor separates the role of internal relay from the
+role of exit relay. That is, some volunteers choose just to relay
+traffic between Tor users and Tor routers, and others choose to also allow
+connections to external Internet resources. Because we don't force all
+volunteers to play both roles, we end up with more relays. This increased
+diversity in turn is what gives Tor its security: the more options the
+user has for her first hop, and the more options she has for her last hop,
+the less likely it is that a given attacker will be watching both ends
+of her circuit [11]. As a bonus, because our design attracts
+more internal relays that want to help out but don't want to deal with
+being an exit relay, we end up providing more options for the first
+hop — the one most critical to being able to reach the Tor network.
+
+
+Fifth, Tor is sustainable. Zero-Knowledge Systems offered the commercial
+but now defunct Freedom Network [2], 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.
+
+
+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 [1,9]. This user base also provides
+something else: hundreds of thousands of different and often-changing
+addresses that we can leverage for our blocking-resistance design.
+
+
+Finally and perhaps most importantly, Tor provides anonymity and prevents any
+single server from linking users to their communication partners. Despite
+initial appearances, distributed-trust anonymity is critical for
+anti-censorship efforts. 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.
+
+
+We discuss and adapt these components further in
+Section 5. But first we examine the strengths and
+weaknesses of other blocking-resistance approaches, so we can expand
+our repertoire of building blocks and ideas.
+
+
+
+
+4 Current proxy solutions
+
+
+
+Relay-based blocking-resistance schemes generally have two main
+components: a relay component and a discovery component. The relay part
+encompasses the process of establishing a connection, sending traffic
+back and forth, and so on — everything that's done once the user knows
+where she's going to connect. Discovery is the step before that: the
+process of finding one or more usable relays.
+
+
+For example, we can divide the pieces of Tor in the previous section
+into the process of building paths and sending
+traffic over them (relay) and the process of learning from the directory
+servers about what routers are available (discovery). With this distinction
+in mind, we now examine several categories of relay-based schemes.
+
+
+
+4.1 Centrally-controlled shared proxies
+
+
+Existing commercial anonymity solutions (like Anonymizer.com) are based
+on a set of single-hop proxies. In these systems, each user connects to
+a single proxy, which then relays traffic between the user and her
+destination. These public proxy
+systems are typically characterized by two features: they control and
+operate the proxies centrally, and many different users get assigned
+to each proxy.
+
+
+In terms of the relay component, single proxies provide weak security
+compared to systems that distribute trust over multiple relays, since a
+compromised proxy can trivially observe all of its users' actions, and
+an eavesdropper only needs to watch a single proxy to perform timing
+correlation attacks against all its users' traffic and thus learn where
+everyone is connecting. Worse, all users
+need to trust the proxy company to have good security itself as well as
+to not reveal user activities.
+
+
+On the other hand, single-hop proxies are easier to deploy, and they
+can provide better performance than distributed-trust designs like Tor,
+since traffic only goes through one relay. They're also more convenient
+from the user's perspective — since users entirely trust the proxy,
+they can just use their web browser directly.
+
+
+Whether public proxy schemes are more or less scalable than Tor is
+still up for debate: commercial anonymity systems can use some of their
+revenue to provision more bandwidth as they grow, whereas volunteer-based
+anonymity systems can attract thousands of fast relays to spread the load.
+
+
+The discovery piece can take several forms. Most commercial anonymous
+proxies have one or a handful of commonly known websites, and their users
+log in to those websites and relay their traffic through them. When
+these websites get blocked (generally soon after the company becomes
+popular), if the company cares about users in the blocked areas, they
+start renting lots of disparate IP addresses and rotating through them
+as they get blocked. They notify their users of new addresses (by email,
+for example). It's an arms race, since attackers can sign up to receive the
+email too, but operators have one nice trick available to them: because they
+have a list of paying subscribers, they can notify certain subscribers
+about updates earlier than others.
+
+
+Access control systems on the proxy let them provide service only to
+users with certain characteristics, such as paying customers or people
+from certain IP address ranges.
+
+
+Discovery in the face of a government-level firewall is a complex and
+unsolved
+topic, and we're stuck in this same arms race ourselves; we explore it
+in more detail in Section 7. 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.
+
+
+
+4.2 Independent personal proxies
+
+
+Personal proxies such as Circumventor [18] and
+CGIProxy [23] use the same technology as the public ones as
+far as the relay component goes, but they use a different strategy for
+discovery. Rather than managing a few centralized proxies and constantly
+getting new addresses for them as the old addresses are blocked, they
+aim to have a large number of entirely independent proxies, each managing
+its own (much smaller) set of users.
+
+
+As the Circumventor site explains, "You don't
+actually install the Circumventor on the computer that is blocked
+from accessing Web sites. You, or a friend of yours, has to install the
+Circumventor on some other machine which is not censored."
+
+
+This tactic has great advantages in terms of blocking-resistance — recall
+our assumption in Section 2 that the attention
+a system attracts from the attacker is proportional to its number of
+users and level of publicity. If each proxy only has a few users, and
+there is no central list of proxies, most of them will never get noticed by
+the censors.
+
+
+On the other hand, there's a huge scalability question that so far has
+prevented these schemes from being widely useful: how does the fellow
+in China find a person in Ohio who will run a Circumventor for him? In
+some cases he may know and trust some people on the outside, but in many
+cases he's just out of luck. Just as hard, how does a new volunteer in
+Ohio find a person in China who needs it?
+
+
+
+
+This challenge leads to a hybrid design-centrally — distributed
+personal proxies — which we will investigate in more detail in
+Section 7.
+
+
+
+4.3 Open proxies
+
+
+Yet another currently used approach to bypassing firewalls is to locate
+open and misconfigured proxies on the Internet. A quick Google search
+for "open proxy list" yields a wide variety of freely available lists
+of HTTP, HTTPS, and SOCKS proxies. Many small companies have sprung up
+providing more refined lists to paying customers.
+
+
+There are some downsides to using these open proxies though. First,
+the proxies are of widely varying quality in terms of bandwidth and
+stability, and many of them are entirely unreachable. Second, unlike
+networks of volunteers like Tor, the legality of routing traffic through
+these proxies is questionable: it's widely believed that most of them
+don't realize what they're offering, and probably wouldn't allow it if
+they realized. Third, in many cases the connection to the proxy is
+unencrypted, so firewalls that filter based on keywords in IP packets
+will not be hindered. Fourth, in many countries (including China), the
+firewall authorities hunt for open proxies as well, to preemptively
+block them. And last, many users are suspicious that some
+open proxies are a little too convenient: are they run by the
+adversary, in which case they get to monitor all the user's requests
+just as single-hop proxies can?
+
+
+A distributed-trust design like Tor resolves each of these issues for
+the relay component, but a constantly changing set of thousands of open
+relays is clearly a useful idea for a discovery component. For example,
+users might be able to make use of these proxies to bootstrap their
+first introduction into the Tor network.
+
+
+
+4.4 Blocking resistance and JAP
+
+
+Köpsell and Hilling's Blocking Resistance
+design [20] is probably
+the closest related work, and is the starting point for the design in this
+paper. In this design, the JAP anonymity system [3] 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.
+
+
+
+4.5 Infranet
+
+
+The Infranet design [14] uses one-hop relays to deliver web
+content, but disguises its communications as ordinary HTTP traffic. Requests
+are split into multiple requests for URLs on the relay, which then encodes
+its responses in the content it returns. The relay needs to be an actual
+website with plausible content and a number of URLs which the user might want
+to access — if the Infranet software produced its own cover content, it would
+be far easier for censors to identify. To keep the censors from noticing
+that cover content changes depending on what data is embedded, Infranet needs
+the cover content to have an innocuous reason for changing frequently: the
+paper recommends watermarked images and webcams.
+
+
+The attacker and relay operators in Infranet's threat model are significantly
+different than in ours. Unlike our attacker, Infranet's censor can't be
+bypassed with encrypted traffic (presumably because the censor blocks
+encrypted traffic, or at least considers it suspicious), and has more
+computational resources to devote to each connection than ours (so it can
+notice subtle patterns over time). Unlike our bridge operators, Infranet's
+operators (and users) have more bandwidth to spare; the overhead in typical
+steganography schemes is far higher than Tor's.
+
+
+The Infranet design does not include a discovery element. Discovery,
+however, is a critical point: if whatever mechanism allows users to learn
+about relays also allows the censor to do so, he can trivially discover and
+block their addresses, even if the steganography would prevent mere traffic
+observation from revealing the relays' addresses.
+
+
+
+4.6 RST-evasion and other packet-level tricks
+
+
+In their analysis of China's firewall's content-based blocking, Clayton,
+Murdoch and Watson discovered that rather than blocking all packets in a TCP
+streams once a forbidden word was noticed, the firewall was simply forging
+RST packets to make the communicating parties believe that the connection was
+closed [7]. They proposed altering operating systems
+to ignore forged RST packets. This approach might work in some cases, but
+in practice it appears that many firewalls start filtering by IP address
+once a sufficient number of RST packets have been sent.
+
+
+Other packet-level responses to filtering include splitting
+sensitive words across multiple TCP packets, so that the censors'
+firewalls can't notice them without performing expensive stream
+reconstruction [27]. This technique relies on the
+same insight as our weak steganography assumption.
+
+
+
+4.7 Internal caching networks
+
+
+Freenet [6] is an anonymous peer-to-peer data store.
+Analyzing Freenet's security can be difficult, as its design is in flux as
+new discovery and routing mechanisms are proposed, and no complete
+specification has (to our knowledge) been written. Freenet servers relay
+requests for specific content (indexed by a digest of the content)
+"toward" the server that hosts it, and then cache the content as it
+follows the same path back to
+the requesting user. If Freenet's routing mechanism is successful in
+allowing nodes to learn about each other and route correctly even as some
+node-to-node links are blocked by firewalls, then users inside censored areas
+can ask a local Freenet server for a piece of content, and get an answer
+without having to connect out of the country at all. Of course, operators of
+servers inside the censored area can still be targeted, and the addresses of
+external servers can still be blocked.
+
+
+
+
+
+The popular Skype voice-over-IP software uses multiple techniques to tolerate
+restrictive networks, some of which allow it to continue operating in the
+presence of censorship. By switching ports and using encryption, Skype
+attempts to resist trivial blocking and content filtering. Even if no
+encryption were used, it would still be expensive to scan all voice
+traffic for sensitive words. Also, most current keyloggers are unable to
+store voice traffic. Nevertheless, Skype can still be blocked, especially at
+its central login server.
+
+
+
+4.9 Tor itself
+
+
+And last, we include Tor itself in the list of current solutions
+to firewalls. Tens of thousands of people use Tor from countries that
+routinely filter their Internet. Tor's website has been blocked in most
+of them. But why hasn't the Tor network been blocked yet?
+
+
+We have several theories. The first is the most straightforward: tens of
+thousands of people are simply too few to matter. It may help that Tor is
+perceived to be for experts only, and thus not worth attention yet. The
+more subtle variant on this theory is that we've positioned Tor in the
+public eye as a tool for retaining civil liberties in more free countries,
+so perhaps blocking authorities don't view it as a threat. (We revisit
+this idea when we consider whether and how to publicize a Tor variant
+that improves blocking-resistance — see Section 9.5
+for more discussion.)
+
+
+The broader explanation is that the maintenance of most government-level
+filters is aimed at stopping widespread information flow and appearing to be
+in control, not by the impossible goal of blocking all possible ways to bypass
+censorship. Censors realize that there will always
+be ways for a few people to get around the firewall, and as long as Tor
+has not publically threatened their control, they see no urgent need to
+block it yet.
+
+
+We should recognize that we're already in the arms race. These
+constraints can give us insight into the priorities and capabilities of
+our various attackers.
+
+
+
+
+5 The relay component of our blocking-resistant design
+
+
+
+Section 3 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.
+
+
+
+
+
+
+
+5.1 Bridge relays
+
+
+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.
+
+
+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 bridge relays (or just bridges
+for short). They can rate limit relayed connections to 10 KB/s (almost
+nothing for a broadband user in a free country, but plenty for a user
+who otherwise has no access at all), and since they are just relaying
+bytes back and forth between blocked users and the main Tor network, they
+won't need to make any external connections to Internet sites. Because
+of this separation of roles, and because we're making use of software
+that the volunteers have already installed for their own use, we expect
+our scheme to attract and maintain more volunteers than previous schemes.
+
+
+As usual, there are new anonymity and security implications from running a
+bridge relay, particularly from letting people relay traffic through your
+Tor client; but we leave this discussion for Section 8.
+
+
+
+
+
+5.2 The bridge directory authority
+
+
+How do the bridge relays advertise their existence to the world? We
+introduce a second new component of the design: a specialized directory
+authority that aggregates and tracks bridges. Bridge relays periodically
+publish 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.
+
+
+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.
+
+
+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 8.5 for more discussion of the public key
+infrastructure and trust chain.
+
+
+Bridges use Tor to publish their descriptors privately and securely,
+so even an attacker monitoring the bridge directory authority's network
+can't make a list of all the addresses contacting the authority.
+Bridges may publish to only a subset of the
+authorities, to limit the potential impact of an authority compromise.
+
+
+
+
+
+
+5.3 Putting them together
+
+
+
+If a blocked user knows the identity keys of a set of bridge relays, and
+he has correct address information for at least one of them, he can use
+that one to make a secure connection to the bridge authority and update
+his knowledge about the other bridge relays. He can also use it to make
+secure connections to the main Tor network and directory 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.
+
+
+So now we've reduced the problem from how to circumvent the firewall
+for all transactions (and how to know that the pages you get have not
+been modified by the local attacker) to how to learn about a working
+bridge relay.
+
+
+There's another catch though. We need to make sure that the network
+traffic we generate by simply connecting to a bridge relay doesn't stand
+out too much.
+
+
+
+
+
+
+
+
+
+6 Hiding Tor's network fingerprint
+
+
+
+
+Currently, Tor uses two protocols for its network communications. The
+main protocol uses TLS for encrypted and authenticated communication
+between Tor instances. The second protocol is standard HTTP, used for
+fetching directory information. All Tor 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.
+
+
+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.
+
+
+However, connecting directly to the directory cache involves a plaintext
+HTTP request. A censor could create a network fingerprint (known as a
+signature in the intrusion detection field) for the request
+and/or its response, thus preventing these connections. To resolve this
+vulnerability, we've modified the Tor protocol so that users can connect
+to the directory cache via the main Tor port — they establish a TLS
+connection with the bridge as normal, and then send a special "begindir"
+relay command to establish an internal connection to its directory cache.
+
+
+Therefore a better way to summarize a bridge's address is by its IP
+address and ORPort, so all communications between the client and the
+bridge will use ordinary TLS. But there are other details that need
+more investigation.
+
+
+What port should bridges pick for their ORPort? We currently recommend
+that they listen on port 443 (the default HTTPS port) if they want to
+be most useful, because clients behind standard firewalls will have
+the best chance to reach them. Is this the best choice in all cases,
+or should we encourage some fraction of them pick random ports, or other
+ports commonly permitted through firewalls like 53 (DNS) or 110
+(POP)? Or perhaps we should use other ports where TLS traffic is
+expected, like 993 (IMAPS) or 995 (POP3S). We need more research on our
+potential users, and their current and anticipated firewall restrictions.
+
+
+Furthermore, we need to look at the specifics of Tor's TLS handshake.
+Right now Tor uses some predictable strings in its TLS handshakes. For
+example, it sets the X.509 organizationName field to "Tor", and it puts
+the Tor 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.
+
+
+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.
+
+
+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.
+
+
+
+
+
+
+6.1 Identity keys as part of addressing information
+
+
+
+We have described a way for the blocked user to bootstrap into the
+network once he knows the IP address and ORPort of a bridge. What about
+local spoofing attacks? That is, since we never learned an identity
+key fingerprint for the bridge, a local attacker could intercept our
+connection and pretend to be the bridge we had in mind. It turns out
+that giving false information isn't that bad — since the Tor client
+ships with trusted keys for the bridge directory authority and the Tor
+network directory authorities, the user can learn whether he's being
+given a real connection to the bridge authorities or not. (After all,
+if the adversary intercepts every connection the user makes and gives
+him a bad connection each time, there's nothing we can do.)
+
+
+What about anonymity-breaking attacks from observing traffic, if the
+blocked user doesn't start out knowing the identity key of his intended
+bridge? The vulnerabilities aren't so bad in this case either — the
+adversary could do similar attacks just by monitoring the network
+traffic.
+
+
+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.
+
+
+So we've shown that it's possible to bootstrap into the network
+just by learning the IP address and ORPort of a bridge, but are there
+situations where it's more convenient or more secure to learn the bridge's
+identity fingerprint as well as instead, while bootstrapping? We keep
+that question in mind as we next investigate bootstrapping and discovery.
+
+
+
+
+7 Discovering working bridge relays
+
+
+
+Tor's modular design means that we can develop a better relay component
+independently of developing the discovery component. This modularity's
+great promise is that we can pick any discovery approach we like; but the
+unfortunate fact is that we have no magic bullet for discovery. We're
+in the same arms race as all the other designs we described in
+Section 4.
+
+
+In this section we describe a variety of approaches to adding discovery
+components for our design.
+
+
+
+
+7.1 Bootstrapping: finding your first bridge.
+
+
+
+In Section 5.3, we showed that a user who knows
+a working bridge address can use it to reach the bridge authority and
+to stay connected to the Tor network. But how do new users reach the
+bridge authority in the first place? After all, the bridge authority
+will be one of the first addresses that a censor blocks.
+
+
+First, we should recognize that most government firewalls are not
+perfect. That is, they may allow connections to Google cache or some
+open proxy servers, or they let file-sharing traffic, Skype, instant
+messaging, or World-of-Warcraft connections through. Different users will
+have different mechanisms for bypassing the firewall initially. Second,
+we should remember that most people don't operate in a vacuum; users will
+hopefully know other people who are in other situations or have other
+resources available. In the rest of this section we develop a toolkit
+of different options and mechanisms, so that we can enable users in a
+diverse set of contexts to bootstrap into the system.
+
+
+(For users who can't use any of these techniques, hopefully they know
+a friend who can — for example, perhaps the friend already knows some
+bridge relay addresses. If they can't get around it at all, then we
+can't help them — they should go meet more people or learn more about
+the technology running the firewall in their area.)
+
+
+By deploying all the schemes in the toolkit at once, we let bridges and
+blocked users employ the discovery approach that is most appropriate
+for their situation.
+
+
+
+7.2 Independent bridges, no central discovery
+
+
+The first design is simply to have no centralized discovery component at
+all. Volunteers run bridges, and we assume they have some blocked users
+in mind and communicate their address information to them out-of-band
+(for example, through Gmail). This design allows for small personal
+bridges that have only one or a handful of users in mind, but it can
+also support an entire community of users. For example, Citizen Lab's
+upcoming Psiphon single-hop proxy tool [13] plans to use this
+social network approach as its discovery component.
+
+
+There are several ways to do bootstrapping in this design. In the simple
+case, the operator of the bridge informs each chosen user about his
+bridge's address information and/or keys. A different approach involves
+blocked users introducing new blocked users to the bridges they know.
+That is, somebody in the blocked area can pass along a bridge's address to
+somebody else they trust. This scheme brings in appealing but complex game
+theoretic properties: the blocked user making the decision has an incentive
+only to delegate to trustworthy people, since an adversary who learns
+the bridge's address and filters it makes it unavailable for both of them.
+Also, delegating known bridges to members of your social network can be
+dangerous: an the adversary who can learn who knows which bridges may
+be able to reconstruct the social network.
+
+
+Note that a central set of bridge directory authorities can still be
+compatible with a decentralized discovery process. That is, how users
+first learn about bridges is entirely up to the bridges, but the process
+of fetching up-to-date descriptors for them can still proceed as described
+in Section 5. 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.
+
+
+
+7.3 Families of bridges, no central discovery
+
+
+Because the blocked users are running our software too, we have many
+opportunities to improve usability or robustness. Our second design builds
+on the first by encouraging volunteers to run several bridges at once
+(or coordinate with other bridge volunteers), such that some
+of the bridges are likely to be available at any given time.
+
+
+The blocked user's Tor client would periodically fetch an updated set of
+recommended bridges from any of the working bridges. Now the client can
+learn new additions to the bridge pool, and can expire abandoned bridges
+or bridges that the adversary has blocked, without the user ever needing
+to care. To simplify maintenance of the community's bridge pool, each
+community could run its own bridge directory authority — reachable via
+the available bridges, and also mirrored at each bridge.
+
+
+
+7.4 Public bridges with central discovery
+
+
+What about people who want to volunteer as bridges but don't know any
+suitable blocked users? What about people who are blocked but don't
+know anybody on the outside? Here we describe how to make use of these
+public bridges in a way that still makes it hard for the attacker
+to learn all of them.
+
+
+The basic idea is to divide public bridges into a set of pools based on
+identity key. Each pool corresponds to a distribution strategy:
+an approach to distributing its bridge addresses to users. Each strategy
+is designed to exercise a different scarce resource or property of
+the user.
+
+
+How do we divide bridges between these strategy pools such that they're
+evenly distributed and the allocation is hard to influence or predict,
+but also in a way that's amenable to creating more strategies later
+on without reshuffling all the pools? We assign a given bridge
+to a strategy pool by hashing the bridge's identity key along with a
+secret that only the bridge authority knows: the first n bits of this
+hash dictate the strategy pool number, where n is a parameter that
+describes how many strategy pools we want at this point. We choose n=3
+to start, so we divide bridges between 8 pools; but as we later invent
+new distribution strategies, we can increment n to split the 8 into
+16. Since a bridge can't predict the next bit in its hash, it can't
+anticipate which identity key will correspond to a certain new pool
+when the pools are split. Further, since the bridge authority doesn't
+provide any feedback to the bridge about which strategy pool it's in,
+an adversary who signs up bridges with the goal of filling a certain
+pool [12] will be hindered.
+
+
+
+
+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 do get blocked. Further,
+remember that we're dealing with different blocking regimes around the
+world that will progress at different rates — so this pool will still
+be useful to some users even as the arms races progress.
+
+
+The second distribution strategy publishes bridge addresses based on the IP
+address of the requesting user. Specifically, the bridge authority will
+divide the available bridges in the pool into a bunch of partitions
+(as in the first distribution scheme), hash the requester's IP address
+with a secret of its own (as in the above allocation scheme for creating
+pools), and give the requester a random bridge from the appropriate
+partition. To raise the bar, we should discard the last octet of the
+IP address before inputting it to the hash function, so an attacker
+who only controls a single "/24" network only counts as one user. A
+large attacker like China will still be able to control many addresses,
+but the hassle of establishing connections from each network (or spoofing
+TCP connections) may still slow them down. Similarly, as a special case,
+we should treat IP addresses that are Tor exit nodes as all being on
+the same network.
+
+
+The third strategy combines the time-based and location-based
+strategies to further constrain and rate-limit the available bridge
+addresses. Specifically, the bridge address provided in a given time
+slot to a given network location is deterministic within the partition,
+rather than chosen randomly each time from the partition. Thus, repeated
+requests during that time slot from a given network are given the same
+bridge address as the first request.
+
+
+The fourth strategy is based on Circumventor's discovery strategy.
+The Circumventor project, realizing that its adoption will remain limited
+if it has no central coordination mechanism, has started a mailing list to
+distribute new proxy addresses every few days. From experimentation it
+seems they have concluded that sending updates every three or four days
+is sufficient to stay ahead of the current attackers.
+
+
+The fifth strategy provides an alternative approach to a mailing list:
+users provide an email address and receive an automated response
+listing an available bridge address. We could limit one response per
+email address. To further rate limit queries, we could require a CAPTCHA
+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.
+
+
+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
+delegation tokens. We run a website next to the bridge authority,
+where users can log in (they connect via Tor, and they don't need to
+provide actual identities, just persistent pseudonyms). Users can delegate
+trust to other people they know by giving them a token, which can be
+exchanged for a new account on the website. Accounts in "good standing"
+then accrue new bridge addresses and new tokens. As usual, reputation
+schemes bring in a host of new complexities [10]: 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 7.7 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 [5] to prevent
+the website from mapping the seeds' social network. We put off deeper
+discussion of the social network reputation strategy for future work.
+
+
+Pools seven and eight are held in reserve, in case our currently deployed
+tricks all fail at once and the adversary blocks all those bridges — so
+we can adapt and move to new approaches quickly, and have some bridges
+immediately available for the new schemes. New strategies might be based
+on some other scarce resource, such as relaying traffic for others or
+other proof of energy spent. (We might also worry about the incentives
+for bridges that sign up and get allocated to the reserve pools: will they
+be unhappy that they're not being used? But this is a transient problem:
+if Tor users are bridges by default, nobody will mind not being used yet.
+See also Section 9.4.)
+
+
+
+
+
+7.5 Public bridges with coordinated discovery
+
+
+We presented the above discovery strategies in the context of a single
+bridge directory authority, but in practice we will want to distribute the
+operations over several bridge authorities — a single point of failure
+or attack is a bad move. The first answer is to run several independent
+bridge directory authorities, and bridges gravitate to one based on
+their identity key. The better answer would be some federation of bridge
+authorities that work together to provide redundancy but don't introduce
+new security issues. We could even imagine designs where the bridge
+authorities have encrypted versions of the bridge's 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.
+
+
+We leave this design question for future work.
+
+
+
+7.6 Assessing whether bridges are useful
+
+
+Learning whether a bridge is useful is important in the bridge authority's
+decision to include it in responses to blocked users. For example, if
+we end up with a list of thousands of bridges and only a few dozen of
+them are reachable right now, most blocked users will not end up knowing
+about working bridges.
+
+
+There are three components for assessing how useful a bridge is. First,
+is it reachable from the public Internet? Second, what proportion of
+the time is it available? Third, is it blocked in certain jurisdictions?
+
+
+The first component can be tested just as we test reachability of
+ordinary Tor 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.
+
+
+The second component can be measured and tracked by the bridge authority.
+By doing periodic reachability tests, we can get a sense of how often the
+bridge is available. More complex tests will involve bandwidth-intensive
+checks to force the bridge to commit resources in order to be counted as
+available. We need to evaluate how the relationship of uptime percentage
+should weigh into our choice of which bridges to advertise. We leave
+this to future work.
+
+
+The third component is perhaps the trickiest: with many different
+adversaries out there, how do we keep track of which adversaries have
+blocked which bridges, and how do we learn about new blocks as they
+occur? We examine this problem next.
+
+
+
+
+7.7 How do we know if a bridge relay has been blocked?
+
+
+
+There are two main mechanisms for testing whether bridges are reachable
+from inside each blocked area: active testing via users, and passive
+testing via bridges.
+
+
+In the case of active testing, certain users inside each area
+sign up as testing relays. The bridge authorities can then use a
+Blossom-like [16] system to build circuits through them
+to each bridge and see if it can establish the connection. But how do
+we pick the users? If we ask random users to do the testing (or if we
+solicit volunteers from the users), the adversary should sign up so he
+can enumerate the bridges we test. Indeed, even if we hand-select our
+testers, the adversary might still discover their location and monitor
+their network activity to learn bridge addresses.
+
+
+Another answer is not to measure directly, but rather let the bridges
+report whether they're being used.
+Specifically, bridges should install a GeoIP database such as the public
+IP-To-Country list [19], and then periodically report to the
+bridge authorities which countries they're seeing use from. This data
+would help us track which countries are making use of the bridge design,
+and can also let us learn about new steps the adversary has taken in
+the arms race. (The compressed GeoIP database is only several hundred
+kilobytes, and we could even automate the update process by serving it
+from the bridge authorities.)
+More analysis of this passive reachability
+testing design is needed to resolve its many edge cases: for example,
+if a bridge stops seeing use from a certain area, does that mean the
+bridge is blocked or does that mean those users are asleep?
+
+
+There are many more problems with the general concept of detecting whether
+bridges are blocked. First, different zones of the Internet are blocked
+in different ways, and the actual firewall jurisdictions do not match
+country borders. Our bridge scheme could help us map out the topology
+of the censored Internet, but this is a huge task. More generally,
+if a bridge relay isn't reachable, is that because of a network block
+somewhere, because of a problem at the bridge relay, or just a temporary
+outage somewhere in between? And last, an attacker could poison our
+bridge database by signing up already-blocked bridges. In this case,
+if we're stingy giving out bridge addresses, users in that country won't
+learn working bridges.
+
+
+All of these issues are made more complex when we try to integrate this
+testing into our social network reputation system above.
+Since in that case we punish or reward users based on whether bridges
+get blocked, the adversary has new attacks to trick or bog down the
+reputation tracking. Indeed, the bridge authority doesn't even know
+what zone the blocked user is in, so do we blame him for any possible
+censored zone, or what?
+
+
+Clearly more analysis is required. The eventual solution will probably
+involve a combination of passive measurement via GeoIP and active
+measurement from trusted testers. More generally, we can use the passive
+feedback mechanism to track usage of the bridge network as a whole — which
+would let us respond to attacks and adapt the design, and it would also
+let the general public track the progress of the project.
+
+
+
+
+
+7.8 Advantages of deploying all solutions at once
+
+
+For once, we're not in the position of the defender: we don't have to
+defend against every possible filtering scheme; we just have to defend
+against at least one. On the flip side, the attacker is forced to guess
+how to allocate his resources to defend against each of these discovery
+strategies. So by deploying all of our strategies at once, we not only
+increase our chances of finding one that the adversary has difficulty
+blocking, but we actually make all of the strategies more robust
+in the face of an adversary with limited resources.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+8 Security considerations
+
+
+
+
+8.1 Possession of Tor in oppressed areas
+
+
+Many people speculate that installing and using a Tor client in areas with
+particularly extreme firewalls is a high risk — and the risk increases
+as the firewall gets more restrictive. This notion certain 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.
+
+
+It's hard to say which of these pressures will ultimately win out,
+but we should keep both sides of the issue in mind.
+
+
+
+
+
+
+
+
+8.2 Observers can tell who is publishing and who is reading
+
+
+
+Tor encrypts traffic on the local network, and it obscures the eventual
+destination of the communication, but it doesn't do much to obscure the
+traffic volume. In particular, a user publishing a home video will have a
+different network fingerprint than a user reading an online news article.
+Based on our assumption in Section 2 that users who
+publish material are in more danger, should we work to improve Tor's
+security in this situation?
+
+
+In the general case this is an extremely challenging task:
+effective end-to-end traffic confirmation attacks
+are known where the adversary observes the origin and the
+destination of traffic and confirms that they are part of the
+same communication [8,24]. Related are
+website fingerprinting attacks, where the adversary downloads
+a few hundred popular websites, makes a set of "fingerprints" for each
+site, and then observes the target Tor client's traffic to look for
+a match [4,21]. But can we do better
+against a limited adversary who just does coarse-grained sweeps looking
+for unusually prolific publishers?
+
+
+One answer is for bridge users to automatically send bursts of padding
+traffic periodically. (This traffic can be implemented in terms of
+long-range drop cells, which are already part of the Tor specification.)
+Of course, convincingly simulating an actual human publishing interesting
+content is a difficult arms race, but it may be worthwhile to at least
+start the race. More research remains.
+
+
+
+8.3 Anonymity effects from acting as a bridge relay
+
+
+Against some attacks, relaying traffic for others can improve
+anonymity. The simplest example is an attacker who owns a small number
+of Tor 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.
+
+
+There are some cases where it doesn't seem to help: if an attacker can
+watch all of the bridge's incoming and outgoing traffic, then it's easy
+to learn which connections were relayed and which started there. (In this
+case he still doesn't know the final destinations unless he is watching
+them too, but in this case bridges are no better off than if they were
+an ordinary client.)
+
+
+There are also some potential downsides to running a bridge. First, while
+we try to make it hard to enumerate all bridges, it's still possible to
+learn about some of them, and for some people just the fact that they're
+running one might signal to an attacker that they place a higher value
+on their anonymity. Second, there are some more esoteric attacks on Tor
+relays that are not as well-understood or well-tested — for example, an
+attacker may be able to "observe" whether the bridge is sending traffic
+even if he can't actually watch its network, by relaying traffic through
+it and noticing changes in traffic timing [25]. On
+the other hand, it may be that limiting the bandwidth the bridge is
+willing to relay will allow this sort of attacker to determine if it's
+being used as a bridge but not easily learn whether it is adding traffic
+of its own.
+
+
+We also need to examine how entry guards fit in. Entry guards
+(a small set of nodes that are always used for the first
+step in a circuit) help protect against certain attacks
+where the attacker runs a few Tor servers and waits for
+the user to choose these servers as the beginning and end of her
+circuit2.
+If the blocked user doesn't use the bridge's entry guards, then the bridge
+doesn't gain as much cover benefit. On the other hand, what design changes
+are needed for the blocked user to use the bridge's entry guards without
+learning what they are (this seems hard), and even if we solve that,
+do they then need to use the guards' guards and so on down the line?
+
+
+It is an open research question whether the benefits of running a bridge
+outweigh the risks. A lot of the decision rests on which attacks the
+users are most worried about. For most users, we don't think running a
+bridge relay will be that damaging, and it could help quite a bit.
+
+
+
+
+8.4 Trusting local hardware: Internet cafes and LiveCDs
+
+
+
+Assuming that users have their own trusted hardware is not
+always reasonable.
+
+
+For Internet cafe Windows computers that let you attach your own USB key,
+a USB-based Tor image would be smart. There's Torpark, and hopefully
+there will be more thoroughly analyzed and trustworthy options down the
+road. Worries remain about hardware or software keyloggers and other
+spyware, as well as and physical surveillance.
+
+
+If the system lets you boot from a CD or from a USB key, you can gain
+a bit more security by bringing a privacy LiveCD with you. (This
+approach isn't foolproof either of course, since hardware
+keyloggers and physical surveillance are still a worry).
+
+
+In fact, LiveCDs are also useful if it's your own hardware, since it's
+easier to avoid leaving private data and logs scattered around the
+system.
+
+
+
+
+
+
+8.5 The trust chain
+
+
+
+Tor's "public key infrastructure" provides a chain of trust to
+let users verify that they're actually talking to the right servers.
+There are four pieces to this trust chain.
+
+
+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 [11]. 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.
+
+
+In the case of blocked users contacting bridges and bridge directory
+authorities, the same logic applies in parallel: the blocked users fetch
+information from both the bridge authorities and the directory authorities
+for the `main' Tor network, and they combine this information locally.
+
+
+How can a user in an oppressed country know that he has the correct
+key fingerprints for the developers? As with other security systems, it
+ultimately comes down to human interaction. The keys are signed by dozens
+of people around the world, and we have to hope that our users have met
+enough people in the PGP web of trust
+that they can learn
+the correct keys. For users that aren't connected to the global security
+community, though, this question remains a critical weakness.
+
+
+
+
+
+
+
+
+
+
+9 Maintaining reachability
+
+
+
+
+9.1 How many bridge relays should you know about?
+
+
+The strategies described in Section 7 talked about
+learning one bridge address at a time. But if most bridges are ordinary
+Tor users on cable modem or DSL connection, many of them will disappear
+and/or move periodically. How many bridge relays should a blocked user
+know about so that she is likely to have at least one reachable at any
+given point? This is already a challenging problem if we only consider
+natural churn: the best approach is to see what bridges we attract in
+reality and measure their churn. We may also need to factor in a parameter
+for how quickly bridges get discovered and blocked by the attacker;
+we leave this for future work after we have more deployment experience.
+
+
+A related question is: if the bridge relays change IP addresses
+periodically, how often does the blocked user need to fetch updates in
+order to keep from being cut out of the loop?
+
+
+Once we have more experience and intuition, we should explore technical
+solutions to this problem too. For example, if the discovery strategies
+give out k bridge addresses rather than a single bridge address, perhaps
+we can improve robustness from the user perspective without significantly
+aiding the adversary. Rather than giving out a new random subset of k
+addresses at each point, we could bind them together into bridge
+families, so all users that learn about one member of the bridge family
+are told about the rest as well.
+
+
+This scheme may also help defend against attacks to map the set of
+bridges. That is, if all blocked users learn a random subset of bridges,
+the attacker should learn about a few bridges, monitor the country-level
+firewall for connections to them, then watch those users to see what
+other bridges they use, and repeat. By segmenting the bridge address
+space, we can limit the exposure of other users.
+
+
+
+
+9.2 Cablemodem users don't usually provide important websites
+
+
+
+Another attacker we might be concerned about is that the attacker could
+just block all DSL and cablemodem network addresses, on the theory that
+they don't run any important services anyway. If most of our bridges
+are on these networks, this attack could really hurt.
+
+
+The first answer is to aim to get volunteers both from traditionally
+"consumer" networks and also from traditionally "producer" networks.
+Since bridges don't need to be Tor exit nodes, as we improve our usability
+it seems quite feasible to get a lot of websites helping out.
+
+
+The second answer (not as practical) would be to encourage more use of
+consumer networks for popular and useful Internet services.
+
+
+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?
+
+
+
+9.3 Scanning resistance: making bridges more subtle
+
+
+If it's trivial to verify that a given address is operating as a bridge,
+and most bridges run on a predictable port, then it's conceivable our
+attacker could scan the whole Internet looking for bridges. (In fact,
+he can just concentrate on scanning likely networks like cablemodem
+and DSL services — see Section 9.2
+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 scanning
+resistance, and it goes along with normalizing Tor's TLS handshake and
+network fingerprint.
+
+
+We could provide a password to the blocked user, and she (or her Tor
+client) provides a nonced hash of this password when she connects. We'd
+need to give her an ID key for the bridge too (in addition to the IP
+address and port — see Section 6.1), 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.
+
+
+How should the bridge behave if accessed without the correct
+authorization? Perhaps it should act like an unconfigured HTTPS server
+("welcome to the default Apache page"), or maybe it should mirror
+and act like common websites, or websites randomly chosen from Google.
+
+
+We might assume that the attacker can recognize HTTPS connections that
+use self-signed certificates. (This process would be resource-intensive
+but not out of the realm of possibility.) But even in this case, many
+popular websites around the Internet use self-signed or just plain broken
+SSL certificates.
+
+
+
+
+
+
+
+
+
+
+9.4 How to motivate people to run bridge relays
+
+
+
+One of the traditional ways to get people to run software that benefits
+others is to give them motivation to install it themselves. An often
+suggested approach is to install it as a stunning screensaver so everybody
+will be pleased to run it. We take a similar approach here, by leveraging
+the fact that these users are already interested in protecting their
+own Internet traffic, so they will install and run the software.
+
+
+Eventually, we may be able to make all Tor users become bridges if they
+pass their self-reachability tests — the software and installers need
+more work on usability first, but we're making progress.
+
+
+In the mean time, we can make a snazzy network graph with
+Vidalia3 that
+emphasizes the connections the bridge user is currently relaying.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+9.5 Publicity attracts attention
+
+
+
+Many people working on this field want to publicize the existence
+and extent of censorship concurrently with the deployment of their
+circumvention software. The easy reason for this two-pronged push is
+to attract volunteers for running proxies in their systems; but in many
+cases their main goal is not to focus on 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.
+
+
+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?
+
+
+
+9.6 The Tor website: how to get the software
+
+
+One of the first censoring attacks against a system like ours is to
+block the website and make the software itself hard to find. Our system
+should work well once the user is running an authentic
+copy of Tor and has found a working bridge, but to get to that point
+we rely on their individual skills and ingenuity.
+
+
+Right now, most countries that block access to Tor block only the main
+website and leave mirrors and the network itself untouched.
+Falling back on word-of-mouth is always a good last resort, but we should
+also take steps to make sure it's relatively easy for users to get a copy,
+such as publicizing the mirrors more and making copies available through
+other media. We might also mirror the latest version of the software on
+each bridge, so users who hear about an honest bridge can get a good
+copy.
+See Section 7.1 for more discussion.
+
+
+
+
+
+
+10 Future designs
+
+
+
+
+10.1 Bridges inside the blocked network too
+
+
+Assuming actually crossing the firewall is the risky part of the
+operation, can we have some bridge relays inside the blocked area too,
+and more established users can use them as relays so they don't need to
+communicate over the firewall directly at all? A simple example here is
+to make new blocked users into internal bridges also — so they sign up
+on the bridge authority as part of doing their query, and we give out
+their addresses
+rather than (or along with) the external bridge addresses. This design
+is a lot trickier because it brings in the complexity of whether the
+internal bridges will remain available, can maintain reachability with
+the outside world, etc.
+
+
+More complex future designs involve operating a separate Tor network
+inside the blocked area, and using hidden service bridges — bridges
+that can be accessed by users of the internal Tor network but whose
+addresses are not published or findable, even by these users — to get
+from inside the firewall to the rest of the Internet. But this design
+requires directory authorities to run inside the blocked area too,
+and they would be a fine target to take down the network.
+
+
+
+
+
+
+11 Next Steps
+
+
+
+Technical solutions won't solve the whole censorship problem. After all,
+the firewalls in places like China are socially very
+successful, even if technologies and tricks exist to get around them.
+However, having a strong technical solution is still necessary as one
+important piece of the puzzle.
+
+
+In this paper, we have shown that Tor provides a great set of building
+blocks to start from. The next steps are to deploy prototype bridges and
+bridge authorities, implement some of the proposed discovery strategies,
+and then observe the system in operation and get more intuition about
+the actual requirements and adversaries we're up against.
+
+
+
+References
+
+
+ - [1]
-
+Alessandro Acquisti, Roger Dingledine, and Paul Syverson.
+ On the economics of anonymity.
+ In Rebecca N. Wright, editor, Financial Cryptography.
+ Springer-Verlag, LNCS 2742, 2003.
+
+
+
+ - [2]
-
+Adam Back, Ian Goldberg, and Adam Shostack.
+ Freedom systems 2.1 security issues and analysis.
+ White paper, Zero Knowledge Systems, Inc., May 2001.
+
+
+
+ - [3]
-
+Oliver Berthold, Hannes Federrath, and Stefan Köpsell.
+ Web MIXes: A system for anonymous and unobservable Internet
+ access.
+ In H. Federrath, editor, Designing Privacy Enhancing
+ Technologies: Workshop on Design Issue in Anonymity and Unobservability.
+ Springer-Verlag, LNCS 2009, 2000.
+
+
+
+ - [4]
-
+George Dean Bissias, Marc Liberatore, and Brian Neil Levine.
+ Privacy vulnerabilities in encrypted http streams.
+ In Proceedings of Privacy Enhancing Technologies workshop (PET
+ 2005), May 2005.
+
+ http://prisms.cs.umass.edu/brian/pubs/bissias.liberatore.pet.2005.pdf.
+
+
+
+ - [5]
-
+David Chaum.
+ Blind signatures for untraceable payments.
+ In D. Chaum, R.L. Rivest, and A.T. Sherman, editors, Advances in
+ Cryptology: Proceedings of Crypto 82, pages 199-203. Plenum Press, 1983.
+
+
+
+ - [6]
-
+Ian Clarke, Oskar Sandberg, Brandon Wiley, and Theodore W. Hong.
+ Freenet: A distributed anonymous information storage and retrieval
+ system.
+ In H. Federrath, editor, Designing Privacy Enhancing
+ Technologies: Workshop on Design Issue in Anonymity and Unobservability,
+ pages 46-66. Springer-Verlag, LNCS 2009, July 2000.
+
+
+
+ - [7]
-
+Richard Clayton, Steven J. Murdoch, and Robert N. M. Watson.
+ Ignoring the great firewall of china.
+ In Proceedings of the Sixth Workshop on Privacy Enhancing
+ Technologies (PET 2006), Cambridge, UK, June 2006. Springer.
+ http://www.cl.cam.ac.uk/~rnc1/ignoring.pdf.
+
+
+
+ - [8]
-
+George Danezis.
+ The traffic analysis of continuous-time mixes.
+ In David Martin and Andrei Serjantov, editors, Privacy Enhancing
+ Technologies (PET 2004), LNCS, May 2004.
+ http://www.cl.cam.ac.uk/users/gd216/cmm2.pdf.
+
+
+
+ - [9]
-
+Roger Dingledine and Nick Mathewson.
+ Anonymity loves company: Usability and the network effect.
+ In Proceedings of the Fifth Workshop on the Economics of
+ Information Security (WEIS 2006), Cambridge, UK, June 2006.
+ http://freehaven.net/doc/wupss04/usability.pdf.
+
+
+
+ - [10]
-
+Roger Dingledine, Nick Mathewson, and Paul Syverson.
+ Reputation in P2P Anonymity Systems.
+ In Proceedings of Workshop on Economics of Peer-to-Peer
+ Systems, June 2003.
+ http://freehaven.net/doc/econp2p03/econp2p03.pdf.
+
+
+
+ - [11]
-
+Roger Dingledine, Nick Mathewson, and Paul Syverson.
+ Tor: The second-generation onion router.
+ In Proceedings of the 13th USENIX Security Symposium, August
+ 2004.
+ http://tor.eff.org/tor-design.pdf.
+
+
+
+ - [12]
-
+Roger Dingledine and Paul Syverson.
+ Reliable MIX Cascade Networks through Reputation.
+ In Matt Blaze, editor, Financial Cryptography. Springer-Verlag,
+ LNCS 2357, 2002.
+
+
+
+ - [13]
-
+Ronald Deibert et al.
+ Psiphon.
+ http://psiphon.civisec.org/.
+
+
+
+ - [14]
-
+Nick Feamster, Magdalena Balazinska, Greg Harfst, Hari Balakrishnan, and David
+ Karger.
+ Infranet: Circumventing web censorship and surveillance.
+ In Proceedings of the 11th USENIX Security Symposium, August
+ 2002.
+ http://nms.lcs.mit.edu/~feamster/papers/usenixsec2002.pdf.
+
+
+
+ - [15]
-
+Gina Fisk, Mike Fisk, Christos Papadopoulos, and Joshua Neil.
+ Eliminating steganography in internet traffic with active wardens.
+ In Fabien Petitcolas, editor, Information Hiding Workshop (IH
+ 2002). Springer-Verlag, LNCS 2578, October 2002.
+
+
+
+ - [16]
-
+Geoffrey Goodell.
+ Perspective Access Networks.
+ PhD thesis, Harvard University, July 2006.
+ http://afs.eecs.harvard.edu/~goodell/thesis.pdf.
+
+
+
+ - [17]
-
+Geoffrey Goodell and Paul Syverson.
+ The right place at the right time: The use of network location in
+ authentication and abuse prevention, 2006.
+ Submitted.
+
+
+
+ - [18]
-
+Bennett Haselton.
+ How to install the Circumventor program.
+
+ http://www.peacefire.org/circumventor/simple-circumventor-instructions.html.
+
+
+
+ - [19]
-
+Ip-to-country database.
+ http://ip-to-country.webhosting.info/.
+
+
+
+ - [20]
-
+Stefan Köpsell and Ulf Hilling.
+ How to achieve blocking resistance for existing systems enabling
+ anonymous web surfing.
+ In Proceedings of the Workshop on Privacy in the Electronic
+ Society (WPES 2004), Washington, DC, USA, October 2004.
+ http://freehaven.net/anonbib/papers/p103-koepsell.pdf.
+
+
+
+ - [21]
-
+Brian N. Levine, Michael K. Reiter, Chenxi Wang, and Matthew Wright.
+ Timing analysis in low-latency mix-based systems.
+ In Ari Juels, editor, Financial Cryptography. Springer-Verlag,
+ LNCS (forthcoming), 2004.
+
+
+
+ - [22]
-
+Rebecca MacKinnon.
+ Private communication, 2006.
+
+
+
+ - [23]
-
+James Marshall.
+ CGIProxy: HTTP/FTP Proxy in a CGI Script.
+ http://www.jmarshall.com/tools/cgiproxy/.
+
+
+
+ - [24]
-
+Nick Mathewson and Roger Dingledine.
+ Practical traffic analysis: Extending and resisting statistical
+ disclosure.
+ In David Martin and Andrei Serjantov, editors, Privacy Enhancing
+ Technologies (PET 2004), LNCS, May 2004.
+ http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf.
+
+
+
+ - [25]
-
+Steven J. Murdoch and George Danezis.
+ Low-cost traffic analysis of tor.
+ In IEEE Symposium on Security and Privacy. IEEE CS, May 2005.
+
+
+
+ - [26]
-
+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, Information Hiding: 7th
+ International Workshop, volume 3727 of LNCS, pages 247-261,
+ Barcelona, Catalonia (Spain), June 2005. Springer-Verlag.
+
+
+
+ - [27]
-
+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.
+
+
+
+ - [28]
-
+Ethan Zuckerman.
+ We've got to adjust some of our threat models.
+ http://www.ethanzuckerman.com/blog/?p=1019.
+
+
+
+
+
+
+
+
+
Footnotes:
+
+
+1So far in places
+ like China, the authorities mainly go after people who publish materials
+ and coordinate organized movements [22].
+ 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 8.2.
+
+2http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ#EntryGuards
+
+3http://vidalia-project.net/
+
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+TEX
+by
+TTH,
+version 3.77.
On 11 May 2007, 21:49.
+
+