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@ -116,24 +116,8 @@ relies on the filtering features of privacy-enhancing
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application-level proxies such as Privoxy \cite{privoxy}, without trying
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to duplicate those features itself.
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\item \textbf{Many TCP streams can share one circuit:} The
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original Onion Routing design built a separate circuit for each
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application-level request. This hurt performance by requiring
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multiple public key operations for every request, and also presented
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a threat to anonymity from building so many different circuits; see
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Section~\ref{sec:maintaining-anonymity}. Tor multiplexes multiple TCP
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streams along each virtual circuit to improve efficiency and anonymity.
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\item \textbf{Leaky-pipe circuit topology:} Through in-band signaling
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within the circuit, Tor initiators can direct traffic to nodes partway
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down the circuit. This novel approach allows for long-range
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padding to frustrate traffic shape and volume attacks at the initiator
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\cite{defensive-dropping}, and
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also allows traffic to exit the circuit from the middle---thus
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frustrating traffic shape and volume attacks based on observing the end
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of the circuit.
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\item \textbf{No mixing, padding, or traffic shaping:} The original Onion
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\item \textbf{No mixing, padding, or traffic shaping yet:} The original
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Onion
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Routing design called for batching and reordering the cells arriving from
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each source. It also included padding between onion routers and, in a
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later design, between onion proxies (that is, users) and onion routers
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@ -148,6 +132,23 @@ have a proven and convenient design for traffic shaping or low-latency
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mixing that will improve anonymity against a realistic adversary, we
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leave these strategies out.
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\item \textbf{Many TCP streams can share one circuit:} The
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original Onion Routing design built a separate circuit for each
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application-level request. This hurt performance by requiring
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multiple public key operations for every request, and also presented
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a threat to anonymity from building so many different circuits; see
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Section~\ref{sec:maintaining-anonymity}. Tor multiplexes multiple TCP
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streams along each virtual circuit to improve efficiency and anonymity.
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\item \textbf{Leaky-pipe circuit topology:} Through in-band signaling
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within the circuit, Tor initiators can direct traffic to nodes partway
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down the circuit. This novel approach allows for long-range padding if
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future research indicates that it can frustrate traffic shape and volume
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attacks at the initiator \cite{defensive-dropping}, and
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also allows traffic to exit the circuit from the middle---again possibly
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frustrating traffic shape and volume attacks based on observing the end
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of the circuit.
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\item \textbf{Congestion control:} Earlier anonymity designs do not
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address traffic bottlenecks. Unfortunately, typical approaches to
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load balancing and flow control in overlay networks involve inter-node
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@ -237,16 +238,19 @@ the cost of introducing comparatively large and variable latencies,
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including {\bf Babel} \cite{babel}, {\bf Mixmaster}
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\cite{mixmaster-spec}, and
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{\bf Mixminion} \cite{minion-design}. Because of this
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decision, these \emph{high-latency} networks are well-suited for anonymous
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email, but introduce too much lag for interactive tasks like web browsing,
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decision, these \emph{high-latency} networks resist strong global
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adversaries,
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but introduce too much lag for interactive tasks like web browsing,
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internet chat, or SSH connections.
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Tor belongs to the second category: \emph{low-latency} designs that
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attempt to anonymize interactive network traffic. These systems handle
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a variety of bidirectional protocols. They also provide more convenient
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mail delivery than the high-latency fire-and-forget anonymous email
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networks, because the remote mail server provides explicit delivery
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confirmation. But because these designs typically
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a variety of bidirectional protocols.
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% They also provide more convenient
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%mail delivery than the high-latency fire-and-forget anonymous email
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%networks, because the remote mail server provides explicit delivery
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%confirmation.
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But because these designs typically
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involve many packets that must be delivered quickly, it is
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difficult for them to prevent an attacker who can eavesdrop both ends of the
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communication from correlating the timing and volume
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@ -482,7 +486,7 @@ suspicion that Alice is
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talking to Bob if the timing and volume patterns of the traffic on the
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connection are distinct enough; active attackers can induce timing
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signatures on the traffic to \emph{force} distinct patterns. Tor
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does not address these \emph{traffic confirmation} attacks.
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does not yet address these \emph{traffic confirmation} attacks.
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Rather, we aim to prevent \emph{traffic
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analysis} attacks, where the adversary uses traffic patterns to learn
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which points in the network he should attack.
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@ -793,8 +797,8 @@ Privoxy safely. But a portable general solution, such as is needed for
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SSH, is
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an open problem. Modifying or replacing the local nameserver
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can be invasive, brittle, and not portable. Forcing the resolver
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library to do its resolution via TCP rather than UDP is
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hard to do right, and also has portability problems. We could provide a
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library to do resolution via TCP rather than UDP is
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hard, and also has portability problems. We could provide a
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tool similar to \emph{dig} to perform a private lookup through the
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Tor network. Our current answer is to encourage the use of
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privacy-aware proxies like Privoxy wherever possible.
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@ -1370,7 +1374,7 @@ acknowledge his existence.
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\Section{Attacks and Defenses}
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\label{sec:attacks}
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% XXX In sec4 we should talk about bandwidth classes, which will
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% XXX In sec9 we should talk about bandwidth classes, which will
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% enable us to accept a lot more ORs than if we continue to
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% require 10mbit connections for all ORs. -RD
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@ -1380,21 +1384,18 @@ design withstands them.
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\subsubsection*{Passive attacks}
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\emph{Observing user traffic patterns.} Observations of connection
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between a user and her first onion router will not reveal to whom
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the user is connecting or what information is being sent. It will
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reveal patterns of user traffic (both sent and received). Simple
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profiling of user connection patterns is not generally possible,
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however, because multiple application streams may be operating
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simultaneously or in series over a single circuit. Thus, further
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processing is necessary to discern even these usage patterns.
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\emph{Observing user traffic patterns.} Observing the connection
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from the user will not reveal her destination or data, but it will
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reveal traffic patterns (both sent and received). Profiling via user
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connection patterns is hampered because multiple application streams may
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be operating simultaneously or in series over a single circuit. Thus,
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further processing is necessary to discern even these usage patterns.
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\emph{Observing user content.} At the user end, content is
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encrypted; however, connections from the network to arbitrary
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websites may not be. Further, a responding website may itself be
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hostile. Filtering content is not a primary goal of
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Onion Routing; nonetheless, Tor can directly make use of Privoxy and
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related filtering services to anonymize application data streams.
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\emph{Observing user content.} While content at the user end is encrypted,
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connections to responders may not be (further, the responding website
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itself may be hostile). Filtering content is not a primary goal of Onion
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Routing; nonetheless, Tor can directly use Privoxy and related
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filtering services to anonymize application data streams.
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\emph{Option distinguishability.} Configuration options can be a
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source of distinguishable patterns. In general there is economic
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@ -1524,12 +1525,6 @@ adversary
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could possibly attract a disproportionately large amount of traffic
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by running an exit node with an unusually permissive exit policy.
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\emph{Compromise entire path.} Anyone compromising both
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endpoints of a circuit can confirm this with high probability. If
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the entire path is compromised, this becomes a certainty; however,
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the added benefit to the adversary of such an attack is small in
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relation to the difficulty.
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\emph{Run a hostile directory server.} Directory servers control
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admission to the network. However, because the network directory
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must be signed by a majority of servers, the threat of a single
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@ -1676,18 +1671,17 @@ by the session key shared by the client and server.
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% There must be a better intro than this! -NM
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In addition to the open problems discussed in
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Section~\ref{subsec:non-goals}, many other questions remain to be
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solved by future research before we can be confident that we
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have built a secure low-latency anonymity service.
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solved by future research before we can be confident of our security.
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Many of these open issues are questions of balance. For example,
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how often should users rotate to fresh circuits? Too-frequent
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rotation is inefficient, expensive, and may lead to intersection attacks,
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rotation is inefficient, expensive, and may lead to intersection attacks
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and predecessor attacks \cite{wright03},
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but too-infrequent rotation
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makes the user's traffic linkable. Instead of opening a fresh
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circuit; clients can also limit linkability by exiting from a middle point
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of the circuit, or by truncating and re-extending the circuit, but
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makes the user's traffic linkable. Along with opening a fresh
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circuit, clients can also limit linkability by exiting from a middle point
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of the circuit, or by truncating and re-extending the circuit; but
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more analysis is needed to determine the proper trade-off.
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%[XXX mention predecessor attacks?]
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A similar question surrounds timing of directory operations:
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how often should directories be updated? With too-infrequent
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@ -1696,7 +1690,6 @@ too-frequent updates the directory servers are overloaded.
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%do different exit policies at different exit nodes trash anonymity sets,
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%or not mess with them much?
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%
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%% Why would they? By routing traffic to certain nodes preferentially?
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%[XXX Choosing paths and path lengths: I'm not writing this bit till
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