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throw down the gauntlet.
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@ -235,6 +235,7 @@ seems overkill (and/or insecure) based on the threat model we've picked.
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% this para should probably move to the scalability / directory system. -RD
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\section{Threat model}
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\label{sec:threat-model}
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Tor does not attempt to defend against a global observer. Any adversary who
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can see a user's connection to the Tor network, and who can see the
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@ -243,8 +244,8 @@ correlation between the two connections to confirm the user's chosen
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communication partners. Defeating this attack would seem to require
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introducing a prohibitive degree of traffic padding between the user and the
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network, or introducing an unacceptable degree of latency (but see
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\ref{subsec:mid-latency} below). Thus, Tor only
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attempts to defend against external observers who can observe both sides of a
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Section \ref{subsec:mid-latency}). Thus, Tor only
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attempts to defend against external observers who cannot observe both sides of a
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user's connection.
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Against internal attackers, who sign up Tor servers, the situation is more
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@ -279,7 +280,7 @@ complicating factors:
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% Sure. In fact, better off, since they seem to scale more easily. -rd
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in practice tor's threat model is based entirely on the goal of dispersal
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and diversity. george and steven describe an attack \cite{draft} that
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and diversity. george and steven describe an attack \cite{attack-tor-oak05} that
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lets them determine the nodes used in a circuit; yet they can't identify
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alice or bob through this attack. so it's really just the endpoints that
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remain secure. and the enclave model seems particularly threatened by
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@ -317,43 +318,75 @@ Tor's interaction with other services on the Internet.
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\subsection{Image and security}
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Image: substantial non-infringing uses. Image is a security parameter,
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since it impacts user base and perceived sustainability.
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A growing field of papers argue that usability for anonymity systems
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contributes directly to their security, because how usable the system
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is impacts the possible anonymity set~\cite{back01,econymics}. Or
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conversely, an unusable system attracts few users and thus can't provide
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much anonymity.
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good uses are kept private, bad uses are publicized. not good.
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This phenomenon has a second-order effect: knowing this, users should
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choose which anonymity system to use based in part on how usable
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\emph{others} will find it, in order to get the protection of a larger
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anonymity set. Thus we might replace the adage ``usability is a security
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parameter''~\cite{back01} with a new one: ``perceived usability is a
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security parameter.'' From here we can better understand the effects
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of publicity and advertising on security: the more convincing your
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advertising, the more likely people will believe you have users, and thus
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the more users you will attract. Perversely, over-hyped systems (if they
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are not too broken) may be a better choice than modestly promoted ones,
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if the hype attracts more users~\cite{usability-network-effect}.
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Public perception, and thus advertising, is a security parameter.
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So it follows that we should come up with ways to accurately communicate
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the available security levels to the user, so she can make informed
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decisions. Dresden's JAP project aims to do this, by including a
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comforting `anonymity meter' dial in the software's graphical interface,
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giving the user an impression of the level of protection for her current
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traffic.
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users do not correlate to anonymity. arma will do this.
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Communicating security levels to the user
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A Tor gui, how jap's gui is nice but does not reflect the security
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they provide.
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However, there's a catch. For users to share the same anonymity set,
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they need to act like each other. An attacker who can distinguish
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a given user's traffic from the rest of the traffic will not be
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distracted by other users on the network. For high-latency systems like
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Mixminion, where the threat model is based on mixing messages with each
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other, there's an arms race between end-to-end statistical attacks and
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counter-strategies~\cite{statistical-disclosure,minion-design,e2e-traffic,trickle02}.
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But for low-latency systems like Tor, end-to-end \emph{traffic
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confirmation} attacks~\cite{danezis-pet2004,SS03,defensive-dropping}
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allow an attacker who watches or controls both ends of a communication
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to use statistics to correlate packet timing and volume, quickly linking
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the initiator to her destination. This is why Tor's threat model is
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based on preventing the adversary from observing both the initiator and
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the responder.
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\subsection{Usability and bandwidth and sustainability and incentives}
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Like Tor, the current JAP implementation does not pad connections
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(apart from using small fixed-size cells for transport). In fact,
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its cascade-based network toplogy may be even more vulnerable to these
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attacks, because the network has fewer endpoints. JAP was born out of
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the ISDN mix design~\cite{isdn-mixes}, where padding made sense because
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every user had a fixed bandwidth allocation, but in its current context
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as a general Internet web anonymizer, adding sufficient padding to JAP
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would be prohibitively expensive.\footnote{Even if they could find and
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maintain extra funding to run higher-capacity nodes, our experience with
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users suggests that many users would not accept the increased per-user
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bandwidth requirements, leading to an overall much smaller user base. But
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see Section \ref{subsec:mid-latency}.} Therefore, since under this threat
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model the number of concurrent users does not seem to have much impact
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on the anonymity provided, we suggest that JAP's anonymity meter is not
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correctly communicating security levels to its users.
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low-pain-threshold users go away until all users are willing to use it
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Sustainability. Previous attempts have been commercial which we think
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adds a lot of unnecessary complexity and accountability. Freedom didn't
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collect enough money to pay its servers; JAP bandwidth is supported by
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continued money, and they periodically ask what they will do when it
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dries up.
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"outside of academia, jap has just lost, permanently"
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Usability: fc03 paper was great, except the lower latency you are the
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less useful it seems it is.
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[nick will write this section]
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On the other hand, while the number of active concurrent users may not
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matter as much as we'd like, it still helps to have some other users
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who use the network. We investigate this issue in the next section.
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\subsection{Reputability}
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Yet another factor in the safety of a given network is its reputability:
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the perception of its social value based on its current users. If I'm
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the only user of a system, it might be socially accepted, but I'm not
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getting any anonymity. Add a thousand Communists, and I'm anonymous,
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but everyone thinks I'm a Commie. Add a thousand random citizens (cancer
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survivors, privacy enthusiasts, and so on) and now I'm hard to profile.
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Another factor impacting the network's security is its reputability:
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the perception of its social value based on its current user base. If I'm
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the only user who has ever downloaded the software, it might be socially
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accepted, but I'm not getting much anonymity. Add a thousand Communists,
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and I'm anonymous, but everyone thinks I'm a Commie. Add a thousand
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random citizens (cancer survivors, privacy enthusiasts, and so on)
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and now I'm harder to profile.
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The more cancer survivors on Tor, the better for the human rights
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activists. The more script kiddies, the worse for the normal users. Thus,
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@ -370,11 +403,30 @@ involved when it comes to anonymity. To follow the above example, a
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network used entirely by cancer survivors might welcome some Communists
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onto the network, though of course they'd prefer a wider variety of users.
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Reputability becomes even more tricky in the case of privacy networks,
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since the good uses of the network (such as publishing by journalists in
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dangerous countries) are typically kept private, whereas network abuses
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or other problems tend to be more widely publicized.
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The impact of public perception on security is especially important
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during the bootstrapping phase of the network, where the first few
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widely publicized uses of the network can dictate the types of users it
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attracts next.
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\subsection{Usability and bandwidth and sustainability and incentives}
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low-pain-threshold users go away until all users are willing to use it
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Sustainability. Previous attempts have been commercial which we think
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adds a lot of unnecessary complexity and accountability. Freedom didn't
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collect enough money to pay its servers; JAP bandwidth is supported by
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continued money, and they periodically ask what they will do when it
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dries up.
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"outside of academia, jap has just lost, permanently"
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[nick will write this section]
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\subsection{Tor and file-sharing}
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[nick will write this section]
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@ -1151,12 +1151,24 @@
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title = {Synchronous Batching: From Cascades to Free Routes},
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author = {Roger Dingledine and Vitaly Shmatikov and Paul Syverson},
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booktitle = {Proceedings of Privacy Enhancing Technologies workshop (PET 2004)},
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editor = {David Martin and Andrei Serjantov},
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year = {2004},
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month = {May},
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series = {LNCS},
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note = {\url{http://freehaven.net/doc/sync-batching/sync-batching.pdf}},
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}
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@InProceedings{e2e-traffic,
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author = "Nick Mathewson and Roger Dingledine",
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title = "Practical Traffic Analysis: Extending and Resisting Statistical Disclosure",
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booktitle= {Privacy Enhancing Technologies (PET 2004)},
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editor = {David Martin and Andrei Serjantov},
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month = {May},
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year = {2004},
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series = {LNCS},
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note = {\url{http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf}},
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}
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@Misc{dtls,
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author = {E. Rescorla and N. Modadugu},
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title = {{Datagram Transport Layer Security}},
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@ -1166,6 +1178,14 @@
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note = {\url{http://www.ietf.org/internet-drafts/draft-rescorla-dtls-02.txt}},
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}
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@InProceedings{usability-network-effect,
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author={Roger Dingledine and Nick Mathewson},
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title={Anonymity Loves Company: Usability and the Network Effect},
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booktitle = {Designing Security Systems That People Can Use},
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year = {2005},
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publisher = {O'Reilly Media},
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}
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%%% Local Variables:
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%%% mode: latex
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%%% TeX-master: "tor-design"
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