tor/doc/tor-design.tex

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\title{Tor: Design of a Second-Generation Onion Router}

%\author{Roger Dingledine \\ The Free Haven Project \\ arma@freehaven.net \and
%Nick Mathewson \\ The Free Haven Project \\ nickm@freehaven.net \and
%Paul Syverson \\ Naval Research Lab \\ syverson@itd.nrl.navy.mil}

\maketitle
\thispagestyle{empty}

\begin{abstract}
We present Tor, a connection-based low-latency anonymous communication
system. Tor is the successor to Onion Routing
and addresses many limitations in the original Onion Routing design.
Tor works in a real-world Internet environment,
% it's user-space too
requires little synchronization or coordination between nodes, and
protects against known anonymity-breaking attacks as well
as or better than other systems with similar design parameters.
% and we present a big list of open problems at the end
% and we present a new practical design for rendezvous points
\end{abstract}

%\begin{center}
%\textbf{Keywords:} anonymity, peer-to-peer, remailer, nymserver, reply block
%\end{center}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\Section{Overview}
\label{sec:intro}

Onion Routing is a distributed overlay network designed to anonymize
low-latency TCP-based applications such as web browsing, secure shell,
and instant messaging. Clients choose a path through the network and
build a \emph{virtual circuit}, in which each node (or ``onion router'') 
in the path knows its
predecessor and successor, but no others. Traffic flowing down the circuit
is sent in fixed-size \emph{cells}, which are unwrapped by a symmetric key
at each node (like the layers of an onion) and relayed downstream. The
original Onion Routing project published several design and analysis
papers
\cite{or-jsac98,or-discex00,or-ih96,or-pet00}. While there was briefly
a wide area Onion Routing network,
% how long is briefly? a day, a month? -RD
the only long-running and publicly accessible
implementation was a fragile proof-of-concept that ran on a single
machine (which nonetheless processed several tens of thousands of connections
daily from thousands of global users).
Many critical design and deployment issues were never resolved,
and the design has not been updated in several years.
Here we describe Tor, a protocol for asynchronous, loosely
federated onion routers that provides the following improvements over
the old Onion Routing design, and over other low-latency anonymity systems:

\begin{tightlist}

\item \textbf{Perfect forward secrecy:} The original Onion Routing
design was vulnerable to a single hostile node recording traffic and later
compromising successive nodes in the circuit and forcing them to
decrypt it. 
Rather than using a single onion to lay each circuit,
Tor now uses an incremental or \emph{telescoping}
path-building design, where the initiator negotiates session keys with
each successive hop in the circuit.  Once these keys are deleted,
subsequently compromised nodes cannot decrypt old traffic.
As a side benefit, onion replay detection is no longer
necessary, and the process of building circuits is more reliable, since
the initiator knows when a hop fails and can then try extending to a new node.

% Perhaps mention that not all of these are things that we invented. -NM

\item \textbf{Separation of protocol cleaning from anonymity:}
The original Onion Routing design required a separate ``application
proxy'' for each
supported application protocol---most
of which were never written, so many applications were never supported.
Tor uses the standard and near-ubiquitous SOCKS
\cite{socks4,socks5} proxy interface, allowing us to support most TCP-based
programs without modification.  This design change allows Tor to
use the filtering features of privacy-enhancing
application-level proxies such as Privoxy \cite{privoxy} without having to
incorporate those features itself.

\item \textbf{Many TCP streams can share one circuit:} The original
Onion Routing design built a separate circuit for each application-level
request.
This hurt performance by requiring multiple public key operations for
every request, and also presented
a threat to anonymity (see Section~\ref{maintaining-anonymity}).
\footnote{The first Onion Routing design \cite{or-ih96} protected against
this threat to some
extent by requiring users to hide network access behind an onion
router/firewall that was also forwarding traffic from other nodes.
However, it is desirable for users to
benefit from Onion Routing even when they can't run their own 
onion routers.
%Such users, especially if they engage in certain unusual
%communication behaviors, may be identifiable \cite{wright03}. 
%To
%complicate the possibility of such attacks Tor multiplexes many
%stream down each circuit, but still rotates the circuit
%periodically to avoid too much linkability from requests on a single
%circuit.
%
% [This digression probably belongs in maintaining-anonymity. -NM
}
The current Tor design multiplexes multiple TCP streams along each virtual
circuit, in order to improve efficiency and anonymity.

\item \textbf{No mixing, padding, or traffic shaping:} The original
Onion Routing design called for mixing of data from each circuit,
plus full link padding both between onion routers and between onion
proxies (that is, users) and onion routers \cite{or-jsac98}. The
later analysis paper \cite{or-pet00} suggested \emph{traffic shaping}
to provide similar protection but use less bandwidth, but did not go
into detail. However, recent research \cite{econymics} and deployment
experience \cite{freedom21-security} suggest that this level of resource
use is not practical or economical; and even full link padding is still
vulnerable \cite{defensive-dropping}. Thus, until we have a proven and
convenient design for traffic shaping or low-latency mixing that will help
anonymity against a realistic adversary, we leave these strategies out.

\item \textbf{Leaky-pipe circuit topology:} Through in-band
  signalling within the
  circuit, Tor initiators can direct traffic to nodes partway down the
  circuit. This not only allows for long-range padding to frustrate traffic
  shape and volume attacks at the initiator \cite{defensive-dropping},
  but because circuits are used by more than one application, it also
  allows traffic to exit the circuit from the middle---thus
  frustrating traffic shape and volume attacks based on observing exit
  points.

\item \textbf{Congestion control:} Earlier anonymity designs do not
address traffic bottlenecks. Unfortunately, typical approaches to load
balancing and flow control in overlay networks involve inter-node control
communication and global views of traffic. Tor's decentralized ack-based
congestion control maintains reasonable anonymity while allowing nodes
at the edges of the network to detect congestion or flooding attacks
and send less data until the congestion subsides.

\item \textbf{Directory servers:} The original Onion Routing design
planned to flood link-state information through the network---an
approach which can be unreliable and
open to partitioning attacks or outright deception. Tor takes a simplified
view towards distributing link-state information. Certain more trusted
onion routers also serve as directory servers; they provide signed
\emph{directories} describing all routers they know about, and which
are currently up. Users periodically download these directories via HTTP.

\item \textbf{End-to-end integrity checking:} Without integrity checking
on traffic going through the network, any onion router on the path
can change the contents of cells as they pass by---for example, to redirect a
connection on the fly so it connects to a different webserver, or to
tag encrypted traffic and look for the tagged traffic at the network
edges \cite{minion-design}.  Tor hampers these attacks by checking data
integrity before it leaves the network.

\item \textbf{Robustness to failed nodes:} A failed node in a traditional
mix network means lost messages, but thanks to Tor's step-by-step
circuit building, users can notice failed
nodes while building circuits and route around them.  Additionally,
liveness information from directories allows users to avoid
unreliable nodes in the first place.
%We further provide a
%simple mechanism that allows connections to be established despite recent
%node failure or slightly dated information from a directory server. Tor
%permits onion routers to have \emph{router twins} --- nodes that share
%the same private decryption key. Note that because connections now have
%perfect forward secrecy, an onion router still cannot read the traffic
%on a connection established through its twin even while that connection
%is active. Also, which nodes are twins can change dynamically depending
%on current circumstances, and twins may or may not be under the same
%administrative authority.
%
%[Commented out; Router twins provide no real increase in robustness
%to failed nodes.  If a non-twinned node goes down, the
%circuit-builder notices this and routes around it.  Circuit-building
%is offline, so there shouldn't even be a latency hit. -NM]

\item \textbf{Variable exit policies:} Tor provides a consistent
mechanism for
each node to specify and advertise a policy describing the hosts and
ports to which it will connect. These exit policies
are critical in a volunteer-based distributed infrastructure, because
each operator is comfortable with allowing different types of traffic
to exit the Tor network from his node.

\item \textbf{Implementable in user-space:} Because it only attempts to
anonymize TCP streams, Tor differs from other anonymity systems like
Freedom \cite{freedom} in that it does not require patches to an operating
system's network stack in order to operate.  Although this approach is less
flexible, it has proven valuable to Tor's portability and deployability.

\item \textbf{Rendezvous points and location-protected servers:} Tor
provides an integrated mechanism for responder anonymity via
location-protected servers.  Previous Onion Routing designs included
long-lived ``reply onions'' which could be used to build virtual
circuits to a hidden server, but this approach is 
brittle because a reply onion becomes useless if any node in the
path goes down or rotates its keys, and it's also
%vulnerable to flooding attacks,
% no it isn't. no more than our rendezvous point approach at least -RD
incompatible with forward security.  In Tor's
current design, clients use {\it introduction points} to negotiate {\it
rendezvous points} to connect with hidden servers; and reply onions
are no longer required.
\end{tightlist}

[XXX carefully mention implementation, emphasizing that experience
deploying isn't there yet, and not all features are implemented.
Mention that it runs, is kinda alpha, kinda deployed, runs on win32.]

We review previous work in Section \ref{sec:background}, describe
our goals and assumptions in Section \ref{sec:assumptions},
and then address the above list of improvements in Sections
\ref{sec:design}-\ref{sec:maintaining-anonymity}. We then summarize
how our design stands up to known attacks, and conclude with a list of
open problems.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\Section{Background and threat model}
\label{sec:background}

\SubSection{Related work}
\label{sec:related-work}
Modern anonymity designs date to Chaum's Mix-Net\cite{chaum-mix} design of
1981.  Chaum proposed hiding sender-recipient connections by wrapping
messages in several layers of public key cryptography, and relaying them
through a path composed of ``Mixes.''  These mixes in turn decrypt, delay,
and re-order messages, before relaying them along the sender-selected
path towards their destinations.

Subsequent relay-based anonymity designs have diverged in two
principal directions.  Some have attempted to maximize anonymity at
the cost of introducing comparatively large and variable latencies,
for example, Babel\cite{babel}, Mixmaster\cite{mixmaster-spec}, and
Mixminion\cite{minion-design}.  Because of this
trade-off, such \emph{high-latency} networks are well-suited for anonymous
email, but introduce too much lag for interactive tasks such as web browsing,
internet chat, or SSH connections.

Tor belongs to the second category: \emph{low-latency} designs that attempt
to anonymize interactive network traffic.  Because these protocols typically
involve a large number of packets that must be delivered quickly, it is
difficult for them to prevent an attacker who can eavesdrop both ends of the
interactive communication from points from correlating the timing and volume
of traffic entering the anonymity network with traffic leaving it.  These
protocols are also vulnerable against certain active attacks in which an
adversary introduces timing patterns into traffic entering the network, and 
looks
for correlated patterns among exiting traffic.
Although some work has been done to frustrate
these attacks,\footnote{
  The most common approach is to pad and limit communication to a constant
  rate, or to limit
  the variation in traffic shape.  Doing so can have prohibitive bandwidth
  costs and/or performance limitations.
  %One can also use a cascade (fixed
  %shared route) with a relatively fixed set of users. This assumes a
  %significant degree of agreement and provides an easier target for an active
  %attacker since the endpoints are generally known.
} most designs protect primarily against traffic analysis rather than traffic
confirmation \cite{or-jsac98}---that is, they assume that the attacker is
attempting to learn who is talking to whom, not to confirm a prior suspicion
about who is talking to whom.

The simplest low-latency designs are single-hop proxies such as the
Anonymizer \cite{anonymizer}, wherein a single trusted server removes
identifying users' data before relaying it.  These designs are easy to
analyze, but require end-users to trust the anonymizing proxy.  Furthermore,
concentrating the traffic to a single point makes traffic analysis easier: an
adversary need only eavesdrop on the proxy in order to become a global
observer against the entire anonymity network.

More complex are distributed-trust, channel-based anonymizing systems.  In
these designs, a user establishes one or more medium-term bidirectional
end-to-end tunnels to exit servers, and uses those tunnels to deliver a
number of low-latency packets to and from one or more destinations per
tunnel.  Establishing tunnels is comparatively expensive and typically
requires public-key cryptography, whereas relaying packets along a tunnel is
comparatively inexpensive.  Because a tunnel crosses several servers, no
single server can learn the user's communication partners.

In some distributed-trust systems, such as the Java Anon Proxy (also known as
JAP or WebMIXes), users
build their tunnels along a fixed shared route or
``cascade.''  Like a single-hop proxy, a single cascade increases anonymity
sets by concentrating concurrent traffic into a single communication pipe.
Concentrating traffic, however, can become a liability: as with a single-hop
proxy, an attacker only needs to observe a limited number of servers (in this
case, both ends of the cascade) in order
to bridge all the system's traffic.
The Java Anon Proxy's design seeks to prevent this by padding
between end users and the head of the cascade \cite{web-mix}. However, the
current implementation does no padding and thus remains vulnerable
to both active and passive bridging.

Systems such as earlier versions of Freedom and the original Onion Routing
build the anonymous channel all at once, using a layered ``onion'' of
public-key encrypted messages, each layer of which provides a set of session
keys and the address of the next server in the channel.  Tor as described
herein, later designs of Freedom, and AnonNet \cite{anonnet} build the
channel in stages, extending it one hop at a time. This approach
makes perfect forward secrecy feasible.

Distributed-trust anonymizing systems differ in how they prevent attackers
from controlling too many servers and thus compromising too many user paths.
Some protocols rely on a centrally maintained set of well-known anonymizing
servers.  The current Tor design falls into this category.
Others (such as Tarzan and MorphMix) allow unknown users to run
servers, while using a limited resource (DHT space for Tarzan; IP space for
MorphMix) to prevent an attacker from owning too much of the network.
Crowds uses a centralized ``blender'' to enforce Crowd membership
policy. For small crowds it is suggested that familiarity with all
members is adequate. For large diverse crowds, limiting accounts in
control of any one party is more complex: 
``(e.g., the blender administrator sets up an account for a user only
after receiving a written, notarized request from that user) and each
account to one jondo, and by monitoring and limiting the number of
jondos on any one net- work (using IP address), the attacker would be
forced to launch jondos using many different identities and on many
different networks to succeed'' \cite{crowds-tissec}.

Another low-latency design that was proposed independently and at
about the same time as the original Onion Routing was PipeNet
\cite{pipenet}.  It provided anonymity protections that were stronger
than Onion Routing's, but at the cost of allowing a single user to
shut down the network simply by not sending. It was also never
implemented or formally published. Low-latency anonymous communication
has also been designed for other types of systems, including
ISDN \cite{isdn-mixes}, and mobile applications such as telephones and
active badging systems \cite{federrath-ih96,reed-protocols97}.

Some systems, such as Crowds \cite{crowds-tissec}, do not rely on changing the
appearance of packets to hide the path; rather they try to prevent an
intermediary from knowing whether it is talking to an initiator
or just another intermediary.  Crowds uses no public-key
encryption, but the responder and all data are visible to all
nodes on the path; so anonymity of the connection initiator depends on
filtering all identifying information from the data stream. Crowds only
supports HTTP traffic.

Hordes \cite{hordes-jcs} is based on Crowds but also uses multicast
responses to hide the initiator. Herbivore \cite{herbivore} and
P5 \cite{p5} go even further, requiring broadcast.
Each uses broadcast in different ways, and trade-offs are made to
make broadcast more practical. Both Herbivore and P5 are designed primarily
for communication between communicating peers, although Herbivore
permits external connections by requesting a peer to serve as a proxy.
Allowing easy connections to nonparticipating responders or recipients
is a practical requirement for many users, e.g., to visit
nonparticipating Web sites or to exchange mail with nonparticipating
recipients.

Tor is not primarily designed for censorship resistance but rather
for anonymous communication. However, Tor's rendezvous points, which
enable connections between mutually anonymous entities, also
facilitate connections to hidden servers.  These building blocks to
censorship resistance and other capabilities are described in
Section~\ref{sec:rendezvous}.  Location-hidden servers are an
essential component for anonymous publishing systems such as
Publius\cite{publius}, Free Haven\cite{freehaven-berk}, and
Tangler\cite{tangler}.


STILL NOT MENTIONED:
real-time mixes\\
rewebbers\\
cebolla\\

Rewebber was mentioned in an earlier version along with Eternity,
which *must* be mentioned if we cite anything at all
in censorship resistance.

[XXX Close by mentioning where Tor fits.]

\Section{Design goals and assumptions}
\label{sec:assumptions}

\subsection{Goals}
Like other low-latency anonymity designs, Tor seeks to frustrate
attackers from linking communication partners, or from linking
multiple communications to or from a single point.  Within this
main goal, however, several design considerations have directed
Tor's evolution.

\begin{description}
\item[Deployability:] The design must be one which can be implemented,
  deployed, and used in the real world.  This requirement precludes designs
  that are expensive to run (for example, by requiring more bandwidth than
  volunteers are willing to provide); designs that place a heavy liability
  burden on operators (for example, by allowing attackers to implicate onion
  routers in illegal activities); and designs that are difficult or expensive
  to implement (for example, by requiring kernel patches, or separate proxies
  for every protocol).  This requirement also precludes systems in which
  users who do not benefit from anonymity are required to run special
  software in order to communicate with anonymous parties.
% XXX Our rendezvous points require clients to use our software to get to
%     the location-hidden servers.
%     Or at least, they require somebody near the client-side running our
%     software. We haven't worked out the details of keeping it transparent
%     for Alice if she's using some other http proxy somewhere. I guess the
%     external http proxy should route through a Tor client, which automatically
%     translates the foo.onion address? -RD
\item[Usability:] A hard-to-use system has fewer users---and because
  anonymity systems hide users among users, a system with fewer users
  provides less anonymity.  Usability is not only a convenience for Tor:
  it is a security requirement \cite{econymics,back01}. Tor
  should work with most of a user's unmodified applications; shouldn't
  introduce prohibitive delays; and should require the user to make as few
  configuration decisions as possible.
\item[Flexibility:] The protocol must be flexible and
  well-specified, so that it can serve as a test-bed for future research in
  low-latency anonymity systems.  Many of the open problems in low-latency
  anonymity networks (such as generating dummy traffic, or preventing
  pseudospoofing attacks) may be solvable independently from the issues
  solved by Tor; it would be beneficial if future systems were not forced to
  reinvent Tor's design decisions.  (But note that while a flexible design
  benefits researchers, there is a danger that differing choices of
  extensions will render users distinguishable.  Thus, implementations should
  not permit different protocol extensions to coexist in a single deployed
  network.)
\item[Conservative design:] The protocol's design and security parameters
  must be conservative.  Because additional features impose implementation
  and complexity costs, Tor should include as few speculative features as
  possible.  (We do not oppose speculative designs in general; however, it is
  our goal with Tor to embody a solution to the problems in low-latency
  anonymity that we can solve today before we plunge into the problems of
  tomorrow.)
  % This last bit sounds completely cheesy.  Somebody should tone it down. -NM 
\end{description}

\subsection{Non-goals}
In favoring conservative, deployable designs, we have explicitly deferred
a number of goals. Many of these goals are desirable in anonymity systems,
but we choose to defer them either because they are solved elsewhere,
or because they present an area of active research lacking a generally
accepted solution.

\begin{description}
\item[Not Peer-to-peer:] Tarzan and Morphmix aim to
  scale to completely decentralized peer-to-peer environments with thousands
  of short-lived servers, many of which may be controlled by an adversary.
  Because of the many open problems in this approach, Tor uses a more
  conservative design.
\item[Not secure against end-to-end attacks:] Tor does not claim to provide a
  definitive solution to end-to-end timing or intersection attacks. Some
  approaches, such as running an onion router, may help; see Section
  \ref{sec:analysis} for more discussion.
\item[No protocol normalization:] Tor does not provide \emph{protocol
  normalization} like Privoxy or the Anonymizer.  In order to make clients
  indistinguishable when they use complex and variable protocols such as HTTP,
  Tor must be layered with a filtering proxy such as Privoxy to hide
  differences between clients, expunge protocol features that leak identity,
  and so on.  Similarly, Tor does not currently integrate tunneling for
  non-stream-based protocols like UDP; this too must be provided by
  an external service.
% Actually, tunneling udp over tcp is probably horrible for some apps.
% Should this get its own non-goal bulletpoint? The motivation for
% non-goal-ness would be burden on clients / portability.
\item[Not steganographic:] Tor does not try to conceal which users are
  sending or receiving communications; it only tries to conceal whom they are
  communicating with.
\end{description}

\SubSection{Adversary Model}
\label{subsec:adversary-model}

A global passive adversary is the most commonly assumed when
analyzing theoretical anonymity designs. But like all practical low-latency
systems, Tor is not secure against this adversary.  Instead, we assume an
adversary that is weaker than global with respect to distribution, but that
is not merely passive.  Our threat model expands on that from
\cite{or-pet00}.

%%%% This is really keen analytical stuff, but it isn't our threat model:
%%%% we just go ahead and assume a fraction of hostile nodes for
%%%% convenience. -NM
%
%% The basic adversary components we consider are:
%% \begin{description}
%% \item[Observer:] can observe a connection (e.g., a sniffer on an
%%   Internet router), but cannot initiate connections. Observations may
%%   include timing and/or volume of packets as well as appearance of
%%   individual packets (including headers and content).
%% \item[Disrupter:] can delay (indefinitely) or corrupt traffic on a
%%   link. Can change all those things that an observer can observe up to
%%   the limits of computational ability (e.g., cannot forge signatures
%%   unless a key is compromised).
%% \item[Hostile initiator:] can initiate (or destroy) connections with
%%   specific routes as well as vary the timing and content of traffic
%%   on the connections it creates. A special case of the disrupter with
%%   additional abilities appropriate to its role in forming connections.
%% \item[Hostile responder:] can vary the traffic on the connections made
%%   to it including refusing them entirely, intentionally modifying what
%%   it sends and at what rate, and selectively closing them. Also a
%%   special case of the disrupter.
%% \item[Key breaker:] can break the key used to encrypt connection
%%   initiation requests sent to a Tor-node.
%% % Er, there are no long-term private decryption keys. They have
%% % long-term private signing keys, and medium-term onion (decryption)
%% % keys. Plus short-term link keys. Should we lump them together or
%% % separate them out? -RD
%% %
%% %  Hmmm, I was talking about the keys used to encrypt the onion skin
%% %  that contains the public DH key from the initiator. Is that what you
%% %  mean by medium-term onion key? (``Onion key'' used to mean the
%% %  session keys distributed in the onion, back when there were onions.)
%% %  Also, why are link keys short-term? By link keys I assume you mean
%% %  keys that neighbor nodes use to superencrypt all the stuff they send
%% %  to each other on a link.  Did you mean the session keys? I had been
%% %  calling session keys short-term and everything else long-term. I
%% %  know I was being sloppy. (I _have_ written papers formalizing
%% %  concepts of relative freshness.) But, there's some questions lurking
%% %  here. First up, I don't see why the onion-skin encryption key should
%% %  be any shorter term than the signature key in terms of threat
%% %  resistance. I understand that how we update onion-skin encryption
%% %  keys makes them depend on the signature keys. But, this is not the
%% %  basis on which we should be deciding about key rotation. Another
%% %  question is whether we want to bother with someone who breaks a
%% %  signature key as a particular adversary. He should be able to do
%% %  nearly the same as a compromised tor-node, although they're not the
%% %  same. I reworded above, I'm thinking we should leave other concerns
%% %  for later. -PS
%% \item[Hostile Tor node:] can arbitrarily manipulate the
%%   connections under its control, as well as creating new connections
%%   (that pass through itself).
%% \end{description}
%
%% All feasible adversaries can be composed out of these basic
%% adversaries. This includes combinations such as one or more
%% compromised Tor-nodes cooperating with disrupters of links on which
%% those nodes are not adjacent, or such as combinations of hostile
%% outsiders and link observers (who watch links between adjacent
%% Tor-nodes).  Note that one type of observer might be a Tor-node. This
%% is sometimes called an honest-but-curious adversary. While an observer
%% Tor-node will perform only correct protocol interactions, it might
%% share information about connections and cannot be assumed to destroy
%% session keys at end of a session.  Note that a compromised Tor-node is
%% stronger than any other adversary component in the sense that
%% replacing a component of any adversary with a compromised Tor-node
%% results in a stronger overall adversary (assuming that the compromised
%% Tor-node retains the same signature keys and other private
%% state-information as the component it replaces).

First, we assume that a threshold of directory servers are honest,
reliable, accurate, and trustworthy.
%% the rest of this isn't needed, if dirservers do threshold concensus dirs
%  To augment this, users can periodically cross-check 
%directories from each directory server (trust, but verify).
%, and that they always have access to at least one directory server that they trust.

Second, we assume that somewhere between ten percent and twenty
percent\footnote{In some circumstances---for example, if the Tor network is
  running on a hardened network where all operators have had background
  checks---the number of compromised nodes could be much lower.} 
of the Tor nodes accepted by the directory servers are compromised, hostile,
and collaborating in an off-line clique.  These compromised nodes can
arbitrarily manipulate the connections that pass through them, as well as
creating new connections that pass through themselves.  They can observe
traffic, and record it for later analysis.  Honest participants do not know
which servers these are.

(In reality, many realistic adversaries might have `bad' servers that are not
fully compromised but simply under observation, or that have had their keys
compromised.  But for the sake of analysis, we ignore, this possibility,
since the threat model we assume is strictly stronger.)

% This next paragraph is also more about analysis than it is about our
% threat model.  Perhaps we can say, ``users can connect to the network and
% use it in any way; we consider abusive attacks separately.'' ? -NM
Third, we constrain the impact of hostile users.  Users are assumed to vary
widely in both the duration and number of times they are connected to the Tor
network. They can also be assumed to vary widely in the volume and shape of
the traffic they send and receive. Hostile users are, by definition, limited
to creating and varying their own connections into or through a Tor
network. They may attack their own connections to try to gain identity
information of the responder in a rendezvous connection. They can also try to
attack sites through the Onion Routing network; however we will consider this
abuse rather than an attack per se (see
Section~\ref{subsec:exitpolicies}). Other than abuse, a hostile user's
motivation to attack his own connections is limited to the network effects of
such actions, such as denial of service (DoS) attacks.  Thus, in this case,
we can view user as simply an extreme case of the ordinary user; although
ordinary users are not likely to engage in, e.g., IP spoofing, to gain their
objectives.

In general, we are more focused on traffic analysis attacks than
traffic confirmation attacks. 
%A user who runs a Tor proxy on his own
%machine, connects to some remote Tor-node and makes a connection to an
%open Internet site, such as a public web server, is vulnerable to
%traffic confirmation.
That is, an active attacker who suspects that
a particular client is communicating with a particular server can
confirm this if she can modify and observe both the
connection between the Tor network and the client and that between the
Tor network and the server. Even a purely passive attacker can
confirm traffic if the timing and volume properties of the traffic on
the connection are unique enough.  (This is not to say that Tor offers
no resistance to traffic confirmation; it does.  We defer discussion
of this point and of particular attacks until Section~\ref{sec:attacks},
after we have described Tor in more detail.)
% XXX We need to say what traffic analysis is:  How about...
On the other hand, we {\it do} try to prevent an attacker from
performing traffic analysis: that is, attempting to learn the communication
partners of an arbitrary user.
% XXX If that's not right, what is?  It would be silly to have a
% threat model section without saying what we want to prevent the
% attacker from doing. -NM
% XXX Also, do we want to mention linkability or building profiles? -NM

Our assumptions about our adversary's capabilities imply a number of
possible attacks against users' anonymity.  Our adversary might try to
mount passive attacks by observing the edges of the network and
correlating traffic entering and leaving the network: either because
of relationships in packet timing; relationships in the volume of data
sent; [XXX simple observation??]; or relationships in any externally
visible user-selected options.  The adversary can also mount active
attacks by trying to compromise all the servers' keys in a
path---either through illegitimate means or through legal coercion in
unfriendly jurisdiction; by selectively DoSing trustworthy servers; by
introducing patterns into entering traffic that can later be detected;
or by modifying data entering the network and hoping that trashed data
comes out the other end.  The attacker can additionally try to
decrease the network's reliability by performing antisocial activities
from reliable servers and trying to get them taken down.
% XXX Should there be more or less?  Should we turn this into a
% bulleted list?  Should we cut it entirely?

We list these attacks and more, and describe our defenses against them
in Section~\ref{sec:attacks}.


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\Section{The Tor Design}
\label{sec:design}

The Tor network is an overlay network; each node is called an onion router
(OR). Onion routers run on normal computers without needing any special
privileges. Each OR maintains a long-term TLS connection to every other
OR (although we look at ways to relax this clique-topology assumption in
section \ref{subsec:restricted-routes}). A subset of the ORs also act as
directory servers, tracking which routers are currently in the network;
see section \ref{subsec:dirservers} for directory server details. Users
run local software called an onion proxy (OP) that fetches directories,
establishes paths (called \emph{virtual circuits}) over the network,
and handles connections from the user applications. Onion proxies accept
TCP streams and multiplex them across the virtual circuit. The onion
router on the other side 
% I don't mean other side, I mean wherever it is on the circuit. But
% don't want to introduce complexity this early? Hm. -RD
of the circuit connects to the destinations of
the TCP streams and relays data.

Onion routers have three types of keys. The first key is the identity
(signing) key. An OR uses this key to sign TLS certificates, to sign its
router descriptor (a summary of its keys, address, bandwidth, exit policy,
etc), and to sign directories if it is a directory server. Changing the
identity key of a router is considered equivalent to creating a new
router. The second key is the onion (decryption) key, which is used
for decrypting requests from users to set up a circuit and negotiate
ephemeral keys. Thirdly, each OR shares link keys (generated by TLS)
with the other ORs it's connected to. We discuss rotating these keys in
Section \ref{subsec:rotating-keys}.

Section \ref{subsec:cells} discusses the structure of the fixed-size
\emph{cells} that are the unit of communication in Tor. We describe
in Section \ref{subsec:circuits} how circuits work, and how they are
built, extended, truncated, and destroyed. Section \ref{subsec:tcp}
discusses the process of opening TCP streams through Tor, and finally
Section \ref{subsec:congestion} talks about congestion control and
fairness issues.

\SubSection{Cells}
\label{subsec:cells}

Traffic passes from node to node in fixed-size cells. Each cell is 256
bytes, and consists of a header and a payload. The header includes the
circuit identifier (ACI) which specifies which circuit the cell refers to
(many circuits can be multiplexed over the single TCP connection between
ORs or between an OP and an OR), and a command to describe what to do
with the cell's payload. Cells are either control cells, meaning they are
intended to be interpreted by the node that receives them, or relay cells,
meaning they carry end-to-end stream data. Controls cells can be one of:
\emph{padding} (currently used for keepalive, but can be used for link
padding), \emph{create} or \emph{created} (to set up a new circuit),
or \emph{destroy} (to tear down a circuit).

Relay cells have an additional header (the relay header) after the
cell header, which specifies the stream identifier (many streams can
be multiplexed over a circuit), an end-to-end checksum for integrity
checking, the length of the relay payload, and a relay command. Relay
commands can be one of: \emph{relay
data} (for data flowing down the stream), \emph{relay begin} (to open a
stream), \emph{relay end} (to close a stream), \emph{relay connected}
(to notify the OP that a relay begin has succeeded), \emph{relay
extend} and \emph{relay extended} (to extend the circuit by a hop,
and to acknowledge), \emph{relay truncate} and \emph{relay truncated}
(to tear down only part of the circuit, and to acknowledge), \emph{relay
sendme} (used for congestion control), and \emph{relay drop} (used to
implement long-range dummies).

We will talk more about each of these cell types below.

% Nick: should there have been a table here? -RD

\SubSection{Circuits and streams}
\label{subsec:circuits}

While the original Onion Routing design built one circuit for each stream,
Tor circuits can be used by many streams. Thus because circuits can
take several tenths of a second to construct due to crypto and network
latency, users construct circuits preemptively. Users build a new circuit
periodically (currently every minute) if the previous one has been used,
and expire old used circuits that are no longer in use. Thus even very
active users spend a negligible amount of time and CPU in building
circuits, but only a limited number of requests can be linked to each
other by a given exit node.

Users set up circuits incrementally, negotiating a symmetric key with
each hop one at a time. To create a new circuit, the user (call her
Alice) sends a \emph{create} cell to the first node in her chosen
path. The payload is the first half of the Diffie-Hellman handshake,
encrypted to the onion key of the OR (call him Bob). Bob responds with a
\emph{created} cell with the second half of the DH handshake, along with
a hash of $K=g^{xy}$. The goal is to get unilateral entity authentication
(Alice knows she's handshaking with Bob, Bob doesn't care who it is ---
recall that Alice has no key and is trying to remain anonymous) and
unilateral key authentication (Alice and Bob agree on a key, and Alice
knows Bob is the only other person who could know it --- if he is
honest, etc.). We also want perfect forward secrecy, key freshness, etc.

\begin{equation}
\begin{aligned}
\mathrm{Alice} \rightarrow \mathrm{Bob}&: E_{PK_{Bob}}(g^x) \\
\mathrm{Bob} \rightarrow \mathrm{Alice}&: g^y, H(K | \mathrm{``handshake"}) \\
\end{aligned}
\end{equation}

The second step shows both that it was Bob
who received $g^x$, and that it was Bob who came up with $y$. We use
PK encryption in the first step (rather than, e.g., using the first two
steps of STS, which has a signature in the second step) because we
don't have enough room in a single cell for a public key and also a
signature. Preliminary analysis with the NRL protocol analyzer shows
the above protocol to be secure (including providing PFS) under the
traditional Dolev-Yao model.
% cite Cathy? -RD
% did I use the buzzwords correctly? -RD

To extend a circuit past the first hop, Alice sends a \emph{relay extend}
cell to the last node in the circuit, specifying the address of the new
OR and an encrypted $g^x$ for it. That node copies the half-handshake
into a \emph{create} cell, and passes it to the new OR to extend the
circuit. When it responds with a \emph{created} cell, the penultimate OR
copies the payload into a \emph{relay extended} cell and passes it back.
% Nick: please fix my "that OR" pronouns -RD

Once Alice has established the circuit (so she shares a key with each
OR on the circuit), she can send relay cells.
%The stream ID in the relay header indicates to which stream the cell belongs.
% Nick: should i include the above line?
% Paul says yes. -PS
Alice can address each relay cell to any of the ORs on the circuit. To
construct a relay cell destined for a given OR, she iteratively
encrypts the cell payload (that is, the relay header and payload)
with the symmetric key of each hop up to that node. Then, at each hop
down the circuit, the OR decrypts the cell payload and checks whether
it recognizes the stream ID. A stream ID is recognized either if it
is an already open stream at that OR, or if it is equal to zero. The
zero stream ID is treated specially, and is used for control messages,
e.g. starting a new stream. If the stream ID is unrecognized, the OR
passes the relay cell downstream. This \emph{leaky pipe} circuit design
allows Alice's streams to exit at different ORs, for example to tolerate
different exit policies, or to keep the ORs from knowing that two streams
originate at the same person.

To tear down a circuit, Alice sends a destroy control cell. Each OR
in the circuit receives the destroy cell, closes all open streams on
that circuit, and passes a new destroy cell forward. But since circuits
can be built incrementally, they can also be torn down incrementally:
Alice can send a relay truncate cell to a node along the circuit. That
node will send a destroy cell forward, and reply with an acknowledgement
(relay truncated). Alice might truncate her circuit so she can extend it
to different nodes without signaling to the first few nodes (or somebody
observing them) that she is changing her circuit. That is, nodes in the
middle are not even aware that the circuit was truncated, because the
relay cells are encrypted. Similarly, if a node on the circuit goes down,
the adjacent node can send a relay truncated back to Alice. Thus the
``break a node and see which circuits go down'' attack is weakened.

\SubSection{Opening and closing streams}
\label{subsec:tcp}

When Alice's application wants to open a TCP connection to a given
address and port, it asks the OP (via SOCKS) to make the connection. The
OP chooses the newest open circuit (or creates one if none is available),
chooses a suitable OR on that circuit to be the exit node (usually the
last node, but maybe others due to exit policy conflicts; see Section
\ref{sec:exit-policies}), chooses a new random stream ID for this stream,
and delivers a relay begin cell to that exit node. It uses a stream ID
of zero for the begin cell (so the OR will recognize it), and the relay
payload lists the new stream ID and the destination address and port.
Once the exit node completes the connection to the remote host, it
responds with a relay connected cell through the circuit. Upon receipt,
the OP notifies the application that it can begin talking.

There's a catch to using SOCKS, though -- some applications hand the
alphanumeric address to the proxy, while others resolve it into an IP
address first and then hand the IP to the proxy. When the application
does the DNS resolution first, Alice broadcasts her destination. Common
applications like Mozilla and ssh have this flaw.

In the case of Mozilla, we're fine: the filtering web proxy called Privoxy
does the SOCKS call safely, and Mozilla talks to Privoxy safely. But a
portable general solution, such as for ssh, is an open problem. We could
modify the local nameserver, but this approach is invasive, brittle, and
not portable. We could encourage the resolver library to do resolution
via TCP rather than UDP, but this approach is hard to do right, and also
has portability problems. Our current answer is to encourage the use of
privacy-aware proxies like Privoxy wherever possible, and also provide
a tool similar to \emph{dig} that can do a private lookup through the
Tor network.

Ending a Tor stream is analogous to ending a TCP stream: it uses a
two-step handshake for normal operation, or a one-step handshake for
errors. If one side of the stream closes abnormally, that node simply
sends a relay teardown cell, and tears down the stream. If one side
% Nick: mention relay teardown in 'cell' subsec? good enough name? -RD
of the stream closes the connection normally, that node sends a relay
end cell down the circuit. When the other side has sent back its own
relay end, the stream can be torn down. This two-step handshake allows
for TCP-based applications that, for example, close a socket for writing
but are still willing to read.

\SubSection{Integrity checking on streams}

In the old Onion Routing design, traffic was vulnerable to a malleability
attack: without integrity checking, an adversary could
guess some of the plaintext of a cell, xor it out, and xor in his own
plaintext. Even an external adversary could do this despite the link
encryption!

For example, an adversary could change a create cell to a
destroy cell; change the destination address in a relay begin cell
to the adversary's webserver; or change a user on an ftp connection
from typing ``dir'' to typing ``delete *''. Any node or observer along
the path can introduce such corruption in a stream.

Tor solves this malleability attack with respect to external adversaries
simply by using TLS. Addressing the insider malleability attack is more
complex.

Rather than doing integrity checking of the relay cells at each hop
(like Mixminion \cite{minion-design}), which would increase packet size
by a function of path length\footnote{This is also the argument against
using recent cipher modes like EAX \cite{eax} --- we don't want the added
message-expansion overhead at each hop, and we don't want to leak the path
length (or pad to some max path length).}, we choose to accept passive
timing attacks, and do integrity
checking only at the edges of the circuit. When Alice negotiates a key
with that hop, they both start a SHA-1 with some derivative of that key,
thus starting out with randomness that only the two of them know. From
then on they each incrementally add all the data bytes flowing across
the stream to the SHA-1, and each relay cell includes the first 4 bytes
of the current value of the hash. 

The attacker must be able to guess all previous bytes between Alice
and Bob on that circuit (including the pseudorandomness from the key
negotiation), plus the bytes in the current cell, to remove or modify the
cell. The computational overhead isn't so bad, compared to doing an AES
% XXX We never say we use AES. Say it somewhere above?
crypt at each hop in the circuit. We use only four bytes per cell to
minimize overhead; the chance that an adversary will correctly guess a
valid hash, plus the payload the current cell, is acceptly low, given
that Alice or Bob tear down the circuit if they receive a bad hash.

%% probably don't need to even mention this, because the randomness
%% covers it:
%The fun SHA1 attack where the bad guy can incrementally add to a hash
%to get a new valid hash doesn't apply to us, because we never show any
%hashes to anybody.

\SubSection{Website fingerprinting attacks}

% this subsection probably wants to move to analysis -RD
old onion routing is vulnerable to website fingerprinting attacks like
david martin's from usenix sec and drew's from pet2002. so is tor. we
need to send some padding or something, including long-range padding
(to foil the first hop), to solve this. let's hope somebody writes
a followup to \cite{defensive-dropping} that tells us what, exactly,
to do, and why, exactly, it helps.

\SubSection{Rate limiting and fairness}

Nodes use a token bucket approach \cite{foo} to limit the number of
bytes they receive. Tokens are added to the bucket each second (when
the bucket is full, new tokens are discarded.) Each token represents
permission to receive one byte from the network --- to receive a byte,
the connection must remove a token from the bucket. Thus if the bucket
is empty, that connection must wait until more tokens arrive. The number
of tokens we add enforces a longterm average rate of incoming bytes, yet
we still permit short-term bursts above the allowed bandwidth. Currently
bucket sizes are set to ten seconds worth of traffic.

Further, we want to avoid starving any Tor streams. Entire circuits
could starve if we read greedily from connections and one connection
uses all the remaining bandwidth. We solve this by dividing the number
of tokens in the bucket by the number of connections that want to read,
and reading at most that number of bytes from each connection. We iterate
this procedure until the number of tokens in the bucket is under some
threshold (eg 10KB), at which point we greedily read from connections.

Because the number of bytes going out of a node is roughly the same
as the number of bytes that have come in, doing rate limiting only on
incoming bytes should be sufficient.

Further, inspired by Rennhard et al's design in \cite{anonnet}, the edges
of the circuit can automatically distinguish interactive streams compared
to bulk streams --- interactive streams supply cells only rarely. We can
get good latency for these streams by giving them preferential service,
while still getting good overall throughput to the bulk streams. Such
preferential treatment can have impact on anonymity, but an adversary
who can observe the stream can already learn this information through
timing attacks.

\SubSection{Congestion control}
\label{subsec:congestion}

Even with bandwidth rate limiting, we still need to worry about
congestion, either accidental or intentional. If enough users choose
the same OR-to-OR connection for their circuits, that connection
will become saturated. For example, an adversary can make a `put'
request through the onion routing network to a webserver he runs,
and then refuse to read any of the bytes at the webserver end of the
circuit. Without some congestion control mechanism, these bottlenecks
can propagate back through the entire network.

\subsubsection{Circuit-level}

To control a circuit's bandwidth usage, each OR keeps track of two
windows. The package window tracks how many relay data cells the OR is
allowed to package (from outside streams) for transmission back to the OP,
and the deliver window tracks how many relay data cells it is willing
to deliver to streams outside the network. Each window is initialized
(say, to 1000 data cells). When a data cell is packaged or delivered,
the appropriate window is decremented. When an OR has received enough
data cells (currently 100), it sends a relay sendme cell towards the OP,
with stream ID zero. When an OR receives a relay sendme cell with stream
ID zero, it increments its packaging window. Either of these cells
increments the corresponding window by 100. If the packaging window
reaches 0, the OR stops reading from TCP connections for all streams
on the corresponding circuit, and sends no more relay data cells until
receiving a relay sendme cell.

The OP behaves identically, except that it must track a packaging window
and a delivery window for every OR in the circuit. If a packaging window
reaches 0, it stops reading from streams destined for that OR.

\subsubsection{Stream-level}

The stream-level congestion control mechanism is similar to the
circuit-level mechanism above. ORs and OPs use relay sendme cells
to implement end-to-end flow control for individual streams across
circuits. Each stream begins with a package window (e.g. 500 cells),
and increments the window by a fixed value (50) upon receiving a relay
sendme cell. Rather than always returning a relay sendme cell as soon
as enough cells have arrived, the stream-level congestion control also
has to check whether data has been successfully flushed onto the TCP
stream; it sends a relay sendme only when the number of bytes pending
to be flushed is under some threshold (currently 10 cells worth).

Currently, non-data relay cells do not affect the windows. Thus we
avoid potential deadlock issues, e.g. because a stream can't send a
relay sendme cell because its packaging window is empty.

\subsubsection{Needs more research}

We don't need to reimplement full TCP windows (with sequence numbers,
the ability to drop cells when we're full and retransmit later, etc),
because the TCP streams already guarantee in-order delivery of each
cell. But we need to investigate further the effects of the current
parameters on throughput and latency, while also keeping privacy in mind;
see Section \ref{sec:maintaining-anonymity} for more discussion.

\Section{Other design decisions}

\SubSection{Resource management and DoS prevention}
\label{subsec:dos}

Describe DoS prevention. cookies before tls begins, rate limiting of
create cells, link-to-link rate limiting, etc.
Mention twins, what the do, what they can't.
How we should do sequencing and acking like TCP so that we can better
tolerate lost data cells.
Mention that designers have to choose what you send across your
  circuit: wrapped IP packets, wrapped stream data, etc.  [Disspell
  TCP-over-TCP misconception.]
Mention that OR-to-OR connections should be highly reliable.  If
  they aren't, everything can stall.

\SubSection{Exit policies and abuse}
\label{subsec:exitpolicies}

Exit abuse is a serious barrier to wide-scale Tor deployment --- we
must block or limit attacks and other abuse that users can do through
the Tor network.

Each onion router's \emph{exit policy} describes to which external
addresses and ports the router will permit stream connections. On one end
of the spectrum are \emph{open exit} nodes that will connect anywhere;
on the other end are \emph{middleman} nodes that only relay traffic to
other Tor nodes, and \emph{private exit} nodes that only connect locally
or to addresses internal to that node's organization. 
This private exit
node configuration is more secure for clients --- the adversary cannot
see plaintext traffic leaving the network (e.g. to a webserver), so he
is less sure of Alice's destination. More generally, nodes can require
a variety of forms of traffic authentication \cite{onion-discex00}.
Most onnion routers will function as \emph{limited exits} that permit
connections to the world at large, but restrict access to certain abuse-prone
addresses and services.

Tor offers more reliability than the high-latency fire-and-forget
anonymous email networks, because the sender opens a TCP stream
with the remote mail server and receives an explicit confirmation of
acceptance. But ironically, the private exit node model works poorly for
email, when Tor nodes are run on volunteer machines that also do other
things, because it's quite hard to configure mail transport agents so
normal users can send mail normally, but the Tor process can only deliver
mail locally. Further, most organizations have specific hosts that will
deliver mail on behalf of certain IP ranges; Tor operators must be aware
of these hosts and consider putting them in the Tor exit policy.

The abuse issues on closed (e.g. military) networks are different
from the abuse on open networks like the Internet. While these IP-based
access controls are still commonplace on the Internet, on closed networks,
nearly all participants will be honest, and end-to-end authentication
can be assumed for anything important.

Tor is harder than minion because tcp doesn't include an abuse
address. you could reach inside the http stream and change the agent
or something, but that's a specific case and probably won't help
much anyway.
And volunteer nodes don't resolve to anonymizer.mit.edu so it never
even occurs to people that it wasn't you.

Preventing abuse of open exit nodes is an unsolved problem. Princeton's
CoDeeN project \cite{darkside} gives us a glimpse of what we're in for.
% This is more speculative than a description of our design. 

but their solutions, which mainly involve rate limiting and blacklisting
nodes which do bad things, don't translate directly to Tor. Rate limiting
still works great, but Tor intentionally separates sender from recipient,
so it's hard to know which sender was the one who did the bad thing,
without just making the whole network wide open.

even limiting most nodes to allow http, ssh, and aim to exit and reject
all other stuff is sketchy, because plenty of abuse can happen over
port 80. but it's a surprisingly good start, because it blocks most things,
and because people are more used to the concept of port 80 abuse not
coming from the machine's owner.

we could also run intrusion detection system (IDS) modules at each tor
node, to dynamically monitor traffic streams for attack signatures. it
can even react when it sees a signature by closing the stream. but IDS's
don't actually work most of the time, and besides, how do you write a
signature for "is sending a mean mail"?

we should run a squid at each exit node, to provide comparable anonymity
to private exit nodes for cache hits, to speed everything up, and to
have a buffer for funny stuff coming out of port 80. we could similarly
have other exit proxies for other protocols, like mail, to check
delivered mail for being spam.

[XXX Um, I'm uncomfortable with this for several reasons.
It's not good for keeping honest nodes honest about discarding
state after it's no longer needed. Granted it keeps an external
observer from noticing how often sites are visited, but it also
allows fishing expeditions. ``We noticed you went to this prohibited
site an hour ago. Kindly turn over your caches to the authorities.''
I previously elsewhere suggested bulk transfer proxies to carve
up big things so that they could be downloaded in less noticeable
pieces over several normal looking connections. We could suggest
similarly one or a handful of squid nodes that might serve up
some of the more sensitive but common material, especially if
the relevant sites didn't want to or couldn't run their own OR.
This would be better than having everyone run a squid which would
just help identify after the fact the different history of that
node's activity. All this kind of speculation needs to move to
future work section I guess. -PS]

A mixture of open and restricted exit nodes will allow the most
flexibility for volunteers running servers. But while a large number
of middleman nodes is useful to provide a large and robust network,
a small number of exit nodes still simplifies traffic analysis because
there are fewer nodes the adversary needs to monitor, and also puts a
greater burden on the exit nodes.
The JAP cascade model is really nice because they only need one node to
take the heat per cascade. On the other hand, a hydra scheme could work
better (it's still hard to watch all the clients).

Discuss importance of public perception, and how abuse affects it.
``Usability is a security parameter''.  ``Public Perception is also a
security parameter.''

Discuss smear attacks.

\SubSection{Directory Servers}
\label{subsec:dirservers}

First-generation Onion Routing designs \cite{or-jsac98,freedom2-arch} did
% is or-jsac98 the right cite here? what's our stock OR cite? -RD
in-band network status updates: each router flooded a signed statement
to its neighbors, which propagated it onward. But anonymizing networks
have different security goals than typical link-state routing protocols.
For example, we worry more about delays (accidental or intentional)
that can cause different parts of the network to have different pictures
of link-state and topology. We also worry about attacks to deceive a
client about the router membership list, topology, or current network
state. Such \emph{partitioning attacks} on client knowledge help an
adversary with limited resources to efficiently deploy those resources
when attacking a target.

Instead, Tor uses a small group of redundant directory servers to
track network topology and node state such as current keys and exit
policies. The directory servers are normal onion routers, but there are
only a few of them and they are more trusted. They listen on a separate
port as an HTTP server, both so participants can fetch current network
state and router lists (a \emph{directory}), and so other onion routers
can upload their router descriptors.

[[mention that descriptors are signed with long-term keys; ORs publish
    regularly to dirservers; policies for generating directories; key
    rotation (link, onion, identity); Everybody already know directory
    keys; how to approve new nodes (advogato, sybil, captcha (RTT));
    policy for handling connections with unknown ORs; diff-based
    retrieval; diff-based consensus; separate liveness from descriptor
    list]]

Of course, a variety of attacks remain. An adversary who controls a
directory server can track certain clients by providing different
information --- perhaps by listing only nodes under its control
as working, or by informing only certain clients about a given
node. Moreover, an adversary without control of a directory server can
still exploit differences among client knowledge. If Eve knows that
node $M$ is listed on server $D_1$ but not on $D_2$, she can use this
knowledge to link traffic through $M$ to clients who have queried $D_1$.

Thus these directory servers must be synchronized and redundant. The
software is distributed with the signature public key of each directory
server, and directories must be signed by a threshold of these keys.

The directory servers in Tor are modeled after those in Mixminion
\cite{minion-design}, but our situation is easier. Firstly, we make the
simplifying assumption that all participants agree on who the directory
servers are. Secondly, Mixminion needs to predict node behavior ---
that is, build a reputation system for guessing future performance of
nodes based on past performance, and then figure out a way to build
a threshold consensus of these predictions. Tor just needs to get a
threshold consensus of the current state of the network.

The threshold consensus can be reached with standard Byzantine agreement
techniques \cite{castro-liskov}.
% Should I just stop the section here? Is the rest crap? -RD
But this library, while more efficient than previous Byzantine agreement
systems, is still complex and heavyweight for our purposes: we only need
to compute a single algorithm, and we do not require strict in-order
computation steps. Indeed, the complexity of Byzantine agreement protocols
threatens our security, because users cannot easily understand it and
thus have less trust in the directory servers. The Tor directory servers
build a consensus directory
through a simple four-round broadcast protocol. First, each server signs
and broadcasts its current opinion to the other directory servers; each
server then rebroadcasts all the signed opinions it has received. At this
point all directory servers check to see if anybody's cheating. If so,
directory service stops, the humans are notified, and that directory
server is permanently removed from the network. Assuming no cheating,
each directory server then computes a local algorithm on the set of
opinions, resulting in a uniform shared directory. Then the servers sign
this directory and broadcast it; and finally all servers rebroadcast
the directory and all the signatures.

The rebroadcast steps ensure that a directory server is heard by either
all of the other servers or none of them (some of the links between
directory servers may be down). Broadcasts are feasible because there
are so few directory servers (currently 3, but we expect to use as many
as 9 as the network scales). The actual local algorithm for computing
the shared directory is straightforward, and is described in the Tor
specification \cite{tor-spec}.
% we should, uh, add this to the spec. oh, and write it. -RD

Using directory servers rather than flooding approaches provides
simplicity and flexibility. For example, they don't complicate
the analysis when we start experimenting with non-clique network
topologies. And because the directories are signed, they can be cached at
all the other onion routers (or even elsewhere). Thus directory servers
are not a performance bottleneck when we have many users, and also they
won't aid traffic analysis by forcing clients to periodically announce
their existence to any central point.
% Mention Hydra as an example of non-clique topologies. -NM, from RD

% also find some place to integrate that dirservers have to actually
% lay test circuits and use them, otherwise routers could connect to
% the dirservers but discard all other traffic.
% in some sense they're like reputation servers in \cite{mix-acc} -RD

\Section{Rendezvous points: location privacy}
\label{sec:rendezvous}

Rendezvous points are a building block for \emph{location-hidden services}
(aka responder anonymity) in the Tor network. Location-hidden services
means Bob can offer a TCP service, such as a webserver, without revealing
the IP of that service. One motivation for location privacy is to provide
protection against DDoS attacks: attackers are forced to attack the
onion routing network as a whole rather than just Bob's IP.

We provide this censorship resistance for Bob by allowing him to
advertise several onion routers (his \emph{Introduction Points}) as his
public location. Alice, the client, chooses a node for her \emph{Meeting
Point}. She connects to one of Bob's introduction points, informs him
about her rendezvous point, and then waits for him to connect to the
rendezvous
point. This extra level of indirection means Bob's introduction points
don't open themselves up to abuse by serving files directly, eg if Bob
chooses a node in France to serve material distateful to the French,
%
% We need a more legitimate-sounding reason here.
%
or if Bob's service tends to get DDoS'ed by script kiddies.
The extra level of indirection also allows Bob to respond to some requests
and ignore others.

We provide the necessary glue so that Alice can view webpages from Bob's
location-hidden webserver with minimal invasive changes. Both Alice and
Bob must run local onion proxies.

The steps of a rendezvous:
\begin{tightlist}
\item Bob chooses some Introduction Points, and advertises them on a
      Distributed Hash Table (DHT).
\item Bob establishes onion routing connections to each of his
      Introduction Points, and waits.
\item Alice learns about Bob's service out of band (perhaps Bob told her,
      or she found it on a website). She looks up the details of Bob's
      service from the DHT.
\item Alice chooses and establishes a Rendezvous Point (RP) for this
      transaction.
\item Alice goes to one of Bob's Introduction Points, and gives it a blob
      (encrypted for Bob) which tells him about herself, the RP
      she chose, and the first half of an ephemeral key handshake. The
      Introduction Point sends the blob to Bob.
\item Bob chooses whether to ignore the blob, or to onion route to RP.
      Let's assume the latter.
\item RP plugs together Alice and Bob. Note that RP can't recognize Alice,
      Bob, or the data they transmit (they share a session key).
\item Alice sends a Begin cell along the circuit. It arrives at Bob's
      onion proxy. Bob's onion proxy connects to Bob's webserver.
\item Data goes back and forth as usual.
\end{tightlist}

When establishing an introduction point, Bob provides the onion router
with a public ``introduction'' key.  The hash of this public key
identifies a unique service, and (since Bob is required to sign his
messages) prevents anybody else from usurping Bob's introduction point
in the future. Bob uses the same public key when establishing the other
introduction points for that service.

The blob that Alice gives the introduction point includes a hash of Bob's
public key to identify the service, an optional initial authentication
token (the introduction point can do prescreening, eg to block replays),
and (encrypted to Bob's public key) the location of the rendezvous point,
a rendezvous cookie Bob should tell RP so he gets connected to
Alice, an optional authentication token so Bob can choose whether to respond,
and the first half of a DH key exchange. When Bob connects to RP
and gets connected to Alice's pipe, his first cell contains the
other half of the DH key exchange.

The authentication tokens can be used to provide selective access to users
proportional to how important it is that they main uninterrupted access
to the service. During normal situations, Bob's service might simply be
offered directly from mirrors; Bob also gives out authentication cookies
to special users. When those mirrors are knocked down by DDoS attacks,
those special users can switch to accessing Bob's service via the Tor
rendezvous system.

\subsection{Integration with user applications}

For each service Bob offers, he configures his local onion proxy to know
the local IP and port of the server, a strategy for authorizating Alices,
and a public key. We assume the existence of a robust decentralized
efficient lookup system which allows authenticated updates, eg
\cite{cfs:sosp01}. (Each onion router could run a node in this lookup
system; also note that as a stopgap measure, we can just run a simple
lookup system on the directory servers.)  Bob publishes into the DHT
(indexed by the hash of the public key) the public key, an expiration
time (``not valid after''), and the current introduction points for that
service. Note that Bob's webserver is unmodified, and doesn't even know
that it's hidden behind the Tor network.

As far as Alice's experience goes, we require that her client interface
remain a SOCKS proxy, and we require that she shouldn't have to modify
her applications. Thus we encode all of the necessary information into
the hostname (more correctly, fully qualified domain name) that Alice
uses, eg when clicking on a url in her browser. Location-hidden services
use the special top level domain called `.onion': thus hostnames take the
form x.y.onion where x encodes the hash of PK, and y is the authentication
cookie. Alice's onion proxy examines hostnames and recognizes when they're
destined for a hidden server. If so, it decodes the PK and starts the
rendezvous as described in the table above.

\subsection{Previous rendezvous work}

Ian Goldberg developed a similar notion of rendezvous points for
low-latency anonymity systems \cite{ian-thesis}. His ``service tag''
is the same concept as our ``hash of service's public key''. We make it
a hash of the public key so it can be self-authenticating, and so the
client can recognize the same service with confidence later on. His
design differs from ours in the following ways though. Firstly, Ian
suggests that the client should manually hunt down a current location of
the service via Gnutella; whereas our use of the DHT makes lookup faster,
more robust, and transparent to the user. Secondly, in Tor the client
and server can share ephemeral DH keys, so at no point in the path is
the plaintext
exposed. Thirdly, our design is much more practical for deployment in a
volunteer network, in terms of getting volunteers to offer introduction
and rendezvous point services. The introduction points do not output any
bytes to the clients, and the rendezvous points don't know the client,
the server, or the stuff being transmitted. The indirection scheme
is also designed with authentication/authorization in mind -- if the
client doesn't include the right cookie with its request for service,
the server doesn't even acknowledge its existence.

\Section{Analysis}

How well do we resist chosen adversary?

How well do we meet stated goals?

Mention jurisdictional arbitrage.

Pull attacks and defenses into analysis as a subsection

\Section{Maintaining anonymity in Tor}
\label{sec:maintaining-anonymity}

I probably should have noted that this means loops will be on at least
five hop routes, which should be rare given the distribution.  I'm    
realizing that this is reproducing some of the thought that led to a  
default of five hops in the original onion routing design.  There were
some different assumptions, which I won't spell out now.  Note that   
enclave level protections really change these assumptions.  If most   
circuits are just two hops, then just a single link observer will be  
able to tell that two enclaves are communicating with high probability.
So, it would seem that enclaves should have a four node minimum circuit
to prevent trivial circuit insider identification of the whole circuit,
and three hop minimum for circuits from an enclave to some nonclave    
responder. But then... we would have to make everyone obey these rules 
or a node that through timing inferred it was on a four hop circuit    
would know that it was probably carrying enclave to enclave traffic.   
Which... if there were even a moderate number of bad nodes in the      
network would make it advantageous to break the connection to conduct  
a reformation intersection attack. Ahhh! I gotta stop thinking         
about this and work on the paper some before the family wakes up.  
On Sat, Oct 25, 2003 at 06:57:12AM -0400, Paul Syverson wrote:
> Which... if there were even a moderate number of bad nodes in the
> network would make it advantageous to break the connection to conduct         > a reformation intersection attack. Ahhh! I gotta stop thinking                > about this and work on the paper some before the family wakes up.             
This is the sort of issue that should go in the 'maintaining anonymity
with tor' section towards the end. :)
Email from between roger and me to beginning of section above. Fix and move.


[Put as much of this as a part of open issues as is possible.]

[what's an anonymity set?]

packet counting attacks work great against initiators. need to do some
level of obfuscation for that. standard link padding for passive link
observers. long-range padding for people who own the first hop. are
we just screwed against people who insert timing signatures into your
traffic?

Even regardless of link padding from Alice to the cloud, there will be
times when Alice is simply not online. Link padding, at the edges or
inside the cloud, does not help for this.

how often should we pull down directories? how often send updated
server descs?

when we start up the client, should we build a circuit immediately,
or should the default be to build a circuit only on demand? should we
fetch a directory immediately?

would we benefit from greater synchronization, to blend with the other
users? would the reduced speed hurt us more?

does the "you can't see when i'm starting or ending a stream because
you can't tell what sort of relay cell it is" idea work, or is just
a distraction?

does running a server actually get you better protection, because traffic
coming from your node could plausibly have come from elsewhere? how
much mixing do you need before this is actually plausible, or is it
immediately beneficial because many adversary can't see your node?

do different exit policies at different exit nodes trash anonymity sets,
or not mess with them much?

do we get better protection against a realistic adversary by having as
many nodes as possible, so he probably can't see the whole network,
or by having a small number of nodes that mix traffic well? is a
cascade topology a more realistic way to get defenses against traffic
confirmation? does the hydra (many inputs, few outputs) topology work
better? are we going to get a hydra anyway because most nodes will be
middleman nodes?

using a circuit many times is good because it's less cpu work.
  good because of predecessor attacks with path rebuilding.
  bad because predecessor attacks can be more likely to link you with a
    previous circuit since you're so verbose.
  bad because each thing you do on that circuit is linked to the other
    things you do on that circuit.
  how often to rotate?
  how to decide when to exit from middle?
  when to truncate and re-extend versus when to start new circuit?

Because Tor runs over TCP, when one of the servers goes down it seems
that all the circuits (and thus streams) going over that server must
break. This reduces anonymity because everybody needs to reconnect
right then (does it? how much?) and because exit connections all break
at the same time, and it also reduces usability. It seems the problem
is even worse in a p2p environment, because so far such systems don't
really provide an incentive for nodes to stay connected when they're
done browsing, so we would expect a much higher churn rate than for
onion routing. Are there ways of allowing streams to survive the loss
of a node in the path?

discuss topologies. Cite George's non-freeroutes paper.  Maybe this
graf goes elsewhere.

discuss attracting users; incentives; usability.

Choosing paths and path lengths.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\Section{Attacks and Defenses}
\label{sec:attacks}

Below we summarize a variety of attacks and how well our design withstands
them.

\begin{enumerate}
\item \textbf{Passive attacks}
\begin{itemize}
\item \emph{Simple observation.}
\item \emph{Timing correlation.}
\item \emph{Size correlation.}
\item \emph{Option distinguishability.}
\end{itemize}

\item \textbf{Active attacks}
\begin{itemize}
\item \emph{Key compromise.}
\item \emph{Iterated subpoena.}
\item \emph{Run recipient.}
\item \emph{Run a hostile node.}
\item \emph{Compromise entire path.}
\item \emph{Selectively DoS servers.}
\item \emph{Introduce timing into messages.}
\item \emph{Tagging attacks.}
the exit node can change the content you're getting to try to
trick you. similarly, when it rejects you due to exit policy,
it could give you a bad IP that sends you somewhere else.
\end{itemize}

we rely on DNS being globally consistent. if people in africa resolve
IPs differently, then asking to extend a circuit to a certain IP can
give away your origin.

\item \textbf{Directory attacks}
\begin{itemize}
\item knock out a dirserver
\item knock out half the dirservers
\item trick user into using different software (with different dirserver
keys)
\item OR connects to the dirservers but nowhere else
\item foo
\end{itemize}

\item \textbf{Attacks against rendezvous points}
\begin{itemize}
\item foo
\end{itemize}

\end{enumerate}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\Section{Future Directions and Open Problems}
\label{sec:conclusion}

% Mention that we need to do TCP over tor for reliability.

Tor brings together many innovations into
a unified deployable system. But there are still several attacks that
work quite well, as well as a number of sustainability and run-time
issues remaining to be ironed out. In particular:

\begin{itemize}
\item \emph{Scalability:} Since Tor's emphasis currently is on simplicity
of design and deployment, the current design won't easily handle more
than a few hundred servers, because of its clique topology. Restricted
route topologies \cite{danezis-pets03} promise comparable anonymity
with much better scaling properties, but we must solve problems like
how to randomly form the network without introducing net attacks.
% [cascades are a restricted route topology too. we must mention
% earlier why we're not satisfied with the cascade approach.]-RD
% [We do. At least 
\item \emph{Cover traffic:} Currently we avoid cover traffic because
it introduces clear performance and bandwidth costs, but and its
security properties are not well understood. With more research
\cite{SS03,defensive-dropping}, the price/value ratio may change, both for
link-level cover traffic and also long-range cover traffic. In particular,
we expect restricted route topologies to reduce the cost of cover traffic
because there are fewer links to cover.
\item \emph{Better directory distribution:} Even with the threshold
directory agreement algorithm described in \ref{subsec:dirservers},
the directory servers are still trust bottlenecks. We must find more
decentralized yet practical ways to distribute up-to-date snapshots of
network status without introducing new attacks.
\item \emph{Implementing location-hidden servers:} While Section
\ref{sec:rendezvous} provides a design for rendezvous points and
location-hidden servers, this feature has not yet been implemented.
We will likely encounter additional issues, both in terms of usability
and anonymity, that must be resolved.
\item \emph{Wider-scale deployment:} The original goal of Tor was to
gain experience in deploying an anonymizing overlay network, and learn
from having actual users. We are now at the point where we can start
deploying a wider network. We will see what happens!
% ok, so that's hokey. fix it. -RD
\item \emph{Further specification review:} Foo.
\end{itemize}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

%\Section{Acknowledgments}
%% commented out for anonymous submission

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\bibliographystyle{latex8}
\bibliography{tor-design}

\end{document}

% Style guide:
%     U.S. spelling
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%     'mix', 'mixes' (as noun)
%     'mix-net'
%     'mix', 'mixing' (as verb)
%     'middleman'  [Not with a hyphen; the hyphen has been optional
%         since Middle English.]
%     'nymserver'
%     'Cypherpunk', 'Cypherpunks', 'Cypherpunk remailer'
%     'Onion Routing design', 'onion router' [note capitalization]
%     'SOCKS'
%    
%
%     'Substitute ``Damn'' every time you're inclined to write ``very;'' your
%     editor will delete it and the writing will be just as it should be.'
%     -- Mark Twain