tor/doc/tor-design.tex

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\title{Tor: The Second-Generation Onion Router}
% Putting the 'Private' back in 'Virtual Private Network'

%\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 circuit-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, requires no special
privileges such as root- or kernel-level access,
requires little synchronization or coordination between nodes, and
provides a reasonable tradeoff between anonymity and usability/efficiency.
We include a new practical design for rendezvous points, as well
as a big list of open problems.
\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-ih96,or-jsac98,or-discex00,or-pet00}. While
a wide area Onion Routing network was deployed for some weeks,
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).
%%Do we really want to say this? It softens our motivation for the paper. -RD
%
% In general, I try to emphasize rather than understate past
% accomplishments so I am giving an accurate comparison,
% which strengthens the claims in the paper. This is true whether
% it is my work or someone else's. 
% This is also the only experimental basic viability result we
% can point to for Onion Routing in general at this point. -PS
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:

\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 from building so many different circuits; see
Section~\ref{sec:maintaining-anonymity}.
Tor multiplexes multiple TCP streams along each virtual
circuit, to improve efficiency and anonymity.

\item \textbf{No mixing, padding, or traffic shaping:} The original
Onion Routing design called for batching and reordering the cells arriving
from each circuit and the ability to do padding between onion routers and,
in a later design, between onion
proxies (that is, users) and onion routers \cite{or-ih96,or-jsac98}.
The tradeoff between padding protection and cost was discussed, but no
general padding scheme was suggested. In
\cite{or-pet00} it was theorized \emph{traffic shaping} would generally
be used, but details were not provided.
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 improve 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 allows for long-range padding to frustrate traffic
  shape and volume attacks at the initiator \cite{defensive-dropping}.
  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 the
  end of the circuit.

\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 congestion
control uses end-to-end acks to maintain 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 act as directory servers: they provide signed
\emph{directories} which describe the routers they know about and mark
those that
are currently up. Users periodically download these directories via HTTP.

\item \textbf{End-to-end integrity checking:} The original Onion Routing
design did no integrity checking on data. Any onion router on the circuit
could 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 the old design
meant that circuit-building failed, 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:} Unlike other anonymity systems
like Freedom \cite{freedom2-arch}, Tor only attempts to anonymize TCP
streams. Thus it does not require patches to an operating system's network
stack (or built-in support) 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 a reply onion becomes useless if any node in
the path goes down or rotates its keys, and it also does not provide
forward security.  In Tor's current design, clients negotiate {\it
rendezvous points} to connect with hidden servers; reply onions are no
longer required.
\end{tightlist}

We have implemented most of the above features. Our source code is
available under a free license, and is not encumbered by patents. We have
recently begun deploying a widespread alpha network to see how well the
design works in practice, to get more experience with usability and users,
and to provide a research platform for experimenting with new ideas.

We review previous work in Section~\ref{sec:related-work}, 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:rendezvous}. We
summarize in Section \ref{sec:analysis}
how our design stands up to known attacks, and conclude with a list of
open problems.

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

\Section{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 linkability by wrapping
messages in 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, these \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
communication from correlating the timing and volume
of traffic entering the anonymity network with traffic leaving it.  These
protocols are also vulnerable against 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.
} 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 strips the
data's origin before relaying it.  These designs are easy to
analyze, but require end-users to trust the anonymizing proxy. 
Concentrating the traffic to a single point increases the anonymity set
(the set of people a given user is hiding among), but it can make traffic
analysis easier: an adversary need only eavesdrop on the proxy to observe
the entire system.

More complex are distributed-trust, circuit-based anonymizing systems.
In these designs, a user establishes one or more medium-term bidirectional
end-to-end circuits, and tunnels TCP streams in fixed-size cells.
Establishing circuits is expensive and typically requires public-key
cryptography, whereas relaying cells is comparatively inexpensive.
Because a circuit crosses several servers, no single server can link a
user to her communication partners.

The Java Anon Proxy (also known
as JAP or Web MIXes) uses fixed shared routes known as
\emph{cascades}.  As with a single-hop proxy, this approach aggregates
users into larger anonymity sets, but again an attacker only needs to
observe both ends of the cascade to bridge all the system's traffic.
The Java Anon Proxy's design provides protection 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.
%XXX fix, yes it does, sort of.

PipeNet \cite{back01, pipenet}, another low-latency design proposed at
about the same time as the original Onion Routing design, provided
stronger anonymity at the cost of allowing a single user to shut
down the network simply by not sending.  Low-latency anonymous
communication has also been designed for other environments such as
ISDN \cite{isdn-mixes}.

In P2P designs like Tarzan \cite{tarzan:ccs02} and MorphMix
\cite{morphmix:fc04}, all participants both generate traffic and relay
traffic for others. Rather than aiming to hide the originator within a
group of other originators, these systems instead aim to prevent a peer
or observer from knowing whether a given peer originated the request
or just relayed it from another peer. While Tarzan and MorphMix use
layered encryption as above, Crowds \cite{crowds-tissec} simply assumes
an adversary who cannot observe the initiator: it uses no public-key
encryption, so nodes on a circuit can read that circuit's traffic. The
anonymity of the initiator relies on filtering all identifying information
from the data stream.

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. They make anonymity
and efficiency tradeoffs to make broadcast more practical.
These systems are designed primarily for communication between peers,
although Herbivore users can make external connections by
requesting a peer to serve as a proxy.  Allowing easy connections to
nonparticipating responders or recipients is important for usability,
for example so users can visit nonparticipating Web sites or exchange
mail with nonparticipating recipients.

Systems like Freedom and the original Onion Routing build the circuit
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 circuit. Tor as described herein, Tarzan, MorphMix,
Cebolla \cite{cebolla}, and AnonNet \cite{anonnet} build the circuit
in stages, extending it one hop at a time. This approach makes perfect
forward secrecy feasible.

Circuit-based anonymity designs must choose which protocol layer
to anonymize. They may choose to intercept IP packets directly, and
relay them whole (stripping the source address) as the contents of
the circuit \cite{tarzan:ccs02,freedom2-arch}.  Alternatively, like
Tor, they may accept TCP streams and relay the data in those streams
along the circuit, ignoring the breakdown of that data into TCP frames
\cite{anonnet,morphmix:fc04}. Finally, they may accept application-level
protocols (such as HTTP) and relay the application requests themselves
along the circuit.  
This protocol-layer decision represents a compromise between flexibility
and anonymity.  For example, a system that understands HTTP can strip
identifying information from those requests; can take advantage of caching
to limit the number of requests that leave the network; and can batch
or encode those requests in order to minimize the number of connections.
On the other hand, an IP-level anonymizer can handle nearly any protocol,
even ones unforeseen by their designers (though these systems require
kernel-level modifications to some operating systems, and so are more
complex and less portable). TCP-level anonymity networks like Tor present
a middle approach: they are fairly application neutral (so long as the
application supports, or can be tunneled across, TCP), but by treating
application connections as data streams rather than raw TCP packets,
they avoid the well-known inefficiencies of tunneling TCP over TCP
\cite{tcp-over-tcp-is-bad}. [XXX what's a better cite?]

Distributed-trust anonymizing systems need to prevent attackers from
adding too many servers and thus compromising too many user paths.
Tor relies on a centrally maintained set of well-known servers. Tarzan
and MorphMix allow unknown users to run servers, and limit an attacker
from becoming too much of the network based on a limited resource such
as number of IPs controlled. Crowds suggests requiring written, notarized
requests from potential crowd members.

Anonymous communication is an essential component of censorship-resistant
systems like Eternity \cite{eternity}, Free Haven \cite{freehaven-berk},
Publius \cite{publius}, and Tangler \cite{tangler}. Tor's rendezvous
points enable connections between mutually anonymous entities; they
are a building block for location-hidden servers, which are needed by
Eternity and Free Haven.

% didn't include rewebbers. No clear place to put them, so I'll leave
% them out for now. -RD

\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 user.  Within this
main goal, however, several design considerations have directed
Tor's evolution.

\textbf{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. (We do not meet this goal for the current rendezvous design,
however; see Section~\ref{sec:rendezvous}.)

\textbf{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 not
require modifying applications; should not introduce prohibitive delays;
and should require the user to make as few configuration decisions
as possible.

\textbf{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 Sybil attacks
\cite{sybil}, may be solvable independently from the issues solved by
Tor. Hopefully future systems will not need 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 make users
distinguishable. Experiments should be run on a separate network.)

\textbf{Conservative design:} The protocol's design and security
parameters must be conservative. Additional features impose implementation
and complexity costs; adding unproven techniques to the design threatens
deployability, readability, and ease of security analysis. Tor aims to
deploy a simple and stable system that integrates the best well-understood
approaches to protecting anonymity.

\SubSection{Non-goals}
\label{subsec:non-goals}
In favoring conservative, deployable designs, we have explicitly deferred
a number of goals, either because they are solved elsewhere, or because
they are an open research question.

\textbf{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.  This approach
is appealing, but still has many open problems.

\textbf{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.

\textbf{No protocol normalization:} Tor does not provide \emph{protocol
normalization} like Privoxy or the Anonymizer. For complex and variable
protocols such as HTTP, Tor must be layered with a filtering proxy such
as Privoxy to hide differences between clients, and expunge protocol
features that leak identity. 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. -RD
% No, leave it as is. -RD

\textbf{Not steganographic:} Tor does not try to conceal which users are
sending or receiving communications; it only tries to conceal with whom
they communicate.

\SubSection{Threat Model}
\label{subsec:threat-model}

A global passive adversary is the most commonly assumed threat when
analyzing theoretical anonymity designs. But like all practical
low-latency systems, Tor does not protect against such a strong
adversary. Instead, we expect an adversary who can observe some fraction
of network traffic; who can generate, modify, delete, or delay traffic
on the network; who can operate onion routers of its own; and who can
compromise some fraction of the onion routers on the network.

%Large adversaries will be able to compromise a considerable fraction
%of the network. (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 quite
%small.) 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.

In low-latency anonymity systems that use layered encryption, the
adversary's typical goal is to observe both the initiator and the
receiver. Passive attackers can confirm a suspicion that Alice is
talking to Bob if the timing and volume properties of the traffic on the
connection are unique enough; active attackers are even more effective
because they can induce timing signatures on the traffic. Tor provides
some defenses against these \emph{traffic confirmation} attacks, for
example by encouraging users to run their own onion routers, but it does
not provide complete protection. Rather, we aim to prevent \emph{traffic
analysis} attacks, where the adversary uses traffic patterns to learn
which points in the network he should attack.

Our adversary might try to link an initiator Alice with any of her
communication partners, or he might try to build a profile of Alice's
behavior. He might 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; or relationships in any externally visible user-selected
options. The adversary can also mount active attacks by compromising
routers or keys; by replaying traffic; by selectively DoSing trustworthy
routers to encourage users to send their traffic through compromised
routers, or DoSing users to see if the traffic elsewhere in the
network stops; or by introducing patterns into traffic that can later be
detected. The adversary might attack the directory servers to give users
differing views of network state. Additionally, he can try to decrease
the network's reliability by attacking nodes or by performing antisocial
activities from reliable servers and trying to get them taken down;
making the network unreliable flushes users to other less anonymous
systems, where they may be easier to attack.

We consider each of these attacks in more detail below, and summarize
in Section~\ref{sec:attacks} how well the Tor design defends against
each of them.

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

\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 as normal user-level processes without needing
any special
privileges.  Currently, each OR maintains a long-term TLS \cite{TLS}
connection to every other
OR.  (We examine some 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) to fetch directories,
establish paths (called \emph{virtual circuits}) across the network,
and handle connections from 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.

Each onion router uses three public keys: a long-term identity key, a
short-term onion key, and a short-term link key.  The identity
(signing) key is used 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 onion (decryption) key is used for decrypting requests
from users to set up a circuit and negotiate ephemeral keys. Finally,
link keys are used by the TLS protocol when communicating between
onion routers.  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 virtual circuits are
built, extended, truncated, and destroyed. Section~\ref{subsec:tcp}
describes how TCP streams are routed through the network, and finally
Section~\ref{subsec:congestion} talks about congestion control and
fairness issues.

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

% I think we should describe connections before cells. -NM

Traffic passes from one OR to another, or between a user's OP and an OR,
in fixed-size cells. Each cell is 256
bytes, and consists of a header and a payload. The header includes an
anonymous circuit identifier (ACI) that specifies which circuit the
% Should we replace ACI with circID ? What is this 'anonymous circuit'
% thing anyway? -RD
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 \emph{control} cells, which are
interpreted by the node that receives them, or \emph{relay} cells,
which carry end-to-end stream data. Controls cells can be one of:
\emph{padding} (currently used for keepalive, but also usable for link
padding); \emph{create} or \emph{created} (used to set up a new circuit);
or \emph{destroy} (to tear down a circuit).
% We need to say that ACIs are connection-specific: each circuit has
% a different ACI along each connection. -NM
% agreed -RD

Relay cells have an additional header (the relay header) after the
cell header, containing 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 describe each of these cell types in more detail below.

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

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

% I think when we say ``the user,'' maybe we should say ``the user's OP.''

The original Onion Routing design built one circuit for each
TCP stream.  Because building a circuit can take several tenths of a
second (due to public-key cryptography delays and network latency),
this design imposed high costs on applications like web browsing that
open many TCP streams.

In Tor, each circuit can be shared by many TCP streams.  To avoid
delays, users construct circuits preemptively.  To limit linkability
among the streams, users rotate connections by building 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 heavy 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. Also, because circuits are built
in the background, failed routers do not affects user experience.

\subsubsection{Constructing a circuit}

Users construct each incrementally, negotiating a symmetric key with
each hop one at a time. To begin creating a new circuit, the user
(call her Alice) sends a \emph{create} cell to the first node in her
chosen path. The cell's 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 containg the second
half of the DH handshake, along with a hash of the negotiated key
$K=g^{xy}$.  This protocol tries to achieve unilateral entity
authentication (Alice knows she's handshaking with Bob, Bob doesn't
care who is opening the circuit---Alice has no key and is trying to
remain anonymous); unilateral key authentication (Alice and Bob
agree on a key, and Alice knows Bob is the only other person who could
know it).  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 \cite{meadows96}
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

% Hm.  I think that this paragraph could go earlier in expository
% order: we describe how to build whole circuit, then explain the
% protocol in more detail.  -NM
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

\subsubsection{Relay cells}
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.
A relay cell can be addressed to any of the ORs on the circuit. To
construct a relay cell addressed to a given OR, Alice iteratively
encrypts the cell payload (that is, the relay header and payload)
with the symmetric key of each hop up to that OR. 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 topology
allows Alice's streams to exit at different ORs on a single circuit.  
Alice may choose different exit points because of their 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 instead send a relay truncate cell to a node along the circuit. That
node will send a destroy cell forward, and reply with an acknowledgment
(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: an attacker could make changes to an encrypted
cell to create corresponding changes to the data leaving the network.
(Even an external adversary could do this, despite link encryption!)

This weakness allowed an adversary to 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 could introduce such corruption in a stream.

Tor prevents external adversaries by mounting this attack simply by
using TLS. Addressing the insider malleability attack, however, is
more complex.

Rather than doing integrity checking of the relay cells at each hop,
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, 
%    (How?  I don't get it.  Do we mean end-to-end traffic
%    confirmation attacks? -NM)
and perform integrity
checking only at the edges of the circuit. When Alice negotiates a key
with the exit 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. Attacks on SHA-1 where the adversary can incrementally add to a
hash to produce a new valid hash don't work,
because all hashes are end-to-end encrypted across the circuit.
The computational overhead isn't so bad, compared to doing an AES
% XXX We never say we use AES. Say it somewhere above? -RD
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.

\SubSection{Rate limiting and fairness}

Volunteers are generally more willing to run services that can limit
their bandwidth usage.  To accomodate them, Tor servers use a token
bucket approach 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 long-term average rate of incoming bytes, while still
permitting short-term bursts above the allowed bandwidth. Current 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 Tor protocol generates roughly the same number of outgoing
bytes as incoming bytes, it is sufficient in practice to rate-limit
incoming bytes.
% Is it?  Fun attack: I send you lots of 1-byte-at-a-time TCP frames.
% In response, you send lots of 256 byte cells.  Can I use this to 
% make you exceed your outgoing bandwidth limit by a factor of 256? -NM
% Can we resolve this by, when reading from edge connections, rounding up
% the bytes read (wrt buckets) to the nearest multiple of 256? -RD

Further, inspired by Rennhard et al's design in \cite{anonnet}, a
circuit's edges heuristically distinguish interactive streams from bulk
streams by comparing the frequency with which they supply cells.  We can
provide good latency for interactive streams by giving them preferential
service, while still getting good overall throughput to the bulk
streams. Such preferential treatment presents a possible end-to-end
attack, but an adversary who can observe both
ends of 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 can become
saturated. For example, an adversary could make a large HTTP 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.  We describe our
responses below.

\subsubsection{Circuit-level}

To control a circuit's bandwidth usage, each OR keeps track of two
windows. The \emph{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 \emph{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}

Providing Tor as a public service provides many opportunities for an
attacker to mount denial-of-service attacks against the network.  While
flow control and rate limiting (discussed in
Section~\ref{subsec:congestion}) prevent users from consuming more
bandwidth than routers are willing to provide, opportunities remain for
users to
consume more network resources than their fair share, or to render the
network unusable for other users.

First of all, there are a number of CPU-consuming denial-of-service
attacks wherein an attacker can force an OR to perform expensive
cryptographic operations.  For example, an attacker who sends a
\emph{create} cell full of junk bytes can force an OR to perform an RSA
decrypt.  Similarly, an attacker can
fake the start of a TLS handshake, forcing the OR to carry out its
(comparatively expensive) half of the handshake at no real computational
cost to the attacker.

Several approaches exist to address these attacks. First, ORs may
demand proof-of-computation tokens \cite{hashcash} before beginning new
TLS handshakes or accepting \emph{create} cells.  So long as these
tokens are easy to verify and computationally expensive to produce, this
approach limits the DoS attack multiplier.  Additionally, ORs may limit
the rate at which they accept create cells and TLS connections, so that
the computational work of processing them does not drown out the (comparatively
inexpensive) work of symmetric cryptography needed to keep cells
flowing.  This rate limiting could, however, allows an attacker
to slow down other users when they build new circuits.

% What about link-to-link rate limiting?

More worrisome are distributed denial of service attacks wherein an
attacker uses a large number of compromised hosts throughout the network
to consume the Tor network's resources.  Although these attacks are not
new to the networking literature, some proposed approaches are a poor
fit to anonymous networks.  For example, solutions based on backtracking
harmful traffic \cite{XXX} could allow an anonymity-breaking
adversary to exploit the backtracking mechanism.

Attackers also have an opportunity to attack the Tor network by mounting
attacks on its hosts and network links. Disrupting a single circuit or
link breaks all currently open streams passing along that part of the
circuit. Indeed, this same loss of service occurs when a router crashes
or its operator restarts it. The current Tor design treats such attacks
as intermittent network failures, and depends on users and applications
to respond or recover as appropriate. A future design could use an
end-to-end based TCP-like acknowledgment protocol, so that no streams are
lost unless the entry or exit point itself is disrupted. This solution
would require more buffering at the network edges, however, and the
performance and anonymity implications from this extra complexity still
require investigation.

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

Exit abuse is a serious barrier to wide-scale Tor deployment.  Not
only does anonymity present would-be vandals and abusers with an
opportunity to hide the origins of their activities---but also,
existing sanctions against abuse present an easy way for attackers to
harm the Tor network by implicating exit servers for their abuse.
Thus, must block or limit attacks and other abuse that travel through
the Tor network.

Also, applications that commonly use IP-based authentication (such
institutional mail or web servers) can be fooled by the fact that
anonymous connections appear to originate at the exit OR.  Rather than
expose a private service, an administrator may prefer to prevent Tor
users from connecting to those services from a local OR.

To mitigate abuse issues, in Tor, 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.  As a compromise,
most onion routers will function as \emph{restricted exits} that
permit connections to the world at large, but prevent access to
certain abuse-prone addresses and services.  on the other end are
\emph{middleman} nodes that only relay traffic to other Tor nodes, and
\emph{private exit} nodes that only connect to a local host or
network.  (Using a private exit (if one exists) is a more secure way
for a client to connect to a given host or network---an external
adversary cannot eavesdrop traffic between the private exit and the
final destination, and so is less sure of Alice's destination and
activities.)  is less sure of Alice's destination. More generally,
nodes can require a variety of forms of traffic authentication
\cite{or-discex00}.

%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 important traffic.

Many administrators will use port restrictions to support only a
limited set of well-known services, such as HTTP, SSH, or AIM.
This is not a complete solution, since abuse opportunities for these
protocols are still well known.  Nonetheless, the benefits are real,
since administrators seem used to  the concept of port 80 abuse not
coming from the machine's owner.

A further solution may be to use proxies to clean traffic for certain
protocols as it leaves the network.  For example, much abusive HTTP
behavior (such as exploiting buffer overflows or well-known script
vulnerabilities) can be detected in a straightforward manner.
Similarly, one could run automatic spam filtering software (such as
SpamAssassin) on email exiting the OR network.  A generic
intrusion detection system (IDS) could be adapted to these purposes.

[XXX Mention possibility of filtering spam-like habits--e.g., many
  recipients. -NM]

ORs may also choose to rewrite exiting traffic in order to append
headers or other information to indicate that the traffic has passed
through an anonymity service.  This approach is commonly used, to some
success, by email-only anonymity systems.  When possible, ORs can also
run on servers with hostnames such as {\it anonymous}, to further
alert abuse targets to the nature of the anonymous traffic.

%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,
having only a small number of exit nodes reduces the number of nodes
an adversary needs to monitor for traffic analysis, and places a
greater burden on the exit nodes.  This tension can be seen in the JAP
cascade model, wherein only one node in each cascade needs to handle
abuse complaints---but an adversary only needs to observe the entry
and exit of a cascade to perform traffic analysis on all that
cascade's users.  The Hydra model (many entries, few exits) presents a
different compromise: only a few exit nodes are needed, but an
adversary needs to work harder to watch all the clients.

Finally, we note that exit abuse must not be dismissed as a peripheral
issue: when a system's public image suffers, it can reduce the number
and diversity of that system's users, and thereby reduce the anonymity
of the system itself.  Like usability, public perception is also a
security parameter.  Sadly, preventing abuse of open exit nodes is an
unsolved problem, and will probably remain an arms race for the
forseeable future.  The abuse problems faced by Princeton's CoDeeN
project \cite{darkside} give us a glimpse of likely issues.

\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, delays (accidental or intentional)
that can cause different parts of the network to have different pictures
of link-state and topology are not only inconvenient---they give
attackers an opportunity to exploit differences in client knowledge.
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 of flooding, Tor uses a small group of redundant, well-known
directory servers to track changes in network topology and node state,
including keys and exit policies.  Directory servers are a small group
of well-known, mostly-trusted onion routers.  They listen on a
separate port as an HTTP server, so that participants can fetch
current network state and router lists (a \emph{directory}), and so
that other onion routers can upload their router descriptors.  Onion
routers now periodically publish signed statements of their state to
the directories only.  The directories themselves combine this state
information with their own views of network liveness, and generate a
signed description of the entire network state whenever its contents
have changed.  Client software is pre-loaded with a list of the
directory servers and their keys, and uses this information to
bootstrap each client's view of the network.

When a directory receives a signed statement from and onion router, it
recognizes the onion router by its identity (signing) key.
Directories do not automatically advertise ORs that they do not
recognize.  (If they did, an adversary could take over the network by
creating many servers \cite{sybil}.)  Instead, new nodes must be
approved by the directory administrator before they are included.
Mechanisms for automated node approval are an area of active research,
and are discussed more in section~\ref{sec:maintaining-anonymity}.
  
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. First, we make the
simplifying assumption that all participants agree on who the
directory servers are. Second, Mixminion needs to predict node
behavior, whereas Tor only needs a threshold consensus of the current
state of the network.
% Cite dir-spec or dir-agreement?

Tor directory servers build a consensus directory through a simple
four-round broadcast protocol.  In round one, each server dates and
signs its current opinion, and broadcasts it to the other directory
servers; then in round two, each server rebroadcasts all the signed
opinions it has received.  At this point all directory servers check
to see whether any server has signed multiple opinions in the same
period. If so, the server is either broken or cheating, so protocol
stops and notifies the administrators, who either remove the cheater
or wait for the broken server to be fixed.  If there are no
discrepancies, each directory server then locally computes algorithm
on the set of opinions, resulting in a uniform shared directory. In
round three servers sign this directory and broadcast it; and finally
in round four the servers rebroadcast the directory and all the
signatures.  If any directory server drops out of the network, its
signature is not included on the file directory.

The rebroadcast steps ensure that a directory server is heard by
either all of the other servers or none of them, assuming that any two
directories can talk directly, or via a third directory (some of the
links between directory servers may be down). Broadcasts are feasible
because there are relatively 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 a straightforward threshold
voting process: we include an OR if a majority of directory servers
believe it to be good.

When a client Alice retrieves a consensus directory, she uses it if it
is signed by a majority of the directory servers she knows.

Using directory servers rather than flooding 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 by other onion routers,
or indeed by any server.  Thus directory servers are not a performance
bottleneck when we have many users, and do not 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} (also known as ``responder anonymity'') in the Tor
network.  Location-hidden services allow a server Bob to a TCP
service, such as a webserver, without revealing the IP of his service.
Besides allowing Bob to provided services anonymously, location
privacy also seeks to provide some protection against DDoS attacks:
attackers are forced to attack the onion routing network as a whole
rather than just Bob's IP.

\subsection{Goals for rendezvous points}
\label{subsec:rendezvous-goals}
In addition to our other goals, have tried to provide the following
properties in our design for location-hidden servers:
\begin{tightlist}
\item[Flood-proof:] An attacker should not be able to flood Bob with traffic
  simply by sending may requests to Bob's public location.  Thus, Bob needs a
  way to filter incoming requests.
\item[Robust:] Bob should be able to maintain a long-term pseudonymous
  identity even in the presence of OR failure.  Thus, Bob's identity must not
  be tied to a single OR.
\item[Smear-resistant:] An attacker should not be able to use rendezvous
  points to smear an OR.  That is, if a social attacker tries to host a 
  location-hidden service that is illegal or disreputable, it should not
  appear---even to a casual observer---that the OR is hosting that service.
\item[Application-transparent:] Although we are willing to require users to
  run special software to access location-hidden servers, we are not willing
  to require them to modify their applications.
\end{tightlist}

\subsection{Rendezvous design}
We provide location-hiding for Bob by allowing him to advertise
several onion routers (his \emph{Introduction Points}) as his public
location.  (He may do this on any robust efficient distributed
key-value lookup system with authenticated updates, such as CFS
\cite{cfs:sosp01}\footnote{
Each onion router could run a node in this lookup
system; also note that as a stopgap measure, we can start by running a
simple lookup system on the directory servers.})  
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
helps Bob's introduction points avoid problems associated with serving
unpopular files directly, as could occur, for example, if Bob chooses
an introduction point in Texas to serve anti-ranching propaganda,
or if Bob's service tends to get DDoS'ed by network vandals.
The extra level of indirection also allows Bob to respond to some requests
and ignore others.

The steps of a rendezvous as follows.  These steps are performed on
behalf of Alice and Bob by their local onion proxies, which they both
must run; application integration is described more fully below.
\begin{tightlist}
\item Bob chooses some introduction ppoints, and advertises them via
      CFS (or some other distributed key-value publication system).
\item Bob establishes a Tor virtual circuit 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 CFS.
\item Alice chooses an OR to serve as a Rendezvous Point (RP) for this
      transaction. She establishes a virtual circuit to her RP, and
      tells it to wait for connections. [XXX how?]
\item Alice opens an anonymous stream to one of Bob's Introduction
      Points, and gives it message (encrypted for Bob) which tells him
      about herself, her chosen RP, and the first half of an ephemeral
      key handshake. The Introduction Point sends the message to Bob.
\item Bob may decide to ignore Alice's request.  [XXX Based on what?]
      Otherwise, he creates a new virtual circuit to Alice's RP, and
      authenticates himself. [XXX how?]
\item If the authentication is successful, the RP connects Alice's
      virtual circuit to Bob's. Note that RP can't recognize Alice,
      Bob, or the data they transmit (they share a session key).
\item Alice now sends a Begin cell along the circuit. It arrives at Bob's
      onion proxy. Bob's onion proxy connects to Bob's webserver.
\item An anonymous stream has been established, and Alice and Bob
      communicate as normal.
\end{tightlist}

[XXX We need to modify the above to refer people down to these next
  paragraphs. -NM]

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 message 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 can also give out authentication cookies
to high-priority users. If those mirrors are knocked down by DDoS attacks,
those 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 authorizing Alices,
and a public key.   Bob publishes
the public key, an expiration
time (``not valid after''), and the current introduction points for
his
service into CFS, all indexed by the hash of the public key
Note that Bob's webserver is unmodified, and doesn't even know
that it's hidden behind the Tor network.

Because Alice's applications must work unchanged, her client interface
remains a SOCKS proxy.  Thus we must encode all of the necessary
information into the fully qualified domain name Alice uses when
establishing her connections.  Location-hidden services use a virtual
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 tags''
play the same role in his design as the hashes of services' public
keys play in ours.  We use public key hashes so that they can be
self-authenticating, and so the client can recognize the same service
with confidence later on. His design also differs from ours in the
following ways: First, Goldberg 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. Second, in Tor the client and server
negotiate ephemeral keys via Diffie-Hellman, so at no point in the
path is the plaintext exposed. Third, our design tries to minimize the
exposure associated with running the service, so as to make volunteers
more willing to offer introduction and rendezvous point services.
Tor's introduction points do not output any bytes to the clients, and
the rendezvous points don't know the client, the server, or the data
being transmitted. The indirection scheme is also designed to include
authentication/authorization---if the client doesn't include the right
cookie with its request for service, the server need not even
acknowledge its existence.

\Section{Analysis}
\label{sec:analysis}

In this section, we discuss how well Tor meets our stated design goals
and its resistance to attacks.

\SubSection{Meeting Basic Goals}
% None of these seem to say very much.  Should this subsection be removed?
\begin{tightlist}
\item [Basic Anonymity:] Because traffic is encrypted, changing in
  appearance, and can flow from anywhere to anywhere within the
  network, a simple observer that cannot see both the initiator
  activity and the corresponding activity where the responder talks to
  the network will not be able to link the initiator and responder.
  Nor is it possible to directly correlate any two communication
  sessions as coming from a single source without additional
  information. Resistance to more sophisticated anonymity threats is
  discussed below.
\item[Deployability:] Tor requires no specialized hardware. Tor
  requires no kernel modifications; it runs in user space (currently
  on Linux, various BSDs, and Windows). All of these imply a low
  technical barrier to running a Tor node. There is an assumption that
  Tor nodes have good relatively persistent net connectivity
  (currently T1 or better);
% Is that reasonable to say? We haven't really discussed it -P.S.
% Roger thinks otherwise; he will fix this. -NM
  however, there is no padding overhead, and operators can limit
  bandwidth on any link.  Tor is freely available under the modified
  BSD license, and operators are able to choose their own exit
  policies, thus reducing legal and social barriers to
  running a node.
  
\item[Usability:] As noted, Tor runs in user space. So does the onion
  proxy, which is comparatively easy to install and run. SOCKS-aware
  applications require nothing more than to be pointed at the onion
  proxy; other applications can be redirected to use SOCKS for their
  outgoing TCP connections by drop-in libraries such as tsocks.
  
\item[Flexibility:] Tor's design and implementation is fairly modular,
  so that,
  for example, a scalable P2P replacement for the directory servers
  would not substantially impact other aspects of the system.  Tor
  runs on top of TCP, so design options that could not easily do so
  would be difficult to test on the current network. However, most
  low-latency protocols are designed to run over TCP. We are currently
  discussing with the designers of MorphMix interoperability of the
  two systems, which seems to be relatively straightforward. This will
  allow testing and direct comparison of the two rather different
  designs.
  % Do we want to say this?  I don't think we should talk about this
  % kind of discussion till we have more positive results.
  
\item[Conservative design:] Tor opts for practicality when there is no
  clear resolution of anonymity tradeoffs or practical means to
  achieve resolution. Thus, we do not currently pad or mix; although
  it would be easy to add either of these. Indeed, our system allows
  long-range and variable padding if this should ever be shown to have
  a clear advantage.  Similarly, we do not currently attempt to
  resolve such issues as Sybil attacks to dominate the network except
  by such direct means as personal familiarity of director operators
  with all node operators.
\end{tightlist}

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

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

[XXX Note that some of these attacks are outside our threat model! -NM]

\subsubsection*{Passive attacks}
\begin{tightlist}
\item \emph{Observing user traffic patterns.} Observations of connection
  between an end user and a first onion router will not reveal to whom
  the user is connecting or what information is being sent. It will
  reveal patterns of user traffic (both sent and received). Simple
  profiling of user connection patterns is not generally possible,
  however, because multiple application connections (streams) may be
  operating simultaneously or in series over a single circuit. Thus,
  further processing is necessary to try to discern even these usage
  patterns.
  
\item \emph{Observing user content.} At the user end, content is
  encrypted; however, connections from the network to arbitrary
  websites may not be. Further, a responding website may itself be
  considered an adversary. Filtering content is not a primary goal of
  Onion Routing; nonetheless, Tor can directly make use of Privoxy and
  related filtering services via SOCKS and thus anonymize their
  application data streams.

\item \emph{Option distinguishability.} Configuration options can be a
  source of distinguishable patterns. In general there is economic
  incentive to allow preferential services \cite{econymics}, and some
  degree of configuration choice is a factor in attracting large
  numbers of users to provide anonymity.  So far, however, we have
  not found a compelling use case in Tor for any client-configurable
  options.  Thus, clients are currently distinguishable only by their
  behavior.
  
\item \emph{End-to-end Timing correlation.}  Tor only minimally hides
  end-to-end timing correlations. If an attacker can watch patterns of
  traffic at the initiator end and the responder end, then he will be
  able to confirm the correspondence with high probability. The
  greatest protection currently against such confirmation is if the
  connection between the onion proxy and the first Tor node is hidden,
  possibly because it is local or behind a firewall.  This approach
  requires an observer to separate traffic originating the onion
  router from traffic passes through it.  We still do not, however,
  predict this approach to be a large problem for an attacker who can
  observe traffic at both ends of an application connection.
  
\item \emph{End-to-end Size correlation.} Simple packet counting
  without timing consideration will also be effective in confirming
  endpoints of a connection through Onion Routing; although slightly
  less so. This is because, even without padding, the leaky pipe
  topology means different numbers of packets may enter one end of a
  circuit than exit at the other.
  
\item \emph{Website fingerprinting.} All the above passive
  attacks that are at all effective are traffic confirmation attacks.
  This puts them outside our general design goals. There is also
  a passive traffic analysis attack that is potentially effective.
  Instead of searching exit connections for timing and volume
  correlations it is possible to build up a database of
  ``fingerprints'' containing file sizes and access patterns for a
  large numbers of interesting websites. If one now wants to
  monitor the activity of a user, it may be possible to confirm a
  connection to a site simply by consulting the database. This attack has
  been shown to be effective against SafeWeb \cite{hintz-pet02}. Onion
  Routing is not as vulnerable as SafeWeb to this attack: There is the
  possibility that multiple streams are exiting the circuit at
  different places concurrently.  Also, fingerprinting will be limited to
  the granularity of cells, currently 256 bytes. Larger cell sizes
  and/or minimal padding schemes that group websites into large sets
  are possible responses.  But this remains an open problem.  Link
  padding or long-range dummies may also make fingerprints harder to
  detect. (Note that
  such fingerprinting should not be confused with the latency attacks
  of \cite{back01}. Those require a fingerprint of the latencies of
  all circuits through the network, combined with those from the
  network edges to the targeted user and the responder website. While
  these are in principal feasible and surprises are always possible,
  these constitute a much more complicated attack, and there is no
  current evidence of their practicality.)

\item \emph{Content analysis.}  Tor explicitly provides no content
  rewriting for any protocol at a higher level than TCP.  When
  protocol cleaners are available, however (as Privoxy is for HTTP),
  Tor can integrate them in order to address these attacks.

\end{tightlist}

\subsubsection*{Active attacks}
\begin{tightlist}
\item \emph{Key compromise.}  We consider the impact of a compromise
  for each type of key in turn, from the shortest- to the
  longest-lived.  If a circuit session key is compromised, the
  attacker can unwrap a single layer of encryption from the relay
  cells traveling along that circuit.  (Only nodes on the circuit can
  see these cells.)  If a TLS session key is compromised, an attacker
  can view all the cells on TLS connection until the key is
  renegotiated.  (These cells are themselves encrypted.)  If a TLS
  private key is compromised, the attacker can fool others into
  thinking that he is the affected OR, but still cannot accept any
  connections.  If an onion private key is compromised, the attacker
  can impersonate the OR in circuits, but only if the attacker has
  also compromised the OR's TLS private key, or is running the
  previous OR in the circuit.  (This compromise affects newly created
  circuits, but because of perfect forward secrecy, the attacker
  cannot hijack old circuits without compromising their session keys.)
  In any case, an attacker can only take advantage of a compromise in
  these mid-term private keys until they expire.  Only by
  compromising a node's identity key can an attacker replace that
  node indefinitely, by sending new forged mid-term keys to the
  directories.  Finally, an attacker who can compromise a
  \emph{directory's} identity key can influence every client's view
  of the network---but only to the degree made possible by gaining a
  vote with the rest of the the directory servers.

\item \emph{Iterated compromise.} A roving adversary who can
  compromise ORs (by system intrusion, legal coersion, or extralegal
  coersion) could march down length of a circuit compromising the
  nodes until he reaches the end.  Unless the adversary can complete
  this attack within the lifetime of the circuit, however, the ORs
  will have discarded the necessary information before the attack can
  be completed.  (Thanks to the perfect forward secrecy of session
  keys, the attacker cannot cannot force nodes to decrypt recorded
  traffic once the circuits have been closed.)  Additionally, building
  circuits that cross jurisdictions can make legal coercion
  harder---this phenomenon is commonly called ``jurisdictional
  arbitrage.''

  
\item \emph{Run a recipient.} By running a Web server, an adversary
  trivially learns the timing patterns of those connecting to it, and
  can introduce arbitrary patterns in its responses.  This can greatly
  facilitate end-to-end attacks: If the adversary can induce certain
  users to connect to connect to his webserver (perhaps by providing
  content targeted at those users), she now holds one end of their
  connection.  Additonally, here is a danger that the application
  protocols and associated programs can be induced to reveal
  information about the initiator.  This is not directly in Onion
  Routing's protection area, so we are dependent on Privoxy and
  similar protocol cleaners to solve the problem.
  
\item \emph{Run an onion proxy.} It is expected that end users will
  nearly always run their own local onion proxy. However, in some
  settings, it may be necessary for the proxy to run
  remotely---typically, in an institutional setting where it was
  necessary to monitor the activity of those connecting to the proxy.
  The drawback, of course, is that if the onion proxy is compromised,
  then all future connections through it are completely compromised.

\item \emph{DoS non-observed nodes.} An observer who can observe some
  of the Tor network can increase the value of this traffic analysis
  if it can attack non-observed nodes to shut them down, reduce
  their reliability, or persuade users that they are not trustworthy.
  The best defense here is robustness.
  
\item \emph{Run a hostile node.}  In addition to the abilties of a
  local observer, an isolated hostile node can create circuits through
  itself, or alter traffic patterns, in order to affect traffic at
  other nodes. Its ability to directly DoS a neighbor is now limited
  by bandwidth throttling. Nonetheless, in order to compromise the
  anonymity of the endpoints of a circuit by its observations, a
  hostile node is only significant if it is immediately adjacent to
  that endpoint. 
  
\item \emph{Run multiple hostile nodes.}  If an adversary is able to
  run multiple ORs, and is able to persuade the directory servers
  that those ORs are trustworthy and independant, then occasionally
  some user will choose one of those ORs for the start and another of
  those ORs as the end of a circuit.  When this happens, the user's
  anonymity is compromised for those circuits.  If an adversary can
  control $m$ out of $N$ nodes, he should be able to correlate at most 
  $\frac{m}{N}$ of the traffic in this way---although an adersary
  could possibly attract a disproportionately large amount of traffic
  by running an exit node with an unusually permisssive exit policy.

\item \emph{Compromise entire path.} Anyone compromising both
  endpoints of a circuit can confirm this with high probability. If
  the entire path is compromised, this becomes a certainty; however,
  the added benefit to the adversary of such an attack is small in
  relation to the difficulty.
  
\item \emph{Run a hostile directory server.} Directory servers control
  admission to the network. However, because the network directory
  must be signed by a majority of servers, the threat of a single
  hostile server is minimized.
  
\item \emph{Selectively DoS a Tor node.} As noted, neighbors are
  bandwidth limited; however, it is possible to open up sufficient
  numbers of circuits that converge at a single onion router to
  overwhelm its network connection, its ability to process new
  circuits or both.

%OK so I noticed that twins are completely removed from the paper above,
% but it's after 5 so I'll leave that problem to you guys. -PS
  
\item \emph{Introduce timing into messages.} This is simply a stronger
  version of passive timing attacks already discussed above.
  
\item \emph{Tagging attacks.} A hostile node could try to ``tag'' a
  cell by altering it. This would render it unreadable, but if the
  connection is, for example, an unencrypted request to a Web site,
  the garbled content coming out at the appropriate time could confirm
  the association. However, integrity checks on cells prevent
  this attack from succeeding.

\item \emph{Replace contents of unauthenticated protocols.}  When a
  relaying an unauthenticated protocol like HTTP, a hostile exit node 
  can impersonate the target server.  Thus, whenever possible, clients
  should prefer protocols with end-to-end authentication.

\item \emph{Replay attacks.} Some anonymity protocols are vulnerable
  to replay attacks.  Tor is not; replaying one side of a handshake
  will result in a different negotiated session key, and so the rest
  of the recorded session can't be used.  
  % ``NonSSL Anonymizer''?

\item \emph{Smear attacks.} An attacker could use the Tor network to
  engage in socially dissapproved acts, so as to try to bring the
  entire network into disrepute and get its operators to shut it down.
  Exit policies can help reduce the possibilities for abuse, but
  ultimately, the network will require volunteers who can tolerate
  some political heat.
\end{tightlist}

\subsubsection*{Directory attacks}
\begin{tightlist}
\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{tightlist}

\subsubsection*{Attacks against rendezvous points}
\begin{tightlist}
\item foo
\end{tightlist}


\Section{Open Questions in Low-latency Anonymity}
\label{sec:maintaining-anonymity}
 
% There must be a better intro than this! -NM
In addition to the open problems discussed in
section~\ref{subsec:non-goals}, many other questions remain to be
solved by future research before we can be truly confident that we
have built a secure low-latency anonymity service.

Many of these open issues are questions of balance.  For example,
how often should users rotate to fresh circuits?  Too-frequent
rotation is inefficient and expensive, but too-infrequent rotation
makes the user's traffic linkable.   Instead of opening a fresh
circuit; clients can also limit linkability exit from a middle point
of the circuit, or by truncating and re-extending the circuit, but
more analysis is needed to determine the proper trade-off.
[XXX mention predecessor attacks?]

A similar question surrounds timing of directory operations:
how often should directories be updated?  With too-infrequent
updates clients receive an inaccurate picture of the network; with
too-frequent updates the directory servers are overloaded.

%do different exit policies at different exit nodes trash anonymity sets,
%or not mess with them much?
%
%% Why would they?  By routing traffic to certain nodes preferentially?

[XXX Choosing paths and path lengths: I'm not writing this bit till
  Arma's pathselection stuff is in. -NM]

%%%% Roger said that he'd put a path selection paragraph into section
%%%% 4 that would replace this.
%
%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.

Throughout this paper, we have assumed that end-to-end traffic
analysis cannot yet be defeated.  But even high-latency anonymity
systems can be vulnerable to end-to-end traffic analysis, if the
traffic volumes are high enough, and if users' habits are sufficiently
distinct \cite{limits-open,statistical-disclosure}.  \emph{What can be
  done to limit the effectiveness of these attacks against low-latency
  systems?}  Tor already makes some effort to conceal the starts and
ends of streams by wrapping all long-range control commands in
identical-looking relay cells, but more analysis is needed.  Link
padding could frustrate passive observer who count packets; long-range
padding could work against observers who own the first hop in a
circuit.  But more research needs to be done in order to find an
efficient and practical approach.  Volunteers prefer not to run
constant-bandwidth padding; but more sophisticated traffic shaping
approaches remain somewhat unanalyzed. [XXX is this so?] Recent work
on long-range padding \cite{defensive-dropping} shows promise.  One
could also try to reduce correlation in packet timing by batching and
re-ordering packets, but it is unclear whether this could improve
anonymity without introducing so much latency as to render the
network unusable.

Even if passive timing attacks were wholly solved, active timing
attacks would remain.  \emph{What can
  be done to address attackers who can introduce timing patterns into
  a user's traffic?}  [XXX mention likely approaches]

%%% I think we cover this by framing the problem as ``Can we make 
%%% end-to-end characteristics of low-latency systems as good as
%%% those of high-latency systems?''  Eliminating long-term
%%% intersection is a hard problem.
%
%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.

In order to scale to large numbers of users, and to prevent an
attacker from observing the whole network at once, it may be necessary
for low-latency anonymity systems to support far more servers than Tor
currently anticipates.  This introduces several issues.  First, if
approval by a centralized set of directory servers is no longer
feasible, what mechanism should be used to prevent adversaries from
signing up many spurious servers? 
Second, if clients can no longer have a complete
picture of the network at all times, how can should they perform
discovery while preventing attackers from manipulating or exploiting
gaps in client knowledge?  Third, if there are to many servers
for every server to constantly communicate with every other, what kind
of non-clique topology should the network use?   Restricted-route
topologies promise comparable anonymity with better scalability
\cite{danezis-pets03}, but whatever topology we choose, we need some
way to keep attackers from manipulating their position within it.
Fourth, since no centralized authority is tracking server reliability,
How do we prevent unreliable servers from rendering the network
unusable?  Fifth, do clients receive so much anonymity benefit from
running their own servers that we should expect them all to do so, or
do we need to find another incentive structure to motivate them?
(Tarzan and MorphMix present possible solutions.)

[[ XXX how to approve new nodes (advogato, sybil, captcha (RTT));]

Alternatively, it may be the case that one of these problems proves
intractable, or that the drawbacks to many-server systems prove
greater than the benefits.  Nevertheless, we may still do well to
consider non-clique topologies.  A cascade topology may provide more
defense against traffic confirmation confirmation.
% Why would it?   Cite.  -NM
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?

%%% Do more with this paragraph once The TCP-over-TCP paragraph is
%%% more integrated into Related works.
%
As mentioned in section\ref{where-is-it-now}, Tor could improve its
robustness against node failure by buffering stream data at the
network's edges, and performing end-to-end acknowledgments.  The
efficacy of this approach remains to be tested, however, and there
may be more effective means for ensuring reliable connections in the
presence of unreliable nodes.

%%% Keeping this original paragraph for a little while, since it 
%%% is not the same as what's written there now.
%
%Because Tor depends on TLS and TCP to provide a reliable transport,
%when one of the servers goes down, all the circuits (and thus streams)
%traveling 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 harms
%usability. It seems the problem is even worse in a peer-to-peer
%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.
%there ways of allowing streams to survive the loss of a node in the
%path?

% Roger or Paul suggested that we say something about incentives,
% too, but I think that's a better candidate for our future work
% section.  After all, we will doubtlessly learn very much about why
% people do or don't run and use Tor in the near future. -NM

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



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

\Section{Future Directions}
\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:

% Many of these (Scalability, cover traffic) are duplicates from open problems.
%
\begin{tightlist}
\item \emph{Scalability:} Tor's emphasis on design simplicity and
  deployability has led us to adopt a clique topology, a
  semi-centralized model for directories and trusts, and a
  full-network-visibility model for client knowledge.  None of these
  properties will scale to more than a few hundred servers, at most.
  Promising approaches to better scalability exist (see
  section~\ref{sec:maintaining-anonymity}), but more deployment
  experience would be helpful in learning the relative importance of
  these bottlenecks.
\item \emph{Cover traffic:} Currently we avoid cover traffic because
  of its clear costs in performance and bandwidth, and because its
  security benefits have 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.
\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.  Also, directory
  retrieval presents a scaling problem, since clients currently
  download a description of the entire network state every 15
  minutes.  As the state grows larger and clients more numerous, we
  may need to move to a solution in which clients only receive
  incremental updates to directory state, or where directories are
  cached at the ORs to avoid high loads on the directory servers.
\item \emph{Implementing location-hidden servers:} While
  Section~\ref{sec:rendezvous} describes a design for rendezvous
  points and location-hidden servers, these feature has not yet been
  implemented.  While doing so, will likely encounter additional
  issues, both in terms of usability and anonymity, that must be
  resolved.
\item \emph{Further specification review:} Although we have a public,
  byte-level specification for the Tor protocols, this protocol has
  not received extensive external review.  We hope that as Tor
  becomes more widely deployed, more people will become interested in
  examining our specification.
\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 in design
  and development where we can start deploying a wider network.  Once
  we have are ready for actual users, we will doubtlessly be better
  able to evaluate some of our design decisions, including our
  robustness/latency tradeoffs, our abuse-prevention mechanisms, and
  our overall usability.
\end{tightlist}

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%% commented out for anonymous submission
%\Section{Acknowledgments}
% Peter Palfrader for editing
% Bram Cohen for congestion control discussions

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

\end{document}

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