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@ -14,7 +14,7 @@
\begin{document} \begin{document}
\title{Challenges in bringing low-latency stream anonymity to the masses (DRAFT)} \title{Challenges in practical low-latency stream anonymity (DRAFT)}
\author{Roger Dingledine and Nick Mathewson} \author{Roger Dingledine and Nick Mathewson}
\institute{The Free Haven Project\\ \institute{The Free Haven Project\\
@ -29,12 +29,10 @@ foo
\section{Introduction} \section{Introduction}
Anonymous communication on the Internet today
Tor is a low-latency anonymous communication overlay network Tor is a low-latency anonymous communication overlay network
\cite{tor-design}. We have been operating a publicly deployed Tor network \cite{tor-design} designed to be practical and usable for securing TCP
since October 2003. streams over the Internet. We have been operating a publicly deployed
Tor network since October 2003.
Tor aims to resist observers and insiders by distributing each transaction Tor aims to resist observers and insiders by distributing each transaction
over several nodes in the network. This ``distributed trust'' approach over several nodes in the network. This ``distributed trust'' approach
@ -48,38 +46,39 @@ who don't want to reveal information to their competitors, and law
enforcement and government intelligence agencies who need enforcement and government intelligence agencies who need
to do operations on the Internet without being noticed. to do operations on the Internet without being noticed.
Tor has been funded by both the U.S. Navy, for use in securing government Tor has been funded by the U.S. Navy, for use in securing government
communications, and also the Electronic Frontier Foundation, for use in communications, and also by the Electronic Frontier Foundation, for use
maintain civil liberties for ordinary citizens online. in maintaining civil liberties for ordinary citizens online. The Tor
The Tor protocol is one of the leading choices protocol is one of the leading choices
to be the anonymizing layer in the European Union's PRIME directive to to be the anonymizing layer in the European Union's PRIME directive to
help maintain privacy in Europe. The University of Dresden in Germany help maintain privacy in Europe. The University of Dresden in Germany
has integrated an independent implementation of the Tor protocol into has integrated an independent implementation of the Tor protocol into
their popular Java Anon Proxy anonymizing client. This wide variety of their popular Java Anon Proxy anonymizing client. This wide variety of
interests helps maintain both the stability and the security of the interests helps maintain both the stability and the security of the
network. network.
Tor has a weaker threat model than many anonymity designs in the
literature. This is because we our primary requirements are to have a
practical and useful network, and from there we aim to provide as much
anonymity as we can.
%need to discuss how we take the approach of building the thing, and then
%assuming that, how much anonymity can we get. we're not here to model or
%to simulate or to produce equations and formulae. but those have their
%roles too.
This paper aims to give the reader enough information to understand the
We deployed this thing called Tor. it's got all these different types of technical and policy issues that Tor faces as we continue deployment,
users. it's been backed by navy and eff, and prime and anonymizer looked at and to lay a research agenda for others to help in addressing some of
it. Because we're this cool, you should believe us when we tell you stuff. these issues. Section \ref{sec:what-is-tor} gives an overview of the Tor
design and ours goals. We go on in Section \ref{sec:related} to describe
In this paper we give the reader an understanding of Tor's context Tor's context in the anonymity space. Sections \ref{sec:crossroads-policy}
in the anonymity space and then we go on to describe the and \ref{sec:crossroads-technical} describe the practical challenges,
practical challenges that stand in the way of moving from a practical both policy and technical respectively, that stand in the way of moving
useful network to a practical useful anonymous network. from a practical useful network to a practical useful anonymous network.
% The goal of the paper is to get the PET-audience reader up to speed
% on all the issues we have with Tor, so he can, if he wants,
% * understand the technical and policy and legal issues and why they're
% tricky in practice
% * help us out with answering some of the technical decisions
% (and in writing it, we'll clarify our own opinions about them)
% * help us out with answering some of the anonymity questions
\section{What Is Tor} \section{What Is Tor}
\label{sec:what-is-tor}
\subsection{Distributed trust: safety in numbers} \subsection{Distributed trust: safety in numbers}
@ -153,6 +152,7 @@ Tor has the following goals.
and we made these assumptions when trying to design the thing. and we made these assumptions when trying to design the thing.
\section{Tor's position in the anonymity field} \section{Tor's position in the anonymity field}
\label{sec:related}
There are many other classes of systems: single-hop proxies, open proxies, There are many other classes of systems: single-hop proxies, open proxies,
jap, mixminion, flash mixes, freenet, i2p, mute/ants/etc, tarzan, jap, mixminion, flash mixes, freenet, i2p, mute/ants/etc, tarzan,
@ -160,15 +160,133 @@ morphmix, freedom. Give brief descriptions and brief characterizations
of how we differ. This is not the breakthrough stuff and we only have of how we differ. This is not the breakthrough stuff and we only have
a page or two for it. a page or two for it.
have a serious discussion of morphmix's assumptions, since they would
seem to be the direct competition. in fact tor is a flexible architecture
that would encompass morphmix, and they're nearly identical except for
path selection and node discovery. and the trust system morphmix has
seems overkill (and/or insecure) based on the threat model we've picked.
\section{Crossroads} \section{Crossroads: Policy issues}
\label{sec:crossroads-policy}
Discuss each item that Tor hasn't solved yet that isn't just coding Bittorrent and dmca. Should we add an IDS to autodetect protocols and
work. Perhaps we'll have so many that we can pick out the best ones to snipe them? Takedowns and efnet abuse and wikipedia complaints and irc
discuss, so it's a bit less of a laundry list. Maybe they'll even fit networks. Should we allow revocation of anonymity if a threshold of
into categories. The trick to making the paper good will be to find servers want to?
the right balance between going into depth and breadth of coverage.
Image: substantial non-infringing uses. Image is a security parameter,
since it impacts user base and perceived sustainability.
good uses are kept private, bad uses are publicized. not good.
Sustainability. Previous attempts have been commercial which we think
adds a lot of unnecessary complexity and accountability. Freedom didn't
collect enough money to pay its servers; JAP bandwidth is supported by
continued money, and they periodically ask what they will do when it
dries up.
How much should Tor aim to do? Applications that leak data. We can say
they're not our problem, but they're somebody's problem.
Logging. Making logs not revealing. A happy coincidence that verbose
logging is our \#2 performance bottleneck. Is there a way to detect
modified servers, or to have them volunteer the information that they're
logging verbosely? Would that actually solve any attacks?
\section{Crossroads: Scaling and Design choices}
\label{sec:crossroads-design}
\subsection{Transporting the stream vs transporting the packets}
We periodically run into ZKS people who tell us that the process of
anonymizing IPs should ``obviously'' be done at the IP layer. Here are
the issues that need to be resolved before we'll be ready to switch Tor
over to arbitrary IP traffic.
1: we still need to do IP-level packet normalization, to stop things
like ip fingerprinting. This is doable.
2: we still need to be easy to integrate with user-level applications,
so they can do application-level scrubbing. So we will still need
application-specific proxies.
3: we need a block-level encryption approach that can provide security despite
packet loss and out-of-order delivery. Freedom allegedly had one, but it was
never publicly specified. (We also believe that the Freedom and Cebolla designs
are vulnerable to tagging attacks.)
4: we still need to play with parameters for throughput, congestion control,
etc -- since we need sequence numbers and maybe more to do replay detection,
and just to handle duplicate frames. so we would be reimplementing some subset of tcp
anyway.
5: tls over udp is not implemented or even specified.
6: exit policies over arbitrary IP packets seems to be an IDS-hard problem. i
don't want to build an IDS into tor.
7: certain protocols are going to leak information at the IP layer anyway. for
example, if we anonymizer your dns requests, but they still go to comcast's dns servers,
that's bad.
8: hidden services, .exit addresses, etc are broken unless we have some way to
reach into the application-level protocol and decide the hostname it's trying to get.
\subsection{Mid-latency}
Mid-latency. Can we do traffic shape to get any defense against George's
PET2004 paper? Will padding or long-range dummies do anything then? Will
it kill the user base or can we get both approaches to play well together?
%\subsection{The DNS problem in practice}
\subsection{Measuring performance and capacity}
How to measure performance without letting people selectively deny service
by distinguishing pings. Heck, just how to measure performance at all. In
practice people have funny firewalls that don't match up to their exit
policies and Tor doesn't deal.
Network investigation: Is all this bandwidth publishing thing a good idea?
How can we collect stats better? Note weasel's smokeping, at
http://seppia.noreply.org/cgi-bin/smokeping.cgi?target=Tor
which probably gives george and steven enough info to break tor?
\subsection{Plausible deniability}
Does running a server help you or harm you? George's Oakland attack.
Plausible deniability -- without even running your traffic through Tor! We
have to pick the path length so adversary can't distinguish client from
server (how many hops is good?).
\subsection{Helper nodes}
When does fixing your entry or exit node help you?
Helper nodes in the literature don't deal with churn, and
especially active attacks to induce churn.
Do general DoS attacks have anonymity implications? See e.g. Adam
Back's IH paper, but I think there's more to be pointed out here.
\subsection{Location-hidden services}
Survivable services are new in practice, yes? Hidden services seem
less hidden than we'd like, since they stay in one place and get used
a lot. They're the epitome of the need for helper nodes. This means
that using Tor as a building block for Free Haven is going to be really
hard. Also, they're brittle in terms of intersection and observation
attacks. Would be nice to have hot-swap services, but hard to design.
%\section{Crossroads: Scaling}
%\label{sec:crossroads-scaling}
%P2P + anonymity issues:
Incentives. Copy the page I wrote for the NSF proposal, and maybe extend
it if we're feeling smart.
Usability: fc03 paper was great, except the lower latency you are the
less useful it seems it is.
A Tor gui, how jap's gui is nice but does not reflect the security
they provide.
Public perception, and thus advertising, is a security parameter.
Peer-to-peer / practical issues: Peer-to-peer / practical issues:
@ -201,124 +319,9 @@ and we maybe we should start with a time-release IP publishing system +
advogato based reputation system, to bound the number of IPs leaked to the advogato based reputation system, to bound the number of IPs leaked to the
adversary. adversary.
\section{The Future}
\label{sec:conclusion}
Policy issues:
Bittorrent and dmca. Should we add an IDS to autodetect protocols and
snipe them? Takedowns and efnet abuse and wikipedia complaints and irc
networks. Should we allow revocation of anonymity if a threshold of
servers want to?
Image: substantial non-infringing uses. Image is a security parameter,
since it impacts user base and perceived sustainability.
Sustainability. Previous attempts have been commercial which we think
adds a lot of unnecessary complexity and accountability. Freedom didn't
collect enough money to pay its servers; JAP bandwidth is supported by
continued money, and they periodically ask what they will do when it
dries up.
Logging. Making logs not revealing. A happy coincidence that verbose
logging is our \#2 performance bottleneck. Is there a way to detect
modified servers, or to have them volunteer the information that they're
logging verbosely? Would that actually solve any attacks?
Anonymity issues:
Transporting the stream vs transporting the packets.
The DNS problem in practice.
Applications that leak data. We can say they're not our problem, but
they're somebody's problem.
How to measure performance without letting people selectively deny service
by distinguishing pings. Heck, just how to measure performance at all. In
practice people have funny firewalls that don't match up to their exit
policies and Tor doesn't deal.
Mid-latency. Can we do traffic shape to get any defense against George's
PET2004 paper? Will padding or long-range dummies do anything then? Will
it kill the user base or can we get both approaches to play well together?
Does running a server help you or harm you? George's Oakland attack.
Plausible deniability -- without even running your traffic through Tor! We
have to pick the path length so adversary can't distinguish client from
server (how many hops is good?).
When does fixing your entry or exit node help you?
Helper nodes in the literature don't deal with churn, and
especially active attacks to induce churn.
Survivable services are new in practice, yes? Hidden services seem
less hidden than we'd like, since they stay in one place and get used
a lot. They're the epitome of the need for helper nodes. This means
that using Tor as a building block for Free Haven is going to be really
hard. Also, they're brittle in terms of intersection and observation
attacks. Would be nice to have hot-swap services, but hard to design.
P2P + anonymity issues:
Incentives. Copy the page I wrote for the NSF proposal, and maybe extend
it if we're feeling smart.
Usability: fc03 paper was great, except the lower latency you are the
less useful it seems it is.
A Tor gui, how jap's gui is nice but does not reflect the security
they provide.
Public perception, and thus advertising, is a security parameter.
Network investigation: Is all this bandwidth publishing thing a good idea?
How can we collect stats better? Note weasel's smokeping, at
http://seppia.noreply.org/cgi-bin/smokeping.cgi?target=Tor
which probably gives george and steven enough info to break tor?
Do general DoS attacks have anonymity implications? See e.g. Adam
Back's IH paper, but I think there's more to be pointed out here.
% need to do somewhere in the paper:
have a serious discussion of morphmix's assumptions, since they would
seem to be the direct competition. in fact tor is a flexible architecture
that would encompass morphmix, and they're nearly identical except for
path selection and node discovery. and the trust system morphmix has
seems overkill (and/or insecure) based on the threat model we've picked.
need to discuss how we take the approach of building the thing, and then
assuming that, how much anonymity can we get. we're not here to model or
to simulate or to produce equations and formulae. but those have their
roles too.
%%%
TCP vs UDP
argument 1: we need to do IP-level packet normalization, to block things like ip
fingerprinting.
argument 2: we still need to be easy to integrate with applications, so they can do
application-level scrubbing.
argument 3: we need a block-level encryption approach that can provide security despite
packet loss and out-of-order delivery. i believe you that such a thing can be created,
but no thing has yet been specified. so specify it for me if you want me to believe it.
(freedom and cebolla are vulnerable to tagging and malleability attacks i believe.)
argument 4: we still need to play with parameters for throughput, congestion control,
etc -- since we need sequence numbers and maybe more to do replay detection,
and just to handle duplicate frames. so we would be reimplementing some subset of tcp
anyway.
argument 5: tls over udp is not implemented or even specified.
argument 6: exit policies over arbitrary IP packets seems to be an IDS-hard problem. i
don't want to build an IDS into tor.
argument 7: certain protocols are going to leak information at the IP layer anyway. for
example, if we anonymizer your dns requests, but they still go to comcast's dns servers,
that's bad.
argument 8: hidden services, .exit addresses, etc are broken unless we have some way to
reach into the application-level protocol and decide the hostname it's trying to get.
\bibliographystyle{plain} \bibliography{tor-design} \bibliographystyle{plain} \bibliography{tor-design}