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Retitle and write section 8.

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doc/tor-design.tex
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@ -476,6 +476,7 @@ Tor's evolution.
\end{description}
\SubSection{Non-goals}
\label{subsec:non-goals}
In favoring conservative, deployable designs, we have explicitly deferred
a number of goals. Many of these goals are desirable in anonymity systems,
but we choose to defer them either because they are solved elsewhere,
@ -1539,124 +1540,161 @@ Mention jurisdictional arbitrage.
Pull attacks and defenses into analysis as a subsection
\Section{Maintaining anonymity in Tor}
\Section{Open Questions in Low-latency Anonymity}
\label{sec:maintaining-anonymity}
\footnote{The first Onion Routing design \cite{or-ih96} protected against
this threat to some
extent by requiring users to hide network access behind an onion
router/firewall that was also forwarding traffic from other nodes.
However, it is desirable for users to
benefit from Onion Routing even when they can't run their own
onion routers.
%Such users, especially if they engage in certain unusual
%communication behaviors, may be identifiable \cite{wright03}.
%To
%complicate the possibility of such attacks Tor multiplexes many
%stream down each circuit, but still rotates the circuit
%periodically to avoid too much linkability from requests on a single
%circuit.
}
% 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.
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.
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.
[Put as much of this as a part of open issues as is possible.]
%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?
[what's an anonymity set?]
[XXX Choosing paths and path lengths: I'm not writing this bit till
Arma's pathselection stuff is in. -NM]
packet counting attacks work great against initiators. need to do some
level of obfuscation for that. standard link padding for passive link
observers. long-range padding for people who own the first hop. are
we just screwed against people who insert timing signatures into your
traffic?
%%%% 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.
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.
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{disclosure,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{long-range-padding} 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.
how often should we pull down directories? how often send updated
server descs?
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]
when we start up the client, should we build a circuit immediately,
or should the default be to build a circuit only on demand? should we
fetch a directory immediately?
%%% 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.
would we benefit from greater synchronization, to blend with the other
users? would the reduced speed hurt us more?
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? (Tarzan and Morphmix present
possible solutions.) Second, if clients can no longer have a complete
picture of the network at all times how do we prevent attackers from
manipulating 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? [XXX cite george's
restricted-routes paper] (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?
does the "you can't see when i'm starting or ending a stream because
you can't tell what sort of relay cell it is" idea work, or is just
a distraction?
does running a server actually get you better protection, because traffic
coming from your node could plausibly have come from elsewhere? how
much mixing do you need before this is actually plausible, or is it
immediately beneficial because many adversary can't see your node?
do different exit policies at different exit nodes trash anonymity sets,
or not mess with them much?
do we get better protection against a realistic adversary by having as
many nodes as possible, so he probably can't see the whole network,
or by having a small number of nodes that mix traffic well? is a
cascade topology a more realistic way to get defenses against traffic
confirmation? does the hydra (many inputs, few outputs) topology work
better? are we going to get a hydra anyway because most nodes will be
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?
using a circuit many times is good because it's less cpu work.
good because of predecessor attacks with path rebuilding.
bad because predecessor attacks can be more likely to link you with a
previous circuit since you're so verbose.
bad because each thing you do on that circuit is linked to the other
things you do on that circuit.
how often to rotate?
how to decide when to exit from middle?
when to truncate and re-extend versus when to start new circuit?
%%% 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.
Because Tor runs over TCP, when one of the servers goes down it seems
that all the circuits (and thus streams) going over that server must
break. This reduces anonymity because everybody needs to reconnect
right then (does it? how much?) and because exit connections all break
at the same time, and it also reduces usability. It seems the problem
is even worse in a p2p environment, because so far such systems don't
really provide an incentive for nodes to stay connected when they're
done browsing, so we would expect a much higher churn rate than for
onion routing. Are there ways of allowing streams to survive the loss
of a node in the path?
%%% 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?
discuss topologies. Cite George's non-freeroutes paper. Maybe this
graf goes elsewhere.
discuss attracting users; incentives; usability.
Choosing paths and path lengths.
% 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
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%