# [or-cvs] i seem to be listing hard problems.

Roger Dingledine arma at seul.org
Tue Oct 21 08:10:40 UTC 2003

Update of /home/or/cvsroot/doc
In directory moria.mit.edu:/home2/arma/work/onion/cvs/doc

Modified Files:
tor-design.tex
Log Message:
i seem to be listing hard problems.
still plenty more hard problems where those came from.

Index: tor-design.tex
===================================================================
RCS file: /home/or/cvsroot/doc/tor-design.tex,v
retrieving revision 1.11
retrieving revision 1.12
diff -u -d -r1.11 -r1.12
--- tor-design.tex	21 Oct 2003 04:27:54 -0000	1.11
+++ tor-design.tex	21 Oct 2003 08:09:55 -0000	1.12
@@ -567,8 +567,64 @@
\Section{Maintaining anonymity sets}
\label{sec:maintaining-anonymity}

-\SubSection{Using a circuit many times}
-\label{subsec:many-messages}
+packet counting attacks work great against initiators. need to do some
+observers. long-range padding for people who own the first hop. are
+we just screwed against people who insert timing signatures into your
+traffic?
+
+Even regardless of link padding from Alice to the cloud, there will be
+times when Alice is simply not online. Link padding, at the edges or
+inside the cloud, does not help for this.
+
+how often should we pull down directories? how often send updated
+server descs?
+
+when we start up the client, should we build a circuit immediately,
+or should the default be to build a circuit only on demand? should we
+fetch a directory immediately?
+
+would we benefit from greater synchronization, to blend with the other
+users? would the reduced speed hurt us more?
+
+does the "you can't see when i'm starting or ending a stream because
+you can't tell what sort of relay cell it is" idea work, or is just
+a distraction?
+
+does running a server actually get you better protection, because traffic
+coming from your node could plausibly have come from elsewhere? how
+much mixing do you need before this is actually plausible, or is it
+
+do different exit policies at different exit nodes trash anonymity sets,
+or not mess with them much?
+
+do we get better protection against a realistic adversary by having as
+many nodes as possible, so he probably can't see the whole network,
+or by having a small number of nodes that mix traffic well? is a
+cascade topology a more realistic way to get defenses against traffic
+confirmation? does the hydra (many inputs, few outputs) topology work
+better? are we going to get a hydra anyway because most nodes will be
+middleman nodes?
+
+using a circuit many times is good because it's less cpu work
+  good because of predecessor attacks with path rebuilding
+  bad because predecessor attacks can be more likely to link you with a
+    previous circuit since you're so verbose
+  bad because each thing you do on that circuit is linked to the other
+    things you do on that circuit
+
+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?
+

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@@ -623,6 +679,8 @@
route topologies \cite{danezis:pet2003} promise comparable anonymity
with much better scaling properties, but we must solve problems like
how to randomly form the network without introducing net attacks.
+% cascades are a restricted route topology too. we must mention
+% earlier why we're not satisfied with the cascade approach.
\item \emph{Cover traffic:} Currently we avoid cover traffic because
it introduces clear performance and bandwidth costs, but and its
security properties are not well understood. With more research
@@ -649,7 +707,8 @@

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

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