Commit 7f1fa9aa authored by Roger Dingledine's avatar Roger Dingledine
Browse files

continue fleshing out the blocking-resistance design doc


svn:r8385
parent bd8362af
......@@ -29,7 +29,16 @@
\begin{abstract}
...
Websites around the world are increasingly being blocked by
government-level firewalls. Many people use anonymizing networks like
Tor to contact sites without letting an attacker trace their activities,
and as an added benefit they are no longer affected by local censorship.
But if the attacker simply denies access to the Tor network itself,
blocked users can no longer benefit from the security Tor offers.
Here we describe a design that uses the current Tor network as a
building block to provide an anonymizing network that resists blocking
by government-level attackers.
\end{abstract}
......@@ -61,7 +70,7 @@ location too.
\section{Adversary assumptions}
\label{sec:adversary}
Three main network attacks currently:
Three main network attacks by censors currently:
\begin{tightlist}
\item Block destination by string matches in TCP packets.
......@@ -71,11 +80,18 @@ Three main network attacks currently:
\item Intercept DNS requests.
\end{tightlist}
Assume the network firewall has very limited CPU [clayton06] %~\cite{clayton06}.
Assume the network firewall has very limited CPU~\cite{clayton06}.
Assume that readers of blocked content will not be punished much
(relative to writers).
Assume that while various different adversaries can coordinate and share
notes, there will be a significant time lag between one attacker learning
how to overcome a facet of our design and other attackers picking it up.
\section{Related schemes}
\subsection{public single-hop proxies}
......@@ -94,6 +110,16 @@ Easier to deploy; might not require client-side software.
\subsection{Tor}
Anonymizing networks such as
Tor~\cite{tor-design}
aim to hide not only what is being said, but also who is
communicating with whom, which users are using which websites, and so on.
These systems have a broad range of users, including ordinary citizens
who want to avoid being profiled for targeted advertisements, corporations
who don't want to reveal information to their competitors, and law
enforcement and government intelligence agencies who need
to do operations on the Internet without being noticed.
Tor provides three security properties:
\begin{tightlist}
\item A local observer can't learn, or influence, your destination.
......@@ -121,19 +147,19 @@ whichever paths work.
\subsection{Tor circuits}
can build arbitrary overlay paths given a set of descriptors [blossom] %~\cite{blossom}
can build arbitrary overlay paths given a set of descriptors~\cite{blossom}
\subsection{Tor directory servers}
\subsection{Tor user base}
\section{The Design}
\section{The Design, version one}
\subsection{Bridge relays}
Some Tor users on the free side of the network will opt to become bridge
relays. They will relay a bit of traffic and don't allow exits. They
sign up on the bridge directory authorities, below.
Some Tor users on the free side of the network will opt to become
bridge relays. They will relay a bit of traffic and won't need to allow
exits. They sign up on the bridge directory authorities, below.
...need to outline instructions for a Tor config that will publish
to an alternate directory authority, and for controller commands
......@@ -147,39 +173,53 @@ answer all queries as usual, except they don't publish network statuses.
So once you know a bridge relay's key, you can get the most recent
server descriptor for it.
XXX need to figure out how to fetch some statuses from the BDA without
fetching all statuses. A new URL to fetch I presume?
XXX need to figure out how to fetch some server statuses from the BDA
without fetching all statuses. A new URL to fetch I presume?
\subsection{Blocked users}
If a blocked user knows about a working bridge relay, then he can make
secure connections to the BDA to update his knowledge about bridge
If a blocked user has a server descriptor for one working bridge relay,
then he can make secure connections to the BDA to update his knowledge
about other bridge
relays, and he can make secure connections to the main Tor network
and directory servers to build circuits and connect to the rest of
the Internet.
So now we've reduced the problem from how to circumvent the firewall
for all transactions (and how to know that the pages you get are the
real ones) to how to learn about a working bridge relay. They can
be distributed in three ways:
\begin{tightlist}
\item IP:dirport, so the user can connect directly to the bridge
relay, learn the associated
server descriptor, and start building circuits. This is great, but what if
the firewall creates signatures for plaintext http requests for server
descriptors, to block them? One option is a workaround that changes the
appearance of the plaintext at each step (I can imagine a simple scheme
where we send a 16 byte key, and then encrypt the rest of the stream with
that key -- it doesn't provide actual confidentiality, but it's hard to
recognize that it's a Tor connection); another option is to conclude that
it will be better to tunnel through a Tor circuit when fetching them.
\item Key fingerprint, which lets you lookup the most recent server
descriptor at the BDA (assuming you can reach it).
\item A blinded token, which can be exchanged at the BDA (assuming you
can reach it) for a new IP:dirport or server descriptor.
\end{tightlist}
See the following section for ways to bootstrap knowledge of your first
for all transactions (and how to know that the pages you get have not
been modified by the local attacker) to how to learn about a working
bridge relay.
The simplest format for communicating information about a bridge relay
is as an IP address and port for its directory cache. From there, the
user can ask the directory cache for an up-to-date copy of that bridge
relay's server descriptor, including its current circuit keys, the port
it uses for Tor connections, and so on.
However, connecting directly to the directory cache involves a plaintext
http request, so the censor could create a firewall signature for the
request and/or its response, thus preventing these connections. If that
happens, the first fix is to use SSL -- not for authentication, but
just for encryption so requests look different every time.
There's another possible attack here: since we only learn an IP address
and port, a local attacker could intercept our directory request and
give us some other server descriptor. But notice that we don't need
strong authentication for the bridge relay. Since the Tor client will
ship with trusted keys for the bridge directory authority and the Tor
network directory authorities, the user can decide if the bridge relays
are lying to him or not.
Once the Tor client has fetched the server descriptor at least once,
it should remember the identity key fingerprint for that bridge relay.
If the bridge relay moves to a new IP address, the client can then
use the bridge directory authority to look up a fresh server descriptor
using this fingerprint.
another option is to conclude
that it will be better to tunnel through a Tor circuit when fetching them.
The following section describes ways to bootstrap knowledge of your first
bridge relay, and ways to maintain connectivity once you know a few
bridge relays.
......@@ -197,6 +237,13 @@ network or other mechanism for learning IP:dirport or key fingerprints
as above, or we assume an account server that allows us to limit the
number of new bridge relays an external attacker can discover.
\section{The Design, version two}
\item A blinded token, which can be exchanged at the BDA (assuming you
can reach it) for a new IP:dirport or server descriptor.
\subsection{The account server}
Users can establish reputations, perhaps based on social network
......@@ -271,6 +318,17 @@ provides improved anonymity against some attacks too:
http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ#ServerAnonymity
\end{verbatim}
\subsection{Cablemodem users don't provide important websites}
...so our adversary could just block all DSL and cablemodem networks,
and for the most part only our bridge relays would be affected.
The first answer is to aim to get volunteers both from traditionally
``consumer'' networks and also from traditionally ``producer'' networks.
The second answer (not so good) would be to encourage more use of consumer
networks for popular and useful websites.
\section{Future designs}
\subsection{Bridges inside the blocked network too}
......
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