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Some stuff on port scanning and a braindumpsortof on directories
svn:r8921
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@ -555,6 +555,7 @@ can't make a list of all the addresses contacting the authority and
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track them that way. Bridges may publish to only a subset of the
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authorities, to limit the potential impact of an authority compromise.
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%\subsection{A simple matter of engineering}
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%
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%Although we've described bridges and bridge authorities in simple terms
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@ -611,6 +612,75 @@ out too much.
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% (See Section~\ref{subsec:first-bridge} for a discussion
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%of exactly what information is sufficient to characterize a bridge relay.)
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\subsubsection{Multiple questions about directory authorities}
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% This dumps many of the notes I had in one place, because I wanted
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% them to get into the tex document, rather than constantly living in
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% a separate notes document. They need to be changed and moved, but
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% now they're in the right document. -PFS
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9. Bridge directories must not simply be a handful of nodes that
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provide the list of bridges. They must flood or otherwise distribute
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information out to other Tor nodes as mirrors. That way it becomes
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difficult for censors to flood the bridge directory servers with
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requests, effectively denying access for others. But, there's lots of
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churn and a much larger size than Tor directories. We are forced to
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handle the directory scaling problem here much sooner than for the
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network in general.
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I think some kind of DHT like scheme would work here. A Tor node is
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assigned a chunk of the directory. Lookups in the directory should be
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via hashes of keys (fingerprints) and that should determine the Tor
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nodes responsible. Ordinary directories can publish lists of Tor nodes
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responsible for fingerprint ranges. Clients looking to update info on
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some bridge will make a Tor connection to one of the nodes responsible
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for that address. Instead of shutting down a circuit after getting
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info on one address, extend it to another that is responsible for that
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address (the node from which you are extending knows you are doing so
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anyway). Keep going. This way you can amortize the Tor connection.
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10. We need some way to give new identity keys out to those who need
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them without letting those get immediately blocked by authorities. One
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way is to give a fingerprint that gets you more fingerprints, as
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already described. These are meted out/updated periodically but allow
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us to keep track of which sources are compromised: if a distribution
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fingerprint repeatedly leads to quickly blocked bridges, it should be
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suspect, dropped, etc. Since we're using hashes, there shouldn't be a
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correlation with bridge directory mirrors, bridges, portions of the
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network observed, etc. It should just be that the authorities know
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about that key that leads to new addresses.
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This last point is very much like the issues in the valet nodes paper,
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which is essentially about blocking resistance wrt exiting the Tor network,
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while this paper is concerned with blocking the entering to the Tor network.
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In fact the tickets used to connect to the IPo (Introduction Point),
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could serve as an example, except that instead of authorizing
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a connection to the Hidden Service, it's authorizing the downloading
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of more fingerprints.
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Also, the fingerprints can follow the hash(q + '1' + cookie) scheme of
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that paper (where q = hash(PK + salt) gave the q.onion address). This
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allows us to control and track which fingerprint was causing problems.
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Note that, unlike many settings, the reputation problem should not be
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hard here. If a bridge says it is blocked, then it might as well be.
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If an adversary can say that the bridge is blocked wrt
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$\mathcal{censor}_i$, then it might as well be, since
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$\mathcal{censor}_i$ can presumaby then block that bridge if it so
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chooses.
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11. How much damage can the adversary do by running nodes in the Tor
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network and watching for bridge nodes connecting to it? (This is
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analogous to an Introduction Point watching for Valet Nodes connecting
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to it.) What percentage of the network do you need to own to do how
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much damage. Here the entry-guard design comes in helpfully. So we
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need to have bridges use entry-guards, but (cf. 3 above) not use
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bridges as entry-guards. Here's a serious tradeoff (again akin to the
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ratio of valets to IPos) the more bridges/client the worse the
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anonymity of that client. The fewer bridges/client the worse the
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blocking resistance of that client.
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\section{Hiding Tor's network signatures}
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\label{sec:network-signature}
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\label{subsec:enclave-dirs}
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@ -693,6 +763,10 @@ differently from, say, instant messaging.
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% Tor cells are 512 bytes each. So TLS records will be roughly
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% multiples of this size? How bad is this?
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% Look at ``Inferring the Source of Encrypted HTTP Connections''
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% by Marc Liberatore and Brian Neil Levine (CCS 2006)
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% They substantially flesh out the numbers for the web fingerprinting
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% attack.
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\subsection{Identity keys as part of addressing information}
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@ -1194,6 +1268,22 @@ adversary can pretend to be the bridge and MITM him to learn the password.
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We could some kind of ID-based knocking protocol, or we could act like an
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unconfigured HTTPS server if treated like one.
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We can assume that the attacker can easily recognize https connections
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to unknown servers. It can then attempt to connect to them and block
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connections to servers that seem suspicious. It may be that password
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protected web sites will not be suspicious in general, in which case
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that may be the easiest way to give controlled access to the bridge.
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If such sites that have no other overt features are automatically
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blocked when detected, then we may need to be more subtle.
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Possibilities include serving an innocuous web page if a TLS encrypted
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request is received without the authorization needed to access the Tor
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network and only responding to a requested access to the Tor network
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of proper authentication is given. If an unauthenticated request to
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access the Tor network is sent, the bridge should respond as if
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it has received a message it does not understand (as would be the
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case were it not a bridge).
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\subsection{How to motivate people to run bridge relays}
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One of the traditional ways to get people to run software that benefits
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