Proposal: Avoiding infinite length circuits
arma at mit.edu
Wed Mar 14 03:15:47 UTC 2007
I figured I'd get up to speed on proposal writing with a rather simple
one, rather than jump in over my head first. :) This design has been
kicking around since Christian Grothoff and I came up with it at PET 2004.
Title: Avoiding infinite length circuits
Author: Roger Dingledine
Right now, an attacker can add load to the Tor network by extending a
circuit an arbitrary number of times. Every cell that goes down the
circuit then adds N times that amount of load in overall bandwidth
use. This vulnerability arises because servers don't know their position
on the path, so they can't tell how many nodes there are before them
on the path.
We propose a new set of relay cells that are distinguishable by
intermediate hops as permitting extend cells. This approach will allow
us to put an upper bound on circuit length relative to the number of
colluding adversary nodes; but there are some downsides too.
The above attack can be used to generally increase load all across the
network, or it can be used to target specific servers: by building a
circuit back and forth between two victim servers, even a low-bandwidth
attacker can soak up all the bandwidth offered by the fastest Tor
The general attacks could be used as a demonstration that Tor isn't
perfect (leading to yet more media articles about "breaking" Tor), and
the targetted attacks will come into play once we have a reputation
system -- it will be trivial to DoS a server so it can't pass its
reputation checks, in turn impacting security.
We should split RELAY cells into two types: RELAY and RELAY_EXTEND.
Relay_extend cells can only be sent in the first K (say, 10) data
cells sent across a circuit, and only relay_extend cells are allowed
to contain extend requests. We still support obscuring the length of
the circuit (if more research shows us what to do), because Alice can
choose how many of the K to mark as relay_extend. Note that relay_extend
cells *can* contain any sort of data cell; so in effect it's actually
the relay type cells that are restricted.
Each intermediate server would pass on the same type of cell that it
received (either relay or relay_extend), and the cell's destination
will be able to learn whether it's allowed to contain an Extend request.
If an intermediate server receives a relay_extend cell after it has
already seen k cells, or if it sees a relay cell that contains an
extend request, then it tears down the circuit (protocol violation).
The upside is that this limits the bandwidth amplification factor to
K: for an individual circuit to become arbitrary-length, the attacker
would need an adversary-controlled node every K hops, and at that
point the attack is no worse than if the attacker creates N/K separate
On the other hand, we want to pick a large enough value of K that we
don't mind the cap.
If we ever want to take steps to hide the number of hops in the circuit
or a node's position in the circuit, this design probably makes that
Phase one: servers should recognize relay_extend cells and pass them
on just like relay cells. Don't do any enforcement of the protocol
yet. We could do this phase in the 0.2.0 timeline.
Phase two: once support in phase one is pervasive, clients could start
using relay_extend cells when all nodes currently in the circuit would
recognize them. We could conceivably do this phase during 0.2.0 too.
Phase three: once clients that don't use relay_extend cells are
obsolete, servers should start enforcing the protocol.
(Another migration plan would be to coordinate this with proposal
105's new link versions. Would that be better/worse? Can somebody
sketch out what it might look like?)
[We can formalize this part once we think the design is a good one.]
Rather than limiting the relay_extend cells to being in the first K
data cells seen, we could instead permit up to K relay_extend cells
in the lifetime of the circuit. This would let us extend the circuit
later on in its life if we decided it was worth doing, though we would
reveal our intent to each node in the circuit when we do.
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