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Proposal: Bring Back PathlenCoinWeight
Title: Pathlen Coin Weight
Author: Mike Perry
The idea is that users should be able to choose a weight which
probabilistically chooses their path lengths to be 2 or 3 hops. This
weight will essentially be a biased coin that indicates an
additional hop (beyond 2) with probability P. The user should be
allowed to choose 0 for this weight to always get 2 hops and 1 to
always get 3.
This value should be modifiable from the controller, and should be
available from Vidalia.
The Tor network is slow and overloaded. Increasingly often I hear
stories about friends and friends of friends who are behind firewalls,
annoying censorware, or under surveillance that interferes with their
productivity and Internet usage, or chills their speech. These people
know about Tor, but they choose to put up with the censorship because
Tor is too slow to be usable for them. In fact, to download a fresh,
complete copy of levine-timing.pdf for the Anonymity Implications
section of this proposal over Tor took me 3 tries.
There are many ways to improve the speed problem, and of course we
should and will implement as many as we can. Johannes's GSoC project
and my reputation system are longer term, higher-effort things that
will still provide benefit independent of this proposal.
However, reducing the path length to 2 for those who do not need the
(questionable) extra anonymity 3 hops provide not only improves
their Tor experience but also reduces their load on the Tor network by
33%, and can be done in less than 10 lines of code. That's not just
Win-Win, it's Win-Win-Win.
Furthermore, when blocking resistance measures insert an extra relay
hop into the equation, 4 hops will certainly be completely unusable
for these users, especially since it will be considerably more
difficult to balance the load across a dark relay net than balancing
the load on Tor itself (which today is still not without its flaws).
It has long been established that timing attacks against mixed
networks are extremely effective, and that regardless of path
length, if the adversary has compromised your first and last
hop of your path, you can assume they have compromised your
identity for that connection.
In , it is demonstrated that for all but the slowest, lossiest
networks, error rates for false positives and false negatives were
very near zero. Only for constant streams of traffic over slow and
(more importantly) extremely lossy network links did the error rate
hit 20%. For loss rates typical to the Internet, even the error rate
for slow nodes with constant traffic streams was 13%.
When you take into account that most Tor streams are not constant,
but probably much more like their "HomeIP" dataset, which consists
mostly of web traffic that exists over finite intervals at specific
times, error rates drop to fractions of 1%, even for the "worst"
Therefore, the user has little benefit from the extra hop, assuming
the adversary does timing correlation on their nodes. The real
protection is the probability of getting both the first and last hop,
and this is constant whether the client chooses 2 hops, 3 hops, or 42.
Partitioning attacks form another concern. Since Tor uses telescoping
to build circuits, it is possible to tell a user is constructing only
two hop paths at the entry node. It is questionable if this data is
actually worth anything though, especially if the majority of users
have easy access to this option, and do actually choose their path
Nick has postulated that exits may also be able to tell that you are
using only 2 hops by the amount of time between sending their
RELAY_CONNECTED cell and the first bit of RELAY_DATA traffic they
see from the OP. I doubt that they will be able to make much use
of this timing pattern, since it will likely vary widely depending
upon the type of node selected for that first hop, and the user's
connection rate to that first hop. It is also questionable if this
data is worth anything, especially if many users are using this
option (and I imagine many will).
Perhaps most seriously, two hop paths do allow malicious guards
to easily fail circuits if they do not extend to their colluding peers
for the exit hop. Since guards can detect the number of hops in a
path, they could always fail the 3 hop circuits and focus on
selectively failing the two hop ones until a peer was chosen.
I believe currently guards are rotated if circuits fail, which does
provide some protection, but this could be changed so that an entry
guard is completely abandoned after a certain number of extend or
general circuit failures, though perhaps this also could be gamed
to increase guard turnover. Such a game would be much more noticeable
than an individual guard failing circuits, though, since it would
affect all clients, not just those who chose a particular guard.
Why not fix Pathlen=2?:
The main reason I am not advocating that we always use 2 hops is that
in some situations, timing correlation evidence by itself may not be
considered as solid and convincing as an actual, uninterrupted, fully
traced path. Are these timing attacks as effective on a real network
as they are in simulation? Would an extralegal adversary or authoritarian
government even care? In the face of these situation-dependent unknowns,
it should be up to the user to decide if this is a concern for them or not.
new_route_len() can be modified directly with a check of the
PathlenCoinWeight option (converted to percent) and a call to
crypto_rand_int(0,100) for the weighted coin.
The Vidalia setting should probably be in the network status window
as a slider, complete with tooltip, help documentation, and perhaps
an "Are you Sure?" checkbox.
The entry_guard_t structure could have a num_circ_failed member
such that if it exceeds N circuit extend failure to a second hop,
it is removed from the entry list. N should be sufficiently high
to avoid churn from normal Tor circuit failure, and could possibly be
represented as a ratio of failed to successful circuits through that
Phase one: Re-enable config and modify new_route_len() to add an
extra hop if coin comes up "heads".
Phase two: Experiment with the proper ratio of circuit failures
used to expire garbage or malicious guards.
Phase three: Make slider or entry box in Vidalia, along with help entry
that explains in layman's terms the risks involved.
Mad Computer Scientist
fscked.org evil labs