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Attempt to address points brought up in #tor flamewar. In particular, moved

Browse Source 
"Who will enable this option?" section towards the top of the proposal, to
attempt to get everyone on the same page right away as far as assumptions
go.

Also, added section on "Consideration of risks for node operators" where
the additional risk of should-be-3-but-actually-2 hop users pose to node
operators is discussed. Upon consideration of this, determined that two hop
users should be made to rotate guards with some frequency on the order of
days (basically, long enough to help scan the network for active adversary
guards, and then move on).

Please re-flame if you feel these or other issues have not been adequately
addressed.



svn:r10498
This commit is contained in:
Mike Perry 2007-06-05 07:44:52 +00:00
parent b6c6dd7e55
commit 6ad4c8a376
1 changed files with 119 additions and 42 deletions
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161
doc/spec/proposals/115-two-hop-paths.txt
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@ -11,7 +11,15 @@ Supersedes: 112
Overview:
The idea is that users should be able to choose if they would like
to have either two or three hop paths through the tor network.
to have either two or three hop paths through the tor network.
Let us be clear: the users who would choose this option should be
those that are concerned with IP obfuscation only: ie they would
not be targets of a resource-intensive multi-node hack. It is often
said that these users should find some other network to use other
than Tor. This is a foolish suggestion: more users improves security
of everyone, and the current small userbase size is a critical
hindrance to anonymity, as is discussed below.
This value should be modifiable from the controller, and should be
available from Vidalia.
@ -46,7 +54,7 @@ Motivation:
but are obviously from a certain institution, location or circumstance,
it is also dangerous to use Tor for risk of being accused of having
something significant enough to hide to be willing to put up with
the horrible performance.
the horrible performance as opposed to using some weaker alternative.
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
@ -60,6 +68,61 @@ Motivation:
security enhancements). That's not just Win-Win, it's Win-Win-Win.
Who will enable this option?
This is the crux of the proposal. Admittedly, there is some anonymity
loss and some degree of decreased investment required on the part of
the adversary to attack 2 hop users versus 3 hop users, even if it is
minimal and limited mostly to up-front costs and false positives.
The key questions are:
1. Are these users in a class such that their risk is significantly
less than the amount of this anonymity loss?
2. Are these users able to identify themselves?
Many many users of Tor are not at risk for an adversary capturing c/n
nodes of the network just to see what they do. These users use Tor to
circumvent aggressive content filters, or simply to keep their IP out of
marketing and search engine databases. Most content filters have no
interest in running Tor nodes to catch violators, and marketers
certainly would never consider such a thing, both on a cost basis and a
legal one.
In a sense, this represents an alternate threat model against these
users who are not at risk for Tor's normal threat model.
It should be evident to these users that they fall into this class. All
that should be needed is a radio button
* "I use Tor for local content filter circumvention and/or IP obfuscation,
not anonymity. Speed is more important to me than high anonymity.
No one will make considerable efforts to determine my real IP."
* "I use Tor for anonymity and/or national-level, legally enforced
censorship. It is possible effort will be taken to identify
me, including but not limited to network surveillance. I need more
protection."
and then some explanation in the help for exactly what this means, and
the risks involved with eliminating the adversary's need for timing
attacks with respect to false positives. Ultimately, the decision is a
simple one that can be made without this information, however. The user
does not need Paul Syverson to instruct them on the deep magic of Onion
Routing to make this decision. They just need to know why they use Tor.
If they use it just to stay out of marketing databases and/or bypass a
local content filter, two hops is plenty. This is likely the vast
majority of Tor users, and many non-users we would like to bring on
board.
So, having established this class of users, let us now go on to
examine theoretical and practical risks we place them at, and determine
if these risks violate the users needs, or introduce additional risk
to node operators who may be subject to requests from law enforcement
to track users who need 3 hops, but use 2 because they enjoy the
thrill of russian roulette.
Theoretical Argument:
It has long been established that timing attacks against mixed
@ -200,6 +263,49 @@ Practical Issues:
be created. TorFlow can scan for failing guards, but after a while,
its unique behavior gives away the fact that its IP is a scanner and
it can be given selective service.
Consideration of risks for node operators:
There is a serious risk for two hop users in the form of guard
profiling. If an adversary running an exit node notices that a
particular site is always visited from a fixed previous hop, it is
likely that this is a two hop user using a certain guard, which could be
monitored to determine their identity. Thus, for the protection of both
2 hop users and node operators, 2 hop users should limit their guard
duration to a sufficient number of days to verify reliability of a node,
but not much more. This duration can be determined experimentally by
TorFlow.
Considering a Tor client builds on average 144 circuits/day (10
minutes per circuit), if the adversary owns c/n% of exits on the
network, they can expect to see 144*c/n circuits from this user, or
about 14 minutes of usage per day per percentage of network penetration.
Since it will take several occurrences of user-identifiable exit content
from the same predecessor hop for the adversary to have any confidence
this is a 2 hop user, it is very unlikely that any sort of demands made
upon the predecessor node would guaranteed to be effective (ie it
actually was a guard), let alone executed in time to apprehend the user
before they rotated guards.
The reverse risk also warrants consideration. If a malicious guard has
orders to surveil Mike Perry, it can determine Mike Perry is using two
hops by observing his tendency to choose a 2nd hop with a viable exit
policy. This can be done relatively quickly, unfortunately, and
probably indicates Mike Perry should spend some of his time building
real 3 hop circuits through the same guards, to require them to at
least wait for him to actually use Tor to determine his style of
operation, rather than collect this information from his passive
building patterns.
However, to actively determine where Mike Perry is going, the guard
will need to require logging ahead of time at multiple exit nodes that
he may use over the course of the few days while he is at that guard,
and correlate the usage times of the exit node with Mike Perry's
activity at that guard for the few days he uses it. At this point, the
adversary is mounting a scale and method of attack (widespread logging,
timing attacks) that works pretty much just as effectively against 3
hops, so exit node operators are exposed to no additional danger than
they otherwise normally are.
Why not fix Pathlen=2?:
@ -227,47 +333,15 @@ Why not fix Pathlen=2?:
for this reason.
Who will enable this option?
This is the crux of the proposal. Admittedly, there is some anonymity
loss and some degree of decreased investment required on the part of
the adversary to attack 2 hop users versus 3 hop users, even if it is
minimal and limited mostly to up-front costs and false positives.
The key questions are:
1. Are these users in a class such that their risk is significantly
less than the amount of this anonymity loss?
2. Are these users able to identify themselves?
Many many users of Tor are not at risk for an adversary capturing c/n
nodes of the network just to see what they do. These users use Tor to
circumvent aggressive content filters, or simply to keep their IP out of
marketing and search engine databases. Most content filters have no
interest in running Tor nodes to catch violators, and marketers
certainly would never consider such a thing, both on a cost basis and a
legal one.
In a sense, this represents an alternate threat model against these
users who are not at risk for Tor's normal threat model.
It should be evident to these users that they fall into this class. All
that should be needed is a radio button
* "I use Tor for censorship resistance and IP obfuscation, not anonymity.
Speed is more important to me than high anonymity."
* "I use Tor for anonymity. I need more protection at the cost of speed."
and then some explanation in the help for exactly what this means, and
the risks involved with eliminating the adversary's need for timing
attacks with respect to false positives.
Implementation:
new_route_len() can be modified directly with a check of the
Pathlen option.
Pathlen option. However, circuit construction logic should be
altered so that both 2 hop and 3 hop users build the same types of
circuits, and the option should ultimately govern circuit selection,
not construction. This improves coverage against guard nodes being
able to passively profile users who aren't even using Tor.
PathlenCoinWeight, anyone? :)
The exit policy hack is a bit more tricky. compare_addr_to_addr_policy
needs to return an alternate ADDR_POLICY_ACCEPTED_WILDCARD or
@ -295,14 +369,17 @@ Migration:
Phase 1: Adjust exit policy checks if Pathlen is set. Modify
new_route_len() to obey a 'Pathlen' config option.
Phase 2: Implement leaky circuit ability.
Phase 2: Implement leaky circuit ability, and 2-3 hop circuit
selection logic.
Phase 3: Experiment to determine the proper ratio of circuit
failures used to expire garbage or malicious guards via TorFlow
(pending Bug #440 backport+adoption).
Phase 4: Implement guard expiration code to kick off failure-prone
guards and warn the user.
guards and warn the user. Cap 2 hop guard duration to a proper number
of days determined sufficient to establish guard reliability (to be
determined by TorFlow).
Phase 5: Make radiobutton in Vidalia, along with help entry
that explains in layman's terms the risks involved.