Re: another reason to keep ExcludeNodes

[Scott Bennett <bennett@xxxxxxxxxx>]

     On Tue, 17 Feb 2009 15:04:13 -0500 Roger Dingledine <arma@xxxxxxx>
wrote:
>On Tue, Feb 17, 2009 at 08:08:19PM +0100, Lexi Pimenidis wrote:
>> > > little bit of investigation it turned out that one particular relay was
>> > > always in a circuit that truncated those files, so I added it to my
>> > > ExcludeNodes list.  And voila' complete images from then on.
>> > 
>> > Would not it be better if you would report this node so that its
>> > problem can be fixed?
>> 
>> This could possibly be used to identify anonymous surfers: imagine an $evil
>> exit node trying to identify somebody surfing on $evil-site1 (which isn't
>> very popular and only a very small subset of people use it). It just needs
>> to modify the output a bit and then wait for somebody to complain about it.
>> 
>> Chances are, the one complaining might give away enough info to identify himself..?
>
>Hey, that brings up another possible attack. What if a website keeps
>giving out partial pages in response to exit nodes that it doesn't like
>(for example because it can't monitor them), to encourage users to
>manually mark them as excludeexit, thus making sure that user won't use
>those exits for other sites either?
>
>It wouldn't break anonymity outright, but it would certainly make the
>probabilities more complex to reason about.

     In the particular case I was describing, the node that was consistently
appearing in the circuits that cut off files happened not to be an exit in
any of the failure cases.  IIRC, it was nearly always in a middleman position,
though I think there was a day when it was an entry guard.  (I don't remember
for sure that it was a guard.  This happened a couple of months or more ago,
and the details have gotten fuzzy since then.)  If it had been an exit for
those circuits, I would have tried excluding it only as an exit first before
eliminating it for all circuit route positions.
>
>Rabbit holes within rabbit holes,

     Yes, indeed.


                                  Scott Bennett, Comm. ASMELG, CFIAG
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