[tor-talk] on the topic of tor's weaknesses

Paul Syverson syverson at itd.nrl.navy.mil
Mon Feb 27 06:10:40 UTC 2012 

On Sun, Feb 26, 2012 at 01:30:25PM -0500, Xinwen Fu wrote:
> Hi Chris,
> 
> On Sun, Feb 26, 2012 at 12:09 PM, Chris Wheeler <grintor at gmail.com> wrote:
> 
> > Dr Fu,
> >
> > About the paper you sent, particularly the section on the simulation
> > using continuous-time mix, by the looks of it much of your degradation came
> > from packets arriving in the wrong order. I know the TCP doesn't handle
> > this well natively but what if a wrapper was written? The TCP stream would
> > travel inside of our wrapper (which would of course itself would be running
> > on TCP across the internet - It would mean transporting a TCP stream over
> > layer 7 of the OSI model, I think this is how PPTP works) The inner TCP
> > stream would be mixed, and the packets out of order but the tor client,
> > before forwarding them on to their destination (a hidden service or, if we
> > are talking about an exit node, the internet) would rearrange them then
> > send them on.
> >
> >
> 
> Are you going to do the reordering link-by-link or end-to-end? TCP over TCP
> is believed a good idea in terms of performance. Also what is the benefit
> for privacy enhancing if the client itself reorders everything back? What
> is the performance overhead?
> 
> Tor is, I believe, a result after considering many possible options,
> including mixes. It evolves from mixes. Mixes use buffer and shuffling in
> various ways.
> 

Actually no. Tor is a flavor of onion routing not mixing. They have
a different design, a different threat model, and a different basis
for security. It is possible to combine them, as we did in the version
of onion routing we did between the first version and Tor. But Tor
does not use any mixing, and it did not evolve from mixing.


> 
> 
> > Another approach to stop correlation attacks might just be to constantly
> > send packets at set time intervals, regardless of if you have any data
> > to forward. Just pad it with zeros if you have nothing to forward and let
> > it get dropped by the client on the other side of the network and of course
> > send real data when you have something to send. For example have a 5Kb
> > buffer, receive packets and place them in that buffer, send the buffer out
> > once per ms, if the buffer is not full pad it with zeros, if you have more
> > than 5Kb to send cache it for the next ms buffer.
> >
> >
> Traffic padding is also an old topic. Tor has an option of using padding. I
> actually did a lot of study on this. The major trouble is the overhead.
> Another question is how you set the overall rate of the padded traffic? You
> can refer to early papers on padding by my group and Matthew Wright at UT
> Arlington.
> 

The main problem, besides the overhead, is that padding doesn't work
if an adversary can do something as trivial as very briefly delaying
It is too easy for an adversary to put a traffic signature on a
circuit in one place, and look for it elsewhere. If he owns, e.g., the
first node and any of the last node, the link to the destination, or
the destination it won't matter what kind of padding is done. There's
lots of published work showing this in various ways. Some already
alluded to in this thread. If nothing else the adversary can just kill
the connection at the first node and see which connection exiting the
network dies.

We did do a design that, if one had a scheme that was successful
against a passive attacker, would resist an active attacker. Though
providing provable guarantees against a class of active attacker, it
is not ready for primetime. See "Preventing Active Timing Attacks in
Low-Latency Anonymous Communication" at http://freehaven.net/anonbib/

aloha,
Paul


> Cheers,
> 
> Xinwen Fu
> 
> 
> 
> > What do you think?
> > Chris
> >
> > On Sat, Feb 25, 2012 at 9:49 PM, Xinwen Fu <xinwenfu at gmail.com> wrote:
> >
> > > Here is a paper of mine on TCP Performance in Flow-Based Mix Networks:
> > > http://www.cs.uml.edu/~xinwenfu/paper/MixPerf_Fu.pdf.
> > >
> > > If you are interested in why Tor performance is still not super, you an
> > > refer to http://www.cs.uml.edu/~xinwenfu/paper/IPDPS08_Fu.pdf.
> > >
> > > There are many other brilliant works on the performance problem.
> > >
> > > Cheers,
> > >
> > > Xinwen Fu
> > >
> > >
> > > On Sat, Feb 25, 2012 at 9:31 PM, Xinwen Fu <xinwenfu at gmail.com> wrote:
> > >
> > > > That is a lot of thinking. Many of the problems have been discussed
> > > > before. If you look at the bibliography on mixes, there are a lot of
> > > > discussions. Here is one of my papers on mixes:
> > > > http://www.cs.uml.edu/~xinwenfu/paper/fu_icdcs05.pdf. An adversary may
> > > > choose different ways for correlation and attacks.
> > > >
> > > > What Tor achieves is phenomenal. It works against many attacks. Tor
> > > > performance has been improving. I'm saying this because a workable
> > system
> > > > is not easy. For example, changing packet orders may not be a good
> > idea.
> > > It
> > > > may not help much on security, but will deteriorate the TCP performance
> > > Tor
> > > > relies on. TCP is very sensitive to packet ordering errors.
> > > >
> > > > Indeed, there are still a lot of things we don't know for sure.
> > However,
> > > > at least Tor is over there and you can test it on the real one or a
> > > private
> > > > Tor.
> > > >
> > > > Cheers,
> > > >
> > > > Xinwen Fu
> > > >
> > > >
> > > >
> > > >
> > > >
> > > > On Sat, Feb 25, 2012 at 3:49 PM, Joe Btfsplk <joebtfsplk at gmx.com>
> > wrote:
> > > >
> > > >> On 2/25/2012 12:41 PM, Xinwen Fu wrote:
> > > >>
> > > >>> Chris,
> > > >>>
> > > >>> An attack may only work under its own threat model (capabilities and
> > > >>> resources an adversary has). If entries and exit are all secure, some
> > > >>> correlation attacks may not work. However, if the adversary can still
> > > >>> observe (not need to compromise Tor routers) the traffic into and out
> > > of
> > > >>> entries and exits, some attacks may still work, e.g., by correlating
> > > the
> > > >>> traffic patterns at the two sides of a circuit. Here is a paper
> > > >>> describing
> > > >>> such possibility:
> > > >>> http://www.cs.uml.edu/~**xinwenfu/paper/TorCellSize_**ICC11_Fu.pdf<
> > > http://www.cs.uml.edu/%7Exinwenfu/paper/TorCellSize_ICC11_Fu.pdf>
> > > >>> .
> > > >>>
> > > >>> Hope it helps a bit.
> > > >>>
> > > >>> Xinwen Fu
> > > >>>
> > > >> Thanks for the link, Xinwen.  I have a more basic question regarding
> > the
> > > >> original question of adversaries somehow getting access to BOTH entry
> > &
> > > >> exit nodes, that lots of users probably are curious about.  Re:
> >  getting
> > > >> enough data (even if they could break encryption, if using * SSL *) to
> > > do
> > > >> any good.  Since Tor / Vidalia changes nodes often (is it at least
> > > every 10
> > > >> min, at most? - & more often if a circuit fails), could an adversary
> > get
> > > >> enough data about ONE user to do any good?
> > > >>
> > > >> In this case, is the threat that the adversary will ONLY be able to
> > > >> identify that one is USING Tor network (not capture the actual data
> > > >> transferred), assuming they have access to the * originating IP
> > > address,*
> > > >> going to the entry node AND also to the exit node?  I suppose (for
> > now),
> > > >> adversaries in repressive countries determining that one is USING Tor
> > > is a
> > > >> big problem.  Not so much in "free" societies, but that could change.
> > > >>
> > > >> What effect, if any, would Tor changing relays more frequently than
> > the
> > > >> current default time, have on ability of adversaries tracking users
> > (in
> > > a
> > > >> meaningful manner) from entry to exit nodes?
> > > >>
> > > >> In Tor network, does the data packet size & order, in & out of entry
> > or
> > > >> exit nodes necessarily HAVE to be the same?  Would it be possible to
> > > make
> > > >> the  in & out packet sizes different sizes, or mix the order of
> > packets
> > > at
> > > >> an exit vs entry node?  (I don't know the technicality of how this
> > would
> > > >> work).  If you d/l a torrent (as an example), you don't receive the
> > file
> > > >> pieces in order.  Is it theoretically possible  the Tor network could
> > > >> develop a way to mix up the packets (of a file), within the network,
> > so
> > > >> that even if an adversary had complete access to a given entry & exit
> > > >> node(s), the data going in one end could never be matched w/ data
> > coming
> > > >> out?  (don't answer too quickly!  Never say never)
> > > >>
> > > >> This is also somewhat similar in concept to the old data correction
> > > >> process, where pieces of a file might be re transferred (due to
> > > >> corruption), much later in the d/l process, after most of the file was
> > > >> already downloaded.
> > > >>
> > > >> The theoretical concept I'm pondering is, could all pieces of data
> > > >> transmission through Tor be scrambled (the order and / or size) on
> > > purpose?
> > > >>  Adversaries generally can't read the actual data because of
> > encryption
> > > (if
> > > >> I understand).  If there was also no correlation of packets (to an
> > > outside
> > > >> observer) at one end vs the other, how could they ever track a user by
> > > >> traffic analysis?  Adversaries would theoretically have to monitor ALL
> > > >> relays, ALL of the time.
> > > >>
> > > >> Even then, how would they track a user, end to end, if the packet
> > order
> > > >> is purposefully & randomly jumbled within the network?  It seems that
> > > the
> > > >> current Tor network model will come under ever increasing attacks /
> > > >> monitoring & needs to change the fundamental way it operates.
> > > >>
> > > >> To some avg users, it might seem there is no way around a determined
> > > >> adversary determining they are using Tor (with current Tor network
> > > >> technology).
> > > >> If an ISP sets up an entry relay or bridge & exit relay, you could be
> > > >> screwed.
> > > >> If a user goes through a proxy to Tor, and an adversary runs the proxy
> > > >> (how do we really know?), you could be screwed.
> > > >>
> > > >> I could go on & on w/ scenarios.  Lots of people throw around the
> > > phrase,
> > > >> "Users have to determine their threat model..."  Quite honestly, most
> > > >> people wouldn't know how.  For avg users, advanced users may as well
> > > say,
> > > >> "We have no idea.  You're own your own.  Don't assume you'll be
> > > anonymous,
> > > >> even if you follow directions exactly for using Tor / TBB."
> > > >>
> > > >> OK, so instead of everyone shooting down my ideas, modify them so they
> > > >> might work, or come up w/ other better ideas, instead of continuing to
> > > put
> > > >> band aids on the current technology that seems to be fraught w/
> > > problems.
> > > >>
> > > >> ______________________________**_________________
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> > > >>
> > > >
> > > >
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