[tor-dev] Proposal 246: Defending Against Guard Discovery Attacks using Vanguards

Aaron Johnson aaron.m.johnson at nrl.navy.mil
Sat Jul 18 04:19:38 UTC 2015

I agree with most of what you said regarding the threat of a targeted observer. What I disagree with is that an adversary running his own relays is less of a threat. Running relays is trivial, and running 5% of the guards is fairly cheap (I estimate ~$3000/month (please ask for details)). If you increase the number of third guards to, say, ten, with average expiration times of, say, 6 hours, then after only 24 hours an adversary with 5% of the bandwidth has an 87% chance of compromising at least one third guard. By reducing the number of third guards to 2 and increasing the expiration time to an average of 24 hours, then it takes 20 days until 87% chance of compromise is reached.

Observing the network traffic of third parties is, it seems to me, actually much more difficult and excludes most adversaries except legal authorities. And legal authorities often do have to follow some legal procedures to observe network traffic, especially if it is being done between different legal jurisdictions.

But ultimately the point of the quickly-rotating third guards is only to *force* the adversary to invest resources into running malicious relays. That is a different type of attack than just traffic observation, and requiring it may stymie some adversaries who don’t have those skills or processes in place. So more and more-quickly rotating third guards would serve that purpose just as well. If the consensus is that an adversary that can observe arbitrary third-party traffic within hours is more of a concern than an adversary that can run 1-5% of the vanguards, then very short-lived third guards would be a reasonable solution, in my opinion. I would not be part of that consensus, but figuring out the speed and size of the adversaries we care about is unfortunately educated guessing at this point.


> On Jul 17, 2015, at 8:11 PM, s7r <s7r at sky-ip.org> wrote:
> Signed PGP part
> On 7/18/2015 12:49 AM, A. Johnson wrote:
> >
> > Not having the third guards be selected by every second guard makes
> > sense when you consider that the adversary may not be able to
> > compromise all relays equally. That was not a consideration I had
> > in mind when I argued for “completely connected” guard sets. The
> > purpose of multiple guards in a given position is (i) to allow
> > onion services that have unusually high load to uses the excess
> > capacity of more than one relay, (ii) to lower the variance of
> > relay performance, and (iii) to recover more quickly from nodes
> > leaving or failing. Note how these are all related to performance.
> > The drawbacks of multiple relays are all related to security, and
> > they are (i) more relays gives the adversary more chances to have
> > his relays selected, (ii) more relays gives the adversary more
> > options and opportunities to compromise relays once they are
> > observed.
> >
> > The best design for security purposes would be a single relay in
> > each guard "set", with the final relay expiring the fastest.
> > Currently, the suggestion was to keep one relay in the first guard
> > position (or whatever NumEntryGuards is), because the security
> > issue there is very serious (it can observe the onion service
> > directly), and also because guards are already selected to have
> > somewhat higher bandwidth and stability. Hopefully (currently
> > unverified) this means that they have more *excess* capacity as
> > well.
> >
> > If you don’t allow all second guards to connect to all third
> > guards, then you have to make a performance / security tradeoff.
> > For example, suppose you give each second guard its own set (or
> > “bucket”) of third guards. You shouldn’t give each second guard the
> > same number of guards that we had planned to have in the third
> > guard set, because that only makes things worse. Now Instead of
> > choosing one set (of size, say, six), you have for each second
> > relay a new chance for the adversary to own one of those relays.
> > There is only a benefit if you reduce the number. It makes sense to
> > me to reduce that number to one, because there is no reason to
> > expect the third guard to be a bottleneck before the second guard
> > (unlike the first guards, which it sounds like may be the only ones
> > requiring the Guard flag). You still have some of the benefit of
> > reducing performance variance and improving failure recovery
> > because you have multiple second guards. And you have gained the
> > benefit that was Mike’s original motivation of giving the adversary
> > that compromises a third guard a view of only one of the second
> > guards, with the hope that maybe this one he finds difficult to
> > compromise.
> >
> > There is an unanswered question of what to do with multiple first
> > guards. The main reason I can see not to have single second guards
> > for each first guard is that the first guard is likely to have more
> > excess capacity and thus not be the bottleneck for high amounts of
> > onion-service traffic. Assuming that's true, then I could imagine
> > having separate second-guard buckets for each first guard as well.
> > However, there is a bad multiplier effect here. For example, with
> > three first guards and three second guards per first guard, there
> > are now *nine* relays in the second-guard position, each of which
> > presents a chance for a malicious relay to be chosen.
> >
> > My current thought is that the following is a better design than
> > the one currently in the proposal: 1. One first guard 2. Two second
> > guards per first guard (so two unless NumEntryGuards is changed) 3.
> > One third guard per second guard (so two unless NumEntryGuards is
> > changed) This will only perform worse than the design suggested in
> > the proposal (which is one first guard, two second guards, and six
> > third guards, with all guards connected to all following guards). I
> > think it might not much worse, though.
> >
> > And I agree with the suggestions to randomize the expiration
> > times.
> >
> > Best, Aaron
> Agreed. The probability to run into a compromised relay will decrease
> dramatically if we use fewer second and third guards. However, this
> will put a penalty over performance, when talking about a popular
> hidden service.
> I still see the third hop (speaking from hidden service server start
> point) is the weak part here. An attacker can connect to a hidden
> service at his malicious relay selected as rendezvous. Before you know
> it, all relays in third_guard_set are enumerated by the attacker. This
> is why I think it's better to have a bigger value for NUM_THIRD_GUARDS
> and a shorter period for THIRD_GUARD_ROTATION.
> Here's why I think so:
> Let's assume a journalist has published some documents offered by a
> whistleblower regarding the abuses done by "Malicious-ISP" and made
> them available via a hidden service to remain anonymous, until
> authorities react to apply the law and punish "Malicious-ISP". Let's
> assume "Malicious-ISP" is a multinational telecom company with
> branches in many countries and good relations with other ISPs around
> the world, who would cooperate with them (unfortunately this is not
> exactly science fiction).
> Currently a big part of Tor network's bandwidth power is concentrated
> in DE,NL,FR. Many times, while browsing with Tor Browser, I had
> circuits with all 3 hops in the same country. Even more times with 2
> out of 3 hops in the same country.
> Think that an attacker has enumerated all the relays in
> third_guard_set, which we know it is trivial to do (for the sake of
> argument, let's pretend we use 2 relays in third_guard_set). Now, it
> all comes down to how fast can this attacker watch the IX point* of
> those 2 relays. Note that the attacker does not necessarily need to
> actually compromise these relays in a special way, just be able to
> watch their traffic somewhere upstream. By watching the traffic
> somewhere upstream, the attacker can enumerate the relays in
> second_guard_set, and so on to eventually first_guard_set.
> If the hidden service is unlucky and picked all hops in the same
> country, its location can be deanon "before dinner". Even if the hops
> are in different countries, depends from case to case, with a simple
> phone call made by the attacker, the IX point* of the previous hop can
> be watched and so on, until the location of the hidden service is
> discovered. The attacker can establish unlimited connections to the
> hidden service while watching all these points and, simple, that's it.
> *IX point - by IX point I don't necessarily mean the internet exchange
> points where the ISP of the relay is connected to, more like the
> master gateway responsible for the traffic of the relay in question.
> I know Tor does not try to protect against a Global Passive adversary,
> but in this scenario there is no need for the adversary to be global -
> this is the problem. The adversary just needs to watch few points on
> the internet, which can be trivially done _before_ our guard rotation
> period expires.
> In short, we give the power which we now think a global passive
> adversary has to a 'local passive adversary' or a 'rather ambitious
> adversary'. What now we think can be accomplished by watching the
> entire network, something we assume is _hard_ to do, will be possible
> just by watching a few parts on the internet. To make it even worse,
> the attacker will even have time to climb up on circuits (time offered
> by our guard rotation period).
> This would be harder to do and require way more effort if the
> third_guard_set was bigger/more diverse and rotated much faster - but
> this increases the chances of a Sybil attack. I think the success
> chances of a Sybil attack are not tragic anyway, and will continue to
> decrease as more 'honest' relays are added to the consensus (this is
> the whole point of Tor anyway, that is why relays can be run by
> volunteers, if the majority of relays aren't honest, then anonymity is
> already killed).
> By not choosing relays in our circuits so often and being paranoid
> that our attacker is 1%, 5%, 10% or whatever (I am not saying it can't
> be) we undermine the concept of onion routing and mixing with as many
> hops as possible. There is a reason why we keep the 10 minutes
> lifetime for a circuit - what's the point of it if after 10 minutes we
> mark a circuit as dirty just to create another one with the very same
> hops. This is intended to protect clients, I know, but in the context
> of a hidden service we assume that _the client is the attacker_.
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