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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_.