Re: [tor-dev] Proposal 247 (Hidden Service Vanguards) Overhaul

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256

Hi,

I like this very much. See comments inline.

On 9/14/2015 2:07 AM, Mike Perry wrote:

I spent some time trying to clean up proposal 247 based on everyone's comments, as well as based on my own thoughts. Please have a look if you commented on the original proposal, and complain if I've not taken your thoughts into account.

(Aaron: In particular, I made several tradeoffs in favor of performance and DoS resistance that may be at odds with some of your suggestions, but I think the end result is still OK after looking into the Sybil rates and thinking about the adversary model in more detail. You may disagree).

I've attached my updated version of the proposal inline in this mail, but the canonical updated proposal is in my remote at: https://gitweb.torproject.org/user/mikeperry/torspec.git/tree/proposals/247-...

Here's a summary of the changes (which are also listed in Git):

• Try to make a coherent threat model and specify its assumptions

• Fold in my comments about using disjoint sets ("buckets") for

the third level guard.

• Make the parameter discussion subsection its own section, and

include tables with far more detail for the Sybil success rates.

• Put the rotation period in a separate subsection from the number

of guards

• Switch to using min(X,X) and max(X,X) for the distribution for

the second and third layer guard lifespans, respectively. Add a subsection describing this distribution (3.2.3)

• Changed the default parameters based on these tables, and based

on my own intuition about Tor's performance properties.

• Move the load balancing, torrc, and other performance

considerations to their own section (Section 5).

• Move "3.2. Distinguishing new HS circuits from normal HS

circuits" to section 4.1.

• Fold in some of "3.3. Circuit nodes can now be linked to specific

hidden services" into 4.1. Some of it I just removed, though, because I did not find it credible.

• Added Roger's concerns about guard linkability to Section 4.2.

• Added a denial of service subsection to Section 4.3.

================================

Filename: 247-hs-guard-discovery.txt Title: Defending Against Guard Discovery Attacks using Vanguards Author: George Kadianakis Created: 2015-07-10 Status: Draft

0. Motivation

A guard discovery attack allow attackers to determine the guard node of a Tor client. The hidden service rendezvous protocol provides an attack vector for a guard discovery attack since anyone can force an HS to construct a 3-hop circuit to a relay (#9001).

Following the guard discovery attack with a compromise and/or coercion of the guard node can lead to the deanonymization of a hidden service.

1. Overview

This document tries to make the above guard discovery + compromise attack harder to launch. It introduces an optional configuration option which makes the hidden service also pin the second and third hops of its circuits for a longer duration.

With this new path selection, we force the adversary to perform a Sybil attack and two compromise attacks before succeeding. This is an improvement over the current state where the Sybil attack is trivial to pull off, and only a single compromise attack is required.

With this new path selection, an attacker is forced to do a one or more node compromise attacks before learning the guard node of a hidden service. This increases the uncertainty of the attacker, since compromise attacks are costly and potentially detectable, so an attacker will have to think twice before beginning a chain of node compromise attacks that he might not be able to complete.

1.1. Visuals

Here is how a hidden service rendezvous circuit currently looks like:

-> middle_1 -> middle_A -> middle_2 -> middle_B -> middle_3 -> middle_C -> middle_4 -> middle_D HS -> guard -> middle_5 -> middle_E -> Rendezvous Point -> middle_6 -> middle_F -> middle_7 -> middle_G -> middle_8 -> middle_H -> ... -> ... -> middle_n -> middle_n

this proposal pins the two middles nodes to a much more restricted set, as follows:

-> guard_3A_A -> guard_2_A -> guard_3A_B -> guard_3A_C -> Rendezvous Point HS -> guard_1 -> guard_3B_D -> guard_2_B -> guard_3B_E -> guard_3B_F -> Rendezvous Point

Note that the third level guards are partitioned into buckets such that they are only used with one specific second-level guard. In this way, we ensure that even if an adversary is able to execute a Sybil attack against the third layer, they only get to learn one of the second-layer Guards, and not all of them. This prevents the adversary from gaining the ability to take their pick of the weakest of the second-level guards for further attack.

2. Design

This feature requires the HiddenServiceGuardDiscovery torrc option to be enabled.

When a hidden service picks its guard nodes, it also picks two additional sets of middle nodes `second_guard_set` and `third_guard_set` of size NUM_SECOND_GUARDS and NUM_THIRD_GUARDS respectively for each hidden service. These sets are unique to each hidden service created by a single Tor client, and must be kept separate and distinct.

When a hidden service needs to establish a circuit to an HSDir, introduction point or a rendezvous point, it uses nodes from `second_guard_set` as the second hop of the circuit and nodes from that second hop's corresponding `third_guard_set` as third hops of the circuit.

A hidden service rotates nodes from the 'second_guard_set' at a random time between MIN_SECOND_GUARD_LIFETIME hours and MAX_SECOND_GUARD_LIFETIME hours.

A hidden service rotates nodes from the 'third_guard_set' at a random time between MIN_THIRD_GUARD_LIFETIME and MAX_THIRD_GUARD_LIFETIME hours.

These extra guard nodes should be picked with the same path selection procedure that is used for regular middle nodes (though see Section 5.1 for performance reasons to restrict this slightly).

Each node's rotation time is tracked independently, to avoid disclosing the rotation times of the primary and second-level guards.

XXX how should proposal 241 ("Resisting guard-turnover attacks") be applied here?

2.1. Security parameters

We set NUM_SECOND_GUARDS to 4 nodes and NUM_THIRD_GUARDS to 16 nodes (ie four sets of four). XXX: 3 and 12 might be another option here, in which case our rotation period for the second guard position can be reduced to 15 days.

4 nodes and 16 nodes are good values, based on your probability table. and 4 second layer guards means more capacity as opposite to 3, meaning this can be applied to higher traffic hidden services easier.

We set MIN_SECOND_GUARD_LIFETIME to 1 day, and MAX_SECOND_GUARD_LIFETIME to 33 days, for an average rotation rate of ~11 days, using the min(X,X) distribution specified in Section 3.2.2.

We set MIN_THIRD_GUARD_LIFETIME to 1 hour, and MAX_THIRD_GUARD_LIFETIME to 18 hours, for an average rotation rate of ~12 hours, using the max(X,X) distribution specified in Section 3.2.2.

I think I like the numbers. As you recall, I have commented on this proposal when submitted by George and suggested a value as high as possible for NUM_THIRD_GUARDS, considering even making the third hop random all the time. The point was to defend against an active observer attack, not a Sybil attack, and I stick to my suggestion that we should not ignore the probability of it. While we want to rotate nodes slow enough in order to make a Sybil attack hard to accomplish, we should also choose nodes from a big pool, in order to make an attacker to watch many places on the internet at once, which we assume it's not trivial to do.

With these numbers in front of me and thinking from another perspective, I think an average lifetime of 12 hours for the third hop is ok. The main argument here is that, right now when second hop and third hop are always random, I am able to keep over 80% of my connections to hidden services alive for > 12 hours (in a single circuit). Please let's not make the average lifetime of the third hop more than 12 hours.

I was thinking of the third hop to be changed faster since we assume the rendezvous point is evil every time (it is selected by the client, and any client could be a potential attacker), and learning one third guard will allow the attacker to learn (at least one of) our second guards and climb on the circuit just by watching the in and out traffic of that node(s), without compromising them in a special way. This is applicable in both cases anyway, so I think having a third guard with an average lifetime of 12 hours is a better defense against a Sybil attack and not a significant drawback against the other attack.

Anyway, if the third hop is random, an attacker can use an evil rendezvous point which only responds and builds circuits with evil middle nodes. This way, an attacker can increase his chances to learn (at least one of) the second guards by connecting to a hidden service at such a rendezvous point (which will only work with the attacker's compromised relays as third hops).

How do we mitigate this with third level guards? If we decide to never drop our third level guards if they can't build circuits with rendezvous points and prefer to scrap circuits entirely and go on, a hidden service would then have a nonzero chance to select bad nodes as third guards which will censor it by not establishing rendezvous circuits at all. If we decide to rotate the third level guards if they fail more than n% of the rendezvous circuits, an attacker can force us to rotate really quick by opening rendezvous circuits at evil rendezvous points that drop circuits from all relays, tricking the client to believe that the third nodes are broken.

XXX make all the above consensus parameters? Yes. Very yes, especially if we decide to change the primary guard lifespan.

See Section 3 for more analysis on these constants.

Yes, very yes on consensus parameters. This offers us the ability to change this numbers with the least effort in case we will understand it better at some time in the future.

Consensus parameters with the possibility to be overwritten by client in torrc, as we currently have for NumEntryGuards.

3. Rationale and Security Parameter Selection

3.1. Threat model, Assumptions, and Goals

Consider an adversary with the following powers:

• Can launch a Sybil guard discovery attack against any node of a

rendezvous circuit. The slower the rotation period of the node, the longer the attack takes. Similarly, the higher the percentage of the network is compromised, the faster the attack runs.

• Can compromise any node on the network, but this compromise

takes time and potentially even coercive action, and also carries risk of discovery.

We also make the following assumptions about the types of attacks:

1. A Sybil attack is noisy. It will require either large amounts of

traffic, multiple test circuits, or both.

2. A Sybil attack against the second or first layer Guards will be

more noisy than a Sybil attack against the third layer guard, since the second and first layer Sybil attack requires a timing side channel in order to determine success, where as the Sybil success is almost immediately obvious to third layer guard, since it will now be returned as a rend point for circuits for the hidden service in question.

3. As soon as the adversary is confident they have won the Sybil

attack, an even more aggressive circuit building attack will allow them to determine the next node very fast (an hour or less).

4. The adversary is strongly disincentivized from compromising

nodes that may prove useless, as node compromise is even more risky for the adversary than a Sybil attack in terms of being noticed.

Given this threat model, our security parameters were selected so that the first two layers of guards should be hard to attack using a Sybil guard discovery attack and hence require a node compromise attack. Ideally, we want the node compromise attacks to carry a non-negligible probability of being useless to the adversary by the time they complete.

On the other hand, the outermost layer of guards should rotate fast enough to _require_ a Sybil attack.

3.2. Parameter Tuning

3.2.1. Sybil rotation counts for a given number of Guards

The probability of Sybil success for Guard discovery can be modeled as the probability of choosing 1 or more malicious middle nodes for a sensitive circuit over some period of time.

P(At least 1 bad middle) = 1 - P(All Good Middles) = 1 - P(One Good middle)^(num_middles) = 1 - (1 - c/n)^(num_middles)

c/n is the adversary compromise percentage

In the case of Vanguards, num_middles is the number of Guards you rotate through in a given time period. This is a function of the number of vanguards in that position (v), as well as the number of rotations (r).

P(At least one bad middle) = 1 - (1 - c/n)^(v*r)

Here's detailed tables in terms of the number of rotations required for a given Sybil success rate for certain number of guards.

1.0% Network Compromise: Sybil Success One Two Three Four Five Six Eight Nine Ten Twelve Sixteen 10% 11 6 4 3 3 2 2 2 2 1 1 15% 17 9 6 5 4 3 3 2 2 2 2 25% 29 15 10 8 6 5 4 4 3 3 2 50% 69 35 23 18 14 12 9 8 7 6 5 60% 92 46 31 23 19 16 12 11 10 8 6 75% 138 69 46 35 28 23 18 16 14 12 9 85% 189 95 63 48 38 32 24 21 19 16 12 90% 230 115 77 58 46 39 29 26 23 20 15 95% 299 150 100 75 60 50 38 34 30 25 19 99% 459 230 153 115 92 77 58 51 46 39 29

5.0% Network Compromise: Sybil Success One Two Three Four Five Six Eight Nine Ten Twelve Sixteen 10% 3 2 1 1 1 1 1 1 1 1 1 15% 4 2 2 1 1 1 1 1 1 1 1 25% 6 3 2 2 2 1 1 1 1 1 1 50% 14 7 5 4 3 3 2 2 2 2 1 60% 18 9 6 5 4 3 3 2 2 2 2 75% 28 14 10 7 6 5 4 4 3 3 2 85% 37 19 13 10 8 7 5 5 4 4 3 90% 45 23 15 12 9 8 6 5 5 4 3 95% 59 30 20 15 12 10 8 7 6 5 4 99% 90 45 30 23 18 15 12 10 9 8 6

10.0% Network Compromise: Sybil Success One Two Three Four Five Six Eight Nine Ten Twelve Sixteen 10% 2 1 1 1 1 1 1 1 1 1 1 15% 2 1 1 1 1 1 1 1 1 1 1 25% 3 2 1 1 1 1 1 1 1 1 1 50% 7 4 3 2 2 2 1 1 1 1 1 60% 9 5 3 3 2 2 2 1 1 1 1 75% 14 7 5 4 3 3 2 2 2 2 1 85% 19 10 7 5 4 4 3 3 2 2 2 90% 22 11 8 6 5 4 3 3 3 2 2 95% 29 15 10 8 6 5 4 4 3 3 2 99% 44 22 15 11 9 8 6 5 5 4 3

The rotation counts in these tables were generated with: def count_rotations(c, v, success): r = 0 while 1-math.pow((1-c), v*r) < success: r += 1 return r

3.2.2. Rotation Period

As specified in Section 3.1, the primary driving force for the third layer selection was to ensure that these nodes rotate fast enough that it is not worth trying to compromise them, because it is unlikely for compromise to succeed and yield useful information before the nodes stop being used. For this reason we chose 1 to 18 hours, with a weighted distribution (Section 3.2.3) causing the expected average to be 12 hours.

From the table in Section 3.2.1, it can be seen that this means that the Sybil attack will complete with near-certainty (99%) in 29*12 hours (14.5 days) for the 1% adversary, 3 days for the 5% adversary, and 1.5 days for the 10% adversary.

Since rotation of each node happens independently, the distribution of when the adversary expects to win this Sybil attack in order to discover the next node up is uniform. This means that on average, the adversary should expect that half of the rotation period of the next node is already over by the time that they win the Sybil.

With this fact, we choose our range and distribution for the second layer rotation to be short enough to cause the adversary to risk compromising nodes that are useless, yet long enough to require a Sybil attack to be noticeable in terms of client activity. For this reason, we choose a minimum second-layer guard lifetime of 1 day, since this gives the adversary a minimum expected value of 12 hours for during which they can compromise a guard before it might be rotated. If the total expected rotation rate is 11 days, then the adversary can expect overall to have 5.5 days remaining after completing their Sybil attack before a second-layer guard rotates away.

3.2.3. Rotation distribution

In order to skew the distribution of the third layer guard towards higher values, we use max(X,X) for the distribution, where X is a random variable that takes on values from the uniform distribution.

In order to skew the distribution of the second layer guard towards low values (to increase the risk of compromising useless nodes) we skew the distribution towards lower values, using min(X,X).

Here's a table of expectation (arithmetic means) for relevant ranges of X (sampled from 0..N).

The current choice for second-layer guards is noted with **, and the current choice for third-layer guards is noted with ***.

Range Min(X,X) Max(X,X) 10 2.85 6.15 11 3.18 6.82 12 3.51 7.49 13 3.85 8.15 14 4.18 8.82 15 4.51 9.49 16 4.84 10.16 17 5.18 10.82*** 18 5.51 11.49 19 5.84 12.16 20 6.18 12.82 21 6.51 13.49 22 6.84 14.16 23 7.17 14.83 24 7.51 15.49 25 7.84 16.16 26 8.17 16.83 27 8.51 17.49 28 8.84 18.16 29 9.17 18.83 30 9.51 19.49 31 9.84 20.16 32 10.17** 20.83 33 10.51 21.49 34 10.84 22.16 35 11.17 22.83 36 11.50 23.50 37 11.84 24.16 38 12.17 24.83 39 12.50 25.50

4. Security concerns and mitigations

4.1. Mitigating fingerprinting of new HS circuits

By pinning the middle nodes of rendezvous circuits, we make it easier for all hops of the circuit to detect that they are part of a special hidden service circuit with varying degrees of certainty.

The Guard node is able to recognize a Vanguard client with a high degree of certainty because it will observe a client IP creating the overwhelming majority of its circuits to just a few middle nodes in any given 10-18 day time period.

The middle nodes will be able to tell with a variable certainty that depends on both its traffic volume and upon the popularity of the service, because they will see a large number of circuits that tend to pick the same Guard and Exit.

The final nodes will be able to tell with a similar level certainty that depends on their capacity and the service popularity, because they will see a lot of rend handshakes that all tend to have the same second hop.

The most serious of these is the Guard fingerprinting issue. When proposal xxx-padding-negotiation is implemented, services that enable this feature should use those padding primitives to create fake circuits to random middle nodes that are not their guards, in an attempt to look more like a client.

Additionally, if Tor Browser implements "virtual circuits" based on SOCKS username+password isolation in order to enforce the re-use of paths when SOCKS username+passwords are re-used, then the number of middle nodes in use during a typical user's browsing session will be proportional to the number of sites they are viewing at any one time. This is likely to be much lower than one new middle node every ten minutes, and for some users, may be close to the number of Vanguards we're considering.

This same reasoning is also an argument for increasing the number of second-level guards beyond just two, as it will spread the hidden service's traffic over a wider set of middle nodes, making it both easier to cover, and behave closer to a client using SOCKS virtual circuit isolation.

Increasing the number of second level guards will indeed increase the false positives for fingerprinting, but it will also increase the success probability of a Sybil attack. So, we should put these in balance somehow.

I think padding is vital important in this context. We need 'fake' second level guards and 'fake' third level guards which we use only for padding circuits and not real data transfer.

Fingerprinting is even easier at third level guards (which are always before an evil rendezvous point and an attacker can have so many evil rendezvous points, feeling great when he has 100% chances to use his evil relays as rendezvous points in any circuit to any hidden service). If you combine this with the netflow data capturing threat, there are chances to find the location of a hidden service after analyzing the historic data and observing what came from where.

4.2. Hidden service linkability

Multiple hidden services on the same Tor instance should use separate second and third level guard sets, otherwise an adversary is trivially able to determine that the two hidden services are co-located by inspecting their current chosen rend point nodes.

Unfortunately, if the adversary is still able to determine that two or more hidden services are run on the same Tor instance through some other means, then they are able to take advantage of this fact to execute a Sybil attack more effectively, since there will now be an extra set of guard nodes for each hidden service in use.

For this reason, if Vanguards are enabled, and more than one hidden service is configured, the user should be advised to ensure that they do not accidentally leak that the two hidden services are from the same Tor instance.

Yes, a notice / warning here makes sense.

4.3. Denial of service

Since it will be fairly trivial for the adversary to enumerate the current set of rend nodes for a hidden service, denial of service becomes a serious risk for Vanguard users.

For this reason, it is important to support a large number of third-level guards, to increase the amount of resources required to bring a hidden service offline by DoSing just a few Tor nodes.

There are multiple denial of service attacks possible in this context which would force a client to rotate quickly the third guards, increasing the Sybil success probability. This is easy to detect by the client, but unfortunately it offers us only 2 options: a) rotate guards; increase Sybil success probability. b) if too many guards fail, don't rotate, wait for our original choices to be able to handle the traffic and be inaccessible until then.

5. Performance considerations

The switch to a restricted set of nodes will very likely cause significant performance issues, especially for high-traffic hidden services. If any of the nodes they select happen to be temporarily overloaded, performance will suffer dramatically until the next rotation period.

5.1. Load Balancing

Since the second and third level "guards" are chosen from the set of all nodes eligible for use in the "middle" hop (as per hidden services today), this proposal should not significantly affect the long-term load on various classes of the Tor network, and should not require any changes to either the node weight equations, or the bandwidth authorities.

Unfortunately, transient load is another matter, as mentioned previously. It is very likely that this scheme will increase instances of transient overload at nodes selected by high-traffic hidden services.

One option to reduce the impact of this transient overload is to restrict the set of middle nodes that we chose from to some percentage of the fastest middle-capable relays in the network. This may have some impact on load balancing, but since the total volume of hidden service traffic is low, it may be unlikely to matter.

5.2. Circuit build timeout

The adaptive circuit build timeout mechanism in Tor is what corrects for instances of transient node overload right now.

The timeout will naturally tend to select the current fastest and least-loaded paths even through this set of restricted routes, but it may fail to behave correctly if there are a very small set of nodes in each guard set, as it is based upon assumptions about the current path selection algorithm, and it may need to be tuned specifically for Vanguards, especially if the set of possible routes is small.

5.3. OnionBalance

At first glance, it seems that this scheme makes multi-homed hidden services such as OnionBalance[1] even more important for high-traffic hidden services.

Unfortunately, if it is equally damaging to the user for any of their multi-homed hidden service locations to be discovered, then OnionBalance is strictly equivalent to simply increasing the number of second-level guard nodes in use, because an active adversary can perform simultaneous Sybil attacks against all of the rend points offered by the multi-homed OnionBalance introduction points.

5.4. Default vs optional behavior

We suggest this torrc option to be optional because it changes path selection in a way that may seriously impact hidden service performance, especially for high traffic services that happen to pick slow guard nodes.

However, by having this setting be disabled by default, we make hidden services who use it stand out a lot. For this reason, we should in fact enable this feature globally, but only after we verify its viability for high-traffic hidden services, and ensure that it is free of second-order load balancing effects.

Even after that point, until Single Onion Services are implemented, there will likely still be classes of very high traffic hidden services for whom some degree of location anonymity is desired, but for which performance is much more important than the benefit of Vanguards, so there should always remain a way to turn this option off.

Of course, we should always have the option to turn this on or off. I think it's better to make it optional, because I don't think clients reusing circuits based on SOCKS username+password authentication will be enough to hide this in the crowd.

6. Future directions

Here are some more ideas for improvements that should be done sooner or later:

• Maybe we should make the size and rotation period of

secondary/third guard sets to be configurable by the user.

I think so, yes. Consensus parameters by default with option to be overwritten by user in torrc (same as we have for NumEntryGuards).

• To make it harder for an adversary, a hidden service MAY extend

the path length of its circuits by an additional static hop. This forces the adversary to use another coercion attack to walk the chain up to the hidden service.

7. Acknowledgments

Thanks to Aaron Johnson, John Brooks, Mike Perry and everyone else who helped with this idea.

https://onionbalance.readthedocs.org/en/latest/design.html#overview