May 2017 - tor-dev - lists.torproject.org

non-anonymous ephemeral onion services with stem
by Micah Lee 30 Jan '18

30 Jan '18

The stem documentation for create_ephemeral_hidden_service [1] says: "Changed in version 1.5.0: Added support for non-anonymous services." But I can't figure out to actually use this feature. There doesn't seem to be a new argument to say if you want your onion service to be non-anonymous. It also says, "Changed in version 1.5.0: Added the basic_auth argument." But there's a new basic_auth argument you can pass into the function to use that. [1] https://stem.torproject.org/api/control.html#stem.control.Controller.create…

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Thoughts on the Counter-RAPTOR paper
by Roger Dingledine 20 Jul '17

20 Jul '17

Hi Prateek, Yixin, (and please involve your other authors as you like), (I'm including tor-dev here too so other Tor people can follow along, and maybe even get involved in the research or the discussion.) I looked through "Counter-RAPTOR: Safeguarding Tor Against Active Routing Attacks": https://arxiv.org/abs/1704.00843 For the tl;dr for others here, the paper: a) comes up with metrics for how to measure resilience of Tor relays to BGP hijacking attacks, and then does the measurements; b) describes a way that clients can choose their guards to be less vulnerable to BGP hijacks, while also considering performance and anonymity loss when guard choice is influenced by client location; and c) builds a monitoring system that takes live BGP feeds and looks for routing table anomalies that could be hijack attempts. Here are some hopefully useful thoughts: ----------------------------------------------------------------------- 0) Since I opted to write these thoughts in public, I should put a little note here in case any journalists run across it and wonder. Yay research! We love research on Tor -- in fact, research like this is the reason Tor is so strong. For many more details about our perspective on Tor research papers, see https://blog.torproject.org/blog/tor-heart-pets-and-privacy-research-commun… ----------------------------------------------------------------------- 1a) The "live BGP feed anomaly detection" part sounds really interesting, since in theory we could start using it really soon now. Have you continued to run it since you wrote the paper? Have you done any more recent analysis on its false positive rate since then? I guess one of the real challenges here is that since most of the alerts are false positives, we really need a routing expert to be able to look at each alert and assess whether we should be worried about it. How hard is it to locate such an expert? Is there even such a thing as an expert in all routing tables, or do we need expertise in "what that part of the network is supposed to look like", which doesn't easily scale to the whole Internet? Or maybe said another way, how much headway can we make on automating the analysis, to make the frequency of alerts manageable? I ask because it's really easy to write a tool that sends a bunch of warnings, and if some of them are false positives, or heck even if they're not but we don't know how to assess how bad they really are, then all we've done is make yet another automated emailer. (We've made a set of these already, to e.g. notice when relays change their identity key a lot: https://gitweb.torproject.org/doctor.git/tree/ but often nobody can figure out whether such an anomaly is really an attack or what, so it's a constant struggle to keep the volume low enough that people don't just ignore the mails.) The big picture question is: what steps remain from what you have now to something that we can actually use? 1b) How does your live-BGP-feed-anomaly-detector compare (either in design, or in closeness to actually being usable ;) to the one Micah Sherr was working on from their PETS 2016 paper? https://security.cs.georgetown.edu/~msherr/reviewed_abstracts.html#tor-data… 1c) Your paper suggests that an alert from a potential hijack attempt could make clients abandon the guard for a while, to keep clients safe from hijack attempts. What about second-order effects of such a design, where the attacker's *goal* is to get clients to abandon a guard, so they add some sketchy routes somewhere to trigger an alert? Specifically, how much easier is it to add sketchy routes that make it look like somebody is attempting an attack, compared to actually succeeding at hijacking traffic? I guess a related question (sorry for my BGP naivete) is: if we're worried about false positives in the alerts, how much authentication and/or attribution is there for sketchy routing table entries in general? Can some jerk drive up our false positive rate, by adding scary entries here and there, in a way that's sustainable? Or heck, can some jerk DDoS parts of the Internet in a way that induces routing table changes that we think look sketchy? These are not reasons to not take the first steps in the arms race, but it's good to know what the later steps might be. ----------------------------------------------------------------------- 2a) Re changing guard selection, you should check out proposal 271, which resulted in the new guard-spec.txt as of Tor 0.3.0.x: https://gitweb.torproject.org/torspec.git/tree/guard-spec.txt I don't fully understand it yet (so many things!), but I bet any future guard selection change proposal should be relative to this design. 2b) Your guard selection algorithm makes the assumption that relays with the Guard flag are the only ones worth choosing from, and then describes a way to choose from among them with different weightings. But you could take a step back, and decide that resilience to BGP hijack should be one of the factors for whether a relay gets the Guard flag in the first place. It sounded from your analysis like some ASes, like OVH, are simply bad news for (nearly) all Tor clients. Your proposed guard selection strategy reduced, but did not eliminate, the chances that clients would get screwed by picking one of these OVH relays. And the tradeoff was that by only reducing the chances, you left the performance changes not as extreme as you might have otherwise. How much of the scariness of a relay is a function of the location of the particular client who is considering using it, and how much is a function of the average (expected) locations of clients? That is, can we identify relays that are likely to be bad news for many different clients, and downplay their weights (or withhold the Guard flag) for everybody? The advantage of making the same decision for all clients is that you can get rid of the "what does guard choice tell you about the client" anonymity question, which is a big win if the rest of the effects aren't too bad. Which leads me to the next topic: ----------------------------------------------------------------------- 3) I think you're right that when analyzing a new path selection strategy, there are three big things to investigate: a) Does the new behavior adequately accomplish the goal that made you want a new path selection strategy (in this case resilience to BGP attacks)? b) What does the new behavior do to anonymity, both in terms of the global effect (e.g. by flattening the selection weights or by concentrating traffic in fewer relays or on fewer networks) and on the individual epistemic side (e.g. by leaking information about the user because of behavior that is a function of sensitive user details)? c) What are the expected changes to performance, and are there particular scenarios (like high load or low load) that have higher or lower impact? I confess that I don't really buy your analysis for 'b' or 'c' in this paper. Average change in entropy doesn't tell me whether particular user populations are especially impacted, and a tiny Shadow simulation with one particular network load and client behavior doesn't tell me whether things will or won't get much worse under other network loads or other client behavior. I can't really fault this paper though, because the structure of an academic research paper means you can only do so much in one paper, and you did a bunch of other interesting things instead. We, the Tor research community, really need better tools for reasoning about the interaction between anonymity and performance. In fact, there sure have been a lot of Tor path selection papers over the past decade which each invent their own ad hoc analysis approach for showing that their proposed change doesn't impact anonymity or performance "too much". Is it time for a Systemization of Knowledge paper on this area -- with the goal of coming up with best practices that future papers can use to provide more convincing analysis? --Roger

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Open topics of prop247: Defending Against Guard Discovery Attacks using Vanguards
by George Kadianakis 04 Jul '17

04 Jul '17

Hello, here is some background information and summarizing of proposal 247 "Defending Against Guard Discovery Attacks using Vanguards" for people who plan to work on this in the short-term future. I include a list of open design topics (probably not exhaustive) and a list of engineering topics. Some engineering stuff can be done parallel to the design stuff. ==================== Background info ==================== * Proposal: https://gitweb.torproject.org/torspec.git/tree/proposals/247-hs-guard-disco… * Discussion: ** Initial prop247 thread: https://lists.torproject.org/pipermail/tor-dev/2015-July/009066.html ** Recent prop247 thread: https://lists.torproject.org/pipermail/tor-dev/2015-September/009497.html ** Reading group notes of prop247: https://lists.torproject.org/pipermail/tor-dev/2016-January/010265.html ==================== Design topics ==================== * Optimize proposal parameters ** Optimize guardset sizes ** Optimize guardset lifetimes and prob distributions (minXX/maxXX/uniform?) ** To take informed decision, we might need a prop247 simulator, or an actual PoC with txtorcon * HOW to choose second-layer and third-layer guards? ** Should they be Guards? middles? Vanguards? Serious security / load balancing implications! ** Can guardsets share guards between them or are they disjoint? Particularly third-layer sets ** background: https://lists.torproject.org/pipermail/tor-dev/2016-January/010265.html * HOW to avoid side-channel guard discovery threats? ** Can IP/RP be the same as first-layer guard? ** Can first-layer guard be the same as third-layer guard? ** background: https://gitweb.torproject.org/user/mikeperry/torspec.git/commit/?h=guard_di… * Change path selection for IP circs to avoid third-layer guard linkability threats. ** Switch from [HS->G1->M->IP] to [HS->G1->G2->G3->IP] or even to [HS->G1->G2->G3->M->IP]. ** Consider the latter option for HSDir circs as well? ** background: https://gitweb.torproject.org/user/mikeperry/torspec.git/commit/?h=guard_di… * Should prop247 be optional or default? ** Consider making it optional for a testing period? * How does prop247 affects network performance and load balancing? ** especially if it's enabled by default? ** Update load balancing proposal? * Correct behavior for multiple HSes on single host? * Does prop247 influence guard fingerprinting (#10969) and should we care enough? ==================== Engineering topics ==================== * What's a good entrynodes API to implement prop247? * What's a good state file API to implement prop247? * Write prop247 simulator to verify security goals and optimize proposal parameters (see above). * Write PoC with txtorcon! * Write PoC with little-t-tor! ============================================================

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Proposal 247 (Hidden Service Vanguards) Overhaul
by Mike Perry 09 Jun '17

09 Jun '17

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. 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. 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. 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. 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. 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. 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. 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. - 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. 1. https://onionbalance.readthedocs.org/en/latest/design.html#overview -- Mike Perry

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Why is my bridge not publishing statistics?
by David Fifield 05 Jun '17

05 Jun '17

I searched for the Snowflake bridge in Atlas, and couldn't find it. Its fingerprint is 2B280B23E1107BB62ABFC40DDCC8824814F80A72. Its torrc is stock "Last updated 9 October 2013 for Tor 0.2.5.2-alpha" except for these settings: ContactInfo David Fifield <dcf(a)torproject.org> SOCKSPort 0 ORPort 9001 BridgeRelay 1 ExtORPort auto ServerTransportPlugin snowflake exec /usr/local/bin/snowflake-server --acme-hostnames snowflake.bamsoftware.com --acme-email dcf(a)torproject.org --log /var/log/tor/snowflake-server.log ServerTransportListenAddr snowflake 0.0.0.0:443 Its ORPort 9001 is blocked by the local firewall, because it is meant to be only a Snowflake bridge, and not a vanilla bridge. (Most of the default Tor Browser obfs4 bridges are configured the same way, with their ORPort blocked.) There are these messages in the log (which I exxpected): [warn] Your server (37.218.242.151:9001) has not managed to confirm that its ORPort is reachable. Please check your firewalls, ports, address, /etc/hosts file, etc. Why is the bridge not appearing in Atlas? I initially suspected https://bugs.torproject.org/18050, whose changelog entry is: - Check that both the ORPort and DirPort (if present) are reachable before publishing a relay descriptor. Otherwise, relays publish a descriptor with DirPort 0 when the DirPort reachability test takes longer than the ORPort reachability test. Closes bug #18050. Reported by "starlight", patch by "teor". Bugfix on 0.1.0.1-rc, commit a1f1fa6ab on 27 Feb 2005. But if it's the case that an unreachable ORPort causes descriptors not to be uploaded, then why do the default obfs4 bridges appear in Atlas? For example: https://atlas.torproject.org/#details/D9C805C955CB124D188C0D44F271E9BE57DE2… https://atlas.torproject.org/#details/D3D4A456FCB5F301F092F6A49ED671B84B432… https://atlas.torproject.org/#details/11A3982C417AF37230F576006405BE5338F41… Actually, now that I look at it, I notice some other default bridges are not present in Atlas, for example the two from https://bugs.torproject.org/21917, which went out in Tor Browser 6.5.2: C8CBDB2464FC9804A69531437BCF2BE31FDD2EE4 0BAC39417268B96B9F514E7F63FA6FBA1A788955 What's going on and how can we fix it? You can find a list of default bridge fingerprints here: https://gitweb.torproject.org/builders/tor-browser-bundle.git/tree/Bundle-D…

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[PATCH] fix GCC 7 implicit fallthroug warnings
by Andreas Stieger 30 May '17

30 May '17

Hello, 6 patches that fix -Wimplicit-fallthrough in GCC 7 on master, by adding __attribute__ ((fallthrough)); or break, as required. Additionally, the patch fixes the same on maint-0.3.0, the warning is no longer present in master. 0001-Fix-implicit-fallthrough-warning-in-GCC-7-in-tor_gzi.patch Please review, Andreas -- Andreas Stieger <astieger(a)suse.com> Project Manager Security SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)

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specific From: for tor-bugs ML emails (like tor-wiki-changes)
by nusenu 25 May '17

25 May '17

Hi, I like how tor-wiki-changes ML emails have a specific From: so you can directly see who made a change without looking into the body of the email. It would be great if we could get the same level of information into the From of tor-bugs ML emails. (currently it is a static string "Tor Bug Tracker & Wiki") regards, nusenu -- https://mastodon.social/@nusenu https://twitter.com/nusenu_

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onionoo.tpo hardly reachable
by nusenu 24 May '17

24 May '17

Hi Karsten, onionoo is hardly reachable since about 17 hours ago. Is this only externally facing or will this also cause onionoo to miss descriptors internally? Any ETA on when this will improve? thanks, nusenu -- https://mastodon.social/@nusenu https://twitter.com/nusenu_

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txtorcon 0.19.3
by meejah 24 May '17

24 May '17

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Fixing another regression, 0.19.3 causes txtorcon to (once again) correctly work with SocksPort lines containing unix socket options when used with txtorcon.connect(). Thas bug was introduced in 0.19.0 which reduced the number of GETINFO and GETCONF calls (to help filtering-proxies). You can download the release from PyPI or GitHub (or of course "pip install txtorcon"): https://pypi.python.org/pypi/txtorcon/0.19.3 https://github.com/meejah/txtorcon/releases/tag/v0.19.3 Releases are also available from the hidden service: http://timaq4ygg2iegci7.onion/txtorcon-0.19.3.tar.gz http://timaq4ygg2iegci7.onion/txtorcon-0.19.3.tar.gz.asc You can verify the sha256sum of both by running the following 4 lines in a shell wherever you have the files downloaded: cat <<EOF | sha256sum --check f73396667909a3c7a98f4dd865edf4ed6a2518ee5a935d92e18b8a479ec244fd txtorcon-0.19.3.tar.gz e0901f7743a43ed6389f0de31c1f547d89b32723a5ae9316d6cae9ad8eacdbea txtorcon-0.19.3-py2.py3-none-any.whl EOF thanks, meejah -----BEGIN PGP SIGNATURE----- iQEcBAEBAgAGBQJZJTJVAAoJEMJgKAMSgGmn5woIAIyFRa7NhSNm/zMMJE92jtgA C3UY55q3FF4VycC7K77EkaudHPMqlr9MA3EeGkx4NVu0KAX7lIT4GFCsZTlnOdXm rglZ7FFYh49KoAWSMGCMJ7LlauYUitXptynte3lWcqbvjGJkCqMd6WuXslTooSI2 68gq/aoTbXoHWNg1C6DtBhR0JqvN+Vq7ARHyqc4bDloh1LXAvEUwsbKvDtrpXTbe eRwMlKTKoUXhXjxO37NtoNSxT6HUFNcOFOYcLnWtBMPty9W90xY2wC1LbWwYymbY Pk97OUceSyoHxAGsBYjKCdTXEWB6TO5lmHE3+t48OqiYt4ELAWkJFUBf5d6Mnto= =ZRcE -----END PGP SIGNATURE-----

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Further New Hope Improvements
by bancfc＠openmailbox.org 24 May '17

24 May '17

New paper released a week ago makes further improvements on New Hope, reducing decryption failure rates, ciphertext size and amount of entropy needed. This new version will be submitted as a NIST PQ competition candidate. https://eprint.iacr.org/2017/424

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