May 2016 - tor-dev - lists.torproject.org

Proposal 247 (Hidden Service Vanguards) Overhaul
by Mike Perry 10 Jun '17

10 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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onionoo.tpo stuck at 2016-05-13 12:00
by nusenu 29 Jan '17

29 Jan '17

Hi Karsten, I was surprised that ornetradar did not send a single email for yesterday's new relays. After looking into it, it turned out it is an onionoo problem. "relays_published":"2016-05-13 12:00:00" https://onionoo.torproject.org/details?limit=4 regards, nusenu

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[Proposal] A simple way to make Tor-Browser-Bundle more portable and secure
by Daniel Simon 31 Oct '16

31 Oct '16

Hello. How it's currently done - The Tor Browser Bundle is dynamically linked against glibc. Security problem - The Tor Browser Bundle has the risk of information about the host system's library ecosystem leaking out onto the network. Portability problem - The Tor Browser Bundle can't be run on systems that don't use glibc, making it unusable due to different syscalls. Solution proposed - Static link the Tor Browser Bundle with musl libc.[1] It is a simple and fast libc implementation that was especially crafted for static linking. This would solve both security and portability issues. What is Tor developers' opinion about this? I personally don't see any drawbacks and would be interested in discussing this further. Sincerely, Daniel [1] https://www.musl-libc.org/

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Re: [tor-dev] Onioncat and Prop224
by str4d 09 Oct '16

09 Oct '16

On 27/04/16 22:31, grarpamp wrote: > On 4/25/16, Tim Wilson-Brown - teor <teor2345(a)gmail.com> wrote: >> >>> On 22 Apr 2016, at 17:03, grarpamp <grarpamp(a)gmail.com> wrote: >>> >>> FYI: The onioncat folks are interested in collaborating >>> with tor folks regarding prop224. >>> >>> https://gitweb.torproject.org/torspec.git/tree/proposals/224-rend-spec-ng.t… >> >> I'm interested in what kind of collaboration onioncat would like to do on >> prop224, next-generation hidden services. >> It would be great to work this out in the next few weeks, as we're coding >> parts of the proposal right now. > > Yep :) And I know Bernhard was hoping to get in touch with Roger > on this before long. > > Basically, prop224 HS being wider than 80 bits will break onioncat's > current HS onion <---> IPv6 addressing mechanism. > > They're looking at various backward compatibility options, as well > as possibly making side use of the HSDir DHT, or even integrating > more directly with the tor client. > Just FYI, I recently migrated all of I2P's spec proposals to the website, and came across a seven-year-old proposal that Bernhard wrote about improving I2P support in GarliCat: https://geti2p.net/spec/proposals/105-garlicat-name-translation I don't know how well it has aged, but given that Tor is now facing the same issues that I2P has, perhaps it can be of some use if resurrected from the dead :) > >> But the tor-onions mailing list is to discuss the technical details running >> onion services. > > Readers of tor-onions / newbies may have been unfamiliar with > onioncat. It's a way to get non-TCP between TorHS onions, thus in > the thread "Hidden datagram service". > > I think there are a nontrivial number of users interested in, and > using, non-strictly-TCP transport over an IPv6 tunnel interface. > For example, look at users of CJDNS... > > For which we should try to continue a way, in v2, to do that over > anonymous overlay network Tor / I2P. > There is already some work on doing this in I2P: https://github.com/majestrate/i2p-tools/tree/master/i2tun https://github.com/majestrate/i2p-tools/tree/master/pyi2tun I2P also natively supports non-TCP protocols if that helps (only datagrams implemented thus far). str4d

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onion moshing
by David Stainton 26 Sep '16

26 Sep '16

I was inspired by onioncat to write a twisted python implementation. Onionvpn doesn't have as many features as onioncat. I've successfully tested that onionvpn and onioncat can talk to each other and play nice. Both onionvpn and onioncat implement a virtual public network. Anyone can send packets to you if they know your onion address or ipv6 address... however injection attacks are unlikely since the attacker cannot know the contents of your traffic without compromising the tor process managing the onion service. I've also tested with mosh; that is, you can use mosh which only works with ipv4 over an ipv4-to-ipv6 tunnel over onionvpn/onioncat. Like this: mosh-client -> udp/ipv4 -> ipv6 -> tun device -> tcp-to-tor -> onion service decodes ipv6 to tun device -> ipv6 -> udp/ipv4 -> mosh-server https://github.com/david415/onionvpn If an onionvpn/onioncat operator were to NAT the onion ipv6 traffic to the Internet then that host essentially becomes a special IPv6 exit node for the tor network. The same can be done for IPv4. Obviously operating such an exit node might be risky due to the potential for abuse... however don't you just love the idea of being about to use low-level network scanners over tor? I wonder if Open Observatory of Network Interference would be interested in this. david

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Tor with collective signatures
by Nicolas Gailly 22 Jul '16

22 Jul '16

Hi, First, thanks a lot both of you for your in-depth comments, I know it takes time, but they've been very helpful! Then I mean to ask you about what you said regarding the fallback directory keys. So keys are not embedded but only hashes, OK. As you pointed out, CoSi would need to download full descriptors in order to safely get the private keys of all the witnesses. This solution would need to clearly be defined as you said in order to be correct. However, there exists another solution and I'd like to know if it looks reasonable in the context of Tor . => Considering the two roles of Fallback directory mirrors are different (serving the consensus document to Tor / collectively signing on it), we thought it would make sense to create new keys for this role and embed those into the Tor source code. These keys would have to be ed25519 keys (same size as the ID), so the size overhead is minimal (32 bytes * 100 = 3.2KB vs 1.5MB). The only reason I can think of why Tor includes hashes instead of public keys is because of local storage: How does Tor consider an increase of storage need by 3.2KB ? Is it acceptable or not ? That would : * Bring down the need for exchanging the descriptors between the fallbacks / no more bandwidth / * No complex scenario such as "fallback key download mechanism" * Simplify the verification on the client side also (no need to get the keys if you already have them). * Add an additional steps for the relays operators where they would need to create that key and inform the Tor people about their public key. Maybe you thought of this solution and there's a reason why you did not mention it, or maybe you did not, then what do you think of it ? Cheers, Nicolas

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Revisiting prop224 time periods and HS descriptor upload/downloads
by George Kadianakis 24 Jun '16

24 Jun '16

Hello, during March we discussed the cell formats of prop224: https://lists.torproject.org/pipermail/tor-dev/2016-March/010534.html The prop224 topic for this month has to do with the way descriptors get uploaded and downloaded, how this is scheduled using time periods and how the shared randomness subsystem interacts with all that. Here are some discussion topics. Lots of text on the first two, less text on the rest: - My main goal was to understand the prop224 sections [TIME-PERIODS] and [TIME-OVERLAP]. Those sections specify a system where hidden services decide in a probabilistic manner _when_ to publish their descriptor so that not all hidden services publish their descriptors at the same moment and cause a thundering herd that stampedes the network. For this to work, time is split into time periods of k hours each. A few hours before each time period, there is an overlap period where hidden services start publishing their _next_ descriptors to HSDirs, so that when the upcoming time period starts, all the HSDirs have already received the descriptors and are ready to serve them. Consider the overlap period at the end of time period #N. During that overlap period, hidden services publish their descriptors for future time period #N+1. In this case, hidden services also need to know the shared random value that will be active during time period #N+1, since it needs to be used to find the responsible HSDirs. This means, that the shared random value for time period #N+1 needs to be published _before_ the overlap period starts. This is not the case in current proposal 224, since time is split into time periods of 25 hours, which means that each day the start time shifts by one hour forward. Since the start/end times of the time periods keep on shifting, there will be cases where the right shared random value will not be accessible when the overlap period starts. So what to do? To fix this, I suggest we change the time period length to a day (24 hours). I also suggest we start time periods every day at 12:00 and finish after 24 hours same time, so that it works well with the current shared randomness schedule (where the new shared random value gets published at 00:00 every day). [It might actually be wiser to actually reverse those schedules: create the SRV at 12:00 and start the time period at 00:00] In any case, this is how this might look like: +------------------------------------------------------------------+ | | | 00:00 12:00 00:00 12:00 00:00 12:00 | | SRV#1 TP#1 SRV#2 TP#2 SRV#3 TP#3 | | | | $ |-----------$-----======|-----------$-----======| | | overlap12 overlap23 | | | +------------------------------------------------------------------+ Legend: [TP#1 = Time Period #1] [SRV#1 = Shared Random Value #1] So, this basically gives a space of 12 hours between the SRV generation and the start of the next time period. We can then easily fit an overlap period of 6 hours before the next time periods starts. In the above diagram, the "equal sign" segments are the overlap periods. 'overlap12' is the overlap period from TP#1 to TP#2. Do you think that's reasonable? And do you see any problems with changing the time period length from 25 hours to 24 hours? - So now that we have ironed out the time period stuff slightly, let's discuss the behavior that hidden services, clients and HSDirs should inherit. This email is quite long already so I'm going to go with examples, instead of formal specification. However, this stuff needs to go formally in the proposal IMO, so any help in formalizing it would be great. + Hidden Service behavior: Example 1: Our hidden service boots up at 14:00 of TP#1. In this case, we are nowhere close to the overlap period, so the hidden service should just publish its TP#1 descriptor to the HSDir hash ring using SRV#1 (which at that point should be in consensuses as "shared-rand-current-value"). The hidden service might also want to calculate its overlap OFFSET (as specified in [TIME-OVERLAP]) and schedule a time callback for publishing its TP#2 descriptors. Example 2: Our hidden service boots up at 03:00 of TP#1. That's outside of the overlap period again, but this time the hidden service needs to use the SRV from "shared-rand-previous-value" because the SRV was rotated at midnight. Example 3: Our hidden service boots up at 09:00 of TP#1. That's inside the overlap period, so the hidden service should calculate its overlap OFFSET and compare it with the current time. If it has not passed, then we are in the exact same case as Example 2. If the overlap OFFSET _has_ passed, then the hidden service needs to act as Example 2, and _also_ publish its TP#2 descriptors to a second set of HSDirs using SRV#2. I think these are all the cases for the hidden service, but I would like to formalize this in a way that can be written in the spec. Particularly, I'm not sure how to formalize which SRV to pick at a given time point. + Client behavior My current intuition with regards to client behavior is that they should always fetch descriptors from the HSDirs of the _current_ time period. They should not concern themselves with the overlap stuff _at all_. The overlap system is there so that by the time the new time period starts, all the HSDirs have received the descriptors and are ready to help the clients. Clients should never notice the overlap stuff happening. For this reason I think we can remove this paragraph from the spec: When a client is looking for a service, it must calculate its key both for the current and for the subsequent period, to decide whether the next period's key is valid yet. What do you think? + HSDir behavior Currently the spec says the following: Hidden service directories should accept descriptors at least [TODO: how much?] minutes before they would become valid, and retain them for at least [TODO: how much?] minutes after the end of the period. After discussion with David, we thought of chopping off the first part of that paragraph and not imposing any such weak restrictions for accepting descriptors (see #18332). We still have not decided about the second part of that paragraph, that is how long descriptors should be retained after the end of the period. We currently think clock skew is the only thing that can bring clients to the wrong HSDir after the end of the period. Maybe an hour is OK? David suggested 12 hours. The current Tor is doing 48 hours... Any ideas? And this half-assedly sums up the behaviors of clients/HSes and HSDirs with regards to descriptor uploads and downloads. What is missing, and do you agree that parts of this should be in the proposal? - We should revert the torspec commit: "prop224: avoid replicas with the same blinded key" https://gitweb.torproject.org/torspec.git/commit/?id=8df8c0584392240aa8fecb… It adds a whole lot of complexity to prop224 with no clear security benefit against realistic adversaries. Furthermore, the time period and descriptor download/upload logic of Tor gets very complicated with it. I discussed this with teor and special and found it reasonable. - The randomized revision-counter logic should also be simplified or even removed: https://gitweb.torproject.org/torspec.git/commit/?id=01119bf1291a40aa309dfb… I haven't looked much into this yet. If someone has thoughts please let me know. - We should use fresh salt every time we rebuild the descriptor, but not for every replica: https://gitweb.torproject.org/torspec.git/commit/?id=01e865d592ffcbb67a0e66… - teor says we should revert the double hashing here, and just use tor's random API: https://gitweb.torproject.org/torspec.git/commit/?id=93f47f4f4e7614d4b3debf… peace

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GSoC: Tails Server
by segfault 15 Jun '16

15 Jun '16

Hi everyone, I'm a Tails contributor since a few months and I'm excited that I will work on the Tails Server [1] project during this year's Google Summer of Code. This project aims at providing a user-friendly interface to start onion services in Tails. My mentors are anonym (of Tails) and asn. I will start with this project four weeks early, which means I start next Monday (April 25th) and finish at the end of July. Most of the discussion on this project will happen on the appropiate Tails communication channels, i.e. the mailing lists tails-dev(a)boum.org and tails-ux(a)boum.org and the tails-dev channels on irc.oftc.net and conference.riseup.net. Regarding Tor specific questions, I will write to this list or ask in the tor channels on irc.oftc.net. I'm segfault1 on OFTC. I will send bi-weekly status reports to this list and tails-dev(a)boum.org. Cheers! segfault [1] https://tails.boum.org/blueprint/tails_server/

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DescripTor 1.2.0 is released
by Karsten Loesing 06 Jun '16

06 Jun '16

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 Hello devs, I just released DescripTor 1.2.0: https://dist.torproject.org/descriptor/1.2.0/ - From the change log: # Changes in version 1.2.0 - 2016-05-31 * Medium changes - Include the hostname in directory source entries of consensuses and votes. - Also accept \r\n as newline in Torperf results files. - Make unrecognized keys of Torperf results available together with the corresponding values, rather than just the whole line. - In Torperf results, recognize all percentiles of expected bytes read for 0 <= x <= 100 rather than just x = { 10, 20, ..., 90 }. - Rename properties for overriding default descriptor source implementation classes. - Actually return the signing key digest in network status votes. - Parse crypto parts in network status votes. - Document all public parts in org.torproject.descriptor and add an Ant target to generate Javadocs. * Minor changes - Include a Torperf results line with more than one unrecognized key only once in the unrecognized lines. - Make "consensus-methods" line optional in network statuses votes, which would mean that only method 1 is supported. - Stop reporting "-----END .*-----" lines in directory key certificates as unrecognized. - Add code used for benchmarking. In particular the full rewrite of Javadocs was painful but hopefully useful to people here, not necessarily just DescripTor users but anyone working with Tor network data. Here's the compiled web version until DescripTor has its own website: https://people.torproject.org/~karsten/volatile/descriptor-docs-2016-05-31/ Many thanks to iwakeh for helping with most of these changes! All the best, Karsten -----BEGIN PGP SIGNATURE----- Comment: GPGTools - http://gpgtools.org iQEcBAEBAgAGBQJXTeNUAAoJEC3ESO/4X7XB208IAKwyJF2jgjvuREfYKkI9UFva ZoMbXgITXfTNJ4hXSc5x30jMw56xVummaWMMgBKwscPyhJAdngUjxQt//8ZOwFx/ hezHbRGxxRZquiROvIMW1mLIfnvSnkZAVL6tPuQmiKfqcR2ExMs3KCZsdCcfI/KR ZB9tHnGsSqhME+XPxQNAhT/OgBNnaq4Y7WFMhLuOHDm4/sCIjeoeix8aF1ve27ue FdOvaxjY1iBipxNdKkup5SXmL1tBmQ7bwTV59EduLatq+30tMnE7Xyat9MeQMyGI Bs9/5h+f29zREzyPp6kPd/m0eN1udvXF8nqa34QqAk0YHECE1BoIRDRo7qkixx4= =xs8m -----END PGP SIGNATURE-----

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adding smartcard support to Tor
by Razvan Dragomirescu 04 Jun '16

04 Jun '16

Hello, I am not sure if this has been discussed before or how hard it would be to implement, but I'm looking for a way to integrate a smartcard with Tor - essentially, I want to be able to host hidden service keys on the card. I'm trying to bind the hidden service to a hardware component (the smartcard) so that it can be securely hosted in a hostile environment as well as impossible to clone/move without physical access to the smartcard. I have Tor running on the USBArmory by InversePath ( http://inversepath.com/usbarmory.html ) and have a microSD form factor card made by Swissbit ( www.swissbit.com/products/security-products/overwiev/security-products-over… ) up and running on it. I am a JavaCard developer myself and I have developed embedded Linux firmwares before but I have never touched the Tor source. Is there anyone that is willing to take on a side project doing this? Would it be just a matter of configuring OpenSSL to use the card (I haven't tried that yet)? Thank you, Razvan -- Razvan Dragomirescu Chief Technology Officer Cayenne Graphics SRL

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