tor-dev March 2016

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75 participants
71 discussions

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 … [View More]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 [View Less]

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onion moshing
by David Stainton 25 Sep '16

25 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 … [View More]

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adding smartcard support to Tor
by Razvan Dragomirescu 03 Jun '16

03 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 ( … [View More]

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Request for feedback/victims: cfc-0.0.2
by Yawning Angel 16 May '16

16 May '16

Hello, Thanks for the feedback so far. [ PEOPLE THAT HAVE BIG SCARY ADVERSARIES IN THEIR THREAT MODEL STILL SHOULD NOT USE THIS. ] New version with changes some that add functionality, some code of quality stuff, hence a version bump to 0.0.2, especially since it'll probably be a bit before I can focus on tackling the TODO items. Source: https://git.schwanenlied.me/yawning/cfc XPI: https://people.torproject.org/~yawning/volatile/cfc-20160327/ Major changes: * Properly deregister … [View More]

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Quantum-safe Hybrid handshake for Tor
by Zhenfei Zhang 29 Apr '16

29 Apr '16

Hi list, This is a proposal to use quantum-safe hybrid handshake for Tor communications. Given NSA's recent announcement on moving towards quantum-safe cryptography, it would be nice to have a quantum-safe feature for Tor. The idea of the quantum-safe hybrid handshake is to combine both classical key exchange and a key encapsulation mechanism (KEM) instantiated by a quantum safe encryption algorithm, so that the combination gives both (classical) authentication and quantum safety. In a bit … [View More]more details, the client and the server agrees on a classic pre-master secret, $c$, using the ntor protocol. In parallel, client generates a public/private key pair of the quantum-safe encryption algorithm, and send the public key to the server. The server picks a random string, $q$, encrypts it with the public key and send the ciphertext back to the client. The final secret is the output of KDF(c|q). This proposal defeats the harvest-then-decrypt attack with a minimum impact to the existing ntor protocol. An adversary needs to be able to break the quantum-safe encryption algorithm to learn q. On the other hand, if the quantum-safe encryption algorithm turns out to be not secure, the protocol is still as secure as ntor protocol. In other words, it will at least do no harm to the current security. In addition, this is a modular design that allows us to use any quantum-safe cryptographic primitives. As a proof of concept, we instantiated the protocol with NTRUEncrypt lattice-based crypto. We implemented the the protocol with NTRU parameters that gives 128 bits security. The code is available at https://github.com/NTRUOpenSourceProject/ntru-tor Please find the attachment for the request to change the feature. The proof of the protocol can be found at: https://eprint.iacr.org/2015/287.pdf Some known issue: 1. cell size. As far as we know, all quantum-safe encryption algorithms have large key and/or ciphertext size that exceeds the cell size ~500. So this protocol needs to transmit multiple cells, no matter which quantum-safe encryption algorithm we chose. This is addressed by "Proposal 249: Allow CREATE cells with >505 bytes of handshake data". 2. quantum-safe authentication: there is no quantum-safe authentication in this protocol. We believe that authentication can wait, as future (quantum) adversary cannot come back to present time and break authentication. Hence, we use ntor authentication to keep the proposal compact and simple. It will be a future work after this proposal. Thanks for your time, and happy holidays! Zhenfei Zhang Security Innovation. [View Less]

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putting 'Nuke MyFamily' to vote (#6676)
by nusenu 17 Apr '16

17 Apr '16

One year after my last thread on that topic [0] I'd like to move forward on the question whether tor should have a MyFamily functionality or not, because it seems to be a blocker of prop242 (#5565) and it is clear that the current MyFamily design is not practical for many - given the amount of improper or undeclared families [6]. prop 242 is also relevant in the context of Sibyls because if MyFamily becomes trivially easy for everyone to configure one might even start to use it to tell "good" … [View More]

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iObfs: obfs4proxy on iOS
by Mike Tigas 14 Apr '16

14 Apr '16

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA512 [Cross-posted to guardian-dev and tor-dev.] Hey, friends. I suppose this serves as a project announcement of sorts, and also as a progress report regarding the current status of this thing. iObfs is an effort to build obfs4proxy for iOS and to also build out some techniques for actually making it usable within other Tor-enabled iOS apps. You may have heard me or n8fr8 discuss the idea at the dev meeting a few weeks ago. I'm not in love with … [View More]the name I gave it (it's a placeholder that stuck around), but such is life. The repository is currently hosted at [1]. [1]: https://github.com/mtigas/iObfs The initial sprint is being supported by a small Guardian Project grant through mid-April. The high-level goals are to: 1) successfully cross-compile obfs4proxy for the iOS environment 2) "library-ify" it and link it into a proof-of-concept iOS app 3) figure out how to get it to work without "managed mode" 4) build out a process for maybe putting it into Onion Browser, iCepa[2], and other Tor iOS efforts [2]: https://github.com/iCepa I've got a bit of work regarding #1. I think I'm close to getting the binary running, thanks to the existence of the gomobile tools[3]. Using gomobile, you easily get[4] an "obfs4proxy.app" and (given a properly configured Xcode and iPhone) you can put it on a device. But it crashes right now because (I believe) the iOS SDK requires apps to declare some user interface (even if it's blank). So that's close, but my relative lack of experience with Go and the Go buildchain have made #1/#2 a bit slow-going. [3]: https://golang.org/x/mobile/cmd/gomobile [4]: https://github.com/mtigas/iObfs/blob/master/buildobfs4.sh This week, I also took the time to figure out how PT "managed mode" actually works and now have a good idea of how I'd get obfs4proxy to work in an environment where tor can't fork it as a subprocess. There were some conversations I had during the dev meeting where we thought I'd have to use the control port and manually build circuits or something crazy like that -- but it turns out that the solution is a lot simpler.[5] [5]: https://github.com/mtigas/iObfs/blob/master/notes/obfs4-nonmanaged.md This essentially solves #3, so the big barrier now is cross-compiling, "library-ifying", and linking -- rather than the software implementation bits. Anyway, that's where things are. Progress will surely ramp up a bit over the next few weeks. Comments welcome. Best, Mike Tigas @mtigas | https://mike.tig.as/ | 0xA993E7156E0E9923 -----BEGIN PGP SIGNATURE----- iQEcBAEBCgAGBQJW9wKpAAoJEGQdTjqn+ftyos4IANK+jKe+mdU3IcDGGWp48QnT 34a8DCk4u17MhGnoiPuRVjyF4R380v6+O23lzDsXpDA3bhorBirn1fdkJpnvTsb1 IiS75Q3falq+TPZ5pYCnijFLE0sj0VphdHcZuoiHnn6n8HlYZ13ih9sfZTUg/4pa nxNGL83H62vpuR2voeLWPgfJi+QRVIJVa9zSOz8xE3nE70sGNhbWpWlTn12UbMvj f3QmclTPNAuLCXGANyEG1QkTu+k6lHWCCOcJh/wQmwgWHgfhu3fr+PEq6WpjFqMl 4Znhn7SvLlE1lADT8FnAbfz9ND1rT+BVmhITEnQ11eXcb0YHJOLPN8Fmua+G/eM= =3Daj -----END PGP SIGNATURE----- [View Less]

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Notes from the prop224 proposal reading group
by George Kadianakis 09 Apr '16

09 Apr '16

Hello, so we had a meeting about the future of "Next Generation Hidden Services" aka prop224. It was a good meeting. We spent most of the time discussing the topics brought up here: https://lists.torproject.org/pipermail/tor-dev/2016-March/010534.html Please read the above mail to get up to speed with the topics of discussion. So these are basically two topics: a) Should we try to maintain backwards compatibility with old intro/rend points? We ended up deciding that it's actually … [View More]worthwhile keeping backwards compatibility if we can help it: Our rationale was that coding the logic for the extra cells that are required to maintain backwards compatibility is probably less painful than coordinating a perfect bug-free incremental deployment of prop224 in such a way that the whole network has upgraded before we enable client support. So our current plan is to bake backwards compat in the code, and also introduce a consensus parameter of "Should we connect to prop224 onion services using legacy IPs/RPs?" which we will switch off when a good percentage of the network has upgraded to support prop224. At that point, we will be able to rip out the backwards compat code as well. As an added point to that, we should implement the backwards compat code in a way that it's not spagghetied with the normal code, so that it's easy to analyze and remove when needed. Furthermore, we should look back in prop224 and see if we can simplify the backwards compat logic even more. FWIW, prop224 does not have backwards compatibility for HSDirs, so we will still have to do careful deployment there, and make sure that a good percentage of HSDirs have upgraded to prop224 before we start using them (maybe a new consensus parameter is needed here). b) In prop224, why do intro points need to know the "intro point encryption key" and also what's the point of UPDATE-KEYS-SUBCMD? Nick told us that the main point of UPDATE-KEYS-SUBCMD is that so hidden services can rotate their intro point encryption key periodically, so that they can reset their replay caches. That's a fair point. The big question here is, is this worth the complexity that MAINT_INTRO and UPDATE-KEYS-SUBCMD add to the protocol logic? Unfortunately we don't know the answer to this yet, because we actually don't have stats on how our replay caches are doing currently. Here are some things we need to look into: i) Speak with facebook or some other big onion hoster, and try to get some information about their replay cache. Is it ready to bust? How much space is it occupying? ii) The above will help us gauge replay caches in terms of normal activity, but we also need to think of how replay caches should act in cases of a DoS. iii) Also think about INTRODUCE1 replay attacks in general, and understand how dangerous they can be, so that we know how robust our replay caches should be. Is it _that_ dangerous if an attacker manages to sneak in a replay attack every once in a while? For example, imagine an attacker who fills up our replay cache on purpose, so that she can sneak in one replay attack when we reset it. If an attacker can send legit INTRODUCE2 cells to fill up our replay cache, what does she need the replays for? What other types of attacker are there? After we learn more about the above, we will understand *how much we care* about onion services being able to rotate intro pint encryption keys on the fly using UPDATE-KEYS-SUBCMD. Here are some possible scenarios here: - We end up deciding that INTRODUCE1 replay attacks are *dangerous* and we need to seriously defend ourselves against them. In this case, having some sort of UPDATE-KEYS-SUBCMD mechanism sort of makes sense, since onion services should be able to rotate keys at will and let their clients know, without them having to fetch a new HS descriptor. - We end up deciding that INTRODUCE1 are a concern but not something we should complicate our design considerably for. In this case, we might want to consider improving the quality of our replay caches, by using more compact designs (= more space for entries), or by using bloom filters or a similar tech. A proposal will need to be made if so. Yawning posted the following relevant paper: https://home.kookmin.ac.kr/~mkyoon/publications/2010TKDE.pdf - We end up deciding it's no big deal if the attacker sneaks in a replay attack every once in a while. In this case, we can just go with the most minimal approach and ditch the UPDATE-KEYS-SUBCMD mechanism for now, and have intro points be oblivious about encryption keys (which further simplifies our cell formats, etc.). If we take this approach, and in the future future we change our minds about the importance of replay attacks, we can still add some sort of UPDATE-KEYS-SUBCMD mechanism using the extensions mechanism of ESTABLISH_INTRO cells. Furthermore on this topic, we discussed some alternate approaches for doing replay detection that would require less heavy replay caches: i) Maybe we can have an increasing counter in the HS descriptor, that clients need to also include in their INTRODUCE1 cell. If an onion service receives an INTRODUCE1 cell with a counter value different than the one in the descriptor, it ignores it. Onion services update the counter and publish a new descriptor when their replay cache is close to getting filled. I believe various subtle engineering details are hidden in this scheme, along with edge cases and race conditions like "What happens if the client fetches a descriptor with counter value N, right before the onion service updates the descriptor with a new counter value?". ii) When an intro point replay cache is almost full, the onion service will make new encryption keys, tear down the old intro circuit and make a new one, and also pass a signed "HS moved to another circuit" message to the intro point. The intro point will forward that message to any clients that ask for the onion service using its old keys, so that clients know the right place to ask. Lots of engineering details hidden in this proposal as well. In the end we decided that Roger might have some useful feedback for us here :] More research is required! Cheers! [View Less]

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Using Let's Encrypt for meek bridges
by David Fifield 09 Apr '16

09 Apr '16

I'm looking for ideas of good ways to handle TLS certificates and their renewal for meek bridges. I want to use Let's Encrypt for this process, and I hope that someone who knows Let's Encrypt well can contribute some ideas. All three of the meek bridges use HTTPS to receive connections from the CDN, so they need TLS certificates. For example, when you use meek-azure, your traffic is forwarded to the bridge at https://meek.bamsoftware.com/. How it works now is I do the usual domain validation … [View More]

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Adding one more (optional) step to Trac's ticket work-flow
by Isabela 06 Apr '16

06 Apr '16

Hello tor-dev! tl;dr; we want to add one more step to Trac ticket work-flow called 'merge_ready' - if you don't want to use it you don't need to During Tor's Dev Meeting in Valencia, the Network Team discussed a code review process[1] for the team and we concluded that for us a two stage review process would make things much better for the team. In order to organize that on our ticket work-flow on Trac, we decided to add the step 'merge_ready'[2] that would come after either 'needs_review' … [View More]

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tor-dev March 2016