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Arti development report: July 7 through July 21
by Nick Mathewson 21 Jul '21

21 Jul '21

# Arti Report 2: July 7 through July 21 It's been a quiet couple of weeks. Most of the team has been on vacation or leave for the last two weeks, so the Arti hacking has been down to me (nick) for this period. Our last report went out on the same day that our [announcement] went public, and since then we've had an influx of attention and volunteers interest. Thanks to everybody who's been trying out the code! Special thanks to `trinity-1686a` for a [stream-isolation] implementation, `lyuyuan` for work on [RESOLVE and RESOLVE_PTR][resolve], and everybody else who's been submitting patches! ## Circuit timeout inference My own work for this period has focused on [circuit timeout inference][cti]. (I decided to start with this because it's part of the Tor protocol that I've worked with the least in the past.) The approach is kind of complicated: in order to adjust our timeout, we need to observe how long circuits actually take to complete... including the circuits that we don't use because they take longer than our current timeout timeout. So we have to keep building circuits for a while even after they time out, so that we can observe how long it takes them to finish. Then we use an estimator based on a Pareto distribution, and use that to determine our actual cutoffs. The core logic is now [implemented in Arti][pareto-code], and we just have a few more pieces ([persistence][ticket-143], [directory notification][ticket-144], and [testing circuits][ticket-145]) to cover before we can call it done. ## Open problems Our timeout tests rely on using a simulated view of time. (Nobody wants to wait 10 seconds for a simulated 10-second timeout!) For that, we have [testing code][rt-mock-time] to override the asynchronous runtime's view of the passage of time. But we have a bug somewhere that makes these tests unreliable. The [WaitFor] future, which is supposed to run a another future to completion while advancing the current (simulated) time, doesn't seem to be reliable. As implemented, it advances the current time even when the future it's testing _isn't_ blocked on time, and sometimes advances time much too far while the tested future makes no progress. I've kludged it up by making it yield a lot and advance time in tiny increments, but that's just a workaround. Does anybody like [debugging Futures code][ticket-149]? ## Next steps In the next two weeks I hope we can wrap up the outstanding issues above in circuit timeouts, including the ones that require some architectural decisions. I'm also hoping that we can make progress on [guards]: that's the last remaining really big security feature we have to do for our 0.0.1 milestone. Once that's done, we can move on to polishing and infrastructure issues for the rest of the time we have for this milestone. Finally, we're going to be expanding our team! Please have a look at our [job-opening], and share it with anybody you know who is interested in Rust and Tor, and who might be interested in a job. Next update in two weeks! [announcement]: https://blog.torproject.org/announcing-arti [cti]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/path-spec.txt#… [cti]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/guard-spec.txt [job-opening]: https://www.torproject.org/about/jobs/rust-dev/ [pareto-code]: https://gitlab.torproject.org/tpo/core/arti/-/blob/main/tor-circmgr/src/tim… [resolve]: https://gitlab.torproject.org/tpo/core/arti/-/merge_requests/39 [rt-mock-time]: https://gitlab.torproject.org/tpo/core/arti/-/blob/main/tor-rtmock/src/time… [stream-isolation]: https://gitlab.torproject.org/tpo/core/arti/-/merge_requests/38 [ticket-143]: https://gitlab.torproject.org/tpo/core/arti/-/issues/143 [ticket-144]: https://gitlab.torproject.org/tpo/core/arti/-/issues/144 [ticket-145]: https://gitlab.torproject.org/tpo/core/arti/-/issues/145 [ticket-149]: https://gitlab.torproject.org/tpo/core/arti/-/issues/149 [WaitFor]: https://gitlab.torproject.org/tpo/core/arti/-/blob/main/tor-rtmock/src/slee…

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Weaving a Faster Tor: paper/video of possible interest
by Ian Goldberg 20 Jul '21

20 Jul '21

Hello tor-dev, Steve Engler (currently part of the Shadow team) and I have a paper of possible interest to appear at ARES 2021 next month. It's a design of a multi-threaded relay architecture, of possible particular interest when considering relay support in arti, for example (though the proof-of-concept implementation is based on the usual C codebase). If you're interested, here are previews of: Paper: https://cs.uwaterloo.ca/~iang/pubs/mttor-ares21.pdf Video: https://www.youtube.com/watch?v=41a6nLUJye8 Code: https://git-crysp.uwaterloo.ca/sengler/tor-parallel-relay-conn https://git-crysp.uwaterloo.ca/sengler/relay-throughput-testing -- Ian Goldberg Canada Research Chair in Privacy Enhancing Technologies Professor, Cheriton School of Computer Science University of Waterloo

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GSoC 2021 - Alexa Top Sites Captcha and Tor Block Monitoring #Update
by Apratim Ranjan Chakrabarty 20 Jul '21

20 Jul '21

Hi everyone! Been quite sometime since the last update but if one wants to see the details in between one could go to the DIAL blogs for the project[1]. As of now, we do have a working project with the following details implemented [2] and further the dotted[consensus module], idea taken from Senser paper[3] haven't been implemented yet, but hopefully I'll implement it within a week or two at most. I personally was tilted towards the similarity of the structure but after some discussions with woswos and Micah Sherr[4], I've thought of implementing the content based approach too. I'll briefly describe both the methods below: + Structure of the website: This was thought of because we don't really know what various changes would be there for a website. More specifically would be useful for dynamic websites, websites with language based on geolocation (Geotargeting). But I have to use a filter list and statistical method to approach the problem. + Content based Approach: Compares the content of the HTML data using tree like structure and hashes to know how the structure is different or similar. Usage of proxies of the same locations as vantage points to get better results. That said, the above mentioned methods are used for the case where websites partially block tor. One good example for this case would be https://dan.me.uk/ which doesn't block tor exit relay nodes completely, but gives an error page (partial block) and no error HTTP response code. The checking of the HTTP response codes being a low-hanging-fruitish algorithm is our first step which is seen performing good and might sometimes result in false positives (Says a website like https://cloudflare.com to be blocked completely, when it returns captcha or is partially blocked). Further for the demo purpose, one can refer to the Experimental code[5] and it's log[6] (Isn't much of a good code and is a bit old but wrote to serve the purpose of backing up the first method (Structure of the website)). Also one could look into the `Analyzer.py`[7,8] which would contain the most recent and improved logic to the analysis. Hope to improve it with every passing day. I also plan to create a FAQ[9] page which would have excerpts of discussions or answers to as why a following approach was taken. Thanks, Apratim (irc: _ranchak_) ** Looking forward for suggestions and comments as to how to improve on it. Also materials like research paper in this domain would be helpful ** References: [1] https://hub.osc.dial.community/t/tor-project-alexa-top-sites-captcha-and-bl… [2] https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/wikis/GSoC-2021#upda… [3] http://people.cs.georgetown.edu/~wzhou/publication/senser-acsac13.pdf [4] https://seclab.cs.georgetown.edu/msherr/ [5] https://github.com/Hackhard/Fetcher/blob/b9f2fa8d09061862cf954537cbaad7921d… [6] https://raw.githubusercontent.com/Hackhard/Fetcher/main/status%20code/test_… [7] Consensus_lite branch: https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/blob/consensus_lite/… [8] Master branch: https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/blob/master/src/capt… [9] https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/wikis/GSoC-2021/Faqs

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Mostly Automatic Censorship Circumvention in Tor Browser
by Richard Pospesel 08 Jul '21

08 Jul '21

Hi Everyone, As part of our Sponsor 30 work, we are looking to improve the new about:torconnect experience by adding automatic tor settings configuration for censorship circumvention. This document outlines and discusses the *technical* challenges associated with this work, and does not go into any great detail on the right UX would be (in terms of easy of use, user trust, etc). Anyway, if you see any pitfalls or problems with anything here, do let us know. ------------------------------8<-------------------------------------- # Mostly Automatic Bridge Configuration to Bypass Internet Censorship Our goal for this work is to enable Tor Browser users to access tor without having to navigate to about:preferences#tor to configure bridges. Technically speaking, this is a trivial problem assuming you know: - which bridge settings work at the user's location - the location of the user ## Circumvention Settings Map For now, it seems sufficient to maintain a map of countries to some data-structure containing information about which censorship circumvention techniques work and which ones do not. A proposed example format can be found here: - https://gitlab.torproject.org/tpo/anti-censorship/state-of-censorship/-/blo… This map would at be distributed and updated through tor-browser releases. ### Problems #### Censorship Changes Invalidate the Map The obvious problem with distributing the censorship-circumvention settings map with Tor Browser is that if the techniques used in a location change such that old settings no longer work, you will be left with a non-functional Tor Browser with no way to update it apart from acquiring a fresh install with the updated settings or by manually configuring Tor Browser's bridge settings (so what users have to do now) A fix for this would be to provide a rules update mechanism whereby updated rules could be fetched outside of tor (via the clearnet, or over moat). Special care would need to be taken to ensure the rule updates from this automatic mechanism actually came from the Tor Project (via some sort of signature verification scheme, for example). Another wrinkle here is that rules would also need to be distributed somewhere that is difficult to censor. It seems likely that we may need different locations and mechanisms for acquiring the rule-set based on the user's location. Whatever the mechanism, updates should happen at least before the user attempts to auto-configure. Otherwise, perhaps we should periodically auto-update the the settings at a reasonable cadence. #### Time Investment to Update Map Another problem with solely distributing the rules through Tor Browser, is that censorship events would now require a Tor Browser release just to push new rules out to people. Publishing new Tor Browser releases is not a simple task, and enabling adversaries to force Tor Browser releases by tweaking their censorship systems seems like a cute way to DDOS the Applications team. An alternate update channel is definitely necessary outside of periodic Tor Browser releases. #### Are Per-Country Entries Granular Enough? One could imagine highly localized censorship events occurring which require special settings that are not needed in the rest of the country. For instance, if there is a clearnet blackout in Minneapolis, would we want to pipe *all* of our US users through the same bridges? Seems like a potential scalability problem for countries with large populations. ## Determining User Location A user's location can be determined by accessing location services through the clearnet. Mozilla offers a such a service ( https://location.services.mozilla.com/ ) with a very simple HTTP interface. Prior to bootstrapping, Tor Browser can access the location service by temporarily enabling network DNS: - network.dns.disabled=false and making an exception for the location service URL to bypass the proxy by: - network.proxy.no_proxies_on="location.services.mozilla.com" The location service would send back a country code in a JSON object which we can use to look up appropriate bridge settings in our map described above. ### Problems So the functionality of this approach is pretty easy to implement: tweak some prefs, make an XMLHttpRequest, change the prefs back. One possible problem we may face is if censors start blocking Mozilla's location services. Maybe we should have a pool of location service providers to make this more difficult (though we would need to do the research and figure out how feasible this is from a cost perspective). It is also possible to add location service functionality to moat, though this would also be a bit of an engineering endeavor. If we move forward with Mozilla's location services, we will need to acquire an API key, but I would not expect this to be an issue. We will also need to make arrangements with them to surpass the current limit of 100,000 daily API requests ( see: https://location.services.mozilla.com/terms ) The big challenge here is engineering the right UX which maintains our users trust. I think we need to be very explicit with this convenience feature, and definitely not just have it silently happen in the background. Users should also be able to opt-out, and manually select their country for the purposes of getting the right settings out of the above mentioned map. It should be very difficult to accidentally enable this automatic lookup. This will likely require a fair bit of iteration on the about:torconnect page design and flow.

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Arti development report: 23 June through July 7
by Nick Mathewson 08 Jul '21

08 Jul '21

# Arti Report 1: 23 June through July 7 We started with a project kickoff meeting. We're planning to continue meeting every 2 weeks, making our meetings increasingly public, and posting them on youtube. We wrote up an announcement to explain what Arti is and why we're doing it. The [announcement](https://blog.torproject.org/announcing-arti) goes live on July 8; it's a good place to start if you're not already familiar with the project, or if you're looking for more helpful links. To get ready for future development, and check the state of our packages we put out an [0.0.0 release](https://crates.io/crates/arti) and uploaded it to crates.io. We found a couple of issues while doing so ([#129][ticket-129], [#130][ticket-130]). We want to switch as much new-feature development to Arti as we can. With that in mind, we're developing a set of guidelines for discouraging features in C -- especially features with the potential to destabilize our code. (See [#128][ticket-128].) We're trying a bottom-up approach here, starting by identifying what kinds of features we've done recently that maybe we should skip in the future. We had an excellent meeting with developers on various Zcash projects, to gather requirements, set expectations, and open lines of communication. Based on their suggestions, we opened a set of new tickets to work on. ([#132][ticket-132] [#133][ticket-133] [#134][ticket-134] [#135][ticket-135] [#136][ticket-136] [#137][ticket-137] [#138][ticket-138] [#139][ticket-139] [#140][ticket-140]) In terms of new code written, things have been a bit slow: we're still ramping up, and a lot of folks take vacations in the summer. We [revised our asynchronous runtime backend][ticket-129] based on issues we found while uploading our packages. More recently, I've started working on an [implementation][ticket-57] of Tor's [circuit timeout inference][cti] code, which is necessary for good performance and security. ## Next steps In our next couple of weeks, I hope we get a first cut at our circuit timeout code finished and working, and figure out a reasonable initial solution for our [persistence needs][ticket-59]. We'll also be ramping up on implementations for [guard nodes][ticket-58] and [stream isolation][ticket-73], both of which are necessary for security. [ticket-57]: https://gitlab.torproject.org/tpo/core/arti/-/issues/57 [ticket-58]: https://gitlab.torproject.org/tpo/core/arti/-/issues/58 [ticket-59]: https://gitlab.torproject.org/tpo/core/arti/-/issues/59 [ticket-73]: https://gitlab.torproject.org/tpo/core/arti/-/issues/73 [ticket-128]: https://gitlab.torproject.org/tpo/core/arti/-/issues/128 [ticket-129]: https://gitlab.torproject.org/tpo/core/arti/-/issues/129 [ticket-130]: https://gitlab.torproject.org/tpo/core/arti/-/issues/130 [ticket-132]: https://gitlab.torproject.org/tpo/core/arti/-/issues/132 [ticket-133]: https://gitlab.torproject.org/tpo/core/arti/-/issues/133 [ticket-134]: https://gitlab.torproject.org/tpo/core/arti/-/issues/134 [ticket-135]: https://gitlab.torproject.org/tpo/core/arti/-/issues/135 [ticket-136]: https://gitlab.torproject.org/tpo/core/arti/-/issues/136 [ticket-137]: https://gitlab.torproject.org/tpo/core/arti/-/issues/137 [ticket-138]: https://gitlab.torproject.org/tpo/core/arti/-/issues/138 [ticket-139]: https://gitlab.torproject.org/tpo/core/arti/-/issues/139 [ticket-140]: https://gitlab.torproject.org/tpo/core/arti/-/issues/140 [cti]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/path-spec.txt#…

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Build HS Circuit
by David Willis 27 Jun '21

27 Jun '21

Hi I'm doing a research project into onions and I'm trying to add a control port command to build a HS circuit. I see EXTENDCIRC can create normal circs, but how would I modify it a to build a HS circuit? Assuming I've already HSFETCHed it. I don't wait to initiate a socks request, just build the circuit and wait. It isn't quite clear to me where in the code base onion circuits are built or how, and how to initiate one. Would be grateful even for high-level instructions. Thanks David University of Singapore Sent with [ProtonMail](https://protonmail.com/) Secure Email.

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[RFC] Proposal: "Res tokens: Anonymous Credentials for Onion Service DoS Resilience"
by George Kadianakis 13 Jun '21

13 Jun '21

Hello all, after lots of investigation on anonymous credentials, we are glad to present you with a draft of the onion services anti-DoS proposal using tokens. While the basic idea of the proposal should remain reasonably solid, there are various XXX sprinkled around the proposal and some of them definitely need to be addressed before the proposal becomes truly usable. We are particularly looking forward to feedback about: - Token issuance services - The anonymous credential scheme chosen - The XXXs and design decisions of the proposal Hope you have a pleasant read! --- ``` Filename: 331-res-tokens-for-anti-dos.md Title: Res tokens: Anonymous Credentials for Onion Service DoS Resilience Author: George Kadianakis, Mike Perry Created: 11-02-2021 Status: Draft ``` +--------------+ +------------------+ | Token Issuer | | Onion Service | +--------------+ +------------------+ ^ ^ | +----------+ | Issuance | 1. | | 2. | Redemption +------->| Alice |<-------+ | | +----------+ # 0. Introduction This proposal specifies a simple anonymous credential scheme based on Blind RSA signatures designed to fight DoS abuse against onion services. We call the scheme "Res tokens". Res tokens are issued by third-party issuance services, and are verified by onion services during the introduction protocol (through the INTRODUCE1 cell). While Res tokens are used for denial of service protection in this proposal, we demonstrate how they can have application in other Tor areas as well, like improving the IP reputation of Tor exit nodes. # 1. Motivation Denial of service attacks against onion services have been explored in the past and various defenses have been proposed: - Tor proposal #305 specifies network-level rate-limiting mechanisms. - Onionbalance allows operators to scale their onions horizontally. - Tor proposal #327 increases the attacker's computational requirements (not implemented yet). While the above proposals in tandem should provide reasonable protection against many DoS attackers, they fundamentally work by reducing the assymetry between the onion service and the attacker. This won't work if the attacker is extremely powerful because the assymetry is already huge and cutting it down does not help. we believe that a proposal based on cryptographic guarantees -- like Res tokens -- can offer protection against even extremely strong attackers. # 2. Overview In this proposal we introduce an anonymous credential scheme -- Res tokens -- that is well fitted for protecting onion services against DoS attacks. We also introduce a system where clients can acquire such anonymous credentials from various types of Token Issuers and then redeem them at the onion service to gain access even when under DoS conditions. In section [TOKEN_DESIGN], we list our requirements from an anonymous credential scheme and provide a high-level overview of how the Res token scheme works. In section [PROTOCOL_SPEC], we specify the token issuance and redemption protocols, as well as the mathematical operations that need to be conducted for these to work. In section [TOKEN_ISSUERS], we provide a few examples and guidelines for various token issuer services that could exist. In section [DISCUSSION], we provide more use cases for Res tokens as well as future improvements we can conduct to the scheme. # 3. Design [TOKEN_DESIGN] In this section we will go over the high-level design of the system, and on the next section we will delve into the lower-level details of the protocol. ## 3.1. Anonymous credentials Anonymous credentials or tokens are cryptographic identifiers that allow their bearer to maintain an identity while also preserving anonymity. Clients can acquire a token in a variety of ways (e.g. registering on a third-party service, solving a CAPTCHA, completing a PoW puzzle) and then redeem it at the onion service proving this way that work was done, but without linking the act of token acquisition with the act of token redemption. ## 3.2. Anonymous credential properties The anonymous credential literature is vast and there are dozens of credential schemes with different properties [REF_TOKEN_ZOO], in this section we detail the properties we care about for this use case: - Public Verifiability: Because of the distributed trust properties of the Tor network, we need anonymous credentials that can be issued by one party (the token issuer) and verified by a different party (in this case the onion service). - Perfect unlinkability: Unlinkability between token issuance and token redemption is vital in private settings like Tor. For this reason we want our scheme to preserve its unlinkability even if its fundamental security assumption is broken. We want unlinkability to be protected by information theoretic security or random oracle, and not just computational security. - Small token size: The tokens will be transfered to the service through the INTRODUCE1 cell which is not flexible and has only a limited amount of space (about 200 bytes) [REF_INTRO_SPACE]. We need tokens to be small. - Quick Verification: Onions are already experiencing resource starvation because of the DoS attacks so it's important that the process of verifying a token should be as quick as possible. In section [TOKEN_PERF] we will go deeper into this requirement. After careful consideration of the above requirements, we have leaned towards using Blind RSA as the primitive for our tokens, since it's the fastest scheme by far that also allows public verifiability. See also Appendix B [BLIND_RSA_PROOF] for a security proof sketch of Blind RSA perfect unlinkability. ## 3.3. Other security considerations Apart from the above properties we also want: - Double spending protection: We don't want Malory to be able to double spend her tokens in various onion services thereby amplifying her attack. For this reason our tokens are not global, and can only be redeemed at a specific destination onion service. - Metadata: We want to encode metadata/attributes in the tokens. In particular, we want to encode the destination onion service and an expiration date. For more information see section [DEST_DIGEST]. For blind RSA tokens this is usually done using "partially blind signatures" but to keep it simple we instead encode the destination directly in the message to be blind-signed and the expiration date using a set of rotating signing keys. - One-show: There are anonymous credential schemes with multi-show support where one token can be used multiple times in an unlinkable fashion. However, that might allow an adversary to use a single token to launch a DoS attack, since revocation solutions are complex and inefficient in anonymous credentials. For this reason, in this work we use one-show tokens that can only be redeemed once. That takes care of the revocation problem but it means that a client will have to get more tokens periodically. ## 3.4. Res tokens overview Throughout this proposal we will be using our own token scheme, named "Res", which is based on blind RSA signatures. In this modern cryptographic world, not only we have the audacity of using Chaum's oldest blind signature scheme of all times, but we are also using RSA with a modulus of 1024 bits... The reason that Res uses only 1024-bits RSA is because we care most about small token size and quick verification rather than the unforgeability of the token. This means that if the attacker breaks the issuer's RSA signing key and issues tokens for herself, this will enable the adversary to launch DoS attacks against onion services, but it won't allow her to link users (because of the "perfect unlinkability" property). Furthermore, Res tokens get a short implicit expiration date by having the issuer rapidly rotate issuance keys every few hours. This means that even if an adversary breaks an issuance key, she will be able to forge tokens for just a few hours before that key expires. For more ideas on future schemes and improvements see section [FUTURE_RES]. ## 3.5. Token performance requirements [TOKEN_PERF] As discussed above, verification performance is extremely important in the anti-DoS use case. In this section we provide some concrete numbers on what we are looking for. In proposal #327 [REF_POW_PERF] we measured that the total time spent by the onion service on processing a single INTRODUCE2 cell ranges from 5 msec to 15 msecs with a mean time around 5.29 msec. This time also includes the launch of a rendezvous circuit, but does not include the additional blocking and time it takes to process future cells from the rendezvous point. We also measured that the parsing and validation of INTRODUCE2 cell ("top half") takes around 0.26 msec; that's the lightweight part before the onion service decides to open a rendezvous circuit and do all the path selection and networking. This means that any defenses introduced by this proposal should add minimal overhead to the above "top half" procedure, so as to apply access control in the lightest way possible. For this reason we implemented a basic version of the Res token scheme in Rust and benchmarked the verification and issuance procedure [REF_RES_BENCH]. We measured that the verification procedure from section [RES_VERIFY] takes about 0.104 ms, which we believe is a reasonable verification overhead for the purposes of this proposal. We also measured that the issuance procedure from [RES_ISSUANCE] takes about 0.614 ms. # 4. Specification [PROTOCOL_SPEC] +--------------+ +------------------+ | Token Issuer | | Onion Service | +--------------+ +------------------+ ^ ^ | +----------+ | Issuance | 1. | | 2. | Redemption +------->| Alice |<-------+ | | +----------+ ## 4.0. Notation Let `a || b` be the concatenation of a with b. Let `a^b` denote the exponentiation of a to the bth power. Let `a == b` denote a check for equality between a and b. Let FDH_N(msg) be a Full Domain Hash (FDH) of 'msg' using SHA256 and stretching the digest to be equal to the size of an RSA modulus N. ## 4.1. Token issuer setup The Issuer creates a set of ephemeral RSA-1024 "issuance keys" that will be used during the issuance protocol. Issuers will be rotating these ephemeral keys every 6 hours. The Issuer exposes the set of active issuance public keys through a REST HTTP API that can be accessed by visiting /issuers.keys. Tor directory authorities periodically fetch the issuer's public keys and vote for those keys in the consensus so that they are readily available by clients. The keys in the current consensus are considered active, whereas the ones that have fallen off have expired. XXX how many issuance public keys are active each time? how does overlapping keys work? clients and onions need to know precise expiration date for each key. this needs to be specified and tested for robustness. XXX every how often does the fetch work? how does the voting work? which issuers are considered official? specify consensus method. XXX An alternative approach: Issuer has a long-term ed25519 certification key that creates expiring certificates for the ephemeral issuance keys. Alice shows the certificate to the service to prove that the token comes from an issuer. The consensus includes the long-term certification key of the issuers to establish ground truth. This way we avoid the synchronization between dirauths and issuers, and the multiple overlapping active issuance keys. However, certificates might not fit in the INTRODUCE1 cell (prop220 certs take 104 bytes on their own). Also certificate metadata might create a vector for linkability attacks between the issuer and the verifier. ## 4.2. Onion service signals ongoing DoS attack When an onion service is under DoS attack it adds the following line in the "encrypted" (inner) part of the v3 descriptor as a way to signal to its clients that tokens are required for gaining access: "token-required" SP token-type SP issuer-list NL [At most once] token-type: Is the type of token supported ("res" for this proposal) issuer: A comma separated list of issuers which are supported by this onion service ## 4.3. Token issuance When Alice visits an onion service with an active "token-required" line in its descriptor it checks whether there are any tokens available for this onion service in its token store. If not, it needs to acquire some and hence the token issuance protocol commences. ### 4.3.1. Client preparation [DEST_DIGEST] Alice first chooses an issuer supported by the onion service depending on her preferences by looking at the consensus and her Tor configuration file for the current list of active issuers. After picking a supported issuer, she performs the following preparation before contacting the issuer: 1) Alice extracts the issuer's public key (N,e) from the consensus 2) Alice computes a destination digest as follows: dest_digest = FDH_N(destination || salt) where: - 'destination' is the 32-byte ed25519 public identity key of the destination onion - 'salt' is a random 32-byte value, 3) Alice samples a blinding factor 'r' uniformly at random from [1, N) 4) Alice computes: blinded_message = dest_digest * r^e (mod N) After this phase is completed, Alice has a blinded message that is tailored specifically for the destination onion service. Alice will send the blinded message to the Token Issuer, but because of the blinding the Issuer does not get to learn the dest_digest value. XXX Is the salt needed? Reevaluate. ### 4.3.3. Token Issuance [RES_ISSUANCE] Alice now initiates contact with the Token Issuer and spends the resources required to get issued a token (e.g. solve a CAPTCHA or a PoW, create an account, etc.). After that step is complete, Alice sends the blinded_message to the issuer through a JSON-RPC API. After the Issuer receives the blinded_message it signs it as follows: blinded_signature = blinded_message ^ d (mod N) where: - 'd' is the private RSA exponent. and returns the blinded_signature to Alice. XXX specify API (JSON-RPC? Needs SSL + pubkey pinning.) ### 4.3.4. Unblinding step Alice verifies the received blinded signature, and unblinds it to get the final token as follows: token = blinded_signature * r^{-1} (mod N) = blinded_message ^ d * r^{-1] (mod N) = (dest_digest * r^e) ^d * r^{-1} (mod N) = dest_digest ^ d * r * r^{-1} (mod N) = dest_digest ^ d (mod N) where: - r^{-1} is the multiplicative inverse of the blinding factor 'r' Alice will now use the 'token' to get access to the onion service. By verifying the received signature using the issuer keys in the consensus, Alice ensures that a legitimate token was received and that it has not expired (since the issuer keys are still in the consensus). ## 4.4. Token redemption ### 4.4.1. Alice sends token to onion service Now that Alice has a valid 'token' it can request access to the onion service. It does so by embedding the token into the INTRODUCE1 cell to the onion service. To do so, Alice adds an extension to the encrypted portion of the INTRODUCE1 cell by using the EXTENSIONS field (see [PROCESS_INTRO2] section in rend-spec-v3.txt). The encrypted portion of the INTRODUCE1 cell only gets read by the onion service and is ignored by the introduction point. We propose a new EXT_FIELD_TYPE value: [02] -- ANON_TOKEN The EXT_FIELD content format is: TOKEN_VERSION [1 byte] ISSUER_KEY [4 bytes] DEST_DIGEST [32 bytes] TOKEN [128 bytes] SALT [32 bytes] where: - TOKEN_VERSION is the version of the token ([0x01] for Res tokens) - ISSUER_KEY is the public key of the chosen issuer (truncated to 4 bytes) - DEST_DIGEST is the 'dest_digest' from above - TOKEN is the 'token' from above - SALT is the 32-byte 'salt' added during blinding This will increase the INTRODUCE1 payload size by 199 bytes since the data above is 197 bytes, the extension type and length is 2 extra bytes, and the N_EXTENSIONS field is always present. According to ticket #33650, INTRODUCE1 cells currently have more than 200 bytes available so we should be able to fit the above fields in the cell. XXX maybe we don't need to pass DEST_DIGEST and we can just derive it XXX maybe with a bit of tweaking we can even use a 1536-bit RSA signature here... ### 4.4.2. Onion service verifies token [RES_VERIFY] Upon receiving an INTRODUCE1 cell with the above extension the service verifies the token. It does so as follows: 1) The service checks its double spend protection cache for an element that matches DEST_DIGEST. If one is found, verification fails. 2) The service checks: DEST_DIGEST == FDH_N(service_pubkey || SALT), where 'service_pubkey' is its own long-term identity pubkey. 3) The service finds the corresponding issuer pubkey 'e' based on ISSUER_KEY from the consensus or its configuration file 4) The service checks: TOKEN ^ e == DEST_DIGEST Finally the onion service adds the DEST_DIGEST to its double spend protection cache to avoid the same token getting redeemed twice. Onion services keep a double spend protection cache by maintaining a sorted array of truncated DEST_DIGEST elements. If any of the above steps fail, the verification process aborts and the introduction request gets discarded. If all the above verification steps have been completed successfully, the service knows that this a valid token issued by the token issuer, and that the token has been created for this onion service specifically. The service considers the token valid and the rest of the onion service protocol carries out as normal. # 5. Token issuers [TOKEN_ISSUERS] In this section we go over some example token issuers. While we can have official token issuers that are supported by the Tor directory authorities, it is also possible to have unofficial token issuers between communities that can be embedded directly into the configuration file of the onion service and the client. In general, we consider the design of token issuers to be independent from this proposal so we will touch the topic but not go too deep into it. ## 5.1. CAPTCHA token issuer A use case resembling the setup of Cloudflare's PrivacyPass would be to have a CAPTCHA service that issues tokens after a successful CAPTCHA solution. Tor Project, Inc runs https://ctokens.torproject.org which serves hCaptcha CAPTCHAs. When the user solves a CAPTCHA the server gives back a list of tokens. The amount of tokens rewarded for each solution can be tuned based on abuse level. Clients reach this service via a regular Tor Exit connection, possibly via a dedicated exit enclave-like relay that can only connect to https://ctokens.torproject.org. Upon receiving tokens, Tor Browser delivers them to the Tor client via the control port, which then stores the tokens into a token cache to be used when connecting to onion services. In terms of UX, most of the above procedure can be hidden from the user by having Tor Browser do most of the things under the scenes and only present the CAPTCHA to the user if/when needed (if the user doesn't have tokens available for that destination). XXX specify control port API between browser and tor ## 5.2. PoW token issuer An idea that mixes the CAPTCHA issuer with proposal#327, would be to have a token issuer that accepts PoW solutions and provides tokens as a reward. This solution tends to be less optimal than applying proposal#327 directly because it doesn't allow us to fine-tune the PoW difficulty based on the attack severity; which is something we are able to do with proposal#327. However, we can use the fact that token issuance happens over HTTP to introduce more advanced PoW-based concepts. For example, we can design token issuers that accept blockchain shares as a reward for tokens. For example, a system like Monero's Primo could be used to provide DoS protection and also incentivize the token issuer by being able to use those shares for pool mining [REF_PRIMO]. ## 5.3. Onion service self-issuing The onion service itself can also issue tokens to its users and then use itself as an issuer for verification. This way it can reward trusted users by giving it tokens for the future. The tokens can be rewarded from within the website of the onion service and passed to the Tor Client through the control port, or they can be provided in an out-of-bands way for future use (e.g. from a journalist to a future source using a QR code). Unfortunately, the anonymous credential scheme specified in this proposal is one-show, so the onion service cannot provide a single token that will work for multiple "logins". In the future we can design multi-show credential systems that also have revocation to further facilitate this use case (see [FUTURE_RES] for more info). # 6. User Experience This proposal has user facing UX consequences. Ideally we want this process to be invisible to the user and things to "just work". This can be achieved with token issuers that don't require manual work by the user (e.g. the PoW issuer, or the onion service itself), since both the token issuance and the token redemption protocols don't require any manual work. In the cases where manual work is needed by the user (e.g. solving a CAPTCHA) it's ideal if the work is presented to the user right before visiting the destination and only if it's absolutely required. An explanation about the service being under attack should be given to the user when the CAPTCHA is provided. # 7. Security In this section we analyze potential security threats of the above system: - An evil client can hoard tokens for hours and unleash them all at once to cause a denial of service attack. We might want to make the key rotation even more frequent if we think that's a possible threat. - A trusted token issuer can always DoS an onion service by forging tokens. - Overwhelming attacks like "top half attacks" and "hybrid attacks" from proposal#327 is valid for this proposal as well. - A bad RNG can completely wreck the linkability properties of this proposal. XXX Actually analyze the above if we think there is merit to listing them # 8. Discussion [DISCUSSION] ## 8.1. Using Res tokens on Exit relays There are more scenarios within Tor that could benefit from Res tokens however we didn't expand on those use cases to keep the proposal short. In the future, we might want to split this document into two proposals: one proposal that specifies the token scheme, and another that specifies how to use it in the context of onion servicves, so that we can then write more proposals that use the token scheme as a primitive. An extremely relevant use case would be to use Res tokens as a way to protect and improve the IP reputation of Exit relays. We can introduce an exit pool that requires tokens in exchange for circuit streams. The idea is that exits that require tokens will see less abuse, and will not have low scores in the various IP address reputation systems that now govern who gets access to websites and web services on the public Internet. We hope that this way we will see less websites blocking Tor. ## 8.2. Future improvements to this proposal [FUTURE_RES] The Res token scheme is a pragmatic scheme that works for the space/time constraints of this use case but it's far from ideal for the greater future (RSA? RSA-1024?). After Tor proposal#319 gets implemented we will be able to pack more data in RELAY cells and that opens the door to token schemes with bigger token sizes. For example, we could design schemes based on BBS+ that can provide more advanced features like multi-show and complex attributes but currently have bigger token sizes (300+ bytes). That would greatly improve UX since the client won't have to solve multiple CAPTCHAs to gain access. Unfortunately, another problem here is that right now pairing-based schemes have significantly worse verification performance than RSA (e.g. in the order of 4-5 ms compared to <0.5 ms). We expect pairing-based cryptography performance to only improve in the future and we are looking forward to these advances. When we switch to a multi-show scheme, we will also need revocation support otherwise a single client can abuse the service with a single multi-show token. To achieve this we would need to use blacklisting schemes based on accumulators (or other primitives) that can provide more flexible revocation and blacklisting; however these come at the cost of additional verification time which is not something we can spare at this time. We warmly welcome research on revocation schemes that are lightweight on the verification side but can be heavy on the proving side. ## 8.3. Other uses for tokens in Tor There is more use cases for tokens in Tor but we think that other token schemes with different properties would be better suited for those. In particular we could use tokens as authentication mechanisms for logging into services (e.g. acquiring bridges, or logging into Wikipedia). However for those use cases we would ideally need multi-show tokens with revocation support. We can also introduce token schemes that help us build a secure name system for onion services. We hope that more research will be done on how to combine various token schemes together, and how we can maintain agility while using schemes with different primitives and properties. # 9. Acknowledgements Thanks to Jeff Burdges for all the information about Blind RSA and anonymous credentials. Thanks to Michele Orrù for the help with the unlinkability proof and for the discussions about anonymous credentials. Thanks to Chelsea Komlo for pointing towards anonymous credentials in the context of DoS defenses for onion services. --- # Appendix A: RSA Blinding Security Proof [BLIND_RSA_PROOF] This proof sketch was provided by Michele Orrù: ``` RSA Blind Sigs: https://en.wikipedia.org/wiki/Blind_signature#Blind_RSA_signatures As you say, blind RSA should be perfectly blind. I tried to look at Boneh-Shoup, Katz-Lindell, and Bellare-Goldwasser for a proof, but didn't find any :( The basic idea is proving that: for any message "m0" that is blinded with "r0^e" to obtain "b" (that is sent to the server), it is possible to freely choose another message "m1" that blinded with another opening "r1^e" to obtain the same "b". As long as r1, r0 are chosen uniformly at random, you have no way of telling if what message was picked and therefore it is *perfectly* blind. To do so: Assume the messages ("m0" and "m1") are invertible mod N=pq (this happens at most with overwhelming probability phi(N)/N if m is uniformly distributed as a result of a hash, or you can enforce it at signing time). Blinding happens by computing: b = m0 * (r0^e). However, I can also write: b = m0 * r0^e = (m1/m1) * m0 * r0^e = m1 * (m0/m1*r0^e). This means that r1 = (m0/m1)^d * r0 is another valid blinding factor for b, and it's distributed exactly as r0 in the group of invertibles (it's unif at random, because r0 is so). ``` --- [REF_TOKEN_ZOO]: https://tokenzoo.github.io/ [REF_INTRO_SPACE]: https://gitlab.torproject.org/legacy/trac/-/issues/33650#note_2350910 [REF_CHAUM]: https://eprint.iacr.org/2001/002.pdf [REF_PRIMO]: https://repo.getmonero.org/selene/primo https://www.monerooutreach.org/stories/RPC-Pay.html [REF_POW_PERF]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/master/proposals/327-… [REF_RES_BENCH]: https://github.com/asn-d6/res_tokens_benchmark

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[RFC] Proposal 332: Vanguards lite
by George Kadianakis 01 Jun '21

01 Jun '21

Hello list, I present you with a simplified version of prop292 which protects against guard discovery attacks. The proposal can also be found in: https://gitlab.torproject.org/asn/torspec/-/commits/vg-lite --- ``` Filename: 332-vanguards-lite.md Title: Vanguards lite Author: George Kadianakis, Mike Perry Created: 2021-05-20 Status: Draft ``` # 0. Introduction & Motivation This proposal specifies a simplified version of Proposal 292 "Mesh-based vanguards" for the purposes of implementing it directly into the C Tor codebase. For more details on guard discovery attacks and how vanguards defend against it, we refer to Proposal 292 [PROP292_REF]. # 1. Overview We propose an identical system to the Mesh-based Vanguards from proposal 292, but with the following differences: - No third layer of guards is used. - The Layer2 lifetime uses the max(x,x) distribution with a minimum of one day and maximum of 12 days. This makes the average lifetime approximately a week. We let NUM_LAYER2_GUARDS=4. - We don't write guards on disk. This means that the guard topology resets when tor restarts. By avoiding a third-layer of guards we reduce the linkability issues of Proposal 292, which means that we don't have to add an extra hop on top of our paths. This simplifies engineering. # 2. Rotation Period Analysis From the table in Section 3.1 of Proposal 292, with NUM_LAYER2_GUARDS=4 it can be seen that this means that the Sybil attack on Layer2 will complete with 50% chance in 18*7 days (126 days) for the 1% adversary, 4*7 days (one month) for the 5% adversary, and 2*7 days (two weeks) for the 10% adversary. # 3. Tradeoffs from Proposal 292 This proposal has several advantages over Proposal 292: By avoiding a third-layer of guards we reduce the linkability issues of Proposal 292, which means that we don't have to add an extra hop on top of our paths. This simplifies engineering and makes paths shorter by default: this means less latency and quicker page load times. This proposal also comes with disadvantages: The lack of third-layer guards makes it easier to launch guard discovery attacks against clients and onion services. Long-lived services are not well protected, and this proposal might provide those services with a false sense of security. Such services should still use the vanguards addon [VANGUARDS_REF]. # 4. References [PROP292_REF]: https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/proposals/292-me… [VANGUARDS_REF]: https://github.com/mikeperry-tor/vanguards

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Renaming the primary branch of tor.git
by Alexander Færøy 25 May '21

25 May '21

Hello! As of today, the primary development branch of tor.git (the Git repository containing the source code of Tor) is now named 'main'. If you use tor.git in your tools, where you refer to the branch names, you probably want to change that to 'main' right away, otherwise your tools may break. All existing Gitlab MR's should have been updated to target the new primary branch name, but contributors should make sure their local checkouts are tracking main now for updates. Tor.git maintainers should re-run the usual scripts and make sure their work tree setup uses 'main'. All the best, Alex. -- Alexander Færøy

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GSoC 2021 - Alexa Top Sites Captcha and Tor Block Monitoring
by Apratim Ranjan Chakrabarty 24 May '21

24 May '21

Hello everyone, I'm Apratim (irc: _ranchak_), and I'm from Haldia Institute of Technology, India, studying Computer Science Engineering. I'll be working this summer on the GSoC project: "Alexa Top Sites Captcha and Tor Block Monitoring" with my mentors Barkin, Pili, Gerog and Roger guiding me through the irc. I'm very excited to be a part of Tor and am interested in working on this project! Briefing about the project: As of now "The Captcha Monitoring" project tracks how often CDN (for ex. Cloudflare, Akamai, Amazon Cloudfront, etc.) fronted webpages return CAPTCHAs to Tor clients. My project will be focussed on tracking the The Alexa Top 500 Websites, to give a detailed perspective of websites mentioned in the Topsites, blocking or returning Captchas to Tor clients. The project aims to do so by fetching webpages over a period of time from both Tor clients/browsers and the non-tor browsers thereby comparing the results. The results will be then collected and provide answers to the different metrics, and also form an understanding of how websites are blocking Tor and affect Internet freedom. Also one could visit: https://gitlab.torproject.org/woswos/CAPTCHA-Monitor/-/wikis/GSoC-2021 for detailed info. As planned and discussed by my mentor, I'll be creating more issues, which could be seen from the sidebar along the wiki page, to record my progress. As of now one could reach out with suggestions and advises in the irc (#tor-dev) as well as here: tpo/community/support/#40013. I also plan to update my wiki page frequently and have all the information required. Hope to have a great learning ahead and enjoy this summer! Regards, Apratim

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