tor-dev March 2020

tor-dev@lists.torproject.org

20 participants
18 discussions

the consequences of deprecating debian alpha repos with every new major branch
by nusenu 21 Dec '21

21 Dec '21

Hi, tldr: - more outdated relays (that is a claim I'm making and you could easily proof me wrong by recreating the 0.3.3.x alpha repos and ship 0.3.3.7 in them and see how things evolve after a week or so) - more work for the tpo website maintainer - less happy relay operators [3][4] - more work for repo maintainers? (since a new repo needs to be created) When the tor 0.3.4 alpha repos (deb.torproject.org) first appeared on 2018-05-23 I was about to submit a PR for the website to include it in the sources.list generator [1] on tpo but didn't do it because I wanted to wait for a previous PR to be merged first. The outstanding PR got merged eventually (2018-06-28) but I still did not submit a PR to update the repo generator for 0.3.4.x nonetheless and here is why. Recently I was wondering why are there so many relays running tor version 0.3.3.5-rc? (see OrNetStats or Relay Search) (> 3.2% CW fraction) Then I realized that this was the last version the tor-experimental-0.3.3.x-* repos were shipping before they got abandoned due to the new 0.3.4.x-* repos (I can no longer verify it since they got removed by now). Peter made it clear in the past that the current way to have per-major-version debian alpha repos (i.e. tor-experimental-0.3.4.x-jessie) will not change [2]: > If you can't be bothered to change your sources.list once or twice a > year, then you probably should be running stable. but maybe someone else would be willing to invoke a "ln" commands everytime a new new alpha repo is born. tor-alpha-jessie -> tor-experimental-0.3.4.x-jessie once 0.3.5.x repos are created the link would point to tor-alpha-jessie -> tor-experimental-0.3.5.x-jessie It is my opinion that this will help reduce the amount of relays running outdated versions of tor. It will certainly avoid having to update the tpo website, which isn't a big task and could probably be automated but it isn't done currently. "..but that would cause relay operators to jump from i.e. 0.3.3.x to 0.3.4.x alphas (and break setups)!" Yes, and I think that is better than relays stuck on an older version because the former repo no longer exists and operators still can choose the old repos which will not jump to newer major versions. [1] https://www.torproject.org/docs/debian.html.en#ubuntu [2] https://trac.torproject.org/projects/tor/ticket/14997#comment:3 [3] https://lists.torproject.org/pipermail/tor-relays/2018-June/015549.html [4] https://trac.torproject.org/projects/tor/ticket/26474 -- https://twitter.com/nusenu_ https://mastodon.social/@nusenu

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tor relay process health data for operators (controlport)
by nusenu 16 Nov '20

16 Nov '20

Hi, every now and then I'm in contact with relay operators about the "health" of their relays. Following these 1:1 discussions and the discussion on tor-relays@ I'd like to rise two issues with you (the developers) with the goal to help improve relay operations and end user experience in the long term: 1) DNS (exits only) 2) tor relay health data 1) DNS ------ Current situation: Arthur Edelstein provides public measurements to tor exit relay operators via his page at: https://arthuredelstein.net/exits/ This page is updated once daily. the process to use that data looks like this: - first they watch Arthur's measurement results - if their failure rate is non-zero they try to tweak/improve/change their setup - wait for another 24 hours (next measurement) This is a somewhat suboptimal and slow feedback loop and is probably also less accurate and less valuable data when compared to the data the tor process can provide. Suggestion for improvement: Exposes the following DNS status information via tor's controlport to help debug and detect DNS issues on exit relays: (total numbers since startup) - amount of DNS queries send to the resolver - amount of DNS queries send to the resolver due to a RESOLVE request - DNS queries send to resolver due to a reverse RESOLVE request - amount of queries that did not result in any answer from the resolver - breakdown of number of responses by response code (RCODE) https://www.iana.org/assignments/dns-parameters/dns-parameters.xhtml#dns-pa… - max amount of DNS queries send per curcuit If this causes a significant performance impact this feature should be disabled by default. 2) general relay health metrics -------------------------------- Compared to other server daemons (webserver, DNS server, ..) tor provides little data for operators to detect operational issues and anomalies. I'd suggest to provide the following stats via the control port: (most of them are already written to logfiles by default but not accessible via the controlport as far as I've seen) - total amount of memory used by the tor process - amount of currently open circuits - circuit handshake stats (TAP / NTor) DoS mitigation stats - amount of circuits killed with too many cells - amount of circuits rejected - marked addresses - amount of connections closed - amount of single hop clients refused - amount of closed/failed circuits broken down by their reason value https://gitweb.torproject.org/torspec.git/tree/tor-spec.txt#n1402 https://gitweb.torproject.org/torspec.git/tree/control-spec.txt#n1994 - amount of closed/failed OR connections broken down by their reason value https://gitweb.torproject.org/torspec.git/tree/control-spec.txt#n2205 If this causes a significant performance impact this feature should be disabled by default. cell stats - extra info cell stats as defined in: https://gitweb.torproject.org/torspec.git/tree/dir-spec.txt#n1072 This data should be useful to answer the following questions: - High level questions: Is the tor relay healthy? - is it hitting any resource limits? - is the tor process under unusual load? - why is tor using more memory? - is it slower than usual at handling circuits? - can the DNS resolver handle the amount of DNS queries tor is sending it? This data could help prevent errors from occurring or provide additional data when trying to narrow down issues. When it comes to the question: **Is it "safe" to make this data accessible via the controlport?** I assume it is safe for all information that current versions of tor writes to logfiles or even publishes as part of its extra info descriptor. Should tor provide this or similar data I'm planing to write scripts for operators to make use of that data (for example a munin plugin that connects to tor's controlport). I'm happy to help write updates for control-spec should these features seem reasonable to you. Looking forward to hearing your feedback. nusenu -- https://twitter.com/nusenu_ https://mastodon.social/@nusenu

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DNS-over-HTTPS (DOH) in Firefox/Torbrowser
by nusenu 01 Jul '20

01 Jul '20

Hi, since Mozilla did tests [0] on DOH [1] in Firefox I was wondering if Torbrowser developers have put any thought into that as well? Note: I'm _not_ suggesting to use DOH in torbrowser I'm just asking because the answer probably matters for exit documentation in the relay guide if clients do DNS themselves over TCP connections instead of relying on the exit (even if torbrowser is not the only tor client). thanks, nusenu [0] https://www.ghacks.net/2018/03/20/firefox-dns-over-https-and-a-worrying-shi… [1] https://datatracker.ietf.org/doc/draft-hoffman-dns-over-https/ -- https://mastodon.social/@nusenu twitter: @nusenu_

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Improving onion service availability during DoS using anonymous credentials
by George Kadianakis 30 Mar '20

30 Mar '20

Hello list, there has been lots of discussions about improving onion service availability under DoS conditions. Many approaches have been proposed [OOO] but only a few have been tried and even fewer show any real improvements to the availability of the service. An approach that we've been considering is the use of anonymous credentials as a way to prioritize good clients from bad clients. The idea is that the service gives tokens to clients it believes to be good, and prioritizes client with tokens over clients without tokens whenever possible. This is a post to start a discussion of how such approaches could work and whether they are worth pursuing futher. == Preliminaries == === When should the access control take place? === Doing DoS defenses with anon credentials is all about enforcing access control at the right point of the protocol so that the amplification factor of evil clients gets cut as early as possible. Very roughly the phases of the onion service protocol are: descriptor fetch phase, intro phase, rendezvous phase. Let's see how those look like for the purposes of access control: - Doing the access control during the descriptor fetch stage is something worth considering because it's the first phase of the protocol and hence the earliest and best place to soak up any damage from evil clients. There is already a form of optional access control implemented here called "client authorization" and it's worth thinking of what's lacking to make it useful against DoS attackers. I'm gonna address this in section [CLIENTAUTH]. - Doing the access control during the introduction phase is another fruitful approach. Blocking bad clients during introduction means that they dont get to force the service to create a costly rendezvous circuit, and since services have a long-term circuit open towards the intro points it makes it easier for services to pass access control related data to the intro point. This is actually the approach we are gonna be talking most about in this post. - Finally, doing the access control during the rendezvous phase is way too late since by that time the onion service has already spent lots of resources catering the evil client, so let's ignore that. === Entities of an anonymous credential system === Anonymous credential systems traditionally have three entities that concern us: - The Issuer: the entity who issues the credentials/tokens - The Prover: the entity who collects tokens and uses them to get access - The Verifier: the entity who verifies that tokens are legit and grants/restricts access In the world of onion services, the Issuer is naturally the onion service, and the Prover is the onion service client. The Verifier could either be the onion service itself or its introduction points. We will see below how this could work and the relevant tradeoffs. +--------+ +------------+ +--------------------+ | Client |<-+-+-+-->|Intro point |<--+---+-->|Onion service | |(Prover)| |(Verifier?) | |(Issuer)(Verifier?) | +--------+ +------------+ +--------------------+ === How do tokens get around? === A main question here is "How do good clients end up with tokens?". For the purposes of this post, we will assume that clients get these tokens in an out of band fashion. For example, a journalist can give tokens to her sources over Signal so they can use them with Securedrop. Or a forum operator can give tokens to old-time members of the forum to be used during a DoS. A natural chicken-and-egg problem occurs here since how is an onion service supposed to give tokens to its users if it's unreachable because of a DoS? We realize this is a big problem and we are not sure exactly how to solve it. This problem naturally limits the use of anonymous credential solutions, and sorta makes them a second-layer of defense since it assumes a first-layer of defense that allows operators to pass tokens to the good people. A first-layer approach here could perhaps look like PrivacyPass where users get tokens by solving CAPTCHAs. == Anonymous credentials == By surveying the anonymous credential literature we have found various types of anonymous credential schemes that are relevant for us: - Discrete-logarithm-based credentials based on blind signatures: This is a class of anon credential schemes that allow us to separate the verifier from the issuer. In particular this means that we can have the service issue the tokens, but the introduction point being the verifier. They are usually based on blind signatures like in the case of Microsoft's U-Prove system [UUU]. - Discrete-logarithm-based credentials based on OPRF: Another approach here is to use OPRF constructions based on the discrete logarithm problem to create an anonymous credential scheme like in the case of PrivacyPass [PPP]. The downside, IIUC, is that in PrivacyPass you can't have a different issuer and verifier so it's the onion service that needs to do the token verification restricting the damage soaking potential. - KVAC (Keyed-Verification Anonymous Credentials): KVAC-based credentails have comparable performance to Dlog-based credentials, and are also easier to create security proofs for due to them being based on symmetric primitives like algebraic MAC constructions [FFF]. The downside is that they assume that the verifier and the issuer is the same entity (the onion service). KVACs are used by Signal for protecting their group chat metadata [GGG]. Which construction and scheme we choose depends on our constraints and the parameters we are trying to minmax. So let's try to explore these constraints and parameters some more. == Space constraints == Let's assume that a client found some anonymous credentials for a service. How will clients present their credentials to the verifier, being either the service or the intro point? The natural approach here is for the client to include their token as part of the INTRODUCE1 cell. === How much space is available in INTRODUCE1 cells? [SPACE_CONSTRAINTS] === >From a brief experiment, I see that the INTRODUCE1 cell payload total size is 498 bytes, and we are already using 310 bytes from it, so that leaves about 188 bytes for use. The cell is extensible using cell extensions that can go either to the intro point [III] or to the service directly [EEE]. === How big are anonymous credentials redemptions? === >From looking at the literature it seems unlikely that those anonymous credentials can be presented given the size limitations from above. In particular here is the size needed for presenting/redeeming an anonymous credential in the various schemes: - Privacy Pass [PPP]: Credential redemption takes 396 bytes - Signal [GGG]: Their paper does not say exactly, but by peeking into their code it seems like presenting the credential takes about 620 bytes [CCC]. So if I'm not mistaken, it seems like we will need another way to present anonymous credentials to the service or intro point. === How can we trasmit these credentials then? === The unfortunate fact here is that presenting those credentials does not fit into the remaining space of an INTRODUCE1 cell, but it also wouldn't even fit if we created an entirely new relay cell for this purpose (say INTRO_REDEEM) since the relay cell payload is about 498 bytes and all the credentials above, except from PrivacyPass (!), are bigger than that. This means that we will either need to use a space-compact anonymous credential scheme, or to implement wide relay cells as suggested in ticket #33650 (which seems like lots of work!). == Computational constraints [COMPUTATIONAL_CONSTRAINTS] == Another big topic for consideration here is the computational resources required for the various operations of the scheme. In particular, how hard it is to issue such tokens, present such tokens and to verify such tokens. Different schemes have different computational profiles. I'm not gonna attempt to summarize the literature here because it will take me more time than I have right now. However, it's clear that our main priority here is a lightweight verification procedure, so that services or intro points spend as little energy as possible validating (potentially evil) tokens. It's imperative that verifying tokens is orders of magnitude more lightweight than the regular job that a service would do if credentials did not exist (i.e. doing path selection, creating a rendezvous circuit, doing the crypto, etc.) Our second priority here is a lighweight token issuance procedure, so that services don't spend too much time issuing tokens, especially if this is done in an automatic real-time manner that can be abused by an adversary. And finally, we don't particularly care about the token presentation procedure being lightweight, since this is a procedure that the attacker will also have to do and hence we want to incur as much costs to them as possible. == Discussion == ==== What's lacking from HSv3 client authorization [CLIENTAUTH] ==== It's really important to understand what's lacking from descriptor-level client authorization [QQQ] with regards to DoS defences before we spend time and energy implementing any anonymous credential approach. To use the descriptor-level client authorization feature for DoS protection the onion service operator creates a mirror of the website that is only reachable using descriptor-level client auth and then they pass client auth tokens to the clients they trust to be good natured. The above system can also be automated and turned into a web service. However this system has a bunch of drawbacks: - Given the implicit and anonymous nature of the descriptor-level client authorization system, if an evil actor gets their hand in a client auth token and they start DoSing the service, then the operator has no means to distinguish that client from any other client. Hence there is no way to identify individual bad clients and revoke them. This is the biggest issue with descriptor-based client authorization for the purposes of DoS defense. - Managing the access list (adding and removing clients) in the descriptor-level client authorization system is an expensive process because it requires uploading a new set of descriptors. Doing this too often (in an automated way) can cause lots of network traffic and also perhaps race conditions with clients who fetch descriptors. - Because the authentication happens by adding new lines on the descriptor, there is a fundamental limit on how many authed clients it can support. In particular, with some back-of-the-envelope calculations [ZZZ] an onion service can support a max of 250 authed clients this way. However, there is nothing stopping the operator from passing the same token to multiple clients and managing them as groups. The above issues would be addressed with intro-level client authorization since the authentication is explicit and done in real-time. It's worth thinking of the above drawbacks more and how we can bypass them to do something useful with the current system before we jump into new complexities and tradeoffs. --- Thanks for reading and looking forward to your feedback :) [OOO]: https://trac.torproject.org/projects/tor/ticket/31223 [ZZZ]: https://lists.torproject.org/pipermail/tor-dev/2016-November/011658.html [QQQ]: https://eprint.iacr.org/2013/516.pdf [PPP]: https://www.petsymposium.org/2018/files/papers/issue3/popets-2018-0026.pdf [UUU]: https://www.microsoft.com/en-us/research/project/u-prove/ [FFF]: https://eprint.iacr.org/2013/516.pdf [GGG]: https://signal.org/blog/signal-private-group-system/ [III]: https://github.com/torproject/torspec/blob/f81b1e6cc53b91abb3ae206807bc371f… [EEE]: https://github.com/torproject/torspec/blob/f81b1e6cc53b91abb3ae206807bc371f… [CCC]: https://github.com/signalapp/zkgroup/blob/4294e428216113d81f5c0cb50f578797d…

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Chutney tests fail for tor/maint-0.3.5 (bug #33677)
by c 30 Mar '20

30 Mar '20

Hi, For the Outreachy.org internship project I decided to take on bug #33677 [1]. I followed steps to run `make test-network-all` both on tor master and maint-0.3.5 branches, using Chutney master branch. I tested before and after tweaking value of HS_WAIT_FOR_UNCHECKED_DIR_INFO just to ensure my testing environment was sane; all tests passed (except for two that were skipped on master: mixed+hs-v23 and mixed+hs-v23-ipv6, and one on 0.3.5: mixed+hs-v2, I assume this is expected though) on the first go, on both branches of Tor. Tests pass on Tor master after setting HS_WAIT_FOR_UNCHECKED_DIR_INFO to 0, but on maint-0.3.5 I've run tests a few times (both with and without CHUTNEY_DEBUG set) each time with a different outcome. Some of the tests which fail: basic-min [2], single-onion-v23 [3], bridges+ipv6-min [4], and ipv6-exit-min [5]. I have excerpted logs at the bottom of this message, narrowed down to test failures; let me know if I need to provide full logs or additional information. I'll keep digging on this issue; I just wanted to show my current progress and share what I hope to be useful information. Caitlin [1]: <https://trac.torproject.org/projects/tor/ticket/33677> "Stop waiting a set time for onion service descriptors" [2]: basic-min.log [...] Connecting: Exit to 127.0.0.1:4747 via client localhost:9005 DEBUG: socket 5 connecting to ('localhost', 9005)... DEBUG: Registering send-data-11 DEBUG: Registering check-12 Transmitting Data: DEBUG: Socks 4a request to 127.0.0.1:4747 DEBUG: Test status: {'send-data-11': 'not done', 'check-12': 'not done'}: 2/0/0 DEBUG: Test status: {'send-data-11': 'not done', 'check-12': 'not done'}: 2/0/0 DEBUG: Done with run(); all_done == False and failure_count == 0 Transmission: Failure [...] chutney verify round 1/1 failed Chutney failed 1 bootstraps; we may have a problem here. FAIL basic-min (exit status: 1) [3]: single-onion-v23.log [...] Connecting: HS to bkjqakom5elyh45a.onion:5858 (127.0.0.1:4747) via client localhost:9005 DEBUG: socket 7 connecting to ('localhost', 9005)... DEBUG: Registering send-data-21 DEBUG: Registering check-22 HS to moymkdharroenjd6zarfurdwmrr55j2zune6k7wafyfakwy2cis327id.onion:5858 (127.0.0.1:4747) via client localhost:9005 DEBUG: socket 8 connecting to ('localhost', 9005)... DEBUG: Registering send-data-23 DEBUG: Registering check-24 Transmitting Data: DEBUG: Socks 4a request to bkjqakom5elyh45a.onion:5858 DEBUG: Socks 4a request to moymkdharroenjd6zarfurdwmrr55j2zune6k7wafyfakwy2cis327id.onion:5858 DEBUG: new client from 127.0.0.1:54314 (fd=9) DEBUG: successfully connected (fd=8) DEBUG: Succeeded send-data-23 DEBUG: successful verification DEBUG: Succeeded check-24 DEBUG: Test status: {'send-data-21': 'not done', 'check-22': 'not done', 'send-data-23': 'success', 'check-24': 'success'}: 2/2/0 DEBUG: Test status: {'send-data-21': 'not done', 'check-22': 'not done', 'send-data-23': 'success', 'check-24': 'success'}: 2/2/0 DEBUG: Done with run(); all_done == False and failure_count == 0 Transmission: Failure [...] chutney verify round 1/1 failed Chutney failed 1 bootstraps; we may have a problem here. FAIL single-onion-v23 (exit status: 1) [4]: bridges+ipv6-min.log [...] Couldn't launch test005bc command '/home/c/git/tor/src/app/tor -f /home/c/git/chutney/net/nodes/005bc/torrc': exit 1, output 'Mar 28 10:47:36.717 [notice] Tor 0.3.5.10-dev (git-38e07b88fac51f2a) running on Linux with Libevent 2.1.8-stable, OpenSSL 1.1.1d, Zlib 1.2.11, Liblzma 5.2.4, and Libzstd 1.3.7. Mar 28 10:47:36.717 [notice] Tor can't help you if you use it wrong! Learn how to be safe at https://www.torproject.org/download/download#warning Mar 28 10:47:36.717 [notice] Read configuration file "/home/c/git/chutney/net/nodes/005bc/torrc". Mar 28 10:47:36.721 [warn] You have used DirAuthority or AlternateDirAuthority to specify alternate directory authorities in your configuration. This is potentially dangerous: it can make you look different from all other Tor users, and hurt your anonymity. Even if you've specified the same authorities as Tor uses by default, the defaults could change in the future. Be sure you know what you're doing. Mar 28 10:47:36.721 [warn] TestingTorNetwork is set. This will make your node almost unusable in the public Tor network, and is therefore only advised if you are building a testing Tor network! Mar 28 10:47:36.722 [notice] Opening Socks listener on 127.0.0.1:9005 Mar 28 10:47:36.722 [warn] Could not bind to 127.0.0.1:9005: Address already in use. Is Tor already running? Mar 28 10:47:36.722 [notice] Opening Control listener on 127.0.0.1:8005 Mar 28 10:47:36.722 [warn] Could not bind to 127.0.0.1:8005: Address already in use. Is Tor already running? Mar 28 10:47:36.722 [notice] Opening Control listener on /home/c/git/chutney/net/nodes/005bc/control Mar 28 10:47:36.722 [notice] Opened Control listener on /home/c/git/chutney/net/nodes/005bc/control Mar 28 10:47:36.722 [notice] Closing partially-constructed Control listener on /home/c/git/chutney/net/nodes/005bc/control:0 Mar 28 10:47:36.722 [warn] Failed to parse/validate config: Failed to bind one of the listener ports. Mar 28 10:47:36.722 [err] Reading config failed--see warnings above. ' [...] Launching chutney using Python 3.7.6 DEBUG: Tor version 0.3.5.10-dev (git-38e07b88fac51f2a). test000a is running with PID 2870: Tor 0.3.5.10-dev (git-38e07b88fac51f2a) test001a is running with PID 2873: Tor 0.3.5.10-dev (git-38e07b88fac51f2a) test002ba is running with PID 2911: Tor 0.3.5.10-dev (git-38e07b88fac51f2a) test003r is running with PID 2879: Tor 0.3.5.10-dev (git-38e07b88fac51f2a) test004br is running with PID 2882: Tor 0.3.5.10-dev (git-38e07b88fac51f2a) test005bc is stopped: Tor 0.3.5.10-dev (git-38e07b88fac51f2a) 5/6 nodes are running [...] bootstrap-network.sh failed Chutney failed 1 bootstraps; we may have a problem here. FAIL bridges+ipv6-min (exit status: 1) [5]: ipv6-exit-min.log [...] Connecting: Exit to 127.0.0.1:4747 via client localhost:9005 DEBUG: socket 5 connecting to ('localhost', 9005)... DEBUG: Registering send-data-11 DEBUG: Registering check-12 Transmitting Data: DEBUG: Socks 4a request to 127.0.0.1:4747 DEBUG: Test status: {'send-data-11': 'not done', 'check-12': 'not done'}: 2/0/0 DEBUG: Test status: {'send-data-11': 'not done', 'check-12': 'not done'}: 2/0/0 DEBUG: Done with run(); all_done == False and failure_count == 0 Transmission: Failure [...] chutney verify round 1/1 failed Chutney failed 1 bootstraps; we may have a problem here. FAIL ipv6-exit-min (exit status: 1)

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Walking onions status update: week 3 notes
by Nick Mathewson 30 Mar '20

30 Mar '20

Walking Onions: week 3 update On our current grant from the zcash foundation, I'm working on a full specification for the Walking Onions design. I'm going to try to send out these updates once a week. My previous updates are linked below: Week 1: formats, preliminaries, git repositories, binary diffs, metaformat decisions, and Merkle Tree trickery. https://lists.torproject.org/pipermail/tor-dev/2020-March/014178.html Week 2: Specifying details of SNIP and ENDIVE formats, in CDDL. https://lists.torproject.org/pipermail/tor-dev/2020-March/014181.html You might like to check out those updates, and their references, if this update doesn't make sense to you. === As you might recall, the SNIPs need to be nice and small, but they need to be completely self-contained. The ENDIVE, on the other hand, needs to be diff-friendly, and compressible. Relays need to derive all the SNIPs from the ENDIVE. The ENDIVE and SNIP formats that I worked on during last week [SNIPFMT] was designed to meet these criteria. This week, I wrote up the algorithms to extract signed SNIPs from an ENDIVE. This is in section 4 [RECONST] of the specs in my work-in-progress repository [GITREPO] This was mainly straightforward consequence from the work I did last week, but there were a few things that turned up. One trickier aspect of this work is that we want to move most of the decision-making to the authorities, and have the relays simply do an expansion algorithm. Updating all the relays would be slow, so relays need to be able to handle new index types and data types without fully understanding them. That means that we need to specify an algorithm that can work flexibly with many future kinds of data. Another tricky aspect is that the data which the relays need to put into SNIPs all needs to be signed, so all of that data needs to come out the same when relays calculate it as the when the authorities calculate it. Thus, all the algorithms for building Merkle trees and indices and SNIPRouterData bodies need to be deterministic. I think that I got all of this relatively right in [RECONST], but there are probably some mistakes hiding in there. I'm hoping that there is a cleaner alternative to the weighted-index reconstruction algorithm -- the one that's there right now puts a lot of requirements on the input data, in order to avoid floating-point operations. === After working on ENDIVE expansion, I moved on to the problem of authorities voting. There's a lot of uncertainty here and decisions I'll need to make. The biggest decision is how the new voting process for ENDIVEs should relate to the current voting process for consensus documents. The choices seem to be: * Have two processes with different kinds of votes. * Expand the information that goes into current votes, and generate ENDIVEs as a new kind of consensus from the current voting process. * Design the ENDIVE voting process so that it can also be used to generate current-style consensuses. Using separate processes might be simplest, but it does carry risk for duplicated code, and for the two voting processes reaching divergent results. It would also be potentially bad if an authority could vote differently in each process. The second approach -- where ENDIVEs are another kind of consensus generated from current-style voting documents -- has some charm to it, but it perpetuates our current consensus-voting system, and makes us specify a second encoding of CBOR objects as text, if we want to vote on them meaningfully. This is a bit nasty, since the standard-specified text encoding of cbor is not a bijection, and doesn't mesh too well with our old directory format. Therefore the third approach seems a little better to me now. In it, we'd specify our votes as a CBOR-based format loosely based on the current vote format, and describe a way to extract from them the same data that is currently used for our voting algorithm. This would let us encode CBOR within the document naturally, while not having to throw away too much of our existing voting specification and voting code. I've got a few more ideas here, though I don't have any specs yet. I've been collecting the ideas in a notes file [VNOTES] so I don't forget them. === While working on voting notes, I found some areas where the ENDIVE and SNIP formats might be better. First, I hadn't thought about authority certificates. Those should go in, so that we can have the ENDIVE-signing keys be signed themselves with the authorities' long-lived identities. Second, there seem to be some opportunities for transmitting indices in a simpler way. Instead of assigning a weight to each router separately, for example, we could derive one weighted index from another weighted index by re-weighting it, as we do with the "bandwidth-weights" values in the current consensus. Teor also made some good suggestions for improving the diff format; I incorporated one and made a note to check the other. === I also spent a little while this week reading about BLS signatures, since they are the leading candidate for how we might do threshold signatures here. === Next week I'm going to try to get voting all specified. This will be a pretty deep dive, since I will need to maintain some backward compatibility with existing voting algorithms. There may be opportunities for simplification, however. === [GITREPO] https://github.com/nmathewson/walking-onions-wip [RECONST] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/04-recon… d [SNIPFMT] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/02-endiv… [VNOTES] https://github.com/nmathewson/walking-onions-wip/blob/master/notes/voting.t…

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Walking onions status update: week 4 notes
by Nick Mathewson 30 Mar '20

30 Mar '20

Walking Onions: week 4 update On our current grant from the zcash foundation, I'm working on a full specification for the Walking Onions design. I'm going to try to send out these updates once a week. My previous updates are linked below: Week 1: formats, preliminaries, git repositories, binary diffs, metaformat decisions, and Merkle Tree trickery. https://lists.torproject.org/pipermail/tor-dev/2020-March/014178.html Week 2: Specifying details of SNIP and ENDIVE formats, in CDDL. https://lists.torproject.org/pipermail/tor-dev/2020-March/014181.html Week 3: Expanding ENDIVEs into SNIPs. https://lists.torproject.org/pipermail/tor-dev/2020-March/014194.html This week was pretty rough for me; the impact of being isolated at home is starting to kick in, and my kid is still trying to find a schedule that works for him. I got a lot less done this week than I had planned. That said, I _did_ get some good design done, in two areas: voting, and circuit extension. == Voting I'm trying to generalize our voting algorithms to minimize the need for new consensus formats in the future. I want most new fields to be self-describing in terms of how to tabulate votes for them. Trying to work this out has taken a bunch of energy, though and I'm not quite sure I'm doing it right. The work here is unfinished. I still think it's reasonable to encode existing votes and new votes in a new cbor-based structure. By using compression, a little redundancy in votes shouldn't hurt us too badly -- though that's something I need to benchmark eventually before we build this. I've come up with a way for voters to use diffs and compression when _uploading_ votes: first, the voter asks for a summary of what mechanisms are supported by the recipient and which older votes it has. Next, the voter can use that information to upload a vote diff with an appropriate compression format. Right now I believe that I have most of the fields for the votes figured out, but I need to clean them up a _lot_. SNIP bodies can work similarly to how microdescriptors work now, if we like -- or they could involve individual votes on input fields. I think I'm going to write up what I can here, and then revisit SNIP bodies once I've worked some on section 6 (the "fancy" client behavior). While working on voting, I found a need for various changes in the already written sections. For one example, the previous description of the ENDIVE format included the SNIP signatures as part of the signed ENDIVE. But we can't actually generate that as part our voting protocol: the authorities can't sign SNIPs until they know the complete ENDIVE contents, and so putting the SNIP signatures in the signed part of the ENDIVE would add an extra round of voting and signatures. Instead, I've moved the SNIP signatures (and related stuff) into the signature header of the ENDIVE. I've also worked somewhat on the so-called "root document" holding the network configuration parameters. I'm hoping that this document can have an effective lifetime of weeks or months for clients, and that clients can download only the parts that they need. The format is in section 02, but needs more description. The work-in-progress voting writeup is in [VOTING], but it is not so close to done. I think it will need a lot of writing before it's ready for even preliminary review; it doesn't make sense as it is, and it contradicts itself in places. == Extending circuits There are a few wrinkles here that we hadn't actually specified in earlier walking onions work! When one relay is told to extending a circuit to a target relay by index, the extending relay actually needs to include part of a SNIP for that second relay: otherwise, the extending relay won't know which onion key the client expects to receive for it. We don't want to send an entire SNIP -- only the parts that the client will expect to match. In addition to wide EXTEND cells, we'll also need wide RENDEZVOUS and INTRODUCE1 and INTRODUCE2 and TRUNCATED cells. All together, that means that having a general "cell fragmentation" mechanism is probably warranted; I've started a separate proposal for that [WIDE_EVERYTHING]. We're going to need a general extension format for CREATE and EXTEND cells, to signal other information, like SNIPs, requests for SNIPs, and so on. We also need a way to ask for SNIPs without actually performing a handshake. I've tried to fit these all together in a fairly logical way in [EXTENDING], but there's more to write here, including examples. == Fun facts The walking onion proposal is now the second-longest that we have ever had, behind only proposal 224 (v3 hidden services). == Next steps In this coming week I plan to try to wrap up section 3 on voting and section 5 on extending circuits. I'm going to go back to the start of section 2 and start revising all I've written so far for consistency. Time permitting, I want to sketch out all the knowns and unknowns for section 6 (on tricky client behavior). [EXTENDING] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/05-exten… [VOTING] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/02-votin… [WIDE_EVERYTHING] https://github.com/nmathewson/walking-onions-wip/blob/master/other-proposal…

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Walking Onions status update: week 2 notes
by Nick Mathewson 29 Mar '20

29 Mar '20

Walking onions -- week 2 update Hi! On our current grant from the zcash foundation, I'm working on a full specification for the Walking Onions design. I'm going to try to send these out thee updates once a week, in case anybody is interested. My previous updates are linked below: Week 1: formats, preliminaries, git repositories, binary diffs, metaformat decisions, and Merkle Tree trickery. https://lists.torproject.org/pipermail/tor-dev/2020-March/014178.html You might like to have a look at that update, and its references, if this update doesn't make sense to you. === This week, I worked specifying the nitty-gritty of the SNIP and ENDIVE document formats. I used the CBOR meta-format [CBOR] to build them, and the CDDL specification language [CDDL] to specify what they should contain. As before, I've been working in a git repository at [GITHUB]; you can see the document I've been focusing on this week at [SNIPFMT]. (That's the thing to read if you want to send me patches for my grammar.) There were a few neat things to do here: * I had to define SNIPs so that clients and relays can be mostly agnostic about whether we're using a merkle tree or a bunch of signatures. * I had to define a binary diff format so that relays can keep on downloading diffs between ENDIVE documents. (Clients don't download ENDIVEs). I did a quick prototype of how to output this format, using python's difflib. * To make ENDIVE diffs as efficient as possible, it's important not to transmit data that changes in every ENDIVE. To this end, I've specified ENDIVEs so that the most volatile parts (Merkle trees and index ranges) are recomputed on the relay side. I still need to specify how these re-computations work, but I'm pretty sure I got the formats right. Doing this calculation should save relays a bunch of bandwidth each hour, but cost some implementation complexity. I'm going to have to come back to this choice going forward to see whether it's worth it. * Some object types are naturally extensible, some aren't. I've tried to err on the size of letting us expand important things in the future, and using maps (key->value mappings) for object that are particularly important. In CBOR, small integers are encoded with a little less space than small strings. To that end, I'm specifying the use of small integers for dictionary keys that need to be encoded briefly, and strings for non-tor and experimental extensions. * This is a fine opportunity to re-think how we handle document liveness. Right now, consensus directories have an official liveness interval on them, but parties that rely on consensuses tolerate larger variance than is specified in the consensus. Instead of that approach, the usable lifetime of each object is now specified in the object, and is ultimately controlled by the authorities. This gives the directory authorities more ability to work around network tolerance issues. Having large lifetime tolerances in the context of walking onions is a little risky: it opens us up to an attack where a hostile relay holds multiple ENDIVEs, and decides which one to use when responding to a request. I think we can address this attack, however, by making sure that SNIPs have a published time in them, and that this time moves monotonically forward. * As I work, I'm identifying other issues in tor that stand in the way of a good efficient walking onion implementation that will require other follow-up work. This week I ran into a need for non-TAP-based v2 hidden services, and a need for a more efficient family encoding. I'm keeping track of these in my outline file. Fun fact: In number of bytes, the walking onions proposal is now the 9th-longest proposal in the Tor proposal repository. And it's still growing! Next week, I'm planning to specify ENDIVE reconstruction, circuit extension, and maybe start on a specification for voting. [CBOR] RFC 7049: "Concise Binary Object Representation (CBOR)" https://tools.ietf.org/html/rfc7049b [CDDL] RFC 8610: "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures" https://tools.ietf.org/html/rfc8610 [GITREPO] https://github.com/nmathewson/walking-onions-wip [SNIPFMT] https://github.com/nmathewson/walking-onions-wip/blob/master/specs/02-endiv…

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Tails vs the capacity of the Meek bridges
by anonym 24 Mar '20

24 Mar '20

Hi, tl;dr: if Tails makes it too easy to use Meek bridges, could it overload the current set of Meek bridges? First some background: during startup Tails can be told to start Tor Launcher so users can e.g. configure any bridges they want. So far we have not provided any pre-configured bridges, i.e. the only option has been to manually enter the bridge information you have obtained yourself. In Tails' threat model it is assumed that adversaries monitor the default bridges provided by the Tor Browser, and that our users want to avoid detection of that, so we are not interested in adding the default bridges to Tails, but we are interested in adding support for Meek [1] (at least because it's the only PT that works in China), since our understanding is that it adds enough plausible deniability to avoid the above problem. So, in summary, Tails would like to provide two options in its (patched) Tor Launcher, Meek or manually providing the bridge info. [2] However, we wonder if the combination of not providing the default bridges while making Meek available could overwhelm the Meek bridges; we expect some significant amount of Tails bridge users to select the Meek option over manual entry simply out of convenience. Let's do some back-of-the-envelope estimations to see what we can expect: (Assumption: Tor Browser users and Tails users are very similar, e.g. similar ratios want to use each PT, similar requirements for "convenience over security", and so on.) Looking at your metrics, there are 1000k daily Tor Browser users [3], overall bridge/PT usage is 50k [4], Meek usage is around 10k [5], so 5% of Tor Browser users use bridges/PTs, and 1% use Meek. On the Tails side, we measure around 30k daily users (from update pings). >From what I said above, I don't think we can expect only 1% of the Tails users to pick Meek; I would expect it to be closer to the 1% of Meek users plus the x% of default bridge users, but I couldn't find stats for that in your metrics. Still, we know that it cannot be more than the percentage of all bridge/PT users, so we know that 1% + x% <= 5%. So, let's just assume the worst case, that 5% of Tails users will use Meek, and we have that we expect 5% of 30k = 1.5k additional Meek users. Also, if we consider places where Meek is the only options, Tails probably has close to zero users there, but once Meek is supported it could grow. I guess China is the only such place (?), so with its 1250 Meek users in China [6] we can expect 1250 * 30k/1000k = 34 new Tails users using Meek. Basically a negligible amount. Even if we consider all Meek users, 10k * 30k/1000k = 300 doesn't change much. So, all-in-all, we can expect Tails to bring up to 1.5k + 300 = 1.8k new Meek users, but since those are upper-bound estimations it would probably be much lower. Looking on Meek usage over time, it seems to fluctuate way more than that, e.g. during the summer of 2019 it was up to over 25k, i.e. more than three times what it is now. So I guess we don't have to worry about shocking the Meek bridges? OTOH, a possible side-effect is that this change in Tails increases usage of Meek outside of China. Perhaps whoever pays the bills for the Meek instances don't want this? Please advice! Also, please let us know if there is something else we haven't thought of! Cheers! [1] https://redmine.tails.boum.org/code/issues/8243 [2] If you are really interested you can check out our PoC/WIP here: https://nightly.tails.boum.org/build_Tails_ISO_feature-8243-meek/builds/las… [3] https://metrics.torproject.org/webstats-tb.html [4] https://metrics.torproject.org/userstats-bridge-transport.html [5] https://metrics.torproject.org/userstats-bridge-transport.html?transport=!%… [6] https://metrics.torproject.org/userstats-bridge-combined.html?start=2019-12…

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Re: [tor-dev] GSOC'20
by Sanjiban Sengupta 23 Mar '20

23 Mar '20

Dear Sir, This is Sanjiban Sengupta, sophomore in Computer Engineering from IIIT Bhubaneswar, India, would like to contribute to Tor Project for GSoC'20, I have practical and working knowledge of C, C++, Python and Java, for web, I am familiar with HTML, CSS, JS, Bootstrap and frameworks such as ReactJS and NodeJS, also i am acquainted with concepts of ML and AI, Linux Kernel and know the technicalities to apply these to solve modern real life problems. On going through the project proposals, I found the projectImplementing Salmon as a bridge distribution mechanism, and Tor Weather interesting to work upon and contribute and thus will be thankful for your kind guidance. Thus I request the mentors to kindly guide me for the beginning processes. Regards, Sanjiban Sengupta On Sat, Mar 21, 2020 at 11:47 AM Sanjiban Sengupta <b518041(a)iiit-bh.ac.in> wrote: > Dear Sir, > > This is Sanjiban Sengupta, sophomore in Computer Engineering from IIIT > Bhubaneswar, India, would like to contribute to Tor Project for GSoC'20, I > have practical and working knowledge of C, C++, Python and Java, for web, I > am familiar with HTML, CSS, JS, Bootstrap and frameworks such as ReactJS > and NodeJS, also i am acquainted with concepts of ML and AI, Linux Kernel > and know the technicalities to apply these to solve modern real life > problems. > > On going through the project proposals, I found the projectImplementing > Salmon as a bridge distribution mechanism, and Tor Weather interesting to > work upon and contribute and thus will be thankful for your kind guidance. > > Thus I request the mentors to kindly guide me for the beginning processes. > > Regards, > Sanjiban Sengupta > > > > On Sat, Mar 21, 2020 at 3:50 AM Sanjiban Sengupta <b518041(a)iiit-bh.ac.in> > wrote: > >> Dear Sir, >> >> This is Sanjiban Sengupta, sophomore in Computer Engineering from IIIT >> Bhubaneswar, India, would like to contribute to Tor Project for GSoC'20, I >> have practical and working knowledge of C, C++, Python and Java, for web, I >> am familiar with HTML, CSS, JS, Bootstrap and frameworks such as ReactJS >> and NodeJS, also i am acquainted with concepts of ML and AI, Linux Kernel >> and know the technicalities to apply these to solve modern real life >> problems. >> >> On going through the project proposals, I found the projectImplementing >> Salmon as a bridge distribution mechanism, and Tor Weather interesting to >> work upon and contribute and thus will be thankful for your kind guidance. >> >> Thus I request the mentors to kindly guide me for the beginning processes. >> >> Regards, >> Sanjiban Sengupta >> >

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tor-dev March 2020