- tor-dev - lists.torproject.org

OTRv4 summit at PETS2019
by Sofia Celi 17 Jun '19

17 Jun '19

Hi, Tor people! My name is Sofía and I'm the person leading the OTRv4 initiative. I don't know if this is the best place to send this, so if it is not, let me know ;) As part of the work on creating the version 4 of the OTR (Off-the-Record) messaging protocol, we are running a summit in Stockholm, Sweden, the 15 of July, in the morning. This summit runs in parallel with the Privacy Enhancing Technology Symposium 2019 (PETS2019, https://petsymposium.org/2019/index.php) It will be a summit around the work done in the field of secure messaging, deniability and privacy on July 15 on the morning (that is, the day before the start of PETS 2019). It will be held at the Royal Institute of Technology (KTH). You can find it on the official PETS2019 programm: https://petsymposium.org/2019/program.php and on the official otr website: http://otr.im/ The OTRv4 summit will feature a mix of a discussions, short talks and presentations which aim to highlight the importance of privacy, security and deniability in a secure messaging setting to researchers, students, activists and developers. The final schedule for the event will be announced closer to the event. The workshop will start at 9 am and end at 12.30 pm. As one of our developers and my current partner, Ola Bini, has been detained in Ecuador with no actual evidence, we will also talk about this. You can read about this in the latest EFF statement: https://www.eff.org/deeplinks/2019/06/eff-ohchr-iachr-cybercrime-law-interc… We will like that you join us there... so let us know if you can! Thank you! The OTRv4 team -- Sofía Celi @claucece Cryptographic research and implementation at CAD: https://autonomia.digital/ EF74 1A5F 5692 E56F 14F6 243C 3992 6144 F89D 996F

2 2

Proposal 304: Extending SOCKS5 Onion Service Error Codes
by David Goulet 06 Jun '19

06 Jun '19

3 2

Dealing with critical sbws tickets
by teor 05 Jun '19

05 Jun '19

Hi juga, I read your meeting notes from this week's network team meeting: juga(offline): Week of 05/20 (planned) - Add Tor version to the bandwidth file (#30196) Week of 05/20 (actual) Week of 06/03 (plan) - Continue with #30406: Refactor header constants in sbws to use Stem's one For the next few weeks, can you focus on fixing critical sbws bugs, and helping with authority deployments? Here's what I think we could do: I would like us to deploy sbws to 3/6 bandwidth authorities some time in June. We can do this deployment as soon as another directory authority operator is ready. To deploy more than 3 sbws instances, we need to fix these critical sbws bugs: We need sbws to generate bandwidth lines for all relays with results, even if they are not Running in the sbws tor client's current consensus. https://trac.torproject.org/projects/tor/ticket/29710#comment:13 We need sbws to use MaxAdvertisedBandwidth from the latest descriptors: https://trac.torproject.org/projects/tor/ticket/30733 We also need to look for any more critical bugs in sbws. Here are some ways we can check for bugs: We need to check if all sbws instances exclude some relays, to help us find any more bugs in sbws: https://trac.torproject.org/projects/tor/ticket/30735 90% of sbws measurement attempts fail. But these are internal errors, not network errors. So it looks like sbws has a relay selection bug: https://trac.torproject.org/projects/tor/ticket/30719#comment:2 After we do these tasks, we can deploy sbws to 4 bandwidth authorities. What do you think? T

2 2

Raising AuthDirMaxServersPerAddr to 4?
by Roger Dingledine 03 Jun '19

03 Jun '19

Hi folks, I've been talking to a longtime exit relay operator, who is in the odd position of having a good 1gbit network connection, but only one IP address. He used to push an average of 500mbit on his exit relay, but then the HSDir DoS flatlined his relay for a while (!), and now, perhaps due to the bwauth variability, his exit relay only recovered to maybe 200mbit. He is running a second exit relay on that IP address, but also perhaps due to the bwauth variability, it hasn't attracted much attention either. The real answer is to fix the bandwidth measurement infrastructure. But while we're patiently waiting for progress there, I've been thinking to raise moria1's AuthDirMaxServersPerAddr to 4, i.e. to allow 4 relays per IP address onto the network. I don't think it would significantly increase our risk due to Sybil attacks, whereas there is a clear benefit in terms of some more 100's of mbits of good exit relay capacity. I will propose this change to the dir-auth list in a bit, but here is your chance to point out surprising impacts that I haven't thought of. Thanks! --Roger

3 14

Release: obfs4proxy-0.0.10
by Yawning Angel 02 Jun '19

02 Jun '19

Hello, I just tagged obfs4proxy-0.0.10. The primary changes are a minor fix to the meek_lite behavior when using `utls` as the TLS implementation, and a series of updates (primarily following upstream) to the `utls` library. Tarball/Signature: https://people.torproject.org/~yawning/releases/obfs4proxy/obfs4proxy-0.0.1… https://people.torproject.org/~yawning/releases/obfs4proxy/obfs4proxy-0.0.1… Changes in version 0.0.10 - 2019-04-12: - Disable behavior distinctive to crypto/tls when using utls. - Bump the version of the utls fork. Regards, -- Yawning Angel

5 6

Should we allow matrix.org users on tor IRC channels?
by teor 01 Jun '19

01 Jun '19

Hi all, JeremyRand has asked us to allow matrix.org users on the tor IRC channels on irc.oftc.net. matrix.org users are rejected at the moment, because they are not registered with OFTC. For details, see: https://trac.torproject.org/projects/tor/ticket/30717 T -- teor ----------------------------------------------------------------------

1 0

Fwd: Orbot 16.1.0-BETA-1-tor-0.4.0.4-rc
by Nathan Freitas 31 May '19

31 May '19

Tor 0.4.0.4 working well on Android. We've updated the tor-android library to 0.4.0.4-rc, and included it in the latest Orbot beta. It is available via Gradle at the usual spot (see below): https://github.com/guardianproject/tor-android/releases/tag/tor-android-bin… maven { url "https://raw.githubusercontent.com/guardianproject/gpmaven/master" } implementation 'org.torproject:tor-android-binary:0.4.0.4-rc' -------- Forwarded Message -------- Subject: Orbot 16.1.0-BETA-1-tor-0.4.0.4-rc Date: Fri, 31 May 2019 16:25:20 -0400 From: Nathan of Guardian <nathan(a)guardianproject.info> Organisation: Guardian Project To: Guardian Dev <guardian-dev(a)lists.mayfirst.org> /** 16.1.0-BETA-1-tor-0.4.0.4-rc / 31 May 2019 / 417e4fcd0720be57328d63a96d2c9fc0e119330f **/ TAG: https://gitlab.com/guardianproject/orbot/tags/16.1.0-BETA-1-tor-0.4.0.4-rc Thanks to @bitmold @sisbell and @eighthave for all the great work, not to mention the core Tor dev team making all things mobile better every day! APKs posted at: https://guardianproject.info/releases/ [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-arm64-v8a-release.apk 2019-05-31 15:17 12M [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-arm64-v8a-release.apk.asc 2019-05-31 15:17 833 [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-armeabi-release.apk 2019-05-31 15:18 12M [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-armeabi-release.apk.asc 2019-05-31 15:18 833 [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-armeabi-v7a-release.apk 2019-05-31 15:18 12M [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-armeabi-v7a-release.apk.asc 2019-05-31 15:18 833 [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-universal-release.apk 2019-05-31 15:18 32M [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-universal-release.apk.asc 2019-05-31 15:18 833 [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-x86-release.apk 2019-05-31 15:18 12M [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-x86-release.apk.asc 2019-05-31 15:18 833 [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-x86_64-release.apk 2019-05-31 15:18 12M [ ] Orbot-16.1.0-BETA-1-tor-0.4.0.4-rc-fullperm-x86_64-release.apk.asc 417e4fcd update version to 16100003 aka 16.1.0-BETA-1-tor-0.4.0.4-rc 7ae000d0 fix pdnsd/VPN support dde1957d Merge pull request #219 from bitmold/delete_minimalperm_manifest a251d52f Merge pull request #218 from bitmold/ndk_app_platform_warning eda464bb remove incorrect torFile assignment a1c5806a update tor-android to 0.4.0.4-rc ad2e875b We no longer use the minimalperm product flavor so there's no need to keep this manifest file in app/src 917e49f5 Removes warning on ndk-build where the target API for NDK (16) is greater than the sdk version defined for the project. Since nothing was specified in the manifest it defaulted to 1 but we can set this to Orbot's minSdkVersion of 16 to get rid of this warning f83a98f4 Merge branch 'sisbell-issue_199' ff7d3dd5 (sisbell-issue_199) Merge branch 'issue_199' of https://github.com/sisbell/orbot into sisbell-issue_199 7c2cfc3e Merge pull request #212 from sisbell/issue_211 1316fd65 Merge branch 'bitmold-no_personalized_learning_kb' e3fd4afa (bitmold-no_personalized_learning_kb) Merge branch 'no_personalized_learning_kb' of https://github.com/bitmold/orbot into bitmold-no_personalized_learning_kb aa8ad867 Merge pull request #204 from bitmold/unreferenced_classes 16826a49 Merge branch 'bitmold-removed_obsolete_version_checks' 17154609 (bitmold-removed_obsolete_version_checks) Merge branch 'removed_obsolete_version_checks' of https://github.com/bitmold/orbot into bitmold-removed_obsolete_version_checks 0e4b42a7 add close bracket 4cee987c Merge branch 'master' into removed_obsolete_version_checks 08c35bd3 Remove unused resources. ae4ce1c9 Fixes #211: Resource Not Found on Command Line Build 2fb7e05a Merge pull request #210 from eighthave/fastlane 57120100 rename all metadata locale dirs after the Fastlane/Play names baced180 setup Fastlane to upload to Google Play d2feefdd Removed Obsolete @TaretApi Annotations for API Levels that are lower than Orbot's minimum, API Level 16 a32452e7 Make text inputs in Orbot declare that they do want to opt out of IME personalized learning. Of course, IMEs may ignore this request, but it's a nudge in the direction of Tor's general philosophy on user privacy, particularly with regards to minimizing the footprint that a Tor app leaves on the user's device. 7d8e41a6 Removed Constraint Layout Dependency 5d04d418 Removed Unused Classes 2b6abd7e Removed Obsolete Version Checks cd6560fa Merge pull request #202 from bitmold/no_constraint_layout a5d5c99f (public/master, gl/master) remove unused launcher art 9257b66f don't shrink or minify for now 56917567 (tag: 16.0.6-BETA-2-tor-0.3.5.8) many small changes to support new binary loading, startup and more - improved handling of port conflicts - fixed loading of tor binary on arm64-v8a - changed how w e execute shell commands 8ed080f9 Removed Constraint Layout Dependency 2a76b7af (tag: 16.0.6-BETA-1-tor-0.3.5.8) updated to SDK 28, added android.permission.FOREGROUND_SERVICE permission fb14c76c fixed strings with two many \\ escapes 45575772 updating to tor-0.3.5.8-rc to add support for Android Q 22d5ffdf update gradle tools 76796fea Merge pull request #200 from eighthave/fastlane-supply 6ba0cecc add .gitlab-ci.yml setup with errorprone 3face00a build gradle to 4.4.1, and make gradlew verify the download 69bd7fe9 move app store graphics into fdroid/fastlane file layout f93c11e7 Merge pull request #190 from SkewedZeppelin/master 8ab13f63 Fixup bad indentation from 6e4b7008 12b91c44 Expose PreferIPv6 and NoIPv4Traffic options 8ad76680 Move Google repo above jcenter d4befad1 cleanup and binary loading fixes 2d4e2a5f update tor to 0.3.5.6-RC 0f314214 (tag: 16.0.5-RC-2-tor-0.3.4.9) update to 16.0.5-RC-2-tor-0.3.4.9 2c59c848 ensure startForeground() is always immediately called - since we use startForegroundServce() we must ensure the receiving service complies!

1 0

Merger and Mainline Handovers
by teor 30 May '19

30 May '19

Hi Nick, George, David, (I'm sending this email to tor-dev so everyone knows how Core Tor merges are going.) Mainline Mergers David is back from leave, so I'm going to stop doing mainline merges. But please let me know if there's a merge I can help with. (Email or Signal is best, IRC has a lot of backlog.) Do we need to do a handover some time? The next team meeting might be a good time. Mainline Merge Ready Tickets I moved my mainline merge trac wiki queries to this page: https://trac.torproject.org/projects/tor/wiki/org/teams/NetworkTeam/Mainlin… That page should show all of the mainline merge_ready tickets, sorted by owner and reviewer. Your name is in bold, so you can work out which tickets you should merge. (We want 3 people to look at every ticket before it merges, except for trivial changes.) Here is our full list of task tracking wiki pages: https://trac.torproject.org/projects/tor/wiki/org/teams/NetworkTeam#TaskTra… When does 0.4.0 stop being mainline? It looks like people aren't merging backports to 0.4.0 any more. That's probably a good idea: we should minimise release candidate changes. When should I start doing 0.4.0 merges as part of the backports? Backport Status We released 0.4.0.4-rc last week, so I'm going to backport some low-risk changes to 0.2.9 and later. Most of these changes have been tested in 0.4.0.3-alpha. I should be able to do the backports tomorrow or Tuesday. Here are the backports for the next few days: https://trac.torproject.org/projects/tor/wiki/user/teor#Backports:0.5dayspe… Here are the backports I will do after I get back from my leave in May: https://trac.torproject.org/projects/tor/wiki/user/teor/HiddenBackports T -- teor ----------------------------------------------------------------------

2 2

Network team: New status page for 0.4.1; help needed on ticket triage
by Nick Mathewson 30 May '19

30 May '19

Hi! This mail is mainly for the network-team folks, but I'm sending it to tor-dev anyway in case it's helpful to anybody else. I've made a new page for our 0.4.1 status, based on Teor's previous 040 status page. Here it is: https://trac.torproject.org/projects/tor/wiki/org/teams/NetworkTeam/CoreTor… Note that this time I'm using two different keywords: 041-must and 041-should. The difference here is that 041 stuff should be things that would really block a release candidate, whereas 041-should stuff is stuff that we might in theory release without fixing. (It might annoy our users or violate our best practices, so we _should_ try to fix it, at least.) At the bottom of the page are all the tickets in the milestone that are *not* marked 041-must or 041-should. Some of these are things we could still fix anyway in 041; many will be removed. Let's triage these together! As discussed at the last meeting, we are currently in feature freeze for 0.4.1. Other than tickets that are marked "postfreeze-ok", we're planning not to take any more features in Tor 0.4.1. (Network team folks, please remember to spend your time on 041-must and 041-should tickets, and on Sponsor 19 tickets. If we make enough progress on these, we can open up 0.4.2 for merging early.) happy hacking, -- Nick

1 1

Proposal 302: Hiding onion service clients using WTF-PAD
by George Kadianakis 27 May '19

27 May '19

Filename: 302-padding-machines-for-onion-clients.txt Title: Hiding onion service clients using padding Author: George Kadianakis, Mike Perry Created: Thursday 16 May 2019 Status: Accepted Ticket: #28634 0. Overview Tor clients use "circuits" to do anonymous communications. There are various types of circuits. Some of them are for navigating the normal Internet, others are for fetching Tor directory information, others are for connecting to onion services, while others are simply for measurements and testing. It's currently possible for MITM type of adversaries (like tor-network-level and local-area-network adversaries) to distinguish Tor circuit types from each other using a wide array of metadata and distinguishers. In this proposal, we study various techniques that can be used to distinguish client-side onion service circuits and provide WTF-PAD circuit padding machines (using prop#254) to hide them against certain adversaries. 1. Motivation We are writing this proposal for various reasons: 1) We believe that in an ideal setting MITM adversaries should not be able to distinguish circuit types by inspecting traffic. Tor traffic should look amorphous to an outside observer to maximize uncertainty and anonymity properties. Client-side onion service circuits are an easy target for this proposal, because we believe we can improve their privacy with low bandwidth overhead. 2) We want to start experimenting with the WTF-PAD subsystem of Tor, and this use-case provides us with a good testbed. 3) We hope that by actually starting to use the WTF-PAD subsystem of Tor, we will encourage more researchers to start experimenting with it. 2. Scope of the proposal [SCOPE] Given the above, this proposal sets forth to use the WTF-PAD system to hide client-side onion service circuits against the classifiers of paper by Kwon et al. above. By client-side onion service circuits we refer to these two types of circuits: - Client-side introduction circuits: Circuit from client to the introduction point - Client-side rendezvous circuits: Circuit from client to the rendezvous point Service-side onion service circuits are not in scope for this proposal, and this is because hiding those would require more bandwidth and also more advanced WTF-PAD features. Furthermore, this proposal only aims to cloak the naive distinguishing features mentioned in the [KNOWN_DISTINGUISHERS] section, and can by no means guarantee that client-side onion service circuits are totally indistinguishable by other means. The machines specified in this proposal are meant to be lightweight and created for a specific purpose. This means that they can be easily extended with additional states to do more advanced hiding. 3. Known distinguishers against onion service circuits [KNOWN_DISTINGUISHERS] Over the past years it's been assumed that motivated adversaries can distinguish onion-service traffic from normal Tor traffic given their special characteristics. As far as we know, there has been relatively little research-level work done to this direction. The main article published in this area is the USENIX paper "Circuit Fingerprinting Attacks: Passive Deanonymization of Tor Hidden Services" by Kwon et al. [0] The above paper deals with onion service circuits in sections 3.2 and 5.1. It uses the following three "naive" circuit features to distinguish circuits: 1) Circuit construction sequence 2) Number of incoming and outgoing cells 3) Duration of Activity ("DoA") All onion service circuits have particularly loud signatures to the above characteristics, but WTF-PAD (prop#254) gives us tools to effectively silence those signatures to the point where the paper's classifiers won't work. 4. Hiding circuit features using WTF-PAD According to section [KNOWN_DISTINGUISHERS] there are three circuit features we are attempting to hide. Here is how we plan to do this using the WTF-PAD system: 1) Circuit construction sequence The USENIX paper uses the directions of the first 10 cells sent in a circuit to fingerprint them. Client-side onion service circuits have unique circuit construction sequences and hence they can be fingeprinted using just the first 10 cells. We use WTF-PAD to destroy this feature of onion service circuits by carefully sending padding cells (relay DROP cells) during circuit construction and making them look exactly like most general tor circuits up till the end of the circuit construction sequence. 2) Number of incoming and outgoing cells The USENIX paper uses the amount of incoming and outgoing cells to distinguish circuit types. For example, client-side introduction circuits have the same amount of incoming and outgoing cells, whereas client-side rendezvous circuits have more incoming than outgoing cells. We use WTF-PAD to destroy this feature by changing the number of cells sent in introduction circuits. We leave rendezvous circuits as is, since the actual rendezvous traffic flow usually resembles well normal Tor circuits. 3) Duration of Activity ("DoA") The USENIX paper uses the period of time during which circuits send and receive cells to distinguish circuit types. For example, client-side introduction circuits are really short lived, wheras service-side introduction circuits are very long lived. OTOH, rendezvous circuits have the same median lifetime as general Tor circuits which is 10 minutes. We use WTF-PAD to destroy this feature of client-side introduction circuits by setting a special WTF-PAD option, which keeps the circuits open for 10 minutes completely mimicking the DoA of general Tor circuits. 4.1. A dive into general circuit construction sequences [CIRCCONSTRUCTION] In this section we give an overview of how circuit construction looks like to a network or guard-level adversary. We use this knowledge to make the right padding machines that can make intro and rend circuits look like these general circuits. In particular, most general Tor circuits used to surf the web or download directory information, start with the following 6-cell relay cell sequence (cells surrounded in [brackets] are outgoing, the others are incoming): [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [BEGIN] -> CONNECTED When this is done, the client has established a 3-hop circuit and also opened a stream to the other end. Usually after this comes a series of DATA cell that either fetches pages, establishes an SSL connection or fetches directory information: [DATA] -> [DATA] -> DATA -> DATA The above stream of 10 relay cells defines the grand majority of general circuits that come out of Tor browser during our testing, and it's what we are gonna use to make introduction and rednezvous circuits blend in. Please note that in this section we only investigate relay cells and not connection-level cells like CREATE/CREATED or AUTHENTICATE/etc. that are used during the link-layer handshake. The rationale is that connection-level cells depend on the type of guard used and are not an effective fingerprint for a network/guard-level adversary. 5. WTF-PAD machines For the purposes of this proposal we will make use of four WTF-PAD machines as follows: - Client-side introduction circuit hiding machine (origin-side) - Client-side introduction circuit hiding machine (relay-side) - Client-side rendezvous circuit hiding machine (origin-side) - Client-side rendezvous circuit hiding machine (relay-side) In the following sections we will analyze these machines. 5.1. Client-side introduction circuit hiding machines [INTRO_CIRC_HIDING] These two machines are meant to hide client-side introduction circuits. The origin-side machine sits on the client and sends padding towards the introduction circuit, whereas the relay-side machine sits on the middle-hop (second hop of the circuit) and sends padding towards the client. The padding from the origin-side machine terminates at the middle-hop and does not get forwarded to the actual introduction point. Both of these machines only get activated for introduction circuits, and only after an INTRODUCE1 cell has been sent out. This means that before the machine gets activated our cell flow looks like this: [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [INTRODUCE1] Comparing the above with section [CIRCCONSTRUCTION], we see that the above cell sequence matches the one from general circuits up to the first 7 cells. However, in normal introduction circuits this is followed by an INTRODUCE_ACK and then the circuit gets teared down, which does not match the sequence from [CIRCCONSTRUCTION]. Hence when our machine is used, after sending an [INTRODUCE1] cell, we also send a [PADDING_NEGOTIATE] cell, which gets answered by a PADDING_NEGOTIATED cell and an INTRODUCE_ACKED cell. This makes us match the [CIRCCONSTRUCTION] sequence up to the first 10 cells. After that, we continue sending padding from the relay-side machine so as to fake a directory download, or an SSL connection setup. We also want to continue sending padding so that the connection stays up longer to destroy the "Duration of Activity" fingerprint. To calculate the padding overhead, we see that the origin-side machine just sends a single [PADDING_NEGOATIATE] cell, wheras the origin-side machine sends a PADDING_NEGOTIATED cell and between 7 to 10 DROP cells. This means that the average overhead of this machine is 11 padding cells. In terms of WTF-PAD terminology, these machines have three states (START, OBF, END). They move from the START to OBF state when the first non-padding cell is received on the circuit, and they stay in the OBF state until all the padding gets depleted. The OBF state is controlled by a histogram which specifies the parameters described in the paragraphs above. After all the padding finishes, it moves to END state. We also set a special WTF-PAD flag which keeps the circuit open even after the introduction is performed. In particular, with this feature the circuit will stay alive for the same durations as normal web circuits before they expire (usually 10 minutes). 5.2. Client-side rendezvous circuit hiding machines The rendezvous circuit machines apply on client-side rendezvous circuits and only after the rendezvous point has been established (REND_ESTABLISHED has been received). Up to that point, the following cell sequence has been observed on the circuit: [EXTEND2] -> EXTENDED2 -> [EXTEND2] -> EXTENDED2 -> [ESTABLISH_REND] -> REND_ESTABLISHED which matches the general circuit construction sequence [CIRCCONSTRUCTION] up to the first 6 cells. However after that, normal rendezvous circuits receive a RENDEZVOUS2 cell followed by a [BEGIN] and a CONNECTED, which does not fit the circuit construction sequence we are trying to imitate. Hence our machine gets activated right after REND_ESTABLISHED is received, and continues by sending a [PADDING_NEGOTIATE] and a [DROP] cell, before receiving a PADDING_NEGOTIATED and a DROP cell, effectively blending into the general circuit construction sequence on the first 10 cells. After that our machine gets deactivated, and we let the actual rendezvous circuit shape the traffic flow. Since rendezvous circuits usually immitate general circuits (their purpose is to surf the web), we can expect that they will look alike. In terms of overhead, this machine is quite light. Both sides send 2 padding cells, for a total of 4 padding cells. 6. Overhead analysis Given the parameters above, intro circuit machines have an overhead of 11 padding cells, and rendezvous circuit machines have an overhead of 4 cpadding ells. . This means that for every intro and rendezvous circuit there will be an overhead of 15 padding cells in average, which is about 7.5kb. In the PrivCount paper [1] we learn that the Tor network sees about 12 million successful descriptor fetches per day. We can use this figure to assume that the Tor network also sees about 12 million intro and rendezvous circuits per day. Given the 7.5kb overhead of each of these circuits, we get that our padding machines infer an additional 94GB overhead per day on the network, which is about 3.9GB per hour. XXX Isn't this kinda intense????? Using the graphs from metrics we see that the Tor network has total capacity of 300 Gbit/s which is about 135000GB per hour, so 3.9GB per hour is not that much, but still... 7. Discussion 7.1. Alternative approaches These machines try to hide onion service client-side circuits by obfuscating their looks. This is a reasonable approach, but if the resulting circuits look unlike any other Tor circuits, they would still be fingerprintable just by that fact. Another approach we could take is make normal client circuits look like onion service circuits, or just make normal clients establish fake onion service circuits periodically. The hope here is that the adversary won't be able to distinguish fake onion service circuits from real ones. This approach has not been taken yet, mainly because it requires additional WTF-PAD features and poses greater overhead risks. 7.2. Future work As discussed in [SCOPE], this proposal only aims to hide some very specific features of client-side onion service circuits. There is lots of work to be done here to see what other features can be used to distinguish such circuits, and also what other classifiers can be built using deep learning and whatnot. --- [0]: https://www.usenix.org/node/190967 https://blog.torproject.org/technical-summary-usenix-fingerprinting-paper [1]: "Understanding Tor Usage with Privacy-Preserving Measurement" by Akshaya Mani, T Wilson-Brown, Rob Jansen, Aaron Johnson, and Micah Sherr In Proceedings of the Internet Measurement Conference 2018 (IMC 2018).

4 5