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Defending against guard discovery attacks by pinning middle nodes
by George Kadianakis 13 Nov '14

13 Nov '14

Hey Nick, this mail is about the schemes we were discussing during the dev meeting on how to protect HSes against guard discovery attacks (#9001). I think we have some ideas on how to offer better protection against such attacks, mainly by keeping our middle nodes more static than we do currently. For example, we could keep our middle nodes for 3-4 days instead of choosing new ones for every circuit. As Roger has suggested, maybe we don't even need to write the static middle nodes on the state file, just use new ones if Tor has restarted. Keeping middle nodes around for longer will make those attacks much slower (it restricts them to one attack attempt every 3-4 days), but are there any serious negative implications? For example, if you were unlucky and you picked an evil middle node, and you keep it for 3-4 days, that middle node will always see your traffic coming through your guard (assuming a single guard per client). If we assume you use a non-popular guard node (with only a few clients using it), the middle guard might be able to think "Ah, the circuit that comes from that guard node is always user X" making your circuits a bit linkable from the PoV of your middle node. What other attacks should we be wary about? Maybe partitioning attacks based on client behavior? And how should we move this forward if we decide it's worth it? Should we start writing a Tor proposal? Thanks!

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Gap between advertised and utilized bandwidth
by Karsten Loesing 12 Nov '14

12 Nov '14

Hi George, I found this in my IRC backlog from November 7, 2014: 20:34 #tor-dev: < asn> karsten: how should one read these graphs https://metrics.torproject.org/bandwidth.html#bandwidth-flags ? 20:34 #tor-dev: < asn> karsten: can the gap between two lines be interpreted as available (unused) bw? 20:35 #tor-dev: < asn> karsten: so the gap between the yellow lines is the available guard bw? 20:35 #tor-dev: < asn> karsten: but there is not that much available exit bw, right? so it's not entirely accurate. These are all fine questions. First of all, the gap between two lines is *supposed* to be interpreted as available (unused) bandwidth. Advertised bandwidth is the number of bytes that all relay together were willing and capable to handle. (It's the minimum of the three bandwidth numbers contained in server descriptors.) Bandwidth history is the number of bytes that all relays together actually handled. (It's the sum of byte histories contained in extra-info descriptors.) So, the difference of the two should be the number of bytes that was left unused, in theory. However, I can't say how accurate this comparison is. If I had the time to investigate, I'd start by downloading the latest descriptor archives from CollecTor and comparing advertised bandwidths and bandwidth histories per relay. If you or somebody else wants to do that I might be able to give them a hand. All the best, Karsten

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tor packet handling
by Mohiuddin Ebna Kawsar 12 Nov '14

12 Nov '14

Hi, I want to develop extension(intrusion detection) for tor. for that i have to extract TCP and IP header from packet. I need to know where and how tor handle packet(TCP/IP). is it in buffer.c / connection.c ? Regards Kawsar

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Re: [tor-dev] Pluggable-transport implementations of your website fingerprinting defenses
by David Fifield 10 Nov '14

10 Nov '14

On Sun, Nov 09, 2014 at 08:23:33PM -0500, Xiang Cai wrote: > I started to work on csbuflo code a long time ago, and I wasn’t using any > version control software back then, so I don’t have file commit history either… > Sorry about that. > > However, I only modified several core files based on openssh-5.9p1 source code: > clientloop.c > serverloop.c > packet.c > misc.c > and related header files. A simple diff between these files and the original > ssh code will tell you what I modified. > > I am not sure if the code is directly useable for your purpose, but I’ll > briefly talk about what my code does, and hopefully, it will give you some help > when reading the code. > > The code actually implements the Glove system, which requires that both the > client and server have a transcript of “super traces”. — I believe the location > of the transcript is hardcoded as ‘/var/tmp/st.txt’ in my code … > When visiting a website that is not shown in the transcript, the system falls > back to use CSBuFLO scheme. I see. Thanks for the references. Building this code into a pluggable transport would be more work than I had originally supposed. If there were a minimal network client and server, only implementing the website fingerprinting defense, then I wouldn't mind spending half a day to make them into a pluggable transport. But as it stands, it looks like it will be more work, essentially a reimplementation. David Fifield

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Re: [tor-dev] Hidden Service authorization UI
by Nathan Freitas 10 Nov '14

10 Nov '14

On Sun, Nov 9, 2014, at 07:50 AM, George Kadianakis wrote: > Hidden Service authorization is a pretty obscure feature of HSes, that > can be quite useful for small-to-medium HSes. ... > For example, it would be interesting if TBB would allow people to > input a password/pubkey upon visiting a protected HS. Protected HSes > can be recognized by looking at the "authentication-required" field of > the HS descriptor. Typing your password on the browser is much more > useable than editing a config file. We have been working on implementing an OnionShare feature for Orbot, as a plugin/add-on. Since the client and the server are both one simple app, it seems like we could easily implement the HS Authorization feature. Since the goal is to share a file with a small audience, the second "client key" approach seems to be the best, most secure approach to me. Any thoughts?

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Re: [tor-dev] HSDir Auth and onion descriptor scraping - actual stats
by Gareth Owen 10 Nov '14

10 Nov '14

OK. curiosity got the better of me. I took a random sample of 20,368 HS descriptors and just 131 were authenticated - that's about 0.6%. The code I used is here: https://github.com/drgowen/tor-research-framework/blob/master/src/main/java… Best Gareth PS - I only took a sample (rather than the whole batch) because each HS descriptor is in its own file and it takes ages to process - nevertheless, because of the way its collected the sample should be representative. -- Dr Gareth Owen Senior Lecturer Forensic Computing Course Leader School of Computing, University of Portsmouth *Office:* BK1.25 *Tel:* +44 (0)2392 84 (6423) *Web*: ghowen.me

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HSDir Auth and onion descriptor scraping
by grarpamp 10 Nov '14

10 Nov '14

> George K: > I suspect that HS authorization is very rare in the current network, > and if we believe it's a useful tool, it might be worthwhile to make > it more useable by people. Is anyone making their HSDir onion descriptor scraping patches available somewhere? I'd suspect the rarity of HS authorization could also be determined with that since some fields would be obfuscated and thus not match patterns. s/scraping/logging/ rend--spec.txt: 2. Authentication and authorization. 2.1. Service with large-scale client authorization 2.2. Authorization for limited number of clients 2.3. Hidden service configuration 2.4. Client configuration

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Running a Separate Tor Network
by Tom Ritter 10 Nov '14

10 Nov '14

Hi all, Not content to let you have all the fun, I decided to run my own Tor network! Kidding ;) But the Directory Authorities, the crappy experiment leading up to Black Hat, and the promise that one can recreate the Tor Network in the event of some catastrophe interests me enough that I decided to investigate it. I'm aware of Chutney and Shadow, but I wanted it to feel as authentic as possible, so I forwent those, and just ran full-featured independent tor daemons. I explicitly wanted to avoid setting TestingTorNetwork. I did have to edit a few other parameters, but very few. [0] I plan on doing a blog post, giving a HOWTO, but I thought I'd write about my experience so far. I've found a number of interesting issues that arise in the bootstrapping of a non-TestingTorNetwork, mostly around reachability testing. ----- One of the first things I ran into was a problem where I could not get any routers to upload descriptors. An OR checks itself to determine reachability before uploading a descriptor by building a circuit - bypassed with AssumeReachable or TestingTorNetwork. This works fine for Chutney and Shadow, as they reach into the OR and set AssumeReachable. But if the Tor Network were to be rebooted... most nodes out there would _not_ have AssumeReachable, and they would not be able to perform self-testing with a consensus consisting of just Directory Authorities. I think nodes left running would be okay, but nodes restarted would be stuck in a startup loop. I imagine what would actually happen is Noisebridge and TorServers and a few other close friends would set the flag, they would get into the consensus, and then the rest of the network would start coming back... (Or possibly a few nodes could anticipate this problem ahead of time, and set it now.) What I had to do was make one of my Directory Authorities an exit - this let the other nodes start building circuits through the authorities and upload descriptors. Maybe an OR should have logic that if it has a valid consensus with no Exit nodes, it should assume it's reachable and send a descriptor - and then let the Directory Authorities perform reachability tests for whether or not to include it? From the POV of an intentional DoS - an OR doesn't have to obey the reachability test of course, so no change there. It could potentially lead to an unintentional DoS where all several thousand routers start slamming the DirAuths as soon as a usable-but-blank consensus is found... but AFAIK routers probe for a consensus based on semi-random timing anyway, so that may mitigate that? ----- Another problem I ran into was that nodes couldn't conduct reachability tests when I had exits that were only using the Reduced Exit Policy - because it doesn't list the ORPort/DirPort! (I was using nonstandard ports actually, but indeed the reduced exit policy does not include 9001 or 9030.) Looking at the current consensus, there are 40 exits that exit to all ports, and 400-something exits that use the ReducedExitPolicy. It seems like 9001 and 9030 should probably be added to that for reachability tests? ----- Continuing in this thread, another problem I hit was that (I believe) nodes expect the 'Stable' flag when conducting certain reachability tests. I'm not 100% certain - it may not prevent the relay from uploading a descriptor, but it seems like if no acceptable exit node is Stable - some reachability tests will be stuck. I see these sorts of errors when there is no stable Exit node (the node generating the errors is in fact a Stable Exit though, so it clearly uploaded its descriptor and keeps running): Oct 13 14:49:46.000 [warn] Making tunnel to dirserver failed. Oct 13 14:49:46.000 [warn] We just marked ourself as down. Are your external addresses reachable? Oct 13 14:50:47.000 [notice] No Tor server allows exit to [scrubbed]:25030. Rejecting. Since ORPort/DirPort are not in the ReducedExitPolicy, this (may?) restrict the number of nodes available for conducting a reachability test. I think the Stable flag is calculated off the average age of the network though, so the only time when this would cause a big problem is when the network (DirAuths) have been running for a little bit and a full exit node hasn't been added - it would have to wait longer for the full exit node to get the Stable flag. ----- Getting a BWAuth running was... nontrivial. Some of the things I found: - SQLAlchemy 0.7.x is no longer supported. 0.9.x does not work, nor 0.8.x. 0.7.10 does. - Several quasi-bugs with the code/documentation (the earliest three commits here: https://github.com/tomrittervg/torflow/commits/tomedits) - The bandwidth scanner actively breaks in certain situations of divide-by-zero (https://github.com/tomrittervg/torflow/commit/053dfc17c0411dac0f6c4e43954f9…) - The scanner will be perpetually stuck if you're sitting on the same /16 and you don't perform the equivalent of EnforceDistinctSubnets 0 [1] Ultimately, while I successfully produced a bandwidth file [2], I wasn't convinced it was meaningful. There is a tremendous amount of code complexity buried beneath the statement 'Scan the nodes and see how fast they are', and a tremendous amount of informational complexity behind 'Weight the nodes so users can pick a good stream'. ----- I tested what it would look like if an imposter DirAuth started trying to participate in the consensus. It generated the warning you would expect: Oct 14 00:04:31.000 [warn] Got a vote from an authority (nickname authimposter, address W.X.Y.Z) with authority key ID Z. This key ID is not recognized. Known v3 key IDs are: A, B, C, D But it also generated a warning you would not expect, and sent me down a rabbit hole for a while: Oct 10 21:44:56.000 [debug] directory_handle_command_post(): Received POST command. Oct 10 21:44:56.000 [debug] directory_handle_command_post(): rewritten url as '"/tor/post/consensus-signature"'. Oct 10 21:44:56.000 [notice] Got a signature from W.X.Y.Z. Adding it to the pending consensus. Oct 10 21:44:56.000 [info] dirvote_add_signatures_to_pending_consensus(): Have 1 signatures for adding to ns consensus. Oct 10 21:44:56.000 [info] dirvote_add_signatures_to_pending_consensus(): Added -1 signatures to consensus. Oct 10 21:44:56.000 [info] dirvote_add_signatures_to_pending_consensus(): Have 1 signatures for adding to microdesc consensus. Oct 10 21:44:56.000 [info] dirvote_add_signatures_to_pending_consensus(): Added -1 signatures to consensus. Oct 10 21:44:56.000 [warn] Unable to store signatures posted by W.X.Y.Z: Mismatched digest. Over on the imposter: Oct 14 00:19:32.000 [warn] http status 400 ("Mismatched digest.") response after uploading signatures to dirserver 'W.X.Y.Z:15030'. Please correct. The imposter DirAuth is sending up a signature for a consensus that is not the same consensus that the rest of the DirAuths computed. Specifically, the imposter DirAuth lists itself as a dir-source and the signature covers this line. (Everything else matches because the imposter has been outvoted and respects that.) I guess the lesson is, if you see the "Mismatched digest" warning in conjunction with the unrecognized key ID - it's just one issue, not two. ----- The notion and problems of an imposter DirAuth also come up during how the network behaves when adding a DirAuth. I started with 4 authorities, then started a fifth (auth5). Not interesting - it behaved as the imposter scenario. I then added auth5 to a single DirAuth (auth1) as a trusted DirAuth. This resulted in a consensus with 3 signatures, as auth1 did not sign the consensus. On auth1 I got warn messages: A consensus needs 3 good signatures from recognized authorities for us to accept it. This one has 2 (auth1 auth5). 3 (auth2 auth3 auth4) of the authorities we know didn't sign it. I then added auth5 to a second DirAuth (auth2) as a trusted DirAuth. This results in a consensus for auth1, auth2, and auth5 - but auth3 and auth4 did not sign it or produce a consensus. Because the consensus was only signed by 2 of the 4 Auths (e.g., not a majority) - it was rejected by the relays (which did not list auth5). At this point something interesting and unexpected happened: The other 2 DirAuths (not knowing about auth5) did not have a consensus. This tricked dirvote_recalculate_timing into thinking we should use the TestingV3AuthInitialVotingInterval parameters, so they got out of sync with the other 3 DirAuths (that did know about auth5). That if/else statement seems very odd, and the parameters seem odd as well. First off, I'm not clear what the parameters are intended to represent. The man page says: TestingV3AuthInitialVotingInterval N minutes|hours Like V3AuthVotingInterval, but for initial voting interval before the first consensus has been created. Changing this requires that TestingTorNetwork is set. (Default: 30 minutes) TestingV3AuthInitialVoteDelay N minutes|hours Like TestingV3AuthInitialVoteDelay, but for initial voting interval before the first consensus has been created. Changing this requires that TestingTorNetwork is set. (Default: 5 minutes) TestingV3AuthInitialDistDelay N minutes|hours Like TestingV3AuthInitialDistDelay, but for initial voting interval before the first consensus has been created. Changing this requires that TestingTorNetwork is set. (Default: 5 minutes) Notice that the first says "Like V3AuthVotingInterval", but the other two just repeat their name? And how there _is no_ V3AuthInitialVotingInterval? And that you can't modify these parameters without turning on TestingTorParameters (despite the fact that they will be used without TestingTorNetwork?) And also, unrelated to the naming, these parameters are a fallback case for when we don't have a consensus, but if they're not kept in sync with V3AuthVotingInterval and their kin - the DirAuth can wind up completely out of sync and be unable to recover (except by luck). It seems like these parameters should be renamed to V3AuthInitialXXX, keep their existing defaults, remove the requirement on TestingTorNetwork, and be documented as needing to be divisors of the V3AuthVotingXXX parameter, to allow a DirAuth who has tripped into them to be able to recover. I have a number of other situations I want to test around adding, subtracting, and manipulating traffic to a DirAuth to see if there are other strange situations that can arise. ----- Other notes: - I was annoyed by TestingAuthDirTimeToLearnReachability several times (as I refused to turn on TestingTorNetwork) - I wanted to override it. I thought maybe that should be an option, but ultimately convinced myself that in the event of a network reboot, the 30 minutes would likely still be needed. - The Directory Authority information is a bit out of date. Specifically, I was most confused by V1 vs V2 vs V3 Directories. I am not sure if the actual network's DirAuths set V1AuthoritativeDirectory or V2AuthoritativeDirectory - but I eventually convinced myself that only V3AuthoritativeDirectory was needed. - It seems like an Authority will not vote for itself as an HSDir or Stable... but I could't find precisely where that was in the code. (It makes sense to not vote itself Stable, but I'm not sure why HSDir...) - The networkstatus-bridges file is not included in the tor man page - I feel like the log message "Consensus includes unrecognized authority" (currently info) is worthy of being upgraded to notice. - While debugging, I feel this patch would be helpful. [3] - I've had my eye on Proposal 164 for a bit, so I'm keeping that in mind - I wanted the https://consensus-health.torproject.org/ page for my network, but didn't want to run the java code, so I ported it to python. This project is growing, and right now I've been editing consensus_health_checker.py as well. https://github.com/tomrittervg/doctor/commits/python-website I have a few more TODOs for it (like download statistics), but it's coming along. ----- Finally, something I wanted to ask after was the idea of a node (an OR, not a client) belonging to two or more Tor networks. From the POV of the node operator, I would see it as a node would add some config lines (maybe 'AdditionalDirServer' to add to, rather than redefining, the default DirServers), and it would upload its descriptors to those as well, fetch a consensus from all AdditionalDirServers, and allow connections from and to nodes in either. I'm still reading through the code to see which areas would be particularly confusing in the context of multiple consensuses, but I thought I'd throw it out there. -tom ----- [0] AuthoritativeDirectory 1 V3AuthoritativeDirectory 1 VersioningAuthoritativeDirectory 1 RecommendedClientVersions [stuff] RecommendedServerVersions [stuff] ConsensusParams [stuff] AuthDirMaxServersPerAddr 0 AuthDirMaxServersPerAuthAddr 0 V3AuthVotingInterval 5 minutes V3AuthVoteDelay 30 seconds V3AuthDistDelay 30 seconds V3AuthNIntervalsValid 3 MinUptimeHidServDirectoryV2 1 hour ----- [1] diff --git a/NetworkScanners/BwAuthority/bwauthority_child.py b/NetworkScanners/BwAuthority/bwauthority_child.py index 28b89c2..e07718f 100755 --- a/NetworkScanners/BwAuthority/bwauthority_child.py +++ b/NetworkScanners/BwAuthority/bwauthority_child.py @@ -60,7 +60,7 @@ __selmgr = PathSupport.SelectionManager( percent_fast=100, percent_skip=0, min_bw=1024, - use_all_exits=False, + use_all_exits=True, uniform=True, use_exit=None, use_guards=False, ----- [2] node_id=$C447A9E99C66A96E775A5EF7A8B0DF96C414D0FE bw=37 nick=relay4 measured_at=1413257649 updated_at=1413257649 pid_error=0.0681488657221 pid_error_sum=0 pid_bw=59064 pid_delta=0 circ_fail=0.0 node_id=$70145044B8C20F46F991B7A38D9F27D157B1CB9D bw=37 nick=relay5 measured_at=1413257649 updated_at=1413257649 pid_error=0.0583026021009 pid_error_sum=0 pid_bw=59603 pid_delta=0 circ_fail=0.0 node_id=$7FAC1066DCCC0C62984B8E579C5AABBBAE8146B2 bw=37 nick=exit2 measured_at=1413257649 updated_at=1413257649 pid_error=0.0307144741235 pid_error_sum=0 pid_bw=55938 pid_delta=0 circ_fail=0.0 node_id=$9838F41EB01BA62B7AA67BDA942AC4DC3B2B0F98 bw=37 nick=exit3 measured_at=1413257649 updated_at=1413257649 pid_error=0.0124944051714 pid_error_sum=0 pid_bw=55986 pid_delta=0 circ_fail=0.0 node_id=$49090AC6DB52AD8FFF95AF1EC1E898126A9E5CA6 bw=37 nick=relay3 measured_at=1413257649 updated_at=1413257649 pid_error=0.0030073731241 pid_error_sum=0 pid_bw=56489 pid_delta=0 circ_fail=0.0 node_id=$F5C43BB6AD2256730197533596930A8DD7BEC367 bw=37 nick=exit1 measured_at=1413257649 updated_at=1413257649 pid_error=-0.0032777385693 pid_error_sum=0 pid_bw=55114 pid_delta=0 circ_fail=0.0 node_id=$3D53FF771CC3CB9DE0A55C33E5E8DA4238C96AB5 bw=37 nick=relay2 measured_at=1413257649 updated_at=1413257649 pid_error=-0.0418210520821 pid_error_sum=0 pid_bw=51021 pid_delta=0 circ_fail=0.0 ----- [3] diff --git a/src/or/networkstatus.c b/src/or/networkstatus.c index 890da0a..4d72add 100644 --- a/src/or/networkstatus.c +++ b/src/or/networkstatus.c @@ -1442,6 +1442,8 @@ networkstatus_note_certs_arrived(void) waiting_body, networkstatus_get_flavor_name(i), NSSET_WAS_WAITING_FOR_CERTS)) { + log_info(LD_DIR, "After fetching certificates, we were able to " + "accept the consensus."); tor_free(waiting_body); } }

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Pluggable-transport implementations of your website fingerprinting defenses
by David Fifield 09 Nov '14

09 Nov '14

NB I'm copying the tor-dev mailing list on this message. At CCS I saw Rishab present these papers: "CS-BuFLO: A Congestion Sensitive Website Fingerprinting Defense" http://www3.cs.stonybrook.edu/~rnithyanand/pubs/wpes2014-csb.pdf "Glove: A Bespoke Website Fingerprinting Defense" http://www3.cs.stonybrook.edu/~rnithyanand/pubs/wpes2014-glove.pdf "A Systematic Approach to Developing and Evaluating Website Fingerprinting Defenses" http://www3.cs.stonybrook.edu/~rnithyanand/pubs/ccs2014.pdf I spoke quite a lot to Rishab and suggested, since these schemes have source code, that they be wrapped in a pluggable transports interface so they can be easily used by tor clients. It is not super difficult to turn a network program into a pluggable transport, as there are libraries like pyptlib and liballium that implement the internal pluggable transports protocol. You might be familiar with wfpadtools (https://bitbucket.org/mjuarezm/obfsproxy-wfpadtools) which aims to make it easy to prototype fingerprinting defenses by specifying them in terms of (e.g. padding) primitives. I looked for the source code mentioned in the papers and I wasn't sure what would be best to use. I found https://crysp.uwaterloo.ca/software/webfingerprint/ which links to some short Python files like https://crysp.uwaterloo.ca/software/webfingerprint/tamaraw.py There is also https://github.com/xiang-cai/CSBuFLO but it doesn't appear to be directly usable. It appears to be a modified OpenSSH, without a commit history showing what was modified. What source code do you recommend for an implementation? David Fifield

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Hi everyone!
by Conny Hermansson 09 Nov '14

09 Nov '14

Hi! I´m new to the Tor project and I´m looking for some easy project to get me started. preferebly in Java, I can do debugging or write some code. Conny

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