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Are we planning to use the "package" mechanism?
by Nick Mathewson 19 Jun '17

19 Jun '17

With proposal 227 in 0.2.6.3-alpha, we added a way for authorities to vote on e.g. the latest versions of the torbrowser package. It appears we aren't actually using that, though. Are we planning to use it in the future? -- Nick

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Extending Tor stats to cover anon OSs?
by bancfc＠openmailbox.org 18 Jun '17

18 Jun '17

@TPO devs Since you do a great job safely collecting useful stats on the network, would you be open to adding a self-identifying anonOS distro option to the protocol? Would this be OK or is it mission creep? On the flip side it would be much more accurate than anything we can do to estimate active user base. Some ideas: *The distro name options could either be hard-coded values (for example TAILS, Whonix...) or a custom one chosen by downstream maintainers. *Distro maintainers would enable it for Tor clients via torrc.d

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[release] metrics-lib/DescripTor 1.8.2 minor bugfix
by iwakeh 16 Jun '17

16 Jun '17

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Hi there! Oh, well, another bugfix release of metrics-lib/DescripTor is available: https://dist.torproject.org/descriptor/1.8.2/ This release fixes a problem introduced in 1.8.0; more detail can be found in the changelog [1]. The error didn't lead to data loss as it was discovered timely. Affected instances of Onionoo and CollecTor (in the realm of torproject.org) are being updated at the moment. Please direct comments and questions to the metrics-team mailing list [2]. Cheers, iwakeh [1] https://gitweb.torproject.org/metrics-lib.git/plain/CHANGELOG.md?id=0917016… [2] https://lists.torproject.org/cgi-bin/mailman/listinfo/metrics-team -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBCAAGBQJZQ/1kAAoJEPeEx9Sa/gviPtYP+QGo4cIdMo9ev8W7E/2SRdC7 gcfEOUZE1RCapm6IkFHboslgb5saEFE952wAhFomWunC9df1TByKTLVPiJsVmOf+ Q7WbQh0zZF1W9BkgwIFGCGLhfm2kuLFwXeNHEJ2h73+aKlOvvKv1Lz13e+VCfzLK jWqsygqVBE3rPloJOOF8CBWWIL0OOzLbWWcp1sUouFUVX1kDpXZZoZ8BsOD3pPH4 oyqyhNbbRwqvGqvt/38RDra80QK2vUtwxi6rxYTildWFRF0l/VLqJEjhgzZBE4z+ kZkc6elnd8Y7F2rlKQY6sHQbDDRbAzcnIsanBkBM8+TzAQVV9crRnoCsuewk93bg K4p2VDqwc9p6ehotnSJ74dCV0ValNSkqRZyawrJi9xViDS6i3yaaswbMog4lzmh/ sMKjMwLYVD3GiV0uIOlz/okyuJBi1DCbV6LgQF5DkFvlhX7eqPQXXSLYeDkYtLcJ +8PZ0fXg3uJjja1I8yrAaaoyh0VRtWGkl0xNMLCY9vLSA6uazWMsk36kME8iODt+ hmvQ2Nml6DylrumhORaOI8jb2Ezy5ME4OkCRbGxTrKCYJ0DN64LGZsumoKOxKqfl m1L4gqIw1UDwL0vGE330rk+ndIoRRKr5F9/I5f0pAClHG4D63UUXNGFniIfb2SLy erda0PO7N8KJ7zKOn/nq =zB9B -----END PGP SIGNATURE-----

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Summer 2017 Internship to Create a Bridge Bandwidth Scanner
by isis agora lovecruft 16 Jun '17

16 Jun '17

Hello all! I have made a brief post on our blog to announce an exciting intership opportunity we have available! https://blog.torproject.org/blog/summer-2017-internship-create-bridge-bandw… Best, -- ♥Ⓐ isis agora lovecruft _________________________________________________________ OpenPGP: 4096R/0A6A58A14B5946ABDE18E207A3ADB67A2CDB8B35 Current Keys: https://fyb.patternsinthevoid.net/isis.txt

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The limits of timing obfuscation in obfs4
by David Fifield 12 Jun '17

12 Jun '17

Web page with graphics: https://people.torproject.org/~dcf/obfs4-timing/ I was experimenting with ways to better obfuscate the timing signature of Tor-in-obfs4, while staying compatible with the obfs4 specification. I tried to make a constant bitrate mode that sends 500 bytes every 100 ms, in which the size and timing of obfs4 packets is independent of the TLS packets underneath. It turns out that obfs4 mostly makes this possible, with one exception: a gap in the client traffic while the client waits for the server's handshake response, during which time the client cannot send anything because it doesn't yet know the shared key. Currently, the implementation of obfs4 sends data only when the underlying process (i.e., tor) has something to send. When tor is quiet, obfs4 is quiet; and when tor wants to send a packet, obfs4 sends a packet without delay. obfs4 does add a random amount of padding to its packets, which slightly alters the packet size signature but not the timing signature. Even the modes that add short interpacket delays (iat-mode=1 and iat-mode=2) only really have an effect on bulk upload/download—they don't have much of an effect on the initial handshake. See the first three rows of the attached graphic—the timing of the first dozen or so packets hardly varies across the three modes. This design, where obfs4 only sends packets when driven by tor, is an implementation choice and isn't inherent in the protocol. obfs4's framing structure [spec §5] allows for frames that contain only padding: +------------+----------+--------+--------------+------------+------------+ | 2 bytes | 16 bytes | 1 byte | 2 bytes | (optional) | (optional) | | Frame len. | Tag | Type | Payload len. | Payload | Padding | +------------+----------+--------+--------------+------------+------------+ \_ Obfs. _/ \___________ NaCl secretbox (Poly1305/XSalsa20) ___________/ obfs4 could send padding frames (at whatever size and rate) during the times when tor is quiet. The current implementation, in pseudocode, works like this (transports/obfs4/obfs4.go obfs4Conn.Write): on recv(data) from tor: send(frame(data)) If it instead worked like this, then obfs4 could choose its own packet scheduling, independent of tor's: on recv(data) from tor: enqueue data on send_buffer func give_me_a_frame(): # never blocks if send_buffer is not empty: dequeue data from send buffer return frame(data) else: return frame(padding) in a separate thread: buf = [] while true: while length(buf) < 500: buf = buf + give_me_a_frame() chunk = buf[:500] buf = buf[500:] send(chunk) sleep(100 ms) The key idea is that give_me_a_frame never blocks: if it doesn't have any application data immediately available, it returns a padding frame instead. The independent sending thread calls give_me_a_frame as often as necessary and obeys its own schedule. Note also that the boundaries of chunks sent by the sending thread are independent of frame boundaries. Yawning points me to this code in basket2 that uses the same idea of independently sending padding according to a schedule: https://git.schwanenlied.me/yawning/basket2/src/72f203e133c90a26f68f0cd33b0… I attach a proof-of-concept patch for obfs4proxy that makes it operate in a constant bitrate mode. You can see its timing signature in the fourth row of the attached graphic. With this proof of concept, I'm not trying to claim that a constant bitrate is good for performance or for censorship resistance. It's just an example of shaping obfs4's traffic pattern in a way that is independent of the underlying stream. The fifth row of the graphic shows a more complicated sine wave pattern—it could be anything. You will however notice an oddity in the fourth and fifth rows of the graphic, a gap in the stream of client packets. This is what I alluded to in the first paragraph, where after the client has sent its client handshake but before it has received the server handshake, the client doesn't yet know the shared key. Because every frame sent after the handshake needs to begin with a tag that depends on the shared key, the client cannot send anything, not even padding, until it receives the server's reponse. During this time, the give_me_a_frame function has no choice but to block. k = ephemeral key p = random amount of padding m = MAC signifying end of padding a = authentication tag d = data frames The obfs4 client handshake looks like this [spec §4]: k | p | m The server doesn't reply until it has verified the client's MAC, which proves that the client knows the bridge's out-of-band secret (this is how obfs4 resists active probing). The server handshake reply is similar, with the addition of an authentication tag: k | a | p | m (different values than in the client handshake) This means that no matter how you schedule packet sending, the traffic will always have this form, with a ...... gap where the client is waiting for the server's handshake to come back: client kpppppm......ddddddddddddddddddddddddddd server .......kapppmddddddddddddddddddddddddddd There isn't a way to completely remove the client gap in obfs4 and still follow the protocol. A future protocol could perhaps remove it (I say perhaps because I haven't thought about the crypto implications) by changing the client's handshake to have a second round of padding, which it could send while waiting for the server to reply: k | p | m1 | p | m2 [spec] https://gitweb.torproject.org/pluggable-transports/obfs4.git/tree/doc/obfs4…

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.onion name resolution with Blockstack
by Jude Nelson 11 Jun '17

11 Jun '17

Hello, I have written a PoC for using Blockstack names to resolve hidden services. Blockstack is a portable decentralized naming system that currently runs on top of the Bitcoin blockchain. Similar to systems like Namecoin, each Blockstack node retains a 100% replica of all name state, including mappings between globally-unique human-readable Blockstack names and the .onion addresses they resolve to. The code and tutorial is hosted here: https://github.com/ jcnelson/blockstack-tor. It works by using the Python stem package to rewrite the target address of a stream from a Blockstack name to a .onion address (similar to how Jesse Victors' OnioNS system works). We plan to add support to the stdin/stdout protocol in Prop279 as well. Best, Jude Nelson -- *JUDE NELSON* *ENGINEERING PARTNER @BlockstackLabs*

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[GSoC17] Ahmia status update #1
by Pushkar Pathak 11 Jun '17

11 Jun '17

Hi everyone, This is the first status update of my work an ahmia.fi. I spent the first week of 'coding period' working on a couple of things. ## Automating blacklisting of onions containing child abuse content. I implemented an automation process which runs once a week and crawls onions having child abuse content from uncensored wiki and bans it from the search results [1]. ## I have been working on the Add page to save added onions on a database and update the elasticsearch servers on a daily basis [2]. During the next two weeks I plan to complete the work left on Add page and replace Polipo with Torsocks5 proxy in ahmia-crawler. That's it for this week, Have a nice day. Pushkar Pathak [1] https://github.com/ahmia/ahmia-index/pull/3 [2] https://github.com/mdhash/ahmia-site

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[release] metrics-lib/DescripTor 1.8.1 minor bugfix
by iwakeh 10 Jun '17

10 Jun '17

-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA256 Hi there! A minor bugfix release of metrics-lib/DescripTor is available: https://dist.torproject.org/descriptor/1.8.1/ This release fixes a regression in parsing microdescriptors and version 1 directories introduced in 1.8.0. The problem only affects CollecTor, which builds on metrics-lib, in so far that the named descriptors were not recognized and caused plenty of log messages, but were stored correctly anyway. Please direct comments and questions to the metrics-team mailing list [2]. Cheers, iwakeh [1] https://gitweb.torproject.org/metrics-lib.git/plain/CHANGELOG.md?id=e0e5d87… [2] https://lists.torproject.org/cgi-bin/mailman/listinfo/metrics-team -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iQIcBAEBCAAGBQJZO/GvAAoJEPeEx9Sa/gviNI0P/1X7wXJesXsFTz0c6BCo6TSQ UcpB0+vYozCJWOc7QznsS+R4zUqLwucSKLMXyBQ8wV5IaDwx0pEvCTkNt85Qbx1b E+SgWGuWpv/dZOcyyOukw1DUq2Flo+OGAN/NL+Rbe3rzm1QciZIHUiyLWCrBYLnK e91/Am3IchGI7UPvVe3xviB70F1g2Ewh2bZ5ePepPUfKhtbymNqsSn6fQ6PKwHjk nSWoYRG8fGpLqQ/8K/ClgLes5C7oT+cvwLuPmPxi+iVXTpZypohDr6SfJBgYxmNJ 7KEcGtQ1LV974vS9F8qBHGKoRDIT6l7LatlB+4j/zAzWz6AJ7POfSnqU7A6+6Od8 trzCRweJVq+MpyIM0ly4K2e3CONRSibyyShrfXXDOcniWamiWPllsVMomiE1bkCB 6RwScQyh0Mkp4l9pA6I9Hn8msaWMjudph7Bv7ZbtqB1sTEvHZTTcZL9XEa7pPwX9 L8RNidaY2AykYe8iWttAB+rrXqWfbyHLTRhANgQPw3XSNlKSHapJQYk2nNyVrP5A T+ohBUSj6lwxNP4zMEDXRcwCYBXqz/PTLirz1ocSMJfAUYWnO6qI9QAOLYxbWExh 6tTOAYhMrKHrqlPdWJPLS+M9Czp8IWM7TYOCotgV/bqbsRIRDzn9wEtDxkVv7c94 9Yi+WrraHC9iVjpqX5wY =5lKs -----END PGP SIGNATURE-----

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[GSoC17] Ahmia status update #1
by Pushkar Pathak 10 Jun '17

10 Jun '17

Hi everyone, This is the first status update of my work an ahmia.fi. I spent the first week of 'coding period' working on a couple of things. ## Automating blacklisting of onions containing child abuse content. I implemented an automation process which runs once a week and crawls onions having child abuse content from uncensored wiki and bans it from the search results [1]. ## I have been working on the Add page to save added onions on a database and update the elasticsearch servers on a daily basis [2]. During the next two weeks I plan to complete the work left on Add page and replace Polipo with Torsocks5 proxy in ahmia-crawler. That's it for this week, Have a nice day. Pushkar Pathak [1] https://github.com/ahmia/ahmia-index/pull/3 [2] https://github.com/mdhash/ahmia-site

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Proposal 247 (Hidden Service Vanguards) Overhaul
by Mike Perry 09 Jun '17

09 Jun '17

I spent some time trying to clean up proposal 247 based on everyone's comments, as well as based on my own thoughts. Please have a look if you commented on the original proposal, and complain if I've not taken your thoughts into account. (Aaron: In particular, I made several tradeoffs in favor of performance and DoS resistance that may be at odds with some of your suggestions, but I think the end result is still OK after looking into the Sybil rates and thinking about the adversary model in more detail. You may disagree). I've attached my updated version of the proposal inline in this mail, but the canonical updated proposal is in my remote at: https://gitweb.torproject.org/user/mikeperry/torspec.git/tree/proposals/247… Here's a summary of the changes (which are also listed in Git): * Try to make a coherent threat model and specify its assumptions * Fold in my comments about using disjoint sets ("buckets") for the third level guard. * Make the parameter discussion subsection its own section, and include tables with far more detail for the Sybil success rates. * Put the rotation period in a separate subsection from the number of guards * Switch to using min(X,X) and max(X,X) for the distribution for the second and third layer guard lifespans, respectively. Add a subsection describing this distribution (3.2.3) * Changed the default parameters based on these tables, and based on my own intuition about Tor's performance properties. * Move the load balancing, torrc, and other performance considerations to their own section (Section 5). * Move "3.2. Distinguishing new HS circuits from normal HS circuits" to section 4.1. * Fold in some of "3.3. Circuit nodes can now be linked to specific hidden services" into 4.1. Some of it I just removed, though, because I did not find it credible. * Added Roger's concerns about guard linkability to Section 4.2. * Added a denial of service subsection to Section 4.3. ================================ Filename: 247-hs-guard-discovery.txt Title: Defending Against Guard Discovery Attacks using Vanguards Author: George Kadianakis Created: 2015-07-10 Status: Draft 0. Motivation A guard discovery attack allow attackers to determine the guard node of a Tor client. The hidden service rendezvous protocol provides an attack vector for a guard discovery attack since anyone can force an HS to construct a 3-hop circuit to a relay (#9001). Following the guard discovery attack with a compromise and/or coercion of the guard node can lead to the deanonymization of a hidden service. 1. Overview This document tries to make the above guard discovery + compromise attack harder to launch. It introduces an optional configuration option which makes the hidden service also pin the second and third hops of its circuits for a longer duration. With this new path selection, we force the adversary to perform a Sybil attack and two compromise attacks before succeeding. This is an improvement over the current state where the Sybil attack is trivial to pull off, and only a single compromise attack is required. With this new path selection, an attacker is forced to do a one or more node compromise attacks before learning the guard node of a hidden service. This increases the uncertainty of the attacker, since compromise attacks are costly and potentially detectable, so an attacker will have to think twice before beginning a chain of node compromise attacks that he might not be able to complete. 1.1. Visuals Here is how a hidden service rendezvous circuit currently looks like: -> middle_1 -> middle_A -> middle_2 -> middle_B -> middle_3 -> middle_C -> middle_4 -> middle_D HS -> guard -> middle_5 -> middle_E -> Rendezvous Point -> middle_6 -> middle_F -> middle_7 -> middle_G -> middle_8 -> middle_H -> ... -> ... -> middle_n -> middle_n this proposal pins the two middles nodes to a much more restricted set, as follows: -> guard_3A_A -> guard_2_A -> guard_3A_B -> guard_3A_C -> Rendezvous Point HS -> guard_1 -> guard_3B_D -> guard_2_B -> guard_3B_E -> guard_3B_F -> Rendezvous Point Note that the third level guards are partitioned into buckets such that they are only used with one specific second-level guard. In this way, we ensure that even if an adversary is able to execute a Sybil attack against the third layer, they only get to learn one of the second-layer Guards, and not all of them. This prevents the adversary from gaining the ability to take their pick of the weakest of the second-level guards for further attack. 2. Design This feature requires the HiddenServiceGuardDiscovery torrc option to be enabled. When a hidden service picks its guard nodes, it also picks two additional sets of middle nodes `second_guard_set` and `third_guard_set` of size NUM_SECOND_GUARDS and NUM_THIRD_GUARDS respectively for each hidden service. These sets are unique to each hidden service created by a single Tor client, and must be kept separate and distinct. When a hidden service needs to establish a circuit to an HSDir, introduction point or a rendezvous point, it uses nodes from `second_guard_set` as the second hop of the circuit and nodes from that second hop's corresponding `third_guard_set` as third hops of the circuit. A hidden service rotates nodes from the 'second_guard_set' at a random time between MIN_SECOND_GUARD_LIFETIME hours and MAX_SECOND_GUARD_LIFETIME hours. A hidden service rotates nodes from the 'third_guard_set' at a random time between MIN_THIRD_GUARD_LIFETIME and MAX_THIRD_GUARD_LIFETIME hours. These extra guard nodes should be picked with the same path selection procedure that is used for regular middle nodes (though see Section 5.1 for performance reasons to restrict this slightly). Each node's rotation time is tracked independently, to avoid disclosing the rotation times of the primary and second-level guards. XXX how should proposal 241 ("Resisting guard-turnover attacks") be applied here? 2.1. Security parameters We set NUM_SECOND_GUARDS to 4 nodes and NUM_THIRD_GUARDS to 16 nodes (ie four sets of four). XXX: 3 and 12 might be another option here, in which case our rotation period for the second guard position can be reduced to 15 days. We set MIN_SECOND_GUARD_LIFETIME to 1 day, and MAX_SECOND_GUARD_LIFETIME to 33 days, for an average rotation rate of ~11 days, using the min(X,X) distribution specified in Section 3.2.2. We set MIN_THIRD_GUARD_LIFETIME to 1 hour, and MAX_THIRD_GUARD_LIFETIME to 18 hours, for an average rotation rate of ~12 hours, using the max(X,X) distribution specified in Section 3.2.2. XXX make all the above consensus parameters? Yes. Very yes, especially if we decide to change the primary guard lifespan. See Section 3 for more analysis on these constants. 3. Rationale and Security Parameter Selection 3.1. Threat model, Assumptions, and Goals Consider an adversary with the following powers: - Can launch a Sybil guard discovery attack against any node of a rendezvous circuit. The slower the rotation period of the node, the longer the attack takes. Similarly, the higher the percentage of the network is compromised, the faster the attack runs. - Can compromise any node on the network, but this compromise takes time and potentially even coercive action, and also carries risk of discovery. We also make the following assumptions about the types of attacks: 1. A Sybil attack is noisy. It will require either large amounts of traffic, multiple test circuits, or both. 2. A Sybil attack against the second or first layer Guards will be more noisy than a Sybil attack against the third layer guard, since the second and first layer Sybil attack requires a timing side channel in order to determine success, where as the Sybil success is almost immediately obvious to third layer guard, since it will now be returned as a rend point for circuits for the hidden service in question. 3. As soon as the adversary is confident they have won the Sybil attack, an even more aggressive circuit building attack will allow them to determine the next node very fast (an hour or less). 4. The adversary is strongly disincentivized from compromising nodes that may prove useless, as node compromise is even more risky for the adversary than a Sybil attack in terms of being noticed. Given this threat model, our security parameters were selected so that the first two layers of guards should be hard to attack using a Sybil guard discovery attack and hence require a node compromise attack. Ideally, we want the node compromise attacks to carry a non-negligible probability of being useless to the adversary by the time they complete. On the other hand, the outermost layer of guards should rotate fast enough to _require_ a Sybil attack. 3.2. Parameter Tuning 3.2.1. Sybil rotation counts for a given number of Guards The probability of Sybil success for Guard discovery can be modeled as the probability of choosing 1 or more malicious middle nodes for a sensitive circuit over some period of time. P(At least 1 bad middle) = 1 - P(All Good Middles) = 1 - P(One Good middle)^(num_middles) = 1 - (1 - c/n)^(num_middles) c/n is the adversary compromise percentage In the case of Vanguards, num_middles is the number of Guards you rotate through in a given time period. This is a function of the number of vanguards in that position (v), as well as the number of rotations (r). P(At least one bad middle) = 1 - (1 - c/n)^(v*r) Here's detailed tables in terms of the number of rotations required for a given Sybil success rate for certain number of guards. 1.0% Network Compromise: Sybil Success One Two Three Four Five Six Eight Nine Ten Twelve Sixteen 10% 11 6 4 3 3 2 2 2 2 1 1 15% 17 9 6 5 4 3 3 2 2 2 2 25% 29 15 10 8 6 5 4 4 3 3 2 50% 69 35 23 18 14 12 9 8 7 6 5 60% 92 46 31 23 19 16 12 11 10 8 6 75% 138 69 46 35 28 23 18 16 14 12 9 85% 189 95 63 48 38 32 24 21 19 16 12 90% 230 115 77 58 46 39 29 26 23 20 15 95% 299 150 100 75 60 50 38 34 30 25 19 99% 459 230 153 115 92 77 58 51 46 39 29 5.0% Network Compromise: Sybil Success One Two Three Four Five Six Eight Nine Ten Twelve Sixteen 10% 3 2 1 1 1 1 1 1 1 1 1 15% 4 2 2 1 1 1 1 1 1 1 1 25% 6 3 2 2 2 1 1 1 1 1 1 50% 14 7 5 4 3 3 2 2 2 2 1 60% 18 9 6 5 4 3 3 2 2 2 2 75% 28 14 10 7 6 5 4 4 3 3 2 85% 37 19 13 10 8 7 5 5 4 4 3 90% 45 23 15 12 9 8 6 5 5 4 3 95% 59 30 20 15 12 10 8 7 6 5 4 99% 90 45 30 23 18 15 12 10 9 8 6 10.0% Network Compromise: Sybil Success One Two Three Four Five Six Eight Nine Ten Twelve Sixteen 10% 2 1 1 1 1 1 1 1 1 1 1 15% 2 1 1 1 1 1 1 1 1 1 1 25% 3 2 1 1 1 1 1 1 1 1 1 50% 7 4 3 2 2 2 1 1 1 1 1 60% 9 5 3 3 2 2 2 1 1 1 1 75% 14 7 5 4 3 3 2 2 2 2 1 85% 19 10 7 5 4 4 3 3 2 2 2 90% 22 11 8 6 5 4 3 3 3 2 2 95% 29 15 10 8 6 5 4 4 3 3 2 99% 44 22 15 11 9 8 6 5 5 4 3 The rotation counts in these tables were generated with: def count_rotations(c, v, success): r = 0 while 1-math.pow((1-c), v*r) < success: r += 1 return r 3.2.2. Rotation Period As specified in Section 3.1, the primary driving force for the third layer selection was to ensure that these nodes rotate fast enough that it is not worth trying to compromise them, because it is unlikely for compromise to succeed and yield useful information before the nodes stop being used. For this reason we chose 1 to 18 hours, with a weighted distribution (Section 3.2.3) causing the expected average to be 12 hours. From the table in Section 3.2.1, it can be seen that this means that the Sybil attack will complete with near-certainty (99%) in 29*12 hours (14.5 days) for the 1% adversary, 3 days for the 5% adversary, and 1.5 days for the 10% adversary. Since rotation of each node happens independently, the distribution of when the adversary expects to win this Sybil attack in order to discover the next node up is uniform. This means that on average, the adversary should expect that half of the rotation period of the next node is already over by the time that they win the Sybil. With this fact, we choose our range and distribution for the second layer rotation to be short enough to cause the adversary to risk compromising nodes that are useless, yet long enough to require a Sybil attack to be noticeable in terms of client activity. For this reason, we choose a minimum second-layer guard lifetime of 1 day, since this gives the adversary a minimum expected value of 12 hours for during which they can compromise a guard before it might be rotated. If the total expected rotation rate is 11 days, then the adversary can expect overall to have 5.5 days remaining after completing their Sybil attack before a second-layer guard rotates away. 3.2.3. Rotation distribution In order to skew the distribution of the third layer guard towards higher values, we use max(X,X) for the distribution, where X is a random variable that takes on values from the uniform distribution. In order to skew the distribution of the second layer guard towards low values (to increase the risk of compromising useless nodes) we skew the distribution towards lower values, using min(X,X). Here's a table of expectation (arithmetic means) for relevant ranges of X (sampled from 0..N). The current choice for second-layer guards is noted with **, and the current choice for third-layer guards is noted with ***. Range Min(X,X) Max(X,X) 10 2.85 6.15 11 3.18 6.82 12 3.51 7.49 13 3.85 8.15 14 4.18 8.82 15 4.51 9.49 16 4.84 10.16 17 5.18 10.82*** 18 5.51 11.49 19 5.84 12.16 20 6.18 12.82 21 6.51 13.49 22 6.84 14.16 23 7.17 14.83 24 7.51 15.49 25 7.84 16.16 26 8.17 16.83 27 8.51 17.49 28 8.84 18.16 29 9.17 18.83 30 9.51 19.49 31 9.84 20.16 32 10.17** 20.83 33 10.51 21.49 34 10.84 22.16 35 11.17 22.83 36 11.50 23.50 37 11.84 24.16 38 12.17 24.83 39 12.50 25.50 4. Security concerns and mitigations 4.1. Mitigating fingerprinting of new HS circuits By pinning the middle nodes of rendezvous circuits, we make it easier for all hops of the circuit to detect that they are part of a special hidden service circuit with varying degrees of certainty. The Guard node is able to recognize a Vanguard client with a high degree of certainty because it will observe a client IP creating the overwhelming majority of its circuits to just a few middle nodes in any given 10-18 day time period. The middle nodes will be able to tell with a variable certainty that depends on both its traffic volume and upon the popularity of the service, because they will see a large number of circuits that tend to pick the same Guard and Exit. The final nodes will be able to tell with a similar level certainty that depends on their capacity and the service popularity, because they will see a lot of rend handshakes that all tend to have the same second hop. The most serious of these is the Guard fingerprinting issue. When proposal xxx-padding-negotiation is implemented, services that enable this feature should use those padding primitives to create fake circuits to random middle nodes that are not their guards, in an attempt to look more like a client. Additionally, if Tor Browser implements "virtual circuits" based on SOCKS username+password isolation in order to enforce the re-use of paths when SOCKS username+passwords are re-used, then the number of middle nodes in use during a typical user's browsing session will be proportional to the number of sites they are viewing at any one time. This is likely to be much lower than one new middle node every ten minutes, and for some users, may be close to the number of Vanguards we're considering. This same reasoning is also an argument for increasing the number of second-level guards beyond just two, as it will spread the hidden service's traffic over a wider set of middle nodes, making it both easier to cover, and behave closer to a client using SOCKS virtual circuit isolation. 4.2. Hidden service linkability Multiple hidden services on the same Tor instance should use separate second and third level guard sets, otherwise an adversary is trivially able to determine that the two hidden services are co-located by inspecting their current chosen rend point nodes. Unfortunately, if the adversary is still able to determine that two or more hidden services are run on the same Tor instance through some other means, then they are able to take advantage of this fact to execute a Sybil attack more effectively, since there will now be an extra set of guard nodes for each hidden service in use. For this reason, if Vanguards are enabled, and more than one hidden service is configured, the user should be advised to ensure that they do not accidentally leak that the two hidden services are from the same Tor instance. 4.3. Denial of service Since it will be fairly trivial for the adversary to enumerate the current set of rend nodes for a hidden service, denial of service becomes a serious risk for Vanguard users. For this reason, it is important to support a large number of third-level guards, to increase the amount of resources required to bring a hidden service offline by DoSing just a few Tor nodes. 5. Performance considerations The switch to a restricted set of nodes will very likely cause significant performance issues, especially for high-traffic hidden services. If any of the nodes they select happen to be temporarily overloaded, performance will suffer dramatically until the next rotation period. 5.1. Load Balancing Since the second and third level "guards" are chosen from the set of all nodes eligible for use in the "middle" hop (as per hidden services today), this proposal should not significantly affect the long-term load on various classes of the Tor network, and should not require any changes to either the node weight equations, or the bandwidth authorities. Unfortunately, transient load is another matter, as mentioned previously. It is very likely that this scheme will increase instances of transient overload at nodes selected by high-traffic hidden services. One option to reduce the impact of this transient overload is to restrict the set of middle nodes that we chose from to some percentage of the fastest middle-capable relays in the network. This may have some impact on load balancing, but since the total volume of hidden service traffic is low, it may be unlikely to matter. 5.2. Circuit build timeout The adaptive circuit build timeout mechanism in Tor is what corrects for instances of transient node overload right now. The timeout will naturally tend to select the current fastest and least-loaded paths even through this set of restricted routes, but it may fail to behave correctly if there are a very small set of nodes in each guard set, as it is based upon assumptions about the current path selection algorithm, and it may need to be tuned specifically for Vanguards, especially if the set of possible routes is small. 5.3. OnionBalance At first glance, it seems that this scheme makes multi-homed hidden services such as OnionBalance[1] even more important for high-traffic hidden services. Unfortunately, if it is equally damaging to the user for any of their multi-homed hidden service locations to be discovered, then OnionBalance is strictly equivalent to simply increasing the number of second-level guard nodes in use, because an active adversary can perform simultaneous Sybil attacks against all of the rend points offered by the multi-homed OnionBalance introduction points. 5.4. Default vs optional behavior We suggest this torrc option to be optional because it changes path selection in a way that may seriously impact hidden service performance, especially for high traffic services that happen to pick slow guard nodes. However, by having this setting be disabled by default, we make hidden services who use it stand out a lot. For this reason, we should in fact enable this feature globally, but only after we verify its viability for high-traffic hidden services, and ensure that it is free of second-order load balancing effects. Even after that point, until Single Onion Services are implemented, there will likely still be classes of very high traffic hidden services for whom some degree of location anonymity is desired, but for which performance is much more important than the benefit of Vanguards, so there should always remain a way to turn this option off. 6. Future directions Here are some more ideas for improvements that should be done sooner or later: - Maybe we should make the size and rotation period of secondary/third guard sets to be configurable by the user. - To make it harder for an adversary, a hidden service MAY extend the path length of its circuits by an additional static hop. This forces the adversary to use another coercion attack to walk the chain up to the hidden service. 7. Acknowledgments Thanks to Aaron Johnson, John Brooks, Mike Perry and everyone else who helped with this idea. 1. https://onionbalance.readthedocs.org/en/latest/design.html#overview -- Mike Perry

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