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Hackathon UDP Support
by Matthew Finkel 14 Apr '21

14 Apr '21

Hello! During the Hackweek, I spent a little time hacking support of UDP-over-Tor. The goal of the project was supporting UDP onion services, and leaving Exit support for another time. I didn't have a working implementation by the end of the week, but life moves one. This is an initial mail about the general idea, and I'll follow up with a concrete proposal in the not-too-distant future. I have a (currently broken) branch `feature_udp_data` in my gitlab repo [0]. I started simplifying/refactoring the hack job, but it's still a work in progress. I'll briefly describe how the working version [1] is designed. Overall, this mostly just-works. While UDP is not a reliable routed protocol, datagrams are not dropped within the network. We delegate drops to the edges' recv/send queues (and any intermediate network equipment). Client Side: The patch continued our dependence on SOCKS5 [2] and (maybe correctly) implemented the UDP ASSOCIATE command. The client sends a normal SOCKS5 handshake, except the new command is 0x03. The tor client then: - Establishes a rendezvous circuit with the specified onion service hostname in the handshake - On completing that connection, creates and binds a datagram socket - Sends the application a Success response and includes the datagram socket's bound address and port in that message. The client then sends all datagrams to the provided (bound) address:port, and the TCP socket used in the SOCKS5 handshake is only kept open as a way of tracking the UDP socket's lifetime. Tor knows that any datagram payload should be transported as a datagram instead of a stream. Tor Internal: The patch introduces two relay cell commands: DATAGRAM and DATAGRAM_FRAG. These relay cell types are stateless. Simply, when a tor client (or general edge, in the future) receives a DATAGRAM_FRAG relay cell, it knows that the payload is only a fragment of an entire datagram. As a result, tor queues the data and does not send it to the application. When a DATAGRAM cell is received, then tor knows the contained payload is either: 1) a complete datagram that fit into a single relay cell 2) the final payload of a fragmented datagram In either case, tor appends the payload to the (possibly empty) queue and then sends it to the application. Tor doesn't need to care which one is the current case. Also, Tor doesn't take into account a MTU mismatch between the client-side and server-side - future work. As an aside, I considered introducing only a single DATAGRAM relay cell, and reserving the first byte for metadata (and using a bit indicating if the payload is a fragment). However, I couldn't convince myself that losing one byte of payload capacity was worth the advantages. I'll leave this as a valuable discussion during the proposal process. Onion Service Side: The onion service is configured normally, except now there is a HiddenServiceType configuration option. When it isn't provided, the default value is "stream", otherwise it should be "dgram". This configuration tells tor that a "stream" opened to this onion service (on the specified port) is actually connectionless, and any payload data should be sent and received as datagrams. Following from the previous section, when a DATAGRAM relay cell is received, the queued payload is packaged as a datagram and sent to the configured onion service address:port. Future design considerations: - Better client-side interface than SOCKS5 UDP Associate? - Relay cell design improvements - Cleaner onion service configuration? - MTU difference on the client/server-sides - What's missing before we can support this on Exits? [0] https://gitlab.torproject.org/sysrqb/tor/ [1] commit ebfb4a97713d49a5e7f61709658b9d55ac21ec95 [2] https://tools.ietf.org/html/rfc1928

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Question about hidden services shared by multiple hosts
by Holmes Wilson 10 Apr '21

10 Apr '21

Hi everyone, We’re working on a peer-to-peer group chat app where peers connect over v3 onion addresses. One issue are groups where there are many users but only a few are online in a given moment. Onion addresses are forever, and existing peers might know every peer in the network, but it will take a while to try connecting to all of them to find one that is online. In this case, it seems helpful for one or more peers to share one or more onion addresses that would serve as reliable “trackers", e.g. 1. All members know the keypairs for these addresses. 2. All online members ping these addresses at random intervals to say they’re online. 3. If they can’t connect to an address, they start hosting it themselves. We’re going to start testing it, but we’re wondering if folks here know the likely outcome of trying to “share” hosting of an onion service in this spontaneous-volunteer sort of way and if there are downsides. I *think* the most important question is how long it takes for the network to stop routing incoming traffic to an offline client when there’s an online one available. How long will the address likely be unreachable in one of these transition moments, assuming some peer immediately detects that a “tracker” onion address has gone offline and begins hosting it themselves? (And does this question make sense?) Thanks! Holmes

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sigstore for improving verification of downloads?
by axel simon 08 Apr '21

08 Apr '21

Hello all, I've been a Tor user for a long time, have helped a bit with translation at times, and been working on issues related to freedom and the Internet for a while (as part of La Quadrature du Net), but i'm writing today with my work hat on, as a member of the security team at Red Hat's office of the CTO. We recently announced a new project called sigstore [1], which could be summarised as "Let's Encrypt, but for software provenance and code signing". If you want to read other people explain it better than me, i found these two articles did a good job: - https://www.darkreading.com/application-security/linux-foundation-debuts-si… - https://www.zdnet.com/article/linux-foundation-announces-new-open-source-so… In any case, the idea is to provide a public good service, heavily inspired by Let's Encrypt, that lets developpers sign packages, containers, code artifacts in general, and lets user verify them in a simple way. It's still early days, but the aim is to make it a lot easier to verify the integrity of software, and to do better and easier than "here's a binary, here's an .asc detached PGP signature, here's a page that says that 0xBLA is our public key", which as we know is a procedure most users simply don't follow. To avoid the pains of key management (and the nasty targets keys quickly become), sigstore uses ephemeral signing keys (which don't ever touch the disk), publishes the signature in a first transparency log, and discards the key after signature. The other part of the system, which connects a signing key to an entity, is based on issuing a certificate to the signer which contains their email address and their pubkey, if they can prove control of the email address using OpenID Connect. This certificate is then stored in a second transpararency log, which provides a second root of trust, directly inspired by the Certificate Transparency project [2]. Using both transparency logs (signatures and certificates), one can then verify that a binary download is the expected one, and that is was signed by someone who controls an email address. Tools are being built to do this automatically, and in a repeatable way. The idea is also to make it easy for developers to monitor the certificate transparency log for their email, which could alert them to untowards actions, if a new certificate pops up for something they never signed. The OpenID Connect part is designed to allow for both large ID providers (like Github or Google) and bring-your-own. As i was saying, it's still early days, but it's coming along nicely and in my mind, Tor (and TAILS) are projects that would very much benefit from sigstore, hence this email. Work is currently under way with the Ruby and WebAssembly communities to help them use sigstore to sign their stuff, and more generally the project has been well received so far. The whole system is, of course, free and open source and i might add, welcomes new contributors. [3] We also have a chat, if anyone is into that kind of thing. [4] Interested in your thoughts, critiques etc. Thanks and hope this is helpful! axel 1. Jointly with Google and Purdue University, https://sigstore.dev 2. https://certificate.transparency.dev/ 3. https://github.com/sigstore 4. https://join.slack.com/t/sigstore/shared_invite/zt-mhs55zh0-XmY3bcfWn4XEyMq… -- axel simon axel(a)redhat.com // axel+tor(a)axelsimon.net

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My tool I made which makes running relays on Windows very simple.
by Keifer Bly 28 Mar '21

28 Mar '21

Hi all, So this is a tool I developed for tor which makes running relays on Windows very simple. https://www.youtube.com/watch?v=Vpk6yvUWQqU Features: Example torrc file which can be configured as wanted. Automatically checks if tor is running the newest version on startup, if not updates to newest version. One button configuration to configure relay to start when Windows starts. Generates a tor log file which can tell tor messages. Drawbacks: Only supports vanilla bridges. Configures Windows Defender Firewall automatically to allow tor traffic, but other firewalls will need to be manually configured. Cheers. --Keifer

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A new idea for email encryption on tor
by Keifer Bly 28 Mar '21

28 Mar '21

Hi there, So I have a new email encryption system which requires that the user has the specific key file generated for a message rather than the password, specifically this software generates a unique key file for a specific message every time a message is created. The user then enters the date and time the message was created. Without the original key file the message can't be opened; https://www.youtube.com/watch?v=R0W7OVdNrOA Here is a video showing the software. I've built it for Windows and Mac OS. I was wondering if this could be implemented in tor. I think it would be an interesting idea for a tor based email system to make the messages unrecoverable after use. OS: Windows, Mac OS What do you think? --Keifer

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Proposal 329: Traffic Splitting (Conflux)
by Mike Perry 26 Mar '21

26 Mar '21

I'm pleased to present for your enjoyment, excitement, and enlightenment: Tor Proposal 329: Traffic Splitting! I've included it inline below. The canonical version for merge exists in a gitlab MR: https://gitlab.torproject.org/tpo/core/torspec/-/merge_requests/34 Please merge that one, as it also has a commit to add a much-deserved Acknowledgement section to Proposal 324, too. Feel free to respond on-thread, though! ================================================================== Filename: 329-traffic-splitting.txt Title: Overcoming Tor's Bottlenecks with Traffic Splitting Author: David Goulet, Mike Perry Created: 2020-11-25 Status: Draft 0. Status This proposal describes the Conflux [CONFLUX] system developed by Mashael AlSabah, Kevin Bauer, Tariq Elahi, and Ian Goldberg. It aims at improving Tor client network performance by dynamically splitting traffic between two circuits. 1. Overview 1.1. Multipath TCP Design Space In order to understand our improvements to Conflux, it is important to properly conceptualize what is involved in the design of multipath algorithms in general. The design space is broken into two orthogonal parts: congestion control algorithms that apply to each path, and traffic scheduling algorithms that decide when to send packets to send on each path. MPTCP specifies 'coupled' congestion control (see [COUPLED]). Coupled congestion control updates single-path congestion control algorithms to account for shared bottlenecks between the paths, so that the combined congestion control algorithms do not overwhelm any bottlenecks that happen to be shared between the multiple paths. Various ways of accomplishing this have been proposed and implemented in the Linux kernel. Because Tor's congestion control only concerns itself with bottnecks in Tor relay queues, and not with any other bottlenecks (such as intermediate Internet routers), we can avoid this complexity merely by specifying that any paths that are constructed should not share any relays. In this way, we can proceed to use the exact same congestion control as specified in Proposal 324, for each path. For this reason, this proposal will focus on the traffic scheduling algorithms, rather than coupling. We propose three candidate algorithms that have been studied in the literature, and will compare their performance using simulation and consensus parameters. 1.2. Divergence from the initial Conflux design The initial [CONFLUX] paper doesn't provide any indications on how to handle the size of out-of-order cell queue, which we consider a potential dangerous memory DoS vector (see [MEMORY_DOS]). It also used RTT as the sole heuristic for selecting which circuit to send on, which may vary depending on the geographical locations of the participant relays, without considering their actual available circuit capacity (which will be available to us via Proposal 324). Additionally, since the publication of [CONFLUX], more modern packet scheduling algorithms have been developed, which aim to reduce out-of-order queue size. We propose mitigations for these issues using modern scheduling algorithms, as well as implementations options for avoiding the out-of-order queue at Exit relays. Additionally, we consider resumption, side channel, and traffic analysis risks and benefits in [RESUMPTION], [SIDE_CHANNELS] and [TRAFFIC_ANALYSIS]. 2. Design The following section describes the Conflux design. Each sub-section is a building block to the multipath design that Conflux proposes. The circuit construction is as follow: Primary Circuit (lower RTT) +-------+ +--------+ |Guard 1|----->|Middle 1|----------+ +---^---+ +--------+ | +-----+ | +--v---+ | OP +------+ | Exit |--> ... +-----+ | +--^---+ +---v---+ +--------+ | |Guard 2|----->|Middle 2|----------+ +-------+ +--------+ Secondary Circuit (higher RTT) Both circuits are built using current Tor path selection, however they SHOULD NOT share the same Guard relay, or middle relay. By avoiding using the same relays in these positions in the path, we ensure additional path capacity, and eliminate the need to use more complicated 'coupled' congestion control algorithms from the MPTCP literature[COUPLED]. This both simplifies design, and improves performance. Then, the OP needs to link the two circuits together, as described in [LINKING_CIRCUITS], [LINKING_EXIT], and [LINKING_SERVICE]. For ease of explanation, the primary circuit is the circuit with lower RTT, and the secondary circuit is the circuit with higher RTT. Initial RTT is measured during circuit linking, as described in [LINKING_CIRCUITS]. RTT is continually measured using SENDME timing, as in Proposal 324. This means that during use, the primary circuit and secondary circuit may switch roles, depending on unrelated network congestion caused by other Tor clients. We also support linking onion service circuits together. In this case, only two rendezvous circuits are linked. Each of these RP circuits will be constructed separately, and then linked. However, the same path constraints apply to each half of the circuits (no shared relays between the legs). Should, by chance, the service and the client sides end up sharing some relays, this is not catastrophic. Multipath TCP researchers we have consulted believe Tor's congestion control from Proposal 324 to be sufficient in this rare case. Only two circuits SHOULD be linked together. However, implementations SHOULD make it easy for researchers to *test* more than two paths, as this has been shown to assist in traffic analysis resistance[WTF_SPLIT]. At minimum, this means not hardcoding only two circuits in the implementation. If the number of circuits exceeds the current number of guard relays, guard relays MAY be re-used, but implementations SHOULD use the same number of Guards as paths. Linked circuits MUST NOT be extended further once linked (ie: 'cannibalization' is not supported). 2.1. Advertising support for conflux We propose a new protocol version in order to advertise support for circuit linking on the relay side: "Relay=4" -- Relay supports an 2 byte sequence number in a RELAY cell header used for multipath circuit which are linked with the new RELAY_CIRCUIT_LINK relay cell command. XXX: Advertise this in onion service descriptor. XXX: Onion service descriptor can advertise more than two circuits? The next section describes how the circuits are linked together. 2.2. Linking circuits [LINKING_CIRCUITS] To link circuits, we propose new relay commands that are sent on both circuits, as well as a response to confirm the join, and an ack of this response. These commands create a 3way handshake, which allows each endpoint to measure the initial RTT of each leg upon link, without needing to wait for any data. All three stages of this handshake are sent on *each* circuit leg to be linked. To save round trips, these cells SHOULD be combined with the initial RELAY_BEGIN cell on the faster circuit leg, using Proposal 325. See [LINKING_EXIT] and [LINKING_SERVICE] for more details on setup in each case. There are other ways to do this linking that we have considered, but they seem not to be significantly better than this method, especially since we can use Proposal 325 to eliminate the RTT cost of this setup before sending data. For those other ideas, see [ALTERNATIVE_LINKING] and [ALTERNATIVE_RTT], in the appendix. The first two parts of the handshake establish the link, and enable resumption: 16 -- RELAY_CIRCUIT_LINK Sent from the OP to the exit/service in order to link circuits together at the end point. 17 -- RELAY_CIRCUIT_LINKED Sent from the exit/service to the OP, to confirm the circuits were linked. These cells have the following contents: VERSION [1 byte] PAYLOAD [variable, up to end of relay payload] The VERSION tells us which circuit linking mechanism to use. At this point in time, only 0x01 is recognized and is the one described by the Conflux design. For version 0x01, the PAYLOAD contains: NONCE [32 bytes] LAST_SEQNO_SENT [8 bytes] LAST_SEQNO_RECV [8 bytes] XXX: Should we let endpoints specify their preferred [SCHEDULING] alg here, to override consensus params? This has benefits: eg low-memory mobile clients can ask for an alg that is better for their reorder queues. But it also has complexity risk, if the other endpoint does not want to support it, because of its own memory issues. The NONCE contains a random 256-bit secret, used to associate the two circuits together. The nonce must not be shared outside of the circuit transmission, or data may be injected into TCP streams. This means it MUST NOT be logged to disk. The two sequence number fields are 0 upon initial link, but non-zero in the case of a resumption attempt (See [RESUMPTION]). If either circuit does not receive a RELAY_CIRCUIT_LINKED response, both circuits MUST be closed. The third stage of the handshake exists to help the exit/service measure initial RTT, for use in [SCHEDULING]: 18 -- RELAY_CIRCUIT_LINKED_RTT_ACK Sent from the OP to the exit/service, to provide initial RTT measurement for the exit/service. For timeout of the handshake, clients should use the normal SOCKS/stream timeout already in use for RELAY_BEGIN. These three relay commands (RELAY_CIRCUIT_LINK, RELAY_CIRCUIT_LINKED, and RELAY_CIRCUIT_LINKED_ACK) are send on *each* leg, to allow each endpoint to measure the initial RTT of each leg. 2.2. Linking Circuits from OP to Exit [LINKING_EXIT] To link exit circuits, two circuits to the same exit are built. The client records the circuit build time of each. If the circuits are being built on-demand, for immediate use, the circuit with the lower build time SHOULD use Proposal 325 to append its first RELAY cell to the RELAY_COMMAND_LINK, on the circuit with the lower circuit build time. The exit MUST respond on this same leg. After that, actual RTT measurements MUST be used to determine future transmissions, as specified in [SCHEDULING]. The RTT times between RELAY_COMMAND_LINK and RELAY_COMMAND_LINKED are measured by the client, to determine each circuit RTT to determine primary vs secondary circuit use, and for packet scheduling. Similarly, the exit measures the RTT times between RELAY_COMMAND_LINKED and RELAY_COMMAND_LINKED_ACK, for the same purpose. 2.3. Linking circuits to an onion service [LINKING_SERVICE] For onion services, we will only concern ourselves with linking rendezvous circuits. To join rendezvous circuits, clients make two introduce requests to a service's intropoint, causing it to create two rendezvous circuits, to meet the client at two separate rendezvous points. These introduce requests MUST be sent to the same intropoint (due to potential use of onionbalance), and SHOULD be sent back-to-back on the same intro circuit. They MAY be combined with Proposal 325. The first rendezvous circuit to get joined SHOULD use Proposal 325 to append the RELAY_BEGIN command, and the service MUST answer on this circuit, until RTT can be measured. Once both circuits are linked and RTT is measured, packet scheduling should be used, as per [SCHEDULING]. 2.4. Congestion Control Application [CONGESTION_CONTROL] The SENDMEs for congestion control are performed per-leg. As data arrives, regardless of its ordering, it is counted towards SENDME delivery. In this way, 'cwnd - package_window' of each leg always reflects the available data to send on each leg. This is important for [SCHEDULING]. The Congestion control Stream XON/XOFF can be sent on either leg, and applies to the stream's transmission on both legs. 2.5. Sequencing [SEQUENCING] With multiple paths for data, the problem of data re-ordering appears. In other words, cells can arrive out of order from the two circuits where cell N + 1 arrives before the cell N. Handling this reordering operates after congestion control for each circuit leg, but before relay cell command processing or stream data delivery. For the receiver to be able to reorder the receiving cells, a sequencing scheme needs to be implemented. However, because Tor does not drop or reorder packets inside of a circuit, this sequence number can be very small. It only has to signal that a cell comes after those arriving on another circuit. To achieve this, we add a small sequence number to the common relay header for all relay cells on linked circuits. This sequence number is meant to signal the number of cells sent on the *other* leg, so that each endpoint knows how many cells are still in-flight on another leg. It is different from the absolute sequence number used in [LINKING_CIRCUITS] and [RESUMPTION], but can be derived from that number, using relative arithmetic. Relay command [1 byte] Recognized [2 bytes] StreamID [2 bytes] Digest [4 bytes] Length [2 bytes] > LongSeq [1 bit] # If this bit is set, use 31 bits for Seq > Sequencing [7 or 31 bits] Data [Remainder] The sequence number is only set for the first cell after the endpoint switches legs. In this case, LongSeq is set to 1, and the Sequencing field is 31 more bits. Otherwise it is a 1 byte 0 value. These fields MUST be present on ALL end-to-end relay cells on each leg that come from the endpoint, following a RELAY_CIRCUIT_LINK command. They are absent on 'leaky pipe' RELAY_COMMAND_DROP and RELAY_COMMAND_PADDING_NEGOTIATED cells that come from middle relays, as opposed to the endpoint, to support padding. When an endpoint switches legs, on the first cell in a new leg, LongSeq is set to 1, and the following 31 bits represent the *total* number of cells sent on the *other* leg, before the switch. The receiver must wait for that number of cells to arrive from the previous leg before delivering that cell. XXX: In the rare event that we send more than 2^31 cells (~1TB) on a single leg, do we force a switch of legs, or expand the field further? An alternative method of sequencing, that assumes that the endpoint knows when it is going to switch, the cell before it switches, is specified in [ALTERNATIVE_SEQUENCING]. Note that that method requires only 1 byte for sequence number and switch signaling, but requires that the sender know that it is planning to switch, the cell before it switches. (This is possible with [BLEST_TOR], but [LOWRTT_TOR] can switch based on RTT change, so it may be one cell late in that case). 2.6. Resumption [RESUMPTION] In the event that a circuit leg is destroyed, they MAY be resumed. Resumption is achieved by re-using the NONCE and method to the same endpoint (either [LINKING_EXIT] or [LINKING_SERVICE]). The resumed path need not use the same middle and guard relays, but should not share any relays with any existing legs(s). To provide resumption, endpoints store an absolute 64bit cell counter of the last cell they have sent on a conflux pair (their LAST_SEQNO_SENT), as well the last sequence number they have delivered in-order to edge connections corresponding to a conflux pair (their LAST_SEQNO_RECV). Additionally, endpoints MAY store the entire contents of unacked inflight cells (ie the 'package_window' from proposal 324), for each leg, along with information corresponding to those cells' absolute sequence numbers. These 64 bit absolute counters can wrap without issue, as congestion windows will never grow to 2^64 cells until well past the Singularity. However, it is possible that extremely long, bulk circuits could exceed 2^64 total sent or received cells, so endpoints SHOULD handle wrapped sequence numbers for purposes of computing retransmit information. (But even this case is unlikely to happen within the next decade or so). Upon resumption, the LAST_SEQNO_SENT and LAST_SEQNO_RECV fields are used to convey the sequence numbers of the last cell the relay sent and received on that leg. The other endpoint can use these sequence numbers to determine if it received the in-flight data or not, or sent more data since that point, up to and including this absolute sequence number. If LAST_SEQNO_SENT has not been received, the endpoint MAY transmit the missing data, if it still has it buffered. Because both endpoints get information about the other side's absolute SENT sequence number, they will know exactly how many re-transmitted packets to expect, should the circuit stay open. Re-transmitters should not re-increment their absolute sent fields while re-transmitting. If it does not have this missing data due to memory pressure, that endpoint should destroy *both* legs, as this represents unrecoverable data loss. Otherwise, the new circuit can be re-joined, and its RTT can be compared to the remaining circuit to determine if the new leg is primary or secondary. It is even possible to resume conflux circuits where both legs have been collapsed using this scheme, if endpoints continue to buffer their unacked package_window data for some time after this close. However, see [TRAFFIC_ANALYSIS] for more details on the full scope of this issue. If endpoints are buffering package_window data, such data should be given priority to be freed in any oomkiller invocation. See [MEMORY_DOS] for more oomkiller information. 3. Traffic Scheduling [SCHEDULING] In order to load balance the traffic between the two circuits, the original conflux paper used only RTT. However, with Proposal 324, we will have accurate information on the instantaneous available bandwidth of each circuit leg, as 'cwnd - package_window' (see Section 3 of Proposal 324). Some additional RTT optimizations are also useful, to improve responsiveness and minimize out-of-order queue sizes. We specify two traffic schedulers from the multipath literature and adapt them to Tor: [LOWRTT_TOR] and [BLEST_TOR]. [LOWRTT_TOR] also has three variants, with different trade offs. However, see the [TRAFFIC_ANALYSIS] sections of this proposal for important details on how this selection can be changed, to reduce website traffic fingerprinting. 3.1. LowRTT Scheduling [LOWRTT_TOR] This scheduling algorithm is based on the original [CONFLUX] paper, with ideas from [MPTCP]'s minRTT/LowRTT scheduler. In this algorithm, endpoints send cells on the circuit with lower RTT (primary circuit). This continues while the congestion window on the circuit has available room: ie whenever cwnd - package_window > 0. Whenever the primary circuit's congestion window becomes full, the secondary circuit is used. We stop reading on the send window source (edge connection) when both congestion windows become full. In this way, unlike original conflux, we switch to the secondary circuit without causing congestion on the primary circuit. This improves both load times, and overall throughput. This behavior matches minRTT from [MPTCP], sometimes called LowRTT. It may be better to stop reading on the edge connection when the primary congestion window becomes full, rather than switch to the secondary circuit as soon as the primary congestion window becomes full. (Ie: only switch if the RTTs themselves change which circuit is primary). This is what was done in the original Conflux paper. This behavior effectively causes us to optimize for responsiveness and congestion avoidance, rather than throughput. For evaluation, we should control this switching behavior with a consensus parameter (see [CONSENSUS_PARAMETERS]). Because of potential side channel risk (see [SIDE_CHANNELS]), a third variant of this algorithm, where the primary circuit is chosen during the [LINKING_CIRCUITS] handshake and never changed, should also be possible to control via consensus parameter. 3.2. BLEST Scheduling [BLEST_TOR] [BLEST] attempts to predict the availability of the primary circuit, and use this information to reorder transmitted data, to minimize head-of-line blocking in the recipient (and thus minimize out-of-order queues there). BLEST_TOR uses the primary circuit until the congestion window is full. Then, it uses the relative RTT times of the two circuits to calculate how much data can be sent on the secondary circuit faster than if we just waited for the primary circuit to become available. This is achieved by computing two variables at the sender: rtts = secondary.currRTT / primary.currRTT primary_limit = primary.cwnd + (rtts-1)/2)*rtts Note: This (rtts-1)/2 factor represents anticipated congestion window growth over this period.. it may be different for Tor, depending on CC alg. If primary_limit < secondary.cwnd - (secondary.package_window + 1), then there is enough space on the secondary circuit to send data faster than we could than waiting for the primary circuit. XXX: Note that BLEST uses total_send_window where we use secondary.cwnd in this check. total_send_window is min(recv_win, CWND). But since Tor does not use receive windows and intead uses stream XON/XOFF, we only use CWND. There is some concern this may alter BLEST's buffer minimization properties, but since receive window should only matter if the application is slower than Tor, and XON/XOFF should cover that case, hopefully this is fine. Otherwise, if the primary_limit condition is not hit, cease reading on source edge connections until SENDME acks come back. Here is the pseudocode for this: while source.has_data_to_send(): if primary.cwnd > primary.package_window: primary.send(source.get_packet()) continue rtts = secondary.currRTT / primary.currRTT primary_limit = (primary.cwnd + (rtts-1)/2)*rtts if primary_limit < secondary.cwnd - (secondary.package_window+1): secondary.send(source.get_packet()) else: break # done for now, wait for SENDME to free up CWND and restart Note that BLEST also has a parameter lambda that is updated whenever HoL blocking occurs. Because it is expensive and takes significant time to signal this over Tor, we omit this. XXX: See [REORDER_SIGNALING] section if we want this lambda feedback. 3.3. Reorder queue signaling [REORDER_SIGNALING] Reordering should be fairly simple task. By following using the sequence number field in [SEQUENCING], endpoints can know how many cells are still in flight on the other leg. To reorder them properly, a buffer of out of order cells needs to be kept. On the Exit side, this can quickly become overwhelming considering ten of thousands of possible circuits can be held open leading to gigabytes of memory being used. There is a clear potential memory DoS vector which means that a tor implementation should be able to limit the size of those queues. Luckily, [BLEST_TOR] and the form of [LOWRTT_TOR] that only uses the primary circuit will minimize or eliminate this out-of-order buffer. XXX: The remainder of this section may be over-complicating things... We only need these concepts if we want to use BLEST's lambda feedback. The default for this queue size is governed by the 'cflx_reorder_client' and 'cflx_reorder_srv' consensus parameters (see [CONSENSUS_PARAMS]). 'cflx_reorder_srv' applies to Exits and onion services. Both parameters can be overridden by Torrc, to larger or smaller than the consensus parameter. (Low memory clients may want to lower it; SecureDrop onion services or other high-upload services may want to raise it). When the reorder queue hits this size, a RELAY_CONFLUX_XOFF is sent down the circuit leg that has data waiting in the queue and use of that leg must cease, until it drains to half of this value, at which point an RELAY_CONFLUX_XON is sent. Note that this is different than the stream XON/XOFF from Proposal 324. XXX: [BLEST] actually does not cease use of a path in this case, but instead uses this signal to adjust the lambda parameter, which biases traffic away from that leg. 4. Security Considerations 4.1. Memory Denial of Service [MEMORY_DOS] Both reorder queues and retransmit buffers inherently represent a memory denial of service condition. For [RESUMPTION] retransmit buffers, endpoints that support this feature SHOULD free retransmit information as soon as they get close to memory pressure. This prevents resumption while data is in flight, but will not otherwise harm operation. For reorder buffers, adversaries can potentially impact this at any point, but most obviously and most severely from the client position. In particular, clients can lie about sequence numbers, sending cells with sequence numbers such that the next expected sequence number is never sent. They can do this repeatedly on many circuits, to exhaust memory at exits. One option is to only allow actual traffic splitting in the downstream direction, towards clients, and always use the primary circuit for everything in the upstream direction. However, the ability to support conflux from the client to the exit shows promise against traffic analysis (see [WTF_SPLIT]). The other option is to use [BLEST_TOR] from clients to exits, as it has predictable interleaved cell scheduling, and minimizes reorder queues at exits. If the ratios prescribed by that algorithm are not followed within some bounds, the other endpoint can close both circuits, and free the queue memory. This still leaves the possibility that intermediate relays may block a leg, allowing cells to traverse only one leg, thus still accumulating at the reorder queue. Clients can also spoof sequence numbers similarly, to make it appear that they are following [BLEST_TOR], without actually sending any data on one of the legs. To handle either of these cases, when a relay is under memory pressure, the circuit OOM killer SHOULD free and close circuits with the oldest reorder queue data, first. This heuristic was shown to be best during the [SNIPER] attack OOM killer iteration cycle. 4.2. Side Channels [SIDE_CHANNELS] Two potential side channels may be introduced by the use of Conflux: 1. RTT leg-use bias by altering SENDME latency 2. Location info leaks through the use of both leg's latencies For RTT and leg-use bias, Guard relays could delay legs to introduce a pattern into the delivery of cells at the exit relay, by varying the latency of SENDME cells (every 100th cell) to change the distribution of traffic to send information. This attack could be performed in either direction of traffic, to bias traffic load off of a particular Guard. If an adversary controls both Guards, it could in theory send a binary signal more easily, by alternating delays on each. However, this risk weighs against the potential benefits against traffic fingerprinting, as per [WTF_SPLIT]. Additionally, even ignoring cryptographic tagging attacks, this side channel provides significantly lower information over time than inter-packet-delay based side channels that are already available to Guards and routers along the path to the Guard. Tor currently provides no defenses against already existing single-circuit delay-based side channels, though both circuit padding and [BACKLIT] are potential options it could conceivably deploy. The [BACKLIT] paper also has an excellent review of the various methods that have been studied for such single circuit side channels, and the [BACKLIT] style RTT monitoring could be used to protect against these conflux side channels as well. Circuit padding can also help to obscure which cells are SENDMEs, since circuit padding is not counted towards SENDME totals. The second class of side channel is where the Exit relay may be able to use the two legs to further infer more information about client location. See [LATENCY_LEAK] for more details. It is unclear at this time how much more severe this is for two paths than just one. We should preserve the ability to disable conflux to and from Exit relays, should these side channels prove more severe, or should it prove possible to mitigate single-circuit side channels, but not conflux side channels. In all cases, all of these side channels appear less severe for onion service traffic, due to the higher path variability due to relay selection, as well as the end-to-end nature of conflux in that case. This indicates that our ability to enable/disable conflux for services should be separate from Exits. 4.3. Traffic analysis [TRAFFIC_ANALYSIS] Even though conflux shows benefits against traffic analysis in [WTF_SPLIT], these gains may be moot if the adversary is able to perform packet counting and timing analysis at guards to guess which specific circuits are linked. In particular, the 3 way handshake in [LINKING_CIRCUITS] may be quite noticeable. As one countermeasure, it may be possible to eliminate the third leg (RELAY_CIRCUIT_LINKED_ACK) by computing the exit/service RTT via measuring the time between CREATED/REND_JOINED and RELAY_CIRCUIT_LINK, but this will introduce cross-component complexity into Tor's protocol that could quickly become unwieldy and fragile. Additionally, the conflux handshake may make onion services stand out more, regardless of the number of stages in the handshake. For this reason, it may be more wise to simply address these issues with circuit padding machines during circuit setup (see padding-spec.txt). Additional traffic analysis considerations arise when combining conflux with padding, for purposes of mitigating traffic fingerprinting. For this, it seems wise to treat the packet schedulers as another piece of a combined optimization problem in tandem with optimizing padding machines, perhaps introducing randomness or fudge factors their scheduling, as a parameterized distribution. For details, see https://github.com/torproject/tor/blob/master/doc/HACKING/CircuitPaddingDev… Finally, conflux may exacerbate forms of confirmation-based traffic analysis that close circuits to determine concretely if they were in use, since closing either leg might cause resumption to fail. TCP RST injection can perform this attack on the side, without surveillance capability. [RESUMPTION] with buffering of the inflight unacked package_window data, for retransmit, is a partial mitigation, if endpoints buffer this data for retransmission for a brief time even if both legs close. This seems more feasible for onion services, which are more vulnerable to this attack. However, if the adversary controls the client, they will notice the resumption re-link, and still obtain confirmation that way. It seems the only way to fully mitigate these kinds of attacks is with the Snowflake pluggable transport, which provides its own resumption and retransmit behavior. Additionally, Snowflake's use of UDP DTLS also protects against TCP RST injection, which we suspect to be the main vector for such attacks. In the future, a DTLS or QUIC transport for Tor such as masque could provide similar RST injection resistance, and resumption at Guard/Bridge nodes, as well. 5. System Interactions - congestion control - EWMA and KIST - CBT and number of guards - Onion service circ obfuscation - Future UDP (may increase need for UDP to buffer before dropping) - Padding (no sequence numbers on padding cells, as per [SEQUENCING]) - Also, any padding machines may need re-tuning - No 'cannibalization' of linked circuits 6. Consensus and Torrc Parameters [CONSENSUS] - conflux_circs - Number of conflux circuits - conflux_sched_exits, conflux_sched_clients, conflux_sched_service - Three forms of LOWRTT_TOR, and BLEST_TOR - ConfluxOnionService - ConfluxOnionCircs 7. Tuning Experiments [EXPERIMENTS] - conflux_sched & conflux_exits - Exit reorder queue size - Responsiveness vs throughput tradeoff? - Congestion control - EWMA and KIST - num guards & conflux_circs Appended A [ALTERNATIVES] A.1 BEGIN/END sequencing [ALTERNATIVE_SEQUENCING] In this method of signaling, we increment the sequence number by 1 only when we switch legs, and use BEGIN/END "bookends" to know that all data on a leg has been received. To achieve this, we add a small sequence number to the common relay header for all relay cells on linked circuits, as well as a field to signal the beginning of a sequence, intermediate data, and the end of a sequence. Relay command [1 byte] Recognized [2 bytes] StreamID [2 bytes] Digest [4 bytes] Length [2 bytes] > Switching [2 bits] # 01 = BEGIN, 00 = CONTINUE, 10 = END > Sequencing [6 bits] Data [PAYLOAD_LEN - 12 - Length bytes] These fields MUST be present on ALL end-to-end relay cells on each leg that come from the endpoint, following a RELAY_CIRCUIT_LINK command. They are absent on 'leaky pipe' RELAY_COMMAND_DROP and RELAY_COMMAND_PADDING_NEGOTIATED cells that come from middle relays, as opposed to the endpoint, to support padding. Sequence numbers are incremented by one when an endpoint switches legs to transmit a cell. This number will wrap; implementations should treat 0 as the next sequence after 2^6-1. Because we do not expect to support significantly more than 2 legs, and much fewer than 63, this is not an issue. The first cell on a new circuit MUST use the BEGIN code for switching. Cells are delivered from that circuit until an END switching signal is received, even if cells arrive first on another circuit with the next sequence number before and END switching field. Recipients MUST only deliver cells with a BEGIN, if their Sequencing number is one more than the last END. A.2 Alternative Link Handshake [ALTERNATIVE_LINKING] The circuit linking in [LINKING_CIRCUITS] could be done as encrypted ntor onionskin extension fields, similar to those used by v3 onions. This approach has at least four problems: i). For onion services, since onionskins traverse the intro circuit and return on the rend circuit, this handshake cannot measure RTT there. ii). Since these onionskins are larger, and have no PFS, an adversary at the middle relay knows that the onionskin is for linking, and can potentially try to obtain the onionskin key for attacks on the link. iii). It makes linking circuits more fragile, since they could timeout due to CBT, or other issues during construction. iv). The overhead in processing this onionskin in onionskin queues adds additional time for linking, even in the Exit case, making that RTT potentially noisy. Additionally, it is not clear that this approach actually saves us anything in terms of setup time, because we can optimize away the linking phase using Proposal 325, to combine initial RELAY_BEGIN cells with RELAY_CIRCUIT_LINK. A.3. Alternative RTT measurement [ALTERNATIVE_RTT] Instead of measuring RTTs during [LINKING_CIRCUITS], we could create PING/PONG cells, whose sole purpose is to allow endpoints to measure RTT. This was rejected for several reasons. First, during circuit use, we already have SENDMEs to measure RTT. Every 100 cells (or 'circwindow_inc' from Proposal 324), we are able to re-measure RTT based on the time between that Nth cell and the SENDME ack. So we only need PING/PONG to measure initial circuit RTT. If we were able to use onionskins, as per [ALTERNATIVE_LINKING] above, we might be able to specify a PING/PONG/PING handshake solely for measuring initial RTT, especially for onion service circuits. The reason for not making a dedicated PING/PONG for this purpose is that it is context-free. Even if we were able to use onionskins for linking and resumption, to avoid additional data in handshake that just measures RTT, we would have to enforce that this PING/PONG/PING only follows the exact form needed by this proposal, at the expected time, and at no other points. If we do not enforce this specific use of PING/PONG/PING, it becomes another potential side channel, for use in attacks such as [DROPMARK]. In general, Tor is planning to remove current forms of context-free and semantic-free cells from its protocol: https://gitlab.torproject.org/tpo/core/torspec/-/issues/39 We should not add more. Appendix B: Acknowledgments Thanks to Per Hurtig for helping us with the framing of the MPTCP problem space. Thanks to Simone Ferlin for clarifications on the [BLEST] paper, and for pointing us at the Linux kernel implementation. Extreme thanks goes again to Toke Høiland-Jørgensen, who helped immensely towards our understanding of how the BLEST condition relates to edge connection pushback, and for clearing up many other misconceptions we had. Finally, thanks to Mashael AlSabah, Kevin Bauer, Tariq Elahi, and Ian Goldberg, for the original [CONFLUX] paper! References: [CONFLUX] https://freehaven.net/anonbib/papers/pets2013/paper_65.pdf [BLEST] https://olivier.mehani.name/publications/2016ferlin_blest_blocking_estimati… https://opus.lib.uts.edu.au/bitstream/10453/140571/2/08636963.pdf https://github.com/multipath-tcp/mptcp/blob/mptcp_v0.95/net/mptcp/mptcp_ble… [WTF_SPLIT] https://www.comsys.rwth-aachen.de/fileadmin/papers/2020/2020-delacadena-tra… [COUPLED] https://datatracker.ietf.org/doc/html/rfc6356 https://www.researchgate.net/profile/Xiaoming_Fu2/publication/230888515_Del… http://staff.ustc.edu.cn/~kpxue/paper/ToN-wwj-2020.04.pdf https://www.thinkmind.org/articles/icn_2019_2_10_30024.pdf https://arxiv.org/pdf/1308.3119.pdf [BACKLIT] https://www.freehaven.net/anonbib/cache/acsac11-backlit.pdf [LATENCY_LEAK] https://www.freehaven.net/anonbib/cache/ccs07-latency-leak.pdf https://www.robgjansen.com/publications/howlow-pets2013.pdf [SNIPER] https://www.freehaven.net/anonbib/cache/sniper14.pdf [DROPMARK] https://www.freehaven.net/anonbib/cache/sniper14.pdf -- Mike Perry

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staking donations
by Jeff Burdges 25 Mar '21

25 Mar '21

I realize this would be the wrong part of the Tor project to which to suggest this, but I know the people on this list and I do not know the people elsewhere in Tor, and there is some small technical content here, so.. There are now a bunch of proof-of-stake cybercoins aka crypto-currencies, like polkadot by my employer, cosmos, cardano, etc. As a rule, these have the property that people who own the currency for one reason or another can lock their currency to help deSybil some special nodes, usually called validators. This earns them some rewards because doing so usually makes them liable for misbehavior of the validator. This is very different from bitcoin where five guys in China and one guy in Island take all the rewards, and those six guys spend a lot of real money doing so. Now quite a lot of people are claiming these reward, and the real work they are doing is running one server, or even merely saying “I know that dude and he seems okay”. This means a lot of people suddenly have an income stream on which they need to pay taxes, but for which the taxes are complex. At least a few of these people do not yet stake because they do not yet know how to handle the taxes, like maybe they need time to set up a company or whatever. At least a few of these should’ve a somewhat flexible key infrastructure like I had us do in polkadot, and in particular users can simply point their rewards at another address besides the one from which they stake. If you’re a non-profit that can avoid the tax complexities, then it’d be a good time to accept donations in these new crypto currencies, because aside from regular donation some people might just skip thinking about their own taxes for a while by point their rewards at someone like Tor who’d spend the money usefully.. at least temporarily. I realize there might be other complexities of course, but if this were interesting then I could likely convince someone around here to implement some partial rewards division thing, which might enable people to contribute longer term. Best, Jeff

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Please reconsider Issue #2667 (tor over tor is much needed)
by maketorgreatagain 23 Mar '21

23 Mar '21

Good day, We am making this request to our bellowed Tor Project developers and relay operators to reconsider issue https://gitlab.torproject.org/tpo/core/tor/-/issues/2667. Tor over tor scenario is not recommended by Tor project but We are forced to use such setup to hide our operations from the powerful adversary which would in case we use bridge between our two tor path most likely possible de-anonymize us and put our life in dangerous. Tor is making our work possible and helping collaborate without restrictions, please let's reconsider this issue one more time. We are running number of relay and we considered not applying latest update to help us at least work with our own relay but adversary observes big part of network and will most likely identify us when 70% of network start dropping tor over tor connections. Tor over tor scenario does not amplify attack power within tor network, the strength of any request is losing it's power right after first hop, but for many people it provides necessary and much required privacy. We do highly sensitive work in research journalism across the globe and only way we found so far not be de-anonymized was tor over tor scenario, probably for reason that two circuits are dynamic and it's very hard for adversary to track that down properly due to nature of tor network. Understandable that you most of time support regular Tor Browser usage and 3 hop circuits but believe us there are situations where our scenario is really necessary. We have collected donations of around 21000 USD in XMR so far(we know it's not much), which we can either donate to Tor project to help fix this issue or otherwise fund relay operators to run not-up-to-date relays just to help us operate under this powerful adversary. The second is worse solution as it would make network more vulnerable, or at least allow relay operators to enable or disable that option and don't make it mandatory. Privacy for the win! Much love for Tor project and you relay operators Best regards

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Circuit Creation Madness: Anyone else still experiencing (extremely) excessive clients / (possibly) modified relays creating millions upon millions of circuits?
by William Kane 22 Mar '21

22 Mar '21

@tor-relays: Sorry for being quite noisy recently but I really need to know how many people are suffering from the same madness I am encountering right now. Quick excerpt from the log: ... Mar 22 09:48:10 <hostname_redacted> tor[pid_redacted]: Mar 22 09:48:10.000 [warn] Your computer is too slow to handle this many circuit creation requests! Please consider using the MaxAdvertisedBandwidth config option or choosing a more restricted exit policy. [12420 similar message(s) suppressed in last 120 seconds] Mar 22 09:49:10 <hostname_redacted> tor[pid_redacted]: Mar 22 09:49:10.000 [warn] Your computer is too slow to handle this many circuit creation requests! Please consider using the MaxAdvertisedBandwidth config option or choosing a more restricted exit policy. [31764 similar message(s) suppressed in last 60 seconds] Mar 22 09:50:10 <hostname_redacted> tor[pid_redacted]: Mar 22 09:50:10.000 [warn] Your computer is too slow to handle this many circuit creation requests! Please consider using the MaxAdvertisedBandwidth config option or choosing a more restricted exit policy. [104748 similar message(s) suppressed in last 60 seconds] Mar 22 09:51:10 <hostname_redacted> tor[pid_redacted]: Mar 22 09:51:10.000 [warn] Your computer is too slow to handle this many circuit creation requests! Please consider using the MaxAdvertisedBandwidth config option or choosing a more restricted exit policy. [364165 similar message(s) suppressed in last 60 seconds] Mar 22 09:52:10 <hostname_redacted> tor[pid_redacted]: Mar 22 09:52:10.000 [warn] Your computer is too slow to handle this many circuit creation requests! Please consider using the MaxAdvertisedBandwidth config option or choosing a more restricted exit policy. [509474 similar message(s) suppressed in last 60 seconds] Mar 22 09:53:10 <hostname_redacted> tor[pid_redacted]: Mar 22 09:53:10.000 [warn] Your computer is too slow to handle this many circuit creation requests! Please consider using the MaxAdvertisedBandwidth config option or choosing a more restricted exit policy. [241332 similar message(s) suppressed in last 60 seconds] ... This then goes on for a while, stopping at a few million suppressed messages / circuit creation attempts. Sorry, but 1 million circuit creation requests in just 5 minutes, there is no way that this is legitimate behavior we are seeing - this is also what was previously used to get my relay oom-killed but that I have fixed so the legitimate clients hopefully don't suffer too much anymore. If any other relay operators are encountering the same log entries or behavior, please don't hesitate to reply. Added tor-dev(a)lists.torproject.org as a CC as they might want to know about this. @tor-dev: I suspect some kind of denial-of-service attack against onion services or a more targeted attack against singular relays for guard discovery / traffic confirmation attacks. Might be smart to add some code which, if this scenario is triggered, lists offenders by hashes of their signing keys (if relay), or IP addresses (if client). There doesn't seem to be a defense against this, and the new connect() rate-limit added through ticket 40253 also won't handle this as the connection is already ACK'd and established, and a malicious relay with custom source code could do whatever it was programmed to do anyway. - William

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Regarding GSoC'21
by Aashim Garg 20 Mar '21

20 Mar '21

Hi everyone, I hope you are all doing great! I am Aashim Garg, a third-year undergrad student studying Computer Science in India.In the past I have made projects using react , next and typescript. I want to work on the OONI: Integration and unit testing of OONI probe Desktop apps . Can you please guide me on how I can start working on it and I would love to be a part of this community! Regards Aashim Garg

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