Filename: xxx-authenticated-sendmes.txt Title: Authenticating sendme cells to mitigate bandwidth attacks Author: Rob Jansen, Roger Dingledine Created: 2016-12-01 Status: open 1. Overview and Motivation In Rob's "Sniper attack", a malicious Tor client builds a circuit, fetches a large file from some website, and then refuses to read any of the cells from the entry guard, yet sends "sendme" (flow control acknowledgement) cells down the circuit to encourage the exit relay to keep sending more cells. Eventually enough cells queue at the entry guard that it runs out of memory and exits [0, 1]. We resolved the "runs out of memory and exits" part of the attack with our Out-Of-Memory (OOM) manager introduced in Tor 0.2.4.18-rc. But the earlier part remains unresolved: a malicious client can launch an asymmetric bandwidth attack by creating circuits and streams and sending a small number of sendme cells on each to cause the target relay to receive a large number of data cells. This attack could be used for general mischief in the network (e.g., consume Tor network bandwidth resources or prevent access to relays), and it could probably also be leveraged to harm anonymity a la the "congestion attack" designs [2, 3]. This proposal describes a way to verify that the client has seen all of the cells that its sendme cell is acknowledging, based on the authenticated sendmes design from [1]. 2. Sniper Attack Variations There are some variations on the attack involving the number and length of the circuits and the number of Tor clients used. We explain them here to help understand which of them this proposal attempts to defend against. We compare the efficiency of these attacks in terms of the number of cells transferred by the adversary and by the network, where receiving and sending a cell counts as two transfers of that cell. 2.1 Single Circuit, without Sendmes The simplest attack is where the adversary starts a single Tor client, creates one circuit and two streams to some website, and stops reading from the TCP connection to the entry guard. The adversary gets 1000 "attack" cells "for free" (until the stream and circuit windows close). The attack data cells are both received and sent by the exit and the middle, while being received and queued by the guard. Adversary: 6 transfers to create the circuit 2 to begin the two exit connections 2 to send the two GET requests --- 10 total Network: 18 transfers to create the circuit 22 to begin the two exit connections (assumes two for the exit TCP connect) 12 to send the two GET requests to the website 5000 for requested data (until the stream and circuit windows close) --- 5052 total 2.2 Single Circuit, with Sendmes A slightly more complex version of the attack in 2.1 is where the adversary continues to send sendme cells to the guard (toward the exit), and then gets another 100 attack data cells sent across the network for every three additional exitward sendme cells that it sends (two stream-level sendmes and one circuit-level sendme). The adversary also gets another three clientward sendme cells sent by the exit for every 100 exitward sendme cells it sends. If the adversary sends N sendmes, then we have: Adversary: 10 for circuit and stream setup N for circuit and stream sendmes --- 10+N Network: 5052 for circuit and stream setup and initial depletion of circuit windows N*100/3*5 for transferring additional data cells from the website N*3/100*4 for transferring sendmes from exit to client --- 5052 + N*166.79 It is important to note that once the adversary stops reading from the guard, it will no longer get feedback on the speed at which the data cells are able to be transferred through the circuit from the exit to the guard. It needs to approximate when it should send sendmes to the exit; if too many sendmes are sent such that the circuit window would open farther than 1000 cells (500 for streams), then the circuit may be closed by the exit. In practice, the adversary could take measurements during the circuit setup process and use them to estimate a conservative sendme sending rate. 2.3 Multiple Circuits The adversary could parallelize the above attacks using multiple circuits. Because the adversary needs to stop reading from the TCP connection to the guard, they would need to do a pre-attack setup phase during which they construct the attack circuits. Then, they would stop reading from the guard and send all of the GET requests across all of the circuits they created. The number of cells from 2.1 and 2.2 would then be multiplied by the number of circuits C that the adversary is able to build and sustain during the attack. 2.4 Multiple Guards The adversary could use the "UseEntryGuards 0" torrc option, or build custom circuits with stem to parallelize the attack across multiple guard nodes. This would slightly increase the bandwidth usage of the adversary, since it would be creating additional TCP connections to guard nodes. 2.5 Multiple Clients The adversary could run multiple attack clients, each of which would choose its own guard. This would slightly increase the bandwidth usage of the adversary, since it would be creating additional TCP connections to guard nodes and would also be downloading directory info, creating testing circuits, etc. 2.6 Short Two-hop Circuits If the adversary uses two-hop circuits, there is less overhead involved with the circuit setup process. Adversary: 4 transfers to create the circuit 2 to begin the two exit connections 2 to send the two GET requests --- 8 Network: 8 transfers to create the circuit 14 to begin the two exit connections (assumes two for the exit TCP connect) 8 to send the two GET requests to the website 5000 for requested data (until the stream and circuit windows close) --- 5030 2.7 Long >3-hop Circuits The adversary could use a circuit longer than three hops to cause more bandwidth usage across the network. Let's use an 8 hop circuit as an example. Adversary: 16 transfers to create the circuit 2 to begin the two exit connections 2 to send the two GET requests --- 20 Network: 128 transfers to create the circuit 62 to begin the two exit connections (assumes two for the exit TCP connect) 32 to send the two GET requests to the website 15000 for requested data (until the stream and circuit windows close) --- 15222 The adversary could also target a specific relay, and use it multiple times as part of the long circuit, e.g., as hop 1, 4, and 7. Target: 54 transfers to create the circuit 22 to begin the two exit connections (assumes two for the exit TCP connect) 12 to send the two GET requests to the website 5000 for requested data (until the stream and circuit windows close) --- 5088 3. Design This proposal aims to defend against the versions of the attack that utilize sendme cells without reading. It does not attempt to handle the case of multiple circuits per guard, or try to restrict the number of guards used by a client, or prevent a sybil attack across multiple client instances. The proposal involves three components: first, the client needs to add a token to the sendme payload, to prove that it knows the contents of the cells that it has received. Second, the exit relay needs to verify this token. Third, to resolve the case where the client already knows the contents of the file so it only pretends to read the cells, the exit relay needs to be able to add unexpected randomness to the circuit. (Note: this proposal talks about clients and exit relays, but since sendmes go in both directions, both sides of the circuit should do these changes.) 3.1. Changing the sendme payload to prove receipt of cells In short: clients put the latest received relay cell digest in the payload of their circuit-level sendme cells. Each relay cell header includes a 4-byte digest which represents the rolling hash of all bytes received on that circuit. So knowledge of that digest is an indication that you've seen the bytes that go into it. We pick circuit-level sendme cells, as opposed to stream-level sendme cells, because we think modifying just circuit-level sendmes is sufficient to accomplish the properties we need, and modifying just stream-level sendmes is not sufficient: a client could send a bunch of begin cells and fake their circuit-level sendmes, but never send any stream-level sendmes, attracting 500*n queued cells to the entry guard for the n streams that it opens. Which digest should the client put in the sendme payload? Right now circuit-level sendmes are sent whenever one window worth of relay cells (100) has arrived. So the client should use the digest from the cell that triggers the sendme. How shall we version the sendme payload so we can change the format of it later? Right now sendme payloads are empty. Here's a simple design: we use five bytes in the payload, where the first byte indicates the sendme payload version (0 in the original design, and 1 once we've implemented this proposal), and the rest of the payload is formatted based on the payload version number: in this case, it's simply the four bytes of digest. Is there a better way to version the payload, e.g. a way that is already standard in other parts of the design, so we aren't adding a new paint color to keep track of on the bike shed? 3.2. Verifying the sendme payload In the current Tor, the exit relay keeps no memory of the cells it has sent down the circuit, so it won't be in a position to verify the digest that it gets back. But fortunately, the exit relay can count also, so it knows which cell is going to trigger the sendme response. Each circuit can have at most 10 sendmes worth of data outstanding. So the exit relay will keep a per-circuit fifo queue of the digests from the appropriate cells, and when a new sendme arrives, it pulls off the next digest in line, and verifies that it matches. If a sendme payload has a payload version of 1 yet its digest doesn't match the expected digest, or if the sendme payload has an unexpected payload version (see below about deployment phases), the exit relay must tear down the circuit. (If we later find that we need to introduce a newer payload version in an incompatible way, we would do that by bumping the circuit protocol version.) 3.3. Making sure there are enough unpredictable bytes in the circuit So far, the design as described fails to a very simple attacker: the client fetches a file whose contents it already knows, and it uses that knowledge to calculate the correct digests and fake its sendmes just like in the original attack. The fix is that the exit relay needs to be able to add some randomness into its cells. It can add this randomness, in a way that's completely orthogonal to the rest of this design, simply by choosing one relay cell every so often and not using the entire relay cell payload for actual data (i.e. using a Length field of less than 498), and putting some random bytes in the remainder of the payload. How many random bytes should the exit relay use, and how often should it use them? There is a tradeoff between security when under attack, and efficiency when not under attack. We think 1 byte of randomness every 1000 cells is a good starting plan, and we can always improve it later without needing to change any of the rest of this design. (Note that the spec currently says "The remainder of the payload is padded with NUL bytes." We think "is" doesn't mean MUST, so we should just be sure to update that part of the spec to reflect our new plans here.) 4. Deployment Plan In phase one, both sides begin remembering their expected digests, and they learn how to parse sendme payloads. When they receive a sendme with payload version 1, they verify its digest and tear down the circuit if it's wrong. But they continue to send and accept payload version 0 sendmes. In phase two, we flip a switch in the consensus, and everybody starts sending payload version 1 sendmes. Payload version 0 sendmes are still accepted. In phase three, we flip a different switch in the consensus, and everybody starts refusing payload version 0 sendmes. (It has to be two separate switches, not one unified one, because otherwise we'd have a race where relays learn about the update before clients know to start the new behavior.) We'll want to do a bunch of testing in chutney before flipping the switches in the real network: I've long suspected we still have bugs in our sendme timing, and this proposal might expose some of them. Alas, this deployment plan leaves a pretty large window until relays are protected from attack. It's not all bad news though, since we could flip the switches earlier than intended if we encounter a network-wide attack. 5. Security Discussion Does our design enable any new adversarial capabilities? An adversarial middle relay could attempt to trick the exit into killing an otherwise valid circuit. An adversarial relay can already kill a circuit, but here it could make it appear that the circuit was killed for a legitimate reason (invalid or missing sendme), and make someone else (the exit) do the killing. There are two ways it might do this: by trying to make a valid sendme appear invalid; and by blocking the delivery of a valid sendme. Both of these depend on the ability for the adversary to guess which exitward cell is a sendme cell, which it could do by counting clientward cells. * Making a valid sendme appear invalid A malicious middle could stomp bits in the exitward sendme so that the exit sendme validation fails. However, bit stomping would be detected at the protocol layer orthogonal to this design, and unrecognized exitward cells would currently cause the circuit to be torn down. Therefore, this attack has the same end result as blocking the delivery of a valid sendme. (Note that, currently, clientward unrecognized cells are dropped but the circuit is not torn down.) * Blocking delivery of a valid sendme A malicious middle could simply drop a exitward sendme, so that the exit is unable to verify the digest in the sendme payload. The following exitward sendme cell would then be misaligned with the sendme that the exit is expecting to verify. The exit would kill the circuit because the client failed to prove it has read all of the clientward cells. The benefits of such an attack over just directly killing the circuit seem low, and we feel that the added benefits of the defense outweigh the risks. 6. Open problems With the proposed defenses in place, an adversary will be unable to successfully use the "continue sending sendmes" part of these attacks. But this proposal won't resolve the "build up many circuits over time, and then use them to attack all at once" issue, nor will it stop sybil attacks like if an attacker makes many parallel connections to a single target relay, or reaches out to many guards in parallel. We spent a while trying to figure out if we can enforce some upper bound on how many circuits a given connection is allowed to have open at once, to limit every connection's potential for launching a bandwidth attack. But there are plausible situations where well-behaving clients accumulate many circuits over time: Ricochet clients with many friends, popular onion services, or even Tor Browser users with a bunch of tabs open. Even though a per-conn circuit limit would produce many false positives, it might still be useful to have it deployed and available as a consensus parameter, as another tool for combatting a wide-scale attack on the network: a parameter to limit the total number of open circuits per conn (viewing each open circuit as a threat) would complement the current work in #24902 to rate limit circuit creates per client address. But we think the threat of parallel attacks might be best handled by teaching relays to react to actual attacks, like we've done in #24902: we should teach Tor relays to recognize when somebody is *doing* this attack on them, and to squeeze down or outright block the client IP addresses that have tried it recently. An alternative direction would be to await research ideas on how guards might coordinate to defend against attacks while still preserving user privacy. In summary, we think authenticating the sendme cells is a useful building block for these future solutions, and it can be (and should be) done orthogonally to whatever sybil defenses we pick later. 7. References [0] https://blog.torproject.org/blog/new-tor-denial-service-attacks-and-defenses [1] https://www.freehaven.net/anonbib/#sniper14 [2] https://www.freehaven.net/anonbib/#torta05 [3] https://www.freehaven.net/anonbib/#congestion-longpaths